Platinum-group-metal catalysts for proton exchange membrane fuel cells: From catalyst design to electrode structure optimization

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Platinum-group-metal catalysts for proton exchange membrane fuel cells: from catalyst design to electrode structure optimization Junbo Hou , Min Yang , Changchun Ke , Guanghua Wei , Cameron Priest , Zhi Qiao , Gang Wu , Junliang Zhang PII: DOI: Reference:

S2589-7780(19)30026-0 https://doi.org/10.1016/j.enchem.2019.100023 ENCHEM 100023

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EnergyChem

Received date: Revised date: Accepted date:

31 August 2019 11 November 2019 14 November 2019

Please cite this article as: Junbo Hou , Min Yang , Changchun Ke , Guanghua Wei , Cameron Priest , Zhi Qiao , Gang Wu , Junliang Zhang , Platinum-group-metal catalysts for proton exchange membrane fuel cells: from catalyst design to electrode structure optimization, EnergyChem (2019), doi: https://doi.org/10.1016/j.enchem.2019.100023

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Platinum-group-metal catalysts for proton exchange membrane fuel cells: from catalyst design to electrode structure optimization

Junbo Hou,a, * Min Yang,b Changchun Ke,a Guanghua Wei, c Cameron Priest,d Zhi Qiao,d Gang Wu,d, * and Junliang Zhang a, *

a

Institute of Fuel Cells, School of Mechanical Engineering, Shanghai Jiao Tong University,

Shanghai 200240, China b

Central Research Institute, Shanghai Electric Group, Shanghai 200070, China

c

SJTU-Paris Tech Elite Institute of Technology, Shanghai Jiao Tong University, Shanghai,

200240, China d

Department of Chemical and Biological Engineering, University at Buffalo, the State

University of New York, Buffalo, NY, 14260, United States

*Correspondence: [email protected] (J. H.); [email protected] (G. W.); [email protected] (J. Z.)

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Highlights 

This review provided a comprehensive understanding on fuel cell electrocatalysis in terms of cathode catalysts and electrode design.

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State of the art Pt based catalysts, carbon supports, proton conductive ionomers, and their structure effects are reviewed

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Important factors of Pt catalyst design are identified with rational understanding

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The catalyst layer structures are discussed to provide insight into the optimization of the critical three-phase interfaces.

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Electrochemistry of the Pt/ionomer interface, as well as interfacial water and sulfonate poisoning are summarized.

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Graphical abstract TOC

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Abstract Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention in the past three decades as a very promising power source for transportation applications and after tremendous effort worldwide, fuel cell vehicles are now being pushed to market. At the vehicle pre-commercialization phase, however, the performance, cost efficiency, and durability of PEM fuel cells are still in need of improvement. To improve the cells, understanding the fundamentals of fuel cell electrocatalysis provides new insight into the choice and design of more durable and better performing materials and components. State of the art Pt based catalysts, carbon supports, proton conductive ionomers, and their structure effects are discussed in this review. The primary effort made in improving fuel cells is focused on designing better cell catalysts because better catalysts increase oxygen reduction reaction (ORR) activity and durability. Low platinum-group metal (PGM) catalysts are promising in this regard. The size effect and a variety of nanostructures (e.g., core-shell, Pt skin, dealloyed, monolayer, polyhedron facets, ligand, and strain effects) are discussed comprehensively in this review to design and synthesize PGM catalysts for the cathode in PEMFCs. Using an ionomer as a binder and proton conductors in the catalyst layer, the catalyst layer structure itself, ink preparation and deposition techniques, and ink drying process are discussed as well. Due to additional local transport resistance observed in fuel cell performance, the morphology and confinement effect of the ionomer thin film are also taken into account. In addition, the electrochemistry of the Pt/ionomer interface, as well as interfacial water and sulfonate poisoning are summarized.

Keywords: Oxygen reduction reaction, PGM catalysts, carbon supports, ionomer thin film, PEM fuel cells

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■ Introduction For the past 30 years, proton exchange membrane fuel cells (PEMFCs) have attracted global attention due to their high energy conversion efficiency and environmentally friendly emissions. The first lines of fuel cell vehicles have now made it to market but despite this pre-marketization activity, large scale fuel cell vehicle deployment is still obstructed by the high initial cost and poor durability of key components in PEMFCs. In work done by Los Alamos National Laboratory 1, 2 on thin film catalyst layer fabrication, the carbon supported Pt nanoparticles blended with a Nafion ionomer were used in the catalyst layer in fuel cells. This led to an improvement in fuel cell performance and thus increased the mass specific activity of Pt significantly. Since this discovery, many efforts have been made to increase cell performance, reduce cost, and increase durability.3-10 According to the U.S. DOE’s 2020 targets for fuel cells, platinum group metal (PGM) loading should not exceed 0.125g/kW,11 which gives an MEA area loading of 0.125 mg/cm2, assuming a rated power of 1.0 W/cm2. For a 100 kW fuel cell vehicle, the PGM loading would be 12.5g, making the fuel cell competitive against a traditional internal combustion engine (ICE) which usually contains about 5-10 g PGM in the catalytic converter for the treatment of exhaust gases. At such a loading level, the fuel cell stack components are anticipated to fall within the optimum cost range stated by Strategic Analysis Inc.12 In this review, at first, the development of the timeline for Pt-based catalysts and electrode for PEMFCs is summarized in Scheme 1.13-24 Then, fuel cell electrocatalysis with low PGM loading is explored through a comprehensive discussion of the fundamentals and basic elements of the electrocatalysis process. With respect to the catalysts, several attempts have been made to decrease Pt loading and improve their mass specific activity. As a result, catalysts have been designed in large part by examining size effects, morphology and structure effects, and ligand and strain effects. Subsequently, carbon support material has also been involved in catalyst 5

design in different aspects, including basics on the Pt-support interaction, new reasoning in catalyst particle distribution containing nucleation, carbon surface properties and pore structure, and the agglomeration-aggregates-diffusion. Finally, using ionomer as the binder and proton provider in the catalyst layer, the catalyst layer structure, ink processing, cutting edge deposition techniques, and the drying process are discussed in depth.

Scheme 1. Development timeline of Pt-based catalysts and electrodes for PEM fuel cells.

■ Pt-based catalysts Why Pt-based catalysts are superior for the ORR Based on the simple dissociation mechanism of O2 adsorbing on the catalyst’s surface followed by electron transfer and protonation to form H2O step-by-step, the binding energies of the intermediates at each step can be calculated by Gibbs free energies.25 Thus, oxygen reduction reaction (ORR) activity can be plotted as function of O or OH binding energy, which gives the well-known “volcano” type plot, as shown in Figure 1.25 For the metals showing smaller binding energy than Pt, such as Ni, they bind O or OH very strongly and thus the proton transfer step becomes slow. For Au, which has a larger binding energy than Pt, O or OH bind on the surface 6

very loosely and therefore almost no transfer of protons and electrons to oxygen occurs. This is the main reason why Pt resides at the top of the volcano plot. As for the Pt based alloys, the same volcano trend is present if the d-band center is used.26, 27 Similar to the dissociation energy for the single metal catalysts, there are two opposing effects found on Pt alloy catalysts: a relatively strong adsorption energy of O2 and reaction intermediates, and a relatively low coverage by adsorbed anions.28 Thus, for Pt, which binds O2 too strongly, the rate of the ORR is limited by the availability of OHad/anion-free Pt sites. When the d-band center is too far away from the Fermi level, as in the case of Pt3V and Pt3Ti, the surface is less covered by OHad and anions, but the adsorption energy of O2 is too low and therefore limits the ORR rate.

Figure 1. Trends in oxygen reduction activity plotted as a function of both the O and the OH binding energy (A). Reproduced with permission.25 Copyright 2004, American Chemical Society; Relationships between experimentally measured specific activity for the ORR on Pt3M surfaces versus the d-band center position for the Pt-skin (B) and Pt-skeleton (C) surfaces. Reproduced with permission.26 Copyright 2007, Macmillan Publishers Limited.

Size effect At the nanoscale, Pt or Pt alloy particles are not only evenly distributed on conductive supports but also provide more available geometric surface area, and thus can possibly help reduce PGM loading. In general, the electrochemical active surface area of Pt or Pt alloys in the liquid 7

electrolytes increase proportionally with the total geometric surface area. However, the question now remains: how will the particle size of Pt or Pt alloys affect the ORR? Here the activity of the catalysts can be classified as either specific activity or mass activity.29 The former is the activity per real surface area of the catalysts, while the latter is the activity per mass. The mass activity is important when considering cost. As previously mentioned, these catalyst materials are being utilized at the nanoscale and have several different factors that affect their performance. Firstly, the particle is composed of a smaller number of atoms at this scale, and this limited number of atoms stack together to show specific crystal facets, terraces and edges. These shape features can be called geometric factors and are associated with the topography of the atom distribution of the catalyst particles. The second factor affecting catalyst performance is the electrochemical properties of the catalysts’ topography, i.e. adsorption-desorption of reactant species on facets, terraces, and edges. These can be considered as electronic factors, that basically are related to the surface electronic structure. By using the CO displacement charge at a controlled potential method, it is found that the potentials of total zero charge (PTZC) shifts are approximately -35 mV when the particle size is decreased from 30 nm down to 1 nm.30 The surface coverage with OH increases by decreasing the particle size, which was demonstrated by examining the CO bulk oxidation. Thus the ORR is hindered with regards to specific activity once the catalysts’ size decreases. Furthermore, the parallel shift in the Tafel plots indicates the same reaction mechanism on different Pt catalysts. The highest mass activity of the ORR at 2.2 nm Pt particles was found in HClO4 solutions.31 Decreasing the particle size further however has a negative effect on performance because at this lower size the edge sites make up the majority of the space in the catalyst particles. These sites show very strong oxygen binding energies, and result in the Pt nanoparticles exhibiting decreased specific activity.31 The {111} facets contribute to the high activity observed on the 2.2 nm Pt particle due to proper oxygen binding energy. Similarly, the specific activity of the oxygen reduction reaction on Pt nanoparticles was found to decrease with decreasing particle size, with a 8

maximum for particles with a diameter of 3 nm.32 The authors implemented vacuum CO temperature programmed desorption (TPD) experiments to correlate the proportion of the terrace sites with ORR activity. It seems that the active sites for the ORR are only located on the terrace sites of the nanoparticles. To further understand particle size effects on the activity of catalytic nanoparticles in an atomic-scale description of the surface microstructure, a model particle was introduced and the fractional population n of the (100), (111) and step surface sites of the particle model were correlated with the particle size33 as shown in Figure 2. The Density functional theory (DFT) model reproduces the experimentally observed trends in both the specific and mass activities for particle sizes in the range between 2 and 30 nm. The mass activity was maximized for particles of a diameter between 2 and 4 nm. In Ref,34 the effect of particle size on ORR was investigated in acid and alkaline mediums. The specific activity toward the ORR rapidly decreases in the order of polycrystalline Pt > unsupported Pt black particles (∼30 nm) > high surface area (HSA) carbon supported Pt nanoparticle catalysts (of various size between 1 and 5 nm) in all three mediums. The absolute reaction rates decrease in the order HClO4 > KOH > H2SO4, which is in line with an increasing anionic adsorption strength in the cases of the acid solutions. Simulation results show a maximum mass activity at 2.4 nm, as shown in Figure 3.34

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Figure 2. Fractional population n of the (100), (111) and step surface sites of the particle model; Insets: Model particle with truncated octahedral shape with dissolved edges and corners. Differently colored atoms correspond to active sites of different activity for the oxygen reduction reaction. Reproduced with permission.33 Copyright 2011, Elsevier.

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Figure 3. Simulation of expected SA (A) and MA (B) at 0.9 VRHE as a function of the ECSA for perchloric and sulfuric acid solutions. It is assumed that only (100) sites are active and that 30% of the particles are covered by the support. Reproduced with permission.34 Copyright 2011, American Chemical Society.

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Based on the above discussion, it can be concluded about size effects that specific activity increases with increasing catalyst particle diameter, and that the maximum mass activity appears within 2~4 nm. However, the particle size effect begins to lose its clarity as the catalyst ages, where Pt dissolution initiated by the formation of irreversible surface oxides results in dissolution/deposition and a broadening of the particle size distribution. Structural evolution under moderate reaction conditions yielded particles of a round geometry, as shown in Figure 4.35 The authors suggested mono-dispersed 7 nm cubo-octahedral Pt NPs would demonstrate a respectable tradeoff between initial mass activity and durability performance. This result is confirmed by others.36 It suggests smaller particles (2.2 and 3.5nm) undergo a dissolution/deposition process during electrochemical cycling which would broaden the size distribution, while larger particles (5.0, 6.7, and 11.3 nm) appear to be stable even after 10000 cycles.36 However, later research found Pt(2 nm)/CB maintained the highest mass activity after 30000 cycles, and implies the carbon support, narrow particle size distribution (10%) and even dispersion over the carbon surface are very important to improve the stability of the Pt NPs.37, 38

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Figure 4. STEM images of Pt nanocube (A) before and (B) after potential cycling, (C) cyclic voltammograms of Pt nanocubes (red line) and corresponding Pt(110) (blue line), Pt(100) (green line) surfaces from 0.5 M H2SO4; HRTEM images of Pt nano-octahedron (D) before and (E) after potential cycling, (F) Cyclic votammograms of Pt nano-octahedrons (blue line) compared with Pt polycrystalline nanoparticles (green line) and Pt(111) (red line) from 0.1 M HClO4. Reproduced with permission.35 Copyright 2014, Royal Society of Chemistry.

Morphology and structure effects Core-shell structures Driven by high activity and durability, the material structure at the nanoscale has been explored for Pt-bimetallic catalysts. Core-shell structures especially show superior electrocatalytic performance for the ORR. The core-shell is a general structure describing the Pt enriched surface of the catalyst particle and can be classified into three distinct forms: Pt skin, Pt mono layer and Pt nanoporous skeleton, as shown in Figure 5.39 The synthesis methods have been summarized in 13

ref29, 40, 41 as dealloying/leaching, thermal and adsorbate segregation, and deposition on seeds for all three types of core-shell structures. Additional coating seems to suppress NP agglomeration and mitigate ECSA decay.42

Figure 5. (A) Core-shell structures: Pt skin, Pt mono layer and Pt dealloyed Pt-bimetallic nanoparticles. Reproduced with permission.39 Copyright 2011, American Chemical Society; (B) Synthesis of Pt-skin, Pt-monolayer and dealloyed Pt-bimetallic nanoparticles with a core–shell structure. Reproduced with permission.29, 40 Copyright 2013, American Chemical Society and Copyright 2016, Macmillan Publishers Limited.

Pt skin Two different polycrystalline alloys of Pt 3Ni and Pt3Co with 75% Pt and 100% Pt on the surfaces were prepared in an ultrahigh vacuum (UHV). The latter is a “Pt-skin” structure and 14

produced by an exchange of Pt and Co in the surface layers.13 It was found that the activity for the “Pt-skin” on Pt3Co in 0.1 M HClO4 was enhanced to 3−4 times greater than that of pure Pt. The reason behind superior performance of the enriched Pt-skin structure is that the alloying of Pt with 3d transition metals tunes the electronic structure of catalyst surfaces. The activity correlates well with the strength of the oxygen–metal bond interaction, which in turn depends on the position of the metal d states relative to the Fermi level.43 This is a now well-known “volcano” type activity graph. The principle used for designing such catalysts is searching for surfaces those bind oxygen a little weaker than Pt. Specifically for Pt skins, the Pt d state must shift downward, thus giving improved ORR performance. To try and further advance the ORR activity, Pt-ternary alloys (Pt 3(MN)1 with M, N = Fe, Co, or Ni) were studied as electrocatalysts. These studies indicate that Pt-ternary alloys achieve higher catalytic activities than bimetallic Pt alloys and boast improvement factors of up to 4 versus monometallic Pt.44 Multi-metallic Au/FePt3 nanoparticles possess both the high catalytic activity of Pt-bimetallic alloys as well as superior durability, with a mass-activity enhancement of more than 1 order of magnitude over Pt catalysts.45 Au@Pt, Pt@Au, and Fe3O4@Au@Pt nanoparticles were synthesized and are catalytically active for ORR. Their electrocatalytic activities depended on the nanoscale spatial arrangement of the metals.46 An unsupported AuPt core-shell catalyst was prepared and demonstrated an outstanding specific activity.47 Core/shell Pd/FePt nanoparticles (NPs) were formed by controlled nucleation of Fe(CO)5 in the presence of a Pt salt and Pd NPs, and showed FePt shell-dependent catalytic properties for ORR.48 Bilayer Ru@Pt NPs were synthesized by a fine-tuned ethanol-based method, and demonstrated a high CO resistant activity towards ORR. 49 A biaxially strained PtPb/Pt core/shell nanoplate with intermetallic core and four uniform layers of Pt shell of the PtPb/Pt was prepared and could sustain 50,000 voltage cycles with negligible activity decay and no apparent structure or composition changes.50

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Pt-monolayer (ML) An effective strategy to reduce the Pt content while retaining the activity of a Pt-based catalyst is to deposit a few atomic layers of Pt atoms on top of nanoscale substrates. Through this strategy, the available active surface area of Pt catalysts is maintained but the PGM loading can decrease dramatically.15, 51-54 Furthermore, if the outermost top layers can be limited to only a few atomic layers, the activity of such layers for ORR may be significantly different when compared to the bulk materials. Pt monolayers on Au(111), Rh(111), Pd(111), Ru(0001), and Ir(111) surfaces were synthesized and investigated with regards to ORR activity.55 Through experimental data and DFT calculations, the ORR results show a volcano-type dependence on the center of metals’ d-bands. Pt-ML/Pd (111) shows improved ORR due to facilitated O-O bond breaking and hydrogenation. Later, the same group used the same core Pd (111) but varied the Pt monolayer composition by adding transition metals (Ir, Ru, Rh, Re, or Os), in which a mixed monolayer of Pt was formed with the transitional metals.52 Some of these catalysts exhibit very high activity, i.e. a 20-fold increase in Pt mass-specific activity. This drastically increased activity and stability can be explained by low OH coverage on Pt due to spacing limitations. To elaborate, the lateral repulsion between the OH adsorbed on Pt and the OH or O adsorbed on neighboring transition metal atoms other than Pt limits the OH coverage on Pt. Based on the Pt-monolayer depositions on Pd and Pd3Co nanoparticles, the thickness of the Pt shell, lattice mismatch, and particle size were studied for their effects on specific and mass activities. It was found that the enhancements in specific activity are largely attributed to the compressive strain effect which reveals the effect of nanosize-induced surface contraction on facet-dependent oxygen binding energy.56 The authors also suggested that moderately compressed (111) facets are most conducive to the oxygen reduction reaction on small nanoparticles. Based on the Pd nanocubes, the number of layers, i.e. 1-6 atomic layers of Pt, were controlled and Pd@PtnL (n = 1−6) was deposited on top of them.57 Both theoretical and experimental studies indicate that the ORR specific activity was maximized for the catalysts based on Pd@Pt2−3L nanocubes. Because of the reduction in Pt content used and the enhancement in specific activity, the 16

Pd@Pt1L nanocubes enhanced the Pt mass activity nearly three-fold when compared to the Pt/C catalyst. The DFT calculations on model (100) surfaces suggest that the enhancement in specific activity can be attributed to the weakening of OH binding, subsequently increasing the rate of OH hydrogenation. Although Pt-ML nanocatalysts have demonstrated potential for highly active behavior in low-Pt fuel cell cathodes for the oxygen reduction reaction (ORR), challenges remain in optimizing their surface and interfacial structures. The main persisting issue is that Pt-ML nanocatalysts often exhibit structural degradation and poor durability. 3.5-5nm of Pd and a Pd9Au1 alloy core/Pt monolayer were developed and no loss of platinum was observed in 200,000 potential cycles.58 Due to the interaction of Pt with Pd whether coordinated (Pd solid) or not well-coordinated (Pd mono layer) and their dissolution potentials ( UPd0 = 0.92 V vs UPt0 = 1.19 V) at different particle sizes and model structures, high stability is proven to be derived from the core that protects the shell from dissolution as shown in Figure 6.58 Later, the same group added a small amount of gold to palladium and formed highly uniform nanoparticle cores. No marked losses in platinum and gold occurred despite the fact that the dissolution of palladium was observed after 100,000 cycles between potentials 0.6 and 1.0 V or even under more severe conditions with a potential range of 0.6–1.4 V.59 Besides the influence of the core composition and structure on the Pt-ML catalyst durability, the outermost mono layer may also form an interphase with the intermediate layer and substrate material. An unsupported nanoporous catalyst with a Pt–Pd shell of sub-nanometer thickness on Au was synthesized and demonstrated stability over 100,000 cycles.60 It is revealed to be an atomic-scale evolution of the shell from an initial Pt–Pd alloy into a bilayer structure with a Pt-rich trimetallic surface, forming a uniform and stable Pt–Pd–Au alloy, as shown in Figure 6. An ordered Pt3Co intermetallic core with a 2–3 atomic-layer-thick platinum shell was synthesized and exhibited more than a 200% increase in mass activity and over a 300% increase in specific activity when compared with the disordered Pt3Co alloy nanoparticles as well as Pt/C. 61 A Pt-monolayer core–shell catalyst (PtML/Pd/C) was studied with a particular focus on high-current-density operation, and extraordinarily the Pt 17

loading was reduced to a level as low as 0.025 mgPt/cm2 without noticeable transport-related losses.62

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Figure 6. (A) Surface models for the nanoparticles and predicted dissolution potentials of a 1 ML Pt shell from nanoparticles and extended surfaces of Pt, PdCPt1, and hPd1Pt1 as a function of particle size. Reproduced with permission.58 Copyright 2010, Wiley-VH; (B) Atomically resolved elemental mapping of the surfaces of NPG–Pd–Pt10,000 and NPG–Pd– Pt30,000 electrocatalysts. a, High-resolution HAADF image of a surface region of NPG– Pd–Pt10,000. b–d, Atomically resolved elemental mapping images of a using the EDS signals of Au, Pd and Pt, respectively. Reproduced with permission.60 Copyright 2017, Macmillan Publishers Limited.

Pt nanoporous skeleton Under prolonged exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt catalysts.39 Strictly, the Pt-skin surface here is actually an electrochemical dealloyed nano-porous structure. Rough or uneven nanoporosity formation on the single catalyst particle by leaching or electrochemical dealloying could cause activity and durability loss depending on the particle size, the composition of the outer layers of the particle, and the particle’s structure after dealloying. It was revealed that nanoporosity formation in particles larger than ca. 10 nm is intrinsically tied to a drastic dissolution of Ni, 39 and, as a result, a rapid drop in intrinsic catalytic activity occurs during ORR testing. This specifically translates to severe catalyst performance degradation. Fortunately, O 2-free acid leaching can suppress this nanoporosity issue. The authors suggest that catalytic stability could be further improved by establishing control over the particle size when below ca. 10 nm and provide another solution to avoid nanoporosity. 63 Specifically, the particle size, the dealloying protocol, and post-acid-treatment annealing were investigated with the intent to identify the correlations with nanoporosity and passivation of the alloy nanoparticles. It was found that smaller size, less-oxidative acid treatment, and annealing significantly reduced Ni leaching and nanoporosity formation, resulting in improved stability and higher catalytic ORR activity. 64 Another important research topic in this area is the clarification of facet dealloying and its influence on ORR. It was demonstrated that the Pt3Ni(111) surface is 10-fold more active for the 19

ORR than the corresponding Pt(111) surface and 90-fold more active than current state-of-the-art Pt/C catalysts for PEMFCs.14 Because of the electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region, the nonreactive oxygenated species interact weakly with the Pt surface atoms and thus increase the number of active sites for O 2 adsorption. Dealloyed Pt-Cu core-shell nanoparticles were synthesized and performed 4 times better than of state-of-the art Pt electrocatalysts.65

Porous structures Bulk porous Pt-based nanostructures with highly ordered networks and narrow pore-size distributions are of particular interest for ORR due to the fact that the porous structures can not only increase the active area and provide efficient mass transfer for reactant molecules, but also improve electron mobility in the solid ligaments.66 The preparation methods have been thoroughly reviewed including template assisted, chemically and electrochemically dealloyed, surfactant assisted and assembly strategy. 66, 67 Nanoporous Pt-Fe alloy nanowires with a diameter of 10-20 nm and ligament diameter of 2-3 nm were by prepared by electrospinning and chemical dealloying techniques. It was found that non-uniform composition in the precursor PtFe5 alloy nanowires facilitated the formation of nanoporous structure, which resulted in a high specific activity 2.3 times that of conventional Pt/C catalysts as well as better durability.68 Mesoporous double gyroid (DG) platinum was synthesized by electrodeposition of Pt into a mesoporous silica film that served as a template atop a glassy carbon (GC) disk, and it exhibited a mass-specific activity within a factor of 2 of the best contemporary Pt/C catalysts.69 Nanostructured Pt and Pd monometallic and Pt xPdy bimetallic aerogels with controlled composition, very high surface area, and high porosity was synthesized and demonstrated about 1.75 times higher mass specific activity than the commercial Pt/C as well as superior durability.70

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Polyhedron-facets It has been verified and widely accepted that ORR activity is highly reliant on Pt single-crystal surface facets and it follows an order of structure-dependent ORR activity: {100} <{111} <{110}.71, 72 After the transition metals were introduced to form Pt-based bimetal, the ORR activity of the catalysts were boosted even further, i.e. PtNi and PtCo on the top of the “volcano” based d band center position of Pt relative to Fermi level. Furthermore, these bimetallic Pt based catalysts also show surface facets that have a preference for ORR, i.e. Pt3Ni {111} demonstrates an ORR activity in HClO4 in the order of {100} < {110} < {111}.14, 26, 43 Therefore, nanoparticles with shape control, specifically polyhedron facets as shown in Figure 7,72 have been synthesized and investigated regarding ORR. Three octahedral Pt xNi1−x alloy nanoparticle electrocatalysts were fabricated featuring a Pt-rich frame along their edges and corners, whereas their Ni atoms were preferentially segregated in their {111} facet region as shown in Figure 7.73 The octahedra leach preferentially in their facet centers and evolve into “concave octahedral”. The mechanism of the octahedra’s anisotropic growth and the evolution of the post-Ni leaching structure change were presented in Figure 7. In particular, a Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase. This M-rich phase is deposited at the concave hexapod surface and forms the octahedral facets.74 Although PtNi octahedra represent an emerging class of electrocatalysts,75 their durability prevents them from seeing practical use in PEM fuel cells. Pt3Ni octahedras were doped with transition metals, including vanadium, chromium, manganese, iron, cobalt, molybdenum (Mo), tungsten, or rhenium, and the ORR activity was measured for each dopant in the octahedras.76 The Mo doped Pt3Ni/C showed the best ORR performance, with a specific activity of 10.3 mA/cm2 and mass activity of 6.98 A/mg Pt, which are 81 and 73‐fold enhancements respectively when compared with the commercial Pt/C catalyst (0.127 mA/cm2 and 0.096 A/mg Pt). Theoretical calculations suggest that Mo prefers subsurface positions near the particle edges in a vacuum and surface vertex/edge sites in oxidizing 21

conditions, where it enhances both the performance and the stability of the Pt 3Ni catalyst as shown in Figure 7.

Figure 7. (A) Illustration of various 3D polyhedral configurations shown as a function of low index {100}, {111}, and/or {110} facets and high index {hkl} facets [notes: C = cube, CO = cuboctahedron, TO = truncated octahedron, O = octahedron, IO = icosahedron, RD = rhombic dodecahedron, CCC = concave cube, CCO = concave octahedron]. Reproduced with permission.72 Copyright 2018, Royal Society of Chemistry; (B) Atomic-scale Z-contrast STEM images and composition profile analysis of PtNi1.5 octahedral nanoparticles. Reproduced with permission.73 Copyright 2013, Macmillan Publishers 22

Limited; (C) Atomic structural models of octahedral Pt bimetallic alloy NCs (Pt-M; M = Ni, Co, etc.) during the solution-phase co-reduction and during the acidic ORR electrocatalysis. Reproduced with permission74 Copyright 2014, AAAS; (D) Mo doping at the edge and corner of octahedron of PtNi particles by Monte Carlo simulation, binding energies for a single oxygen atom on all fcc and hcp sites on the (111) facet and the change of binding energies once adding Mo. Reproduced with permission.76 Copyright 2018 American Chemical Society.

Nano structures Nanocage Designing a hollow structure for a Pt catalyst offers a great opportunity to enhance its electrocatalytic performance and maximize the use of precious Pt. Hollow structures can be obtained from 4 different methods: heteroepitaxial growth on templates (which is usually with flat facets i.e. cubic template), Non-epitaxial/random growth which is generally on a sphere template, Galvanic replacement, and the Kirkendall effect.16 Based on the Kirkendall effect, compact and smooth hollow Pt nanocrystals were fabricated and exhibited an enhancement in Pt mass activity for ORR.77 The authors ascribe this enhancement to the induced hollow lattice contraction and high surface area per mass. Heteroepitaxial growth of Pt layers on Pd templates and Pt nanocages with {100} (cubic nanocages) and {111} (octahedral nanocages) facets, shown in Figure 8, exhibited distinctive catalytic activity toward oxygen reduction.78 By using a similar method, Pt-based icosahedral nanocages with {111} facets and six atomic layers of Pt atoms were synthesized, as shown in Figure 8, and the catalysts show a specific activity of 3.50 mA cm–2, larger than both the Pt-based octahedral nanocages (1.98 mA cm–2) and a state-of-the-art commercial Pt/C catalyst (0.35 mA cm–2). After 5,000 cycles of accelerated durability testing, the mass activity of the Pt-based icosahedral nanocages drops from 1.28 to 0.76 A mg –1 Pt, which is still about four times greater than that of the original Pt/C catalyst (0.19 A mg–1 Pt).79 23

Ultrathin icosahedral Pt-enriched nanocages were fabricated using Pd icosahedral seeds.80 This catalyst shows extraordinary ORR activity when compared against the commercial Pt/C catalyst, having specific and mass activities that are 10 and 7 times higher respectively which outperforms the cubic and octahedral nanocages reported above.

Figure 8. TEM and HAADF-STEM of Pt cubic and octahedral nanocages (A-D). Reproduced with permission.78 Copyright 2015, AAAS; TEM and low-magnification HAADF-STEM images and EDX mapping of the Pt icosahedral nanocages (E-F). Reproduced with permission.79 Copyright 2016, American Chemical Society.

Nanoframe Starting from the crystalline PtNi3 polyhedra, the edges of the Pt-rich PtNi3 polyhedra are maintained in the final Pt 3Ni nanoframes after being immersed in a corrosive medium.17, 81 As shown in Figure 9, the nanoframe catalysts achieved enhancement factors of 36 and 22 in mass 24

and specific activities respectively. 65 Both the interior and exterior catalytic surfaces of this open-framework structure are composed of the nanosegregated Pt-skin structure, granting it enhanced oxygen reduction reaction (ORR) activity. Using a facet-controlled Pt@Ni core–shell octahedron nanoparticle allows the nanoscale phase segregation to have directionality in addition to allowing it to be geometrically controlled.82 Geometric control can be used to produce a Ni octahedron that is penetrated by Pt atoms along three orthogonal Cartesian axes and coated by Pt atoms along its edges. The selective removal of the Ni-rich phase by etching then results in a structurally fortified Pt-rich skeletal PtNi alloy nanostructure framework. Electrochemical evaluation of this hollow nanoframe suggests that the ORR activity is greatly improved compared to conventional Pt catalysts.82 PtCu NFs with nanothorns protruding from their edges were synthesized using hydrothermal methods.83 Pure Cu nanodecahedra are formed first followed by the galvanic replacement reaction between Cu nanodecahedra and Pt precursors. Finally, the co-deposition of Pt and Cu atoms are responsible for the formation of highly anisotropic five-fold-twinned PtCu NFs, which show impressive ORR activity. 83

Figure 9 (A) Initial solid PtNi3 polyhedra. (B) PtNi intermediates. (C) Final hollow Pt 3Ni nanoframes. (D) Annealed Pt3Ni nanoframes with Pt (111)-skin–like surfaces dispersed on high–surface area carbon. Reproduced with permission.65 Copyright 2014, AAAS. 25

Nanowires Single-crystalline Pt nanowires were synthesized on carbon black by chemical reduction method at room temperature in aqueous solution, and exhibited a 50% higher mass activity and three-fold higher specific activity than those of the state-of-the-art Pt nanoparticle catalyst.84 Ultra-thin PtxFey-NWs (PtxFey-NWs/C) with a diameter of 2–3 nm were successfully prepared through a solution-phase reduction method, and demonstrated both higher oxygen reduction reaction (ORR) activity and better electrochemical durability than conventional Pt/C catalysts. This is proven by examining the electrochemical surface area (ECSA) of Pt 2Fe1-NW/C which dropped to 46%, two times better than the conventional Pt/C catalyst after 1,000 cycles of 0–1.3 V.85 Ultrathin Pt monolayer shell-Pd nanowire cores maintained outstanding area and mass specific activities of 0.77 mA/cm2 and 1.83 A/mgPt, respectively even after ozone treatment. 86 Advanced organic-phase synthesis of thin FePt and CoPt alloy nanowires (NWs) was reported, and the surface specific and mass activities of the FePt NW catalysts reached 1.53 mAcm-2 and 844 mAmg-1 Pt, which are 4.7 and 5.5 times higher than those of the commercial BASF 3.2 nm Pt catalyst.87 1D platinum–cobalt alloy nanowires (PtCoNWs) were prepared by electrospinning, they exhibited a nearly 7-fold specific activity increase in comparison to commercial Pt/C catalysts, along with improved electrochemically active surface area retention through 1,000 potential cycles.88 FeNiPt/FePt core/shell NWs were prepared through a seed-mediated growth of FePt over the FeNiPt NWs and the surface profile of the core/shell NWs were controlled through acid etching and thermal annealing to be either Pt-skeleton or Pt-skin with their 1D morphology, thus demonstrating superior mass activity.89 Pt3Fe zigzag-like nanowires (Pt-skin Pt3Fe z-NWs) with stable high-index facets (HIFs) and nanosegregated Pt-skin structure were synthesized, and showed a mass activity of 2.11 A mg−1 and a specific activity of 4.34 mA cm−2 for ORR at 0.9 V versus reversible hydrogen electrode, the highest in all reported PtFe-based ORR catalysts.90 In a three step process, Pt/NiO core/shell nanowire solutions were first synthesized and then were 26

converted into PtNi alloy nanowires through a thermal annealing process. Finally the jagged Pt nanowires as shown in Figure 10 were obtained by electrochemical dealloying.18 The jagged nanowires exhibit an ECSA of 118 m2/gPt and a specific activity of 11.5 mA/cm2 for ORR, yielding a mass activity of 13.6 A/mgPt, as shown in Figure 10. This is the second highest ORR activity stated in the previous report, exceeded only by PtNi {111}. It is suggested that highly stressed, under-coordinated rhombus-rich surface configurations of the jagged nanowires yield greater ORR activity as opposed to more relaxed surfaces.

Figure 10. (A) HRTEM characterization of the J-PtNW Structure, and (B) the ORR activity comparison. Reproduced with permission.18 Copyright 2016, AAAS.

Composition effects Introducing heterogeneous atoms into Pt metal forms bimetallic or multimetallic Pt based catalysts and results in electronic changes to the Pt metal, thus modifying the chemical properties of the catalyst surface.91 Two critical effects contribute to the modification of the electronic properties of a metal in a bimetallic surface. The first is called the strain effect or geometric 27

effect (shown in Figure 11) which is usually caused by the atomic arrangement of surface atoms.92 It generally includes compressed or expanded arrangements of surface atoms. The second effect is called the ligand effect or electronic effect (Figure 11), which is introduced by the atomic proximity of two dissimilar surface metal atoms.93 It generally involves electron transfer between the two metal atoms and both strain and ligand effects affect the electronic band structure of the Pt based catalysts. By DFT calculations not considering the strain effect, it was found that the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals.93 This resulted in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces, with the magnitude of the adsorption energy decrease being largest for the early 3d transition metals and the smallest for the late 3d transition metals.93 After that, the same authors showed how the combination of strain and ligand effects modify the electronic and surface chemical properties of Ni, Pd, and Pt monolayers supported on other transition metals.94 Strain and ligand effects are shown to change the width of the surface d band, which subsequently moves up or down in energy to maintain constant band filling. By tuning the core– shell structure and composition of the catalyst, the strain effect was demonstrated experimentally.95 The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and the weakening of the chemisorption of oxygenated species. Pt5La was synthesized first and a 3.5- to 4.5-fold improvement in activity over Pt was demonstrated in the range of 0.9 to 0.87 V. The strain and ligand effects were used to understand the activity of Pt5La.96 Thermally equilibrated Pt multilayers on Ru(0001) were pseudomorphic up to a thickness of at least five atomic layers and thus subject to lateral lattice compression with respect to the structurally similar Pt(111) surface. Together with vertical ligand effects due to the underlying Ru(0001) substrate, this compression leads to weaker H ad adsorption than on Pt(111), with the first mono layer being the most weakly adsorbing. 97

PtCo/HSC and PtCo 3/HSC

nanoparticle (NP) catalysts possessed similar Pt–Pt and Pt–Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The attenuated strain and/or 28

ligand effects caused by Co dissolution were presumably counterbalanced by particle size effects with particle growth, which likely accounted for the constant specific activity of the catalysts with voltage cycling.98 It was also found that without heat treatment, the catalysts exhibited an activity increase for the ORR mainly due to a strain effect, whereas an improved performance due to a combined strain and ligand effect required thermal treatment of the catalyst. 99 Ternary Pt–Au–M (M = 3d transition metal) nanoparticles showed reduced OH adsorption energies and improved activity compared to pure Pt nanoparticles. The strain and ligand effects in nanoparticles are decoupled by density functional theory and correlated with extended Pt(111) through the ternary metal in the core which allowed for the tuning of catalytic activity through strain effects.100 By using a very clever method, a NiTi shape memory alloy was selected as the substrate to strain engineer the deposited Pt nanofilm in both compressive and tensile strained states by taking advantage of the two-way shape memory effect.101 Using a nominal Pt layer by layer deposition method, imperfect layers of Pt on Au could be obtained, on which the ORR decreased with increasing Pt thickness. By DFT calculations, the activity trend to strain, ligand, and ensemble effects were correlated. 102

Figure 11. Strain effect and its influence on d band. Reproduced with permission.92 Copyright 2005, Wiley-VCH; Ligand effect with early and later transition metals. Reproduced with permission.93 Copyright 2004, American Institute of Physics.

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Although platinum group metal (PGM)-free electrocatalysts have been explored for decades103-114 and transition metal and nitrogen co-doped carbons (M-N-C) are developed as the best PGM-free catalysts115-118, their relatively low cell performance still prevents their practical application. Thus, such a topic is beyond the scope of this review paper.

■ Catalyst supports Pt-based catalysts for PEM fuel cells have been loaded onto high surface area nanomaterials to increase the amount of electrochemically active surface area (ECSA). Various carbon materials including carbon blacks, mesoporous carbon, carbon nanotubes (CNT), and graphene have been used as support materials for fuel cell catalysts due to their high electronic conductivities and surface areas.119 Furthermore, stimulated by carbon corrosion and Pt electrochemical active surface area loss, metal oxides i.e., TiO2, WC and others are employed as the support materials and the corresponding catalysts exhibit excellent fuel cell performance and extremely high stability under accelerated stress testing conditions.120-123 A systematical and comprehensive review was given regarding non-carbon supports available in open literatures. 124 It is well-known that the non-carbon supports are usually less electronically conductive compared to carbon type supports, not chemically and electrochemically inert enough for fuel cell environments, and have comparably low surface area, which is extremely detrimental for the distribution of Pt nanoparticles on their surface, especially for the high loading of Pt supported catalysts. Due to these three disadvantages, non-carbon supports are still rarely used in practical applications. Therefore, we will limit our discussion to the carbon supports. In this section, we will explore more fundamentals and basics underneath carbon supported Pt based electrocatalysts and their application in the catalyst layer of PEM fuel cells. Firstly, Pt-support interactions will be discussed in order to investigate a possible synergistic effect for ORR and also how Pt loss relates to durability issues in PEM fuel cells. Then, we will explore the catalyst particle 30

distribution on the carbon supports, a property which significantly influences the activity and durability of carbon supported electrocatalysts. By analyzing the mechanisms and processes of nucleation and growth, surface properties such as hydrophobicity, functional groups and charge of the carbon materials, as well as the morphology and structure properties such as surface area, pore size distribution and pore volume, the factors impacting the Pt distribution on carbon will be examined extensively. Following this, we will review the agglomerates-aggregates-diffusion which not only reflects the morphology and structuring of the carbon supported Pt clusters but also plays an extremely critical role in the preparation, performance, and durability of the catalyst layer. We hope this investigation inspires deeper study and understanding on these topics which were not considered in great detail in the previous R&D, in order to help design more efficient supported catalysts. We also hope that our analysis not only serves to stimulate the design of increasingly active and durable catalysts themselves, but also to better understand the relation between materials morphology, structure and performance, and better control the progress of materials properties-processing-performance of key components and devices for PEMFCs.

Pt-support interaction Support materials strongly interact with NP catalysts and can influence catalytic activity, a claim that has been widely explored and accepted in heterogeneous catalysis.125-128 For example, the support materials can influence the mechanical stability of the NPs, modify their morphology, induce strain on the NP due to lattice mismatch, and change the electronic structure via charge transfer processes. Furthermore, the supports can play an important role in NP stability control because support materials which bind strongly to the NPs can help prevent detachment. However, this interaction between supports and catalyst NPs and the influence of supports on electrochemical activity are seldom explored regarding carbon supported Pt based catalysts that 31

see widespread use in fuel cells. It has been highlighted in a review paper that since carbon is an electronically conductive material, it could possibly raise the electronic density in catalysts, which could in turn lower the Fermi level and change the Galvani potential. For these reasons carbon supported Pt catalysts could prove valuable in improving the electrode process.129 From an electrochemical perspective, it is postulated that there is an interface between the catalyst NP and its support which forms an electrical double layer. Due to the difference in the electronic work functions of platinum (5.4 eV) and the carbon support (4.7 eV), the electron density on the platinum will increase.129 Yet, the rise in electron density is only significant if the particle size of the catalyst is comparable to the thickness of the previously mentioned electrical double layer.130 By comparing the XPS of Pt and Pt/C, it was also found that there are electron interactions between Pt and C.131 Although lacking a comprehensive and systematic investigation of the interaction between Pt based catalyst NP and carbon supports, different carbon materials could impact the particle size distribution of Pt based catalyst NPs, thus resulting in different electrochemically active surface areas of the catalyst. Various types of carbon support materials do affect ORR activity to some extent, indicating that there is a Pt-support interaction. However, how the synergy behaves and how significant contributions are made to ORR activity still require further exploration. Another issue associated with the Pt-support interaction is the stability and durability of the supported catalysts, a topic which is being deeply investigated in the fuel cell research community. To elaborate, the carbon supported Pt-based catalysts will experience degradation through Pt dissolution, particle detachment, agglomeration, Ostwald Ripening, and carbon corrosion.132 As shown in Figure 12,132 if the Pt NPs do not adhere strongly onto the carbon surface, particle detachment will occur. For the second case, Pt NPs will agglomerate through surface diffusion, a process which depends largely on the distance between two NPs and the interaction of Pt NPs with the carbon surface. The third case is Oswald Ripening, which occurs when the atoms originally deposited on the carbon surface diffuse through 2D pathways to the 32

larger NPs. These three cases of performance degradation directly support the evident importance of Pt-support interactions. Another situation worth noticing is the corrosion of the carbon under the NPs. Once this area of carbon is corroded, the Pt NPs will be lost. This kind of Pt-support interaction involves the charge transfer process and potential change at the electrochemical interface/interphase. Deep fundamental understanding of these phenomena will provide insight into more effective engineering of carbon supported Pt catalysts by improving their stability and durability.

Figure 12. Possible degradation pathways. Reproduced with permission.132 Copyright 2014, Beilstein.

Catalyst particle distribution Referring to the meaning of the term, “supported catalysts”, one might ask how these Pt nanoparticles are distributed on the supports for which the catalysts are named. In fact, the catalyst particle distribution not only determines the activity of the supported catalysts, but also significantly impacts their durability especially when transitioning from low percentage to high percentage loading such as from 20% Pt/C to 60% Pt/C. Even worse, much higher percentage loading cannot practically be achieved, a fact ascribed to the limitation between the geometric size of carbon supports and Pt particles. In this section, we will analyze more of the 33

fundamentals beneath catalyst particle distribution with regards to three main topics, nucleation and growth, surface properties of carbon supports, and their surface morphologies. Nucleation Various types of carbon materials have been utilized as supports for fuel cell catalysts. Regarding electrochemical catalysis, the first important question that arises is if there is any synergistic effect from the support materials, which has been discussed in a previous section. As the carbon support materials are introduced, the second sensible question related to the catalysis is how the catalyst NPs distribute on the carbon surface, and how the carbon support materials impact the particle size and particle size distribution of the catalyst NPs. Unfortunately, it seems that detailed systematic study in this area is sorely lacking. To answer these questions, the most important process that must be clarified is the nucleation and growth mechanism of the catalyst NPs that deposit on the carbon support materials. For example, for the scenario of metal deposition on planar substrates in UHV, nucleation determines the particle size, particle size distribution and particle distribution on the surface of oxides and carbon substrates.133-136 The nucleation and particle growth are briefly described and shown in Figure 12. The first and most important step in the nucleation is the adsorption of catalyst atoms on the surface. In order to adsorb, the atoms must become thermally accommodated to the surface or else they would be elastically scattered. After this common first step, there exist two different cases of nucleation that can occur. The first is heterogeneous nucleation, in which the adatoms are trapped at the defect sites on the surface forming nuclei for subsequent growth processes. The second is homogeneous nucleation, where a stable nucleus is generated by aggregation of at least two adatoms on regular sites. By further addition of adatoms, these nuclei will then grow. The growth mechanism of these nuclei can be split into three different modes: island or Volmer-Weber, layer plus island or Stranski-Krastanov, and layer or Frank-van der Merwe. From the surface free energy or the wetting point of view (γadatom + γadatom-substrate interface > γsubstrate), it is believed that the nuclei grow according to the Volmer-Weber mode. This might explain why supported catalysts 34

always form 3D particles and not film like structures. However, further investigation is needed to discern a detailed understanding of the nucleation process. The stable clusters and particles eventually grow from critical nuclei until a maximum number density is reached. Afterwards, coalescence happens with further growth. Referring back to the formation of Pt-based catalyst NPs on carbon surfaces, it is worth noting that the resource of adsorption atoms for this particular formation process might be quite different from the UHV preparation of supported catalysts. Shown in Figure 13, the synthesis of Pt-based catalysts in PEM fuel cells can be generally classified into four methods with respect to the state of Pt resources. The first method being that Pt precursors dispersed in solution are reduced by reductants which causes Pt to deposit simultaneously on carbon supports. The second is the impregnation method, i.e. the carbon support being mixed with the solution of Pt precursors followed by thermal heat treatment and gas reduction, where Pt precursors are in the solid state. The third method is similar to the UHV case, i.e. sputtering, PVD and so on. Here we use the term “gas” in the UHV case to describe the source of the Pt adatoms. The fourth method is quite different from the previous three, and commonly used in catalyst synthesis research groups. Nucleation and particle growth occur in the solution, and the Pt NPs become suspended in the solution in this fourth method. Then, the NPs are mixed with carbon support materials to fabricate the supported Pt based catalysts. For these four methods, the first step of nucleation, the adsorption of the Pt atom, is quite different. To authors’ knowledge so far, there are very few reports on the growth mechanism of each of these four synthesis methods. At present we cannot go into too much detail on each case because, as we used primarily simulated results to support our research, there is an absence of direct experimental data. Generally, we will provide leads for the research community to follow with the intent of inspiring further exploration and more research on understanding the fundamentals of the growth mechanism. For the “liquid” synthesis method, the carbon dispersion in the solution, the interaction of Pt precursors with the functional group on the carbon surface, the precursor concentration, the diffusion layer thickness, and Fick’s law govern 35

the adsorption. For the “solid” method, the dispersion or distribution of the Pt precursor on the carbon surface, the temperature of the heat treatment, and the reducibility of the reducing gas, are factors that might be responsible for the Pt adatom adsorption and surface diffusion of Pt atoms. Also, paying attention to the type of the Pt precursors could be useful in understanding the possible CVD effect during the reduction process. For the “gas” synthesis method, it can generally adopt the same method from the heterogeneous catalysis. As with the fourth case, it is a direct nucleation in the solution where the particle grows. The fundamentals and theory are still emerging and very scarcely understood for this method, but to the authors’ points of view, the Pt-support interaction is not as strong as the previous three methods, which does not bode well for the stability and durability of Pt-based catalysts. Another concern is that further NP growth or particle coalescence happen during the deposition of preformed Pt NPs on carbon surfaces, which will give a lower electrochemically active surface area compared to the NPs suspended in the solution. It is clear that the particle size is critically dependent on the respective nucleation conditions during the deposition process, and these conditions depend on the surface properties of the support. As indicated in Ref.133, support properties such as E, the adsorption energy of an adatom; Ed, the surface diffusion energy of an adatom; a, the adatom hop distance; f, the surface hop frequency in any direction; N, the number density of preferred nucleation (defects and surface functional groups) sites; and a number of additional nucleation and growth parameters (the limit from the adsorption step during the nucleation process) are essential growth parameters to be studied. As with the preferred nucleation sites, it is well known that there are oxygen containing groups such as carboxylic, lactone, phenol, carbonyl, anhydride, ether and quinone groups on the carbon surfaces. Especially when treated by acids or oxidants, the C-C bond will be attacked and broken, causing carbon to be bonded with the functional groups instead.137 Yet again, there still remains a lack of comprehensive investigation on the surface defects on carbon materials due to limited experimental apparatuses. Investigative and experimental limitations 36

aside, the particle size and particle size distribution of the Pt based NPs are a function of the following variables: {E, Ed, a, f, N, adsorption related diffusion}. As for the carbon or support material structures, morphology and their influence on particle distribution will be discussed below.

Figure 13. (A) Pt-nucleation on the surface; (B) Pt nucleates from different sources: “liquid” (chemical reduction); Pt nucleates from “solid” (impregnation); Pt nucleates from “gas” (UHV deposit); Pt nucleates already in the solution and mixed with carbon.

Surface properties Interaction with solvents, whether polar or nonpolar, is particularly interesting for the dispersion of carbon materials when preparing for the synthesis of supported catalysts. Because most carbon materials are hydrophobic, the surface has a very low affinity for polar solvents such as water 37

and high affinity for nonpolar solvents such as an acetone, as shown Figure 14.138 The metal precursor will be located mostly at the external surface of the carbon particle when using water, but will penetrate to the interior of the porosity when using acetone, thus leading to a more uniform distribution throughout the particle.138 This might trigger further exploration of the wetting properties of different solvents at the interface of the carbon support and the solvent. Interaction with Pt precursors was briefly discussed above from the surface functional group point of view. Another aspect of the interaction is the surface charge of the carbon in different solutions, especially with different pH values and different Pt salts. Much like how metals inserted in different solutions exhibit various surface potentials, the carbon materials will also exhibit different behavior depending on the solution, especially when the carbon materials shrink to a nanoscale. Amorphous or graphite type structures make it more complicated and difficult to evaluate the support materials. Surface property changes after chemical and heat treatments are also worth noting because these treatments will change the types and numbers of the surface functional groups and may provide more graphite structure at the carbon surface. This would introduce the effect of π electron clouds interacting with H2O and other ions.139

Figure 14. (A) Platinum concentration profiles in grains of activated carbon support impregnated with a solution of hexachloroplatinic acid. Reproduced with permission.138 Copyright 1981, Wiley-VCH; (B) the skeptic map for surface potential and ζ potential. 38

Surface area, pore size distribution, pore volume The study of cutting edge support materials for supported catalysts in PEM fuel cells still focuses heavily on carbon materials. This is because most metal oxides/carbides have low electronic conductivity and high density, resulting in dispersion difficulty when preparing supported catalysts and catalyst layer ink. Although graphite materials are very hydrophobic and surface inert, making it hard to anchor the Pt and Pt alloy nanoparticles, the fuel cell industry has found the solution in utilizing high surface area graphitized carbon (HSGC) as the support material. 140 Therefore, this section will only focus on carbon based support materials, which is believed to be helpful for paving the way in the exploration of other support materials. High surface area and well-developed porosity are essential for achieving large metal dispersions, which usually results in high catalytic activity. When dispersing Pt particles on highly microporous carbons it is found that dispersion increases linearly with an increasing number of pores in a given pore dimension at the mesopore range of 9-11 nm.139 Later, the relationship between the average Pt particle diameter and the surface area of different carbon supports (Table 1) was investigated to find that particle diameter decreased with an increase of the carbon support surface area.141 Additionally, it is important to know that for the most frequently used carbon materials for PEM fuel cell catalyst supports, the particle size is smaller than 40nm, and the surface area of the carbon materials is inversely proportional to particle size.

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Table 1. Properties of carbon blacks as catalyst supports141 S (m2g-1)

Carbon black

Type

Maker

Denka black

Acetylene black

Denkikagaku kogyo 58

40

Exp sample

Acetylene black

Denkikagaku kogyo 835

30

Conductex 975

Furnace black

Columbian carbon

250

24

Vulcan XC-72R

Furnace black

Cabot

254

30

Black pearls 2000 Furnace black

Cabot

1475

15

3950

Mitsubishi Kasei

1500

16

Furnace black

d (nm)

Further transmission electron micrograph (TEM) measurements revealed that exchanged platinum particles are extremely small (0.75-1.7 nm) and highly dispersed on the surface of the carbon black.19 The uneven Pt particle size distribution and large particle size were observed more on the smaller surface area of the carbon support material (Vulcan XC72R, shown in Figure 15A) rather than the larger one (Black pearl 2000, as shown in Figure 15B).19 The pore size distribution was then investigated on Ketjen black and Vulcan XC72R as shown in Figure 15C.20 The two carbon materials have similar pore size distributions, and the calculated BET surface areas are 890±64.3 m2/g for Ketjen Black and 228±3.57 m2/g for Vulcan XC-72 (estimated in the range of partial pressures 0.05-0.2 P/P0), while the total pore volumes are 1.02±0.15 and 0.40±0.02 cm3/g, respectively (calculated at a partial pressure of 0.98 P/P0).20 The peak volume occurs at about 55nm for both carbon materials. Because the mean diameter for Vulcan XC-72R is 30nm and the Ketjen black particles are even smaller, pores larger than the mean particle diameter should come from the grouping of these particles. As indicated in Figure 15C,20 the filling of <2 nm pores occurs in the primary carbon particles, and are thought to exist 40

between either the graphite planes of the crystallites or the edges of two crystallites. The primary carbon particles arrange into 100-300 nm agglomerates. Within these agglomerates, mesopores of diameter 2-20 nm exist. The agglomerates then coalesce into chainlike aggregates and a continuous network of pores >20 nm is formed in the interstices.

Figure 15. Transmission electron micrographs of platinum catalyst samples loaded on A) Vulcan XC72R and B) Black Pearls 2000. Reproduced with permission.19 Copyright 1995, Royal Society of Chemistry; C) pore size distribution of Vulcan XC-72 and Ketjen black, and the schematic of the primary particle, agglomerate and aggregate. Reproduced with permission.20 Copyright 2010, American Chemical Society.

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One question that arises here is, what does the structure of the primary carbon particle look like? When investigating the solvent effect on the dispersion of carbon for the preparation of the catalyst inks, a cryo-TEM was taken to image the carbon particles and agglomerates.142 It seems that there is a different contrast at the edge and center of the carbon particles. Later based on XC72 and Black Pearl 2000, the SEM morphology was examined regarding the structure changes both before and after the steam etching.143 From Figure 16, it is clear that the strongest carbon corrosion occurs at the center of the XC72 particle rather than on the surface, and the weight loss at 800 ◦C for 3 h is 28.43%. The disorderly portion found in a BP2000 particle is far more prevalent than that in an XC72 particle. This is because no bright features are apparent in the center of the BP2000 particles after etching. The weight loss of a BP2000 particle at 800 ◦C for 3 h is 54.82%. It is reasonably apparent that the significant difference between the fresh XC72 and BP2000 is that the BP2000 graphite layer planes are not as concentrically parallel to each other toward the center as opposed to the XC72 counterpart. These results suggest that the XC72 and BP2000 have differences in the structure of their primary particles, and by extension may also have different surface and center structures.

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Figure 16. TEM micrographs of the steam etched samples A-D) XC72, E-H) Black pearl 2000, A, B, E, F are original ones, C, D, G, H are after steam etched samples; B, D, F, H are magnified ones. Reproduced with permission.142, 143 Copyright 2013, American Electrochemical Society and Copyright 2010, Elsevier.

Further evidence is drawn from the decoration of Pt NPs on the different carbon supports. This is possible because Pt deposition or the distribution of the Pt NPs can indicate the carbon structure to some extent. The two commercial carbon supported catalysts, TEC10V50E and TEC10E50E from TKK, were compared and investigated using TEM tomography shown in Figure 17.144 It was found that the TEC10V50E had Pt catalytic nanoparticles located only at the substrate surface, while the TEC10E50E showed Pt nanoparticles both inside and at the surface of the carbon substrate. Small pores are visible inside the Ketjen black, and the total number of the Pt particles in the TEMT image was 433, 61 of which were the external particles. Thus, 85% of the Pt particles were inside the carbon substrates. By using SEM and TEM, the Pt particle size distributions both on the exterior or interior were studied from four different carbon supports, 43

namely c-Pt/CB, c-Pt/GCB, c-Pt/GCB-HT and n-Pt/GCB.145 Two graphitized carbon materials show that the Pt NPs only decorated on the surface of the carbon primary particles.

Figure 17. (A) TEM tomography characterization of TEC10V50E and TEC10E50E. Reproduced with permission.144 Copyright 2011, Elsevier; Pt particle-size distributions at both the interior and exterior surfaces at different catalysts: (a) c-Pt/CB, (b) c-Pt/GCB, (c) c-Pt/GCB-HT and (d) n-Pt/GCB. Reproduced with permission.145 Copyright 2013, Royal Society of Chemistry; (B) schematic structures of high surface area of carbon, i.e. black pearl and ketjen black, and XC-72R.

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Based on these results, it is very reasonable to say that high surface area carbon materials have quite a different structure of primary particles compared to XC72R, the material frequently used in fuel cells. It can also be speculated that XC72R has fractured graphite-like layers on its outer surface while the interior is amorphous. As for Ketjen black or Black Pearl carbons, their primary particle sizes are smaller than that of XC72R and their structure is less ordered, shown in Figure 17. The graphite layer of Ketjen black and Black Pearl carbons is more loose and packed with the amorphous region. This structure can provide very high surface area and higher pore volume. However, there are no dense graphite-like layers to protect the material from either steam etching or solution penetration during the Pt decoration. A large portion of the amorphous carbon region should exist in the primary particle. A recent report based on direct and quantitative 3D study of Pt dispersions on carbon supports (high surface area carbon (HSAC), Vulcan XC-72, and graphitized carbon) shows that there is quite a difference in tomography with increasing Pt loadings from 5 to 40 wt%.146 The HSAC provides the best Pt NPs distribution even at 40 wt% loading, and the severe agglomeration of Pt NPs is observed from XC72 to graphitized carbon, especially at high Pt loading (3-D TEM tomography shown in Figure 18). Pt NPs agglomeration occurs predominantly at junctions and edges of aggregated graphitized carbon particles, leading to poor Pt dispersion in the as prepared catalysts and increased coalescence during ASTs.

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Figure 18. 3D reconstructions from electron tomography of as-prepared Pt/C catalysts (segmented Pt surfaces appear in red over gray/translucent C: Catalysts with 5, 20, and 40 wt % Pt loading (left to right) are shown for (A−C) Pt/HSAC, (D−F) Pt/V, and (G−I) Pt/LSAC. Reproduced with permission.146 Copyright 2017, American Chemical Society.

Novel carbon materials are another hot topic for the supported catalysts of PEM fuel cells because of advances made in their activity and durability. Carbon nano-onion (CNO)147 and hollow graphitic spheres (HGS)148 with high surface area and precisely controlled pore structure have been developed as support materials, as shown in Figure 19. The superior ORR activity and durability of these novel carbon materials are mainly due to carbon geometry effects. Such effects include (1) its islands-by-islands configuration to isolate each Pt nanoparticle from its 46

neighbors; (2) highly tortuous void structure of the configuration to suppress Ostwald ripening; and (3) the strong curvature-induced interaction between CNO and Pt.

Figure 19. (A) Islands-by-islands for Pt/CNO VS Pt/CB, Reproduced with permission.147 Copyright 2017, American Chemical Society; (B) a representative Pt@HGS particle. Reproduced with permission.148 Copyright 2012, American Chemical Society.

Agglomerates-aggregates-diffusion As discussed above, although the primary carbon nanoparticles differ greatly, they will pack together, to an extent, in clusters of a few tens of particles that are covalently linked and considered agglomerated. Such an agglomerated structure is usually governed through Van der Waals interactions. The schematic demonstration of carbon materials with various structures and surface areas and the commonly used carbon materials for catalyst supports in PEM fuel cells are shown in Figure 20.149 The primary carbon nanoparticles will be linked through covalent bonds to form the agglomeration structure in which the agglomerations pack together to form the aggregates. Regarding the specific weight percentage of aggregates in four shape categories for various carbon blacks including spheroidal, ellipsoidal, linear and branched, there is insufficient data. There are micropores from the primary particles, mesopores within the agglomeration and/or within the aggregates, and macropores from aggregates. This particular structure of the 47

carbon prevents the formation of dead end pores, provides high structure and high surface area, and promotes good electric conductivity. Because the Pt decoration on carbon supports is usually processed by wet chemistry plus the fact that the catalyst layer deposition requires the preparation of the catalyst ink in PEM fuel cells,150, 151

it is necessary to briefly discuss the Zeta potential regarding the dispersion of the carbon

materials in the solvents and the carbon supported catalyst dispersion in the solutions. Once the carbon powder is dispersed in the liquid solvent, the primary particle surface will interact with the ions in the solvent, causing the surface charge to change. Furthermore, the previously formed agglomerations and aggregates at the dry state have a high likelihood of becoming unpacked. Here, the ζ potential will be a good index for the stabilization of the suspension. However, so far there are few reports in open literature investigating the ζ potentials of the different carbon materials with different surface areas and structures in either water, short chain alcohols, and their mixtures. The ζ potential will be very helpful in understanding the link between ink processing and the catalyst layer structures, and also bridging the gap between the dry carbon powder structure and catalyst layer morphology in PEM fuel cells.

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Figure 20. Schematic demonstration of carbon materials with various structures and surface areas; the common used carbon materials for the catalyst supports in PEM fuel cells. Reproduced with permission.149 Copyright 1999, Elsevier.

Here we will use two examples to inspire further work in this area. Stimulated by the durability of the carbon supports in PEM fuel cells, the responses of platinized Vulcan XC-72 nanoparticles to corrosion under thermal and electrochemical conditions were monitored by transmission 49

electron microscopy TEM. This work revealed four types of structural changes: i) total removal of structurally weak aggregates, ii) breakdown of aggregates via neck-breaking, iii) center-hollowed primary particles caused by an inside-out corrosion starting from the center to outer region, and iv) gradual decrease in the size of primary particles caused by a uniform removal of material from the surface.152 These structural changes took place in sequence or simultaneously depending on the competition of dynamic carbon corrosion processes. The results obtained from this work provide insight into carbon corrosion and its effects on the long-term performance and durability of fuel cells. Another research topic for us was the characterization of the catalyst ink using ultra small angle X-ray scattering (USAX) and cryogenic TEM techniques.21 The dispersion of catalyst ink depends not only on the solvent but also on the interaction of Nafion and carbon particles in the ink. The results suggest that the particle size and size distribution of the carbon supported Pt is quite different when using the different solvents and forms of Nafion. The slope P2 = 3.08 shows the radius of gyration to be 103.0 nm (206.0 nm in diameter). As apparent from the cryo-TEM image (Figure 21), which corresponds to this medium q region, some spherical carbon particles form an aggregate with a diameter of around 200 nm, like the one shown in the red dashed circle. The fractal factor of the third level is P = 2.25 with an infinite radius of gyration. This scattering level is likely attributed to the system of large agglomerates formed by the CB particles whose radius of gyration is beyond the range of the measurement, as can be seen from the cryo-TEM image.21

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Figure 21. (A-D) Gas-cell TEM micrographs showing three Vulcan XC-72 carbon aggregates and the structure evolution during the corrosion process. Reproduced with permission.152 Copyright 2010, American Electrochemical Society; (E) USAXS pattern of CB dispersion and the insets are cryo-TEM images of CB aggregates; (F) the schematic map for the CB aggregates. Reproduced with permission.21 Copyright 2017, American Chemical Society.

The gas transport mechanisms in nanopores include viscous flow, Knudsen flow, and the slip flow that is between them. Viscous flow is caused by collision between molecules. Knudsen flow is caused by collision between molecules and the nanopore wall. The Knudsen number Kn is introduced, which is defined as the ratio between the mean free path of gas molecules to the pore diameter. When Kn is larger than 10, the flow is Knudsen flow.153 Based on the Maxwell– 52

Boltzmann distribution law for molecular velocities, the free path length for O 2, N2 and H2 can be calculated as 63.3, 58.8 and 110.6 nm, respectively. Thus, when the pore size is about 6.3 nm, which is the case for the carbon primary particle and the carbon agglomerations as well as the aggregates, the Knudsen diffusion occurs within the catalyst layer in PEM fuel cells. Thus, the four possible types of pore diffusion happening in PEM fuel cells are illustrated in Figure 22.154 The first three, pure molecular diffusion, pure Knudsen diffusion, and Knudsen and molecular combined diffusion, are based on diffusion within straight and cylindrical pores that are aligned in a parallel array. The fourth involves diffusion via “tortuous paths” that exist within the compacted solid.154

Figure 22. Types of porous diffusion. Shaded areas represent non-porous solids. Reproduced with permission.154 Copyright 2007, Wiley-VCH.

■ Proton conductive Ionomer

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In fuel cell environments, the use of proton conductive ionomers like Nafion® has two main purposes, providing proton pathways and binding effects, which differentiates the ORR in PEM fuel cells from the normal rotating disk electrode based wet electrochemistry. As a result, investigations into fuel cell catalysis must examine the proton conductive ionomer. In this section, we will first present a big picture perspective of the catalyst layer introducing the structure in the presence of the ionomer, ink processing and state of the art preparation techniques, and the drying process. Then, we will narrow our scope to a focus on the ionomer thin film in the catalyst layer by analyzing the morphology of such ionomer thin film, the confinement effect from such thin film and transport issues with reduced Pt loading. We will then dig even deeper, with more focus on the Pt-ionomer interface, of which we will profile from the aspect of the electrochemical double layer. Additionally, we will probe the role of interfacial water present at the interface and summarize the poisoning effect from the sulfonates in the Nafion® ionomer. Based on our analysis and by revisiting previous literature, we hope to stimulate more interest in studying the importance of ionomer thin films in the present structure of PEM fuel cell catalyst layers. Catalyst layer The state of the art catalyst layer in current industrial fuel cells consists of a carbon supported catalyst and Nafion® ionomer. To identify the issues with the ionomer, it is believed that a matrix must be introduced. Thus, we will first picture the ionomer in the catalyst layer, summarize the morphology and structure of the ionomer in the catalyst ink, and examine the drying process which will influence the final structures in the catalyst layer. Catalyst layer structure in the presence of the Ionomer The thin film and flooded agglomerate model depicting the catalyst layer structure has been widely accepted in the fuel cell research community.155-158 The microstructure of the catalyst layer is described to be catalyzed carbon particles flooded with electrolyte, forming agglomerates 54

and covered with a thin film of electrolyte. Firstly, the reactant gas passes through the channels among the agglomerates, then diffuses through the ionomer thin film and subsequently in the agglomerates until finally reaching the reaction sites.22, 159-161 According to the extraordinary work done by LANL and Protech, the thin film based catalyst layer exhibited excellent performance, as shown in Figure 23A.1 By varying the Nafion content in the catalyst layer, the cell performance first improves and then decreases (Figure 23B), which has been well-known with an optimum Nafion content at roughly 33%.162 The interactions between constituent materials, key structural features, and their impact on transport and reaction can be observed in Figure 24. The agglomeration of the carbon supported catalysts and pores formed between these agglomerations are evident.163 While only the access ionomer is applied in the catalyst layer, the ionomer strands can be observed and the ionomer fills the pockets of the agglomerations.164 The membrane electrode assembly catalyst layer prepared from recast Nafion and carbon supported catalyst has a different pore structure, which is determined by the type of catalyst used, the preparation method, and the total Nafion content. The catalyst layer has two distinctive pore categories with a boundary of 17 nm in diameter. The specific pore volume of both the primary pores and the secondary pores decreases with increasing Nafion content.165 A bimodal pore-size distribution (PSD) is observed for the catalyst layer with 6-20 nm pores within agglomerates and larger 20-100 nm pores between aggregates of agglomerates. The ionomer is originally believed to be distributed within the secondary pores, forming a complex three dimensional network.141, 166

In addition, catalyst layers (CLs) with a range of Nafion ionomer loadings were studied in

order to evaluate the effect of ionomer on the CL microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm. These covered pores are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).20 The 3D structure of the catalyst layer was also modeled by numerical simulation167 and numerical reconstruction of CL by a dual beam Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) 168. As expected, increasing 55

the i/C ratio makes the smaller pores disappear and the number of isolated pores increase. The tortuosity of the pores and the ionomer exhibit a trade-off relationship depending on the ionomer volume. Regarding the inaccessible pores within the agglomerations and among the aggregates, the most sensitive influences to the CL structure are the effective O2 and H2O diffusivities and the effective proton conductivity. 1.2

1.0

Nafion content 14% 25% 33% 50% 66%

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Figure 23 (A) The comparison of cell performance, empty: Los Alamos National Lab thin film 0.15mg/cm2; solid: Prototech 0.3mg/cm2 + 50nm Pt, Reproduced with permission.1 Copyright 1992, Springer; (B) Cell performance by varying Nafion content. Reproduced with permission.162 Copyright 2001, Elsevier.

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Figure 24. The catalyst layer structure: (A) SEM images of the cathode catalyst layer; (B) SEM image showing the ionomer strands; (C) TEM images of the cathode catalyst layer with the membrane. Reproduced with permission.163-165 Copyright 2004, 2006, 2012, American Electrochemical Society.

Ink processing and state of the art deposition techniques Improper ink processing can cause a large agglomeration of the carbon supported catalyst, which deceases catalyst utilization and inhibits mass transport in the catalyst layer. This can result in poor performance and shorten the fuel cell’s lifetime.169 The catalyst ink for PEM fuel cells is a multi-component and multi-phase complex system, which contains carbon supported catalysts, ionomers and a mixture of solvents. This means that multiple bi-interactions exist including Pt/C-Pt/C, ionomer-ionomer, Pt/C-ionomer, Pt/C-solvent, and ionomer-solvent. Unfortunately, there has been very little research done in ink processing fundamentals. This section will present some of the few reports available in the literature that will serve to inspire further systematic explorations. The effects of carbon microstructure and ionomer loading in the catalyst ink on the catalyst layers (CLs) of PEM fuel cells were investigated.170 It is found Ketjen and Vulcan give different CL structures. When increasing ionomer content, the reaction sites are blocked by the ionomer and water as shown in Figure 25. It is found that pores <20 nm diameter facilitate water uptake by capillary condensation in the intermediate range of relative humidity. A broad pore size distribution (PSD) is found to enhance water retention in Ketjen Black-based CLs whereas the narrower mesoporous PSD of Vulcan CLs is shown to have an enhanced water repelling action.170

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Figure 25. Ionomer and water distribution in the CL with 5, 30, and 50 wt. % ionomers. Reproduced with permission.170 Copyright 2011, American Chemical Society.

The effect of the solvents on ionomer dispersion in fuel cell CL inks was studied by measuring the dispersion’s pH.171 Dispersions in water-rich solvents are more acidic than those in propanol rich solvents, which shows that the difference between 90% and 30% water dispersion can have a measured proton deviation of up to 55%. Furthermore, because electrostatic interactions are a function of pH, these differences affect how particles aggregate in solution, shown in Figure 26.171

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Figure 26. Schematic of 2D slice of potential structure representing individual chains and aggregates of Nafion, showing the side-chain orientation differences (pH differences) as a function of aggregation and solvent content. Reproduced with permission.171 Copyright 2018, American Chemical Society.

The effect of the solvent on the Pt/C agglomerates and aggregates was investigated, shown in Figure 27.172 According to the cryo-SEM images, if there is a large amount of alcohol in the catalyst ink then the number of small agglomerates increases. It was likely that the ionomer was dispersed and therefore would cover the agglomerate well. In contrast, in the case of a water-rich catalyst ink, the size of the agglomerates increased. The fabricated CLs show similar structure trends to their corresponding catalyst inks.

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Figure 27. Comparison of the catalyst ink structures and corresponding catalyst layer structures using different solvents. Reproduced with permission.172 Copyright 2015, Wiley-VCH.

The state of the art techniques used for depositing the fuel cell catalyst layer ink in order to form the catalyst layer include, but are not limited to, painting, doctor blade, ink-jet, slot-die, and spray coating (Figure 28).173 Each of these processes can be transformed into, or already exist at, a roll-to-roll scale. Each processing technique might require a particular ink viscosity which implies that the rheology of the catalyst layer ink must be considered.173 Furthermore, the ink contains the ionomer, and the macromolecule transformation of the polymer during the deposition process must be considered under either shear flow or extensional flow, which might impact the catalyst structures with regards to the ionomer. 60

Figure 28. (A) Examples of different processing methods relevant to electrode manufacturing; (B) the subsequent shear regimes exposed to the ink High shear-rate processing; (C) characteristics of spray coating applications can lead to extension flow dynamics. Reproduced with permission.173 Copyright 2017, Royal Society of Chemistry.

Drying process The solidification of the catalyst ink on the catalyst layer structure and the structural evolution of catalyst inks were investigated by contrast-variation small-angle neutron scattering.174 It indicates that a catalyst ink is formed with carbon agglomerates of core radius of ~42 nm and an ionomer shell of thickness >8 nm. Interestingly, it was discovered that both sizes of the carbon agglomerates and the thickness of the ionomer shells decrease with increasing ink concentration, a behavior that is attributed to the exclusion of solvent molecules from the carbon and ionomer agglomerates during the drying process (Figure 29).174 In addition, the coffee ring effect 175, 176 during the solidification of the catalyst ink may also require further investigation. Evaporation 61

over the film draws fluid towards the solidification front. Ahead of the drying front is a transition region where the film height, solid volume fraction, and particle/fluid velocities vary. Especially when the catalyst layer thins, the dynamics of the process must be considered.

Figure 29. Schematic illustration of the structural change of catalyst ink during the drying process. The carbon agglomerates shrink and the thickness of the Nafion shell decreases upon drying for the Nafion system. Reproduced with permission.174 Copyright 2015, Macmillan Publishers Limited.

Ionomer thin film It has been a long and arduous task for the PEM fuel cell research community to directly apply such ionomer thin films in catalyst layers, even though the agglomerate-thin film model was first introduced back in 1990.22 However, due to current advanced SEM and TEM techniques and 3D imaging software, such thin film ionomers could become much more common in fuel cell technology. Further exploration also found that such nanoscale thin film exhibits completely different properties when compared to the bulk materials, which of course indicates that the previously established viewpoints and understanding must be corrected due to these new findings 62

in ionomer film morphology and the confinement effect. Besides these two new findings, there is another problem that emerges when further reducing the Pt loading in the catalyst layer, an obstacle which seriously impedes industrial cost reduction. We reiterate that such an additional mass transport resistance is highly related to ionomer thin films. Therefore, we will comprehensively review these three issues in following discussions.

Morphology of ionomer thin film The ionomer in the catalyst layer plays two key roles: binding the catalyst powder together and delivering the proton through the entire catalyst layer. However, very little is known about the ionomer morphology and its distribution in the electrode. Electron tomography was used to reveal the 3D morphology of the Nafion thin layer surrounding the carbon particles.177 It was found that the thickness of the ionomer thin film is about 7 nm and remains unchanged even after doubling the amount of Nafion in the electrode, as shown in Figure 30.177 By material-sensitive and conductive atomic force microscopy (AFM), the distribution and size of the ionomer phase at the surface of the catalytic layer were retrieved from adhesion force mappings.178, 179 The average ionomer layer thickness varied between 7 and 13 nm for three different prepared samples and the thin film shows a lamellar structure. It was also found that the film becomes thinner after long term operation. Through a coarse-grained molecular dynamics study of the catalyst layer structure, it seems that the ionomer forms a thin adhesive film which partially covers agglomerates of Pt/carbon. Densely arranged charged side chains of the ionomer form a highly ordered array on the ionomer film surface. The preferential orientation of these charged side chains depends on the surface wetting properties of the agglomerates, which is believed to strongly correlate with the flooding phenomenon in PEM fuel cells.180 Furthermore, classical molecular dynamic simulations were conducted to explore both the molecular-level structure as well as the structure−property relationships of the ionomer film. The results demonstrate that the 63

ionomer forms irregular patches instead of a continuous film on the carbon surface. It was also discovered that the water uptake for these ionomer films is lower than the bulk ionomer membranes.180 Hydroxylation enhances the adhesion of the film relative to a pristine surface, but oxidation of the carbon support can result in partial delamination of the film. Another factor that affects the morphology of the ionomer film is the presence of Pt or PtO nanoparticles which could impact the distribution of water and the ionomer.181 To tune ionomer distribution in the catalyst layer, mixtures of dipropylene glycol (DPG) and water were used for the ionomer dispersion. Dynamic light scattering and molecular dynamics simulations demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is reduced exponentially because of the higher affinity of DPG for Nafion ionomers. The authors suggest that when the ionomer aggregates in the dispersion with a size similar to the Pt/C aggregates at a DPG content of 50 wt. %, the catalyst layer shows the best power performance.182

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Figure 30. (A) Electron tomography imaging of the Nafion layer deposited on CB in the electrodes of PEMFCs. Reproduced with permission.177 Copyright 2014, Macmillan Publishers Limited; (B) 3D current mode AFM images in the catalyst layer. Reproduced with permission.178 Copyright 2015, Elsevier; (C) Adhesive map in the catalyst layer and diagram distribution of the thin film. Reproduced with permission.179 Copyright 2016, American Chemical Society.

Confinement effect The ionomer thin film is only a few nanometers thick in the catalyst layer, and is very close to the size of the water phase domain in the bulk membrane.183 At such a scale, there might be a size confinement effect for thin films which would be significantly different from the properties of the bulk membrane. The structure, swelling, water solubility, and water transport kinetics for confined Nafion films thinner than 222 nm were studied using X-ray reflectivity, neutron reflectivity, grazing incidence small-angle X-ray scattering, quartz crystal microbalance, and polarization-modulation infrared reflection−absorption spectroscopy.24 It was found that the humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity are nearly constant and comparable to those of the bulk membrane once the films are thicker than ca. 60 nm. The effective water diffusivity is about 2 orders of magnitude smaller at 20 nm thickness than that at 200 nm, while the water volume fraction was halved after going from 200 nm to 20 nm thickness. These water property changes were further confirmed by using time-resolved in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).184 These results evidently show that the thin film has specific properties that differ from their corresponding bulk materials. By surface plasma resonance and neutron reflectivity, the water sorption kinetics were investigated in Nafion thin film.185 It is interesting to note that the asymptotic swelling ratios are 1.05, 1.26, and 1.41, and coincide with the transition points of different hydration states in the bulk Nafion. This implies that the hydration process for the thin film experiences three steps: water binding to sulfonic acid groups, the formation of sphere-like 65

ionic clusters, and bridge formation between clusters. However, the swelling is much slower in thin films than in the bulk membrane due to the mobility restriction of Nafion near the substrate. Based on the two findings above, it is safe to say there are still hydrophilic and hydrophobic domains for the hydrated thin film and the water uptake is significantly lower compared to the bulk membrane. This would mean that there is still phase segregation within the thin film and the water phase domain there should be much smaller than that in the bulk membrane. Besides the thickness of the thin film, the equivalent weight (EW) also plays a key role in influencing confinement-driven structural changes, with phase separation becoming weaker as the film thickness is reduced below 25 nm or as EW is increased, as shown in Figure 31. For the lower-EW 3M PFSA ionomers, confinement appears to induce even stronger phase separation accompanied by domain alignment parallel to the substrate.186 The influence of film thickness (70-600nm) and hydration number on proton dissociation, proton transfer from photoexcited 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS) to the surrounding solvation environment, and the local mobility of 9-(2-carboxy-2-cyanovinyl) julolidine (CCVJ) in the samples were investigated.187 The smaller water domains form in the thin film, and the confined domain size not only limits the polymer side chain mobility but also the solvation environment for accepting and transporting protons, as shown in Figure 31. This is confirmed by acidity measurements from the fluorescent molecule 2-(2’-pyridyl)benzimidazole (2PBI) and it is found that the acidity of these films is less than that of the Nafion 117 membrane at hydrated conditions.188 Later, substrate overlayer attenuated total reflection (SO-ATR) together with FTIR research189 and a solid-state 1H NMR spectroscopy study190 indicate an ordered domain structure and re-oriented H bonded network for the thin film. As a result of the water uptake, water transport, water absorption kinetics, phase separation, and domain structures, the proton conductivity for the thin film should decrease compared to the bulk materials, a prediction confirmed by impedance spectroscopy.191 Interestingly, the capacitance of the thin film is found to increase exponentially with decreasing film thickness. Referencing Young’s modulus, higher 66

values for the thin film within 60 nm are shown, and at the particular thickness that the confinement effect was observed.192 Inspired by the hot-pressing during the MEA preparation, the annealing effect on the thin film’s surface and structure was investigated by contact angle and conductivity measurements.193 A dramatic change in free-surface wettability from superhydrophilic to hydrophobic and a significant depression in proton conductivity with increasing treatment temperature were observed.

Figure 31. (A) Phase separation diagram at different thin film thickness, Reproduced with permission.186 Copyright 2016, Wiley-VCH; (B) protonation and de-protonation within bulk and thin film of Nafion. Reproduced with permission.187 Copyright 2013, American Chemical Society.

Based on the observation of bimodal surface wettability investigated by water contact angle, a multi-lamellar model was introduced.194 The sub-55 nm films were found to exhibit a hydrophilic surface whereas the thicker films showed a hydrophobic surface similar to those 67

reported for Nafion membranes. This model is shown in Figure 32.194 The model is further confirmed by another group that investigated water uptake and swelling hysteresis in a 15 nm Nafion film at different relative humidities (RH) by neutron reflectometry.195 The authors observed alternating water-rich and water-depleted layers due to a multilamellar microphase-segregated structure aligned parallel to the substrate. Furthermore, a restructuring at the near interface of the ionomer and air is speculated for the water uptake hysteresis phenomenon during heating and cooling. However, the molecular dynamics results show that the substrate has a tremendous effect on the structure of the thin film.196 Specifically, the thin film will transform from irregular inverse-capsules to a phase-separated configuration, with polymer backbone accumulating at the top as the substrate becomes more hydrophilic. Interestingly, the water diffusion coefficient has almost no change along the length of the film. However, through the film’s face, the coefficient will decrease dramatically at the top of the film and increase when close to the interface between the thin film and substrate. Deep clarification the lamellar structure of such a thin film is very important in modeling and understanding the mass transport within the catalyst layer, and will be discussed in the next section. The substrate effect on the thin film structure was also demonstrated by using carbon, Au, and Pt.197 Decreased swelling and less structural order are observed on gold for spin-cast films compared to self-assembled films, and the opposite effect is observed for films on carbon.

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Figure 32. The model for the Multi-lamellar structure of the thin ionomer film at different thickness. Reproduced with permission.194 Copyright 2013, American Chemical Society.

Transport with reduced Pt loading In order for the large scale commercialization of fuel cell vehicles to become a reality, normalized fuel cell platinum group-metal (PGM) performance must be further improved. More specifically the performance of PGM in the catalyst layer must approach the DOE’s 2020 target of 0.125 kWrated/gPGM. This means the PGM loading in the catalyst layer must be reduced to 0.125mg/cm2 assuming 1.0 Wrated/cm2. However, when further reducing the PGM loading, an additional mass transport resistance was observed alongside general mass transport effects.198-200 This additional resistance causes the fuel cell performance to deteriorate further and must be analyzed extensively if PGM fuel cell catalysts are to meet the aforementioned DOE target. This phenomenon has triggered recent work on catalyst layer transport properties in industry, with Toyota,23, 201-203 Nissan204, 205 and GM163, 206 all performing research on this topic (Figure 33A). It 69

was found that the conventional agglomerate-thin film model can’t predict the experimental data very well unless a O2 dissolution resistance from the gas phase onto the ionomer surface is introduced.201 Later, a more comprehensive model was constructed that considered the molecular diffusion in the diffusion layer, Knudsen diffusion in the diffusion layer and the catalyst layer, dissolution and transport in the thin film covering the agglomerates, and diffusion within the agglomerates. It was found that the ionomer-film resistance is dominant, especially at lower temperatures and lower Pt loadings. However, the calculated film properties through the ionomer suggest that the resistance is even higher.23 Applying this model along with the reduced effective ionomer film area (aioneff), either due to catalyst degradation202 or the reduced Pt loading in the catalyst layer,203 successfully explains the previously mystifying experimental results. It was concluded that the hindrance of oxygen transport in the catalyst layer is due to slow oxygen dissolution at the gas/ionomer interface, rather than the presence of large agglomerates. The research group from Nissan introduced two factors into the transmission line model including Knudsen diffusion resistance within secondary pores in the catalyst layer and the local transport resistance into ionomer thin film towards a Pt surface. This proved successful and the Nissan group accurately predicted the cell performance with reduced Pt loading.204 Later the Nissan group varied the EW values of ionomers and the Pt loading in the catalyst layers and then compared the results with models and previous experiments. Their findings revealed that the local transport resistance did not arise from the diffusion within the thin ionomer film. Rather, the local transport resistance is related to interfacial phenomena of the ionomer such as the coverage of absorbed water on the film surface, the oxygen dissolution rate in the ionomer, or the blockage of the Pt surface by sulfonic acid groups. This significant performance loss due to transport limitations at the Pt surface was also confirmed by research done at GM.163, 206 GM concluded that the performance loss associated with low cathode Pt loading stemmed from oxygen flux through the gas/ionomer interface to the Pt surface, a claim that is supported by two review papers.207, 208 The catalyst layer structure is sketched in Figure 34 and shows that the 70

additional local transport resistance stems from the ionomer thin-film surrounding the catalyst sites where confinement and substrate interactions dominate. Figure 34 shows the work performed by the GM researchers in which they measured the Non-Fickian O2 transport resistance from different membrane electrode assemblies (MEA) with various catalyst loadings. This local transport resistance is highly related to the ionomer thin film, because nanostructured thin film (NSTF) electrodes without ionomers in their catalyst layers do not show additional mass transport resistance whereas other electrodes of similar electrochemically active surface areas with ionomers show a large transport hindrance. However, as long as the electrochemical active surface area is larger than 50 m2/gPt and assuming the PGM loading is 0.1mg/cm2, there are no additional transport effects regardless of the type of the catalysts and their related weight ratios. This implies that the structuring in the catalyst layer also plays an important role for such transport resistance. Further understanding the confinement effect of the ionomer thin film and transport properties in alternative ionomers will be instrumental in solving the issue of local transport resistance with reduced Pt loading. From the interface point of view, there exist three resistances to transport: gas-ionomer interfacial resistance, diffusion resistance within the ionomer thin film, and ionomer-Pt interface resistance. The former interfacial resistance dominates the O2 solubility in the ionomer and the latter interfacial resistance introduces the interfacial permeation resistivity. Based on electrochemical measurements and molecular dynamics investigation, it is evident that the interfacial resistance, especially the interfacial resistance at the Nafion-Pt interface, is the factor limiting O2 transport. Additionally, it was found that the interfacial resistance is equivalent to 30–70 nm of the Nafion film, a value far exceeding the 7 nm observed in the catalyst layer.209, 210 Alternatively tuning the interaction of the ionomer with carbon supported Pt and thus modifying the structuring effect in the catalyst layer is also being explored to improve fuel cell performance. Recent work has revealed that, by functionalizing carbon surface with –NHx groups, the ionomer can be homogeneously distributed throughout the catalyst layer thanks to the coulombic attraction between the sulfonate anions of 71

the ionomer and NHx surface groups, leading to high H2/air performance.211 By carefully depositing Pt at different locations on the carbon surface, the interaction of the ionomer thin film with Pt particles can be fine-tuned and causes the local transport resistance to change accordingly.212 An alternative method is to choose a carbon support material with a suitable pore size distribution in which the internal pores are large enough to host Pt particles, protecting them from direct contact with the ionomer while still allowing reasonable access to protons and O2. Such a carbon support material is screened out and provides about 1.9A/cm2 @0.67V with a Pt loading of 0.125mg MEA/cm2, as shown in Figure 33B.213, 214

Figure 33 (A) I-V curves for different cathode Pt loading indicating there is additional mass transport effect at high current density region by Nissan, Reproduced with permission. 204 Copyright 2011, Elsevier; (B) Fuel cell performance comparison of PtCo catalysts on different carbons. Anode Pt loadings were 0.025 mgPt/cm2, and various cathode Pt loadings by General Motor. Reproduced with permission.214 Copyright 2018, American Chemical Society.

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Figure 34. (A) The catalyst layer structure and the transport therein and the local transport to a Pt site through the ionomer film, Reproduced with permission.207 Copyright 2014, Royal Society of Chemistry; (B) Non-Fickian O2 transport resistance (RNF) as a function of total Pt area on an MEA cathode. Reproduced with permission.208 Copyright 2016, American Chemical Society.

Pt-ionomer interface Strictly speaking, the electrochemical double layer in 3D porous electrode might be quite different from the traditional theories of electrochemical double layers based on planar models. With the solid state of ionomer involved, it further complicates the interface. In this section, we will cover this topic of the electrochemical double layer from three points: the Pt/ionomer interface, interfacial water, and sulfonate poisoning which we believe to be highly related to fuel cell catalysis. Pt/ionomer interface Unlike the liquid solution that is commonly used in laboratories to characterize the activity of the electrochemical catalysts, the electrolyte/ionomer actually implemented in fuel cells is a solid. A solid electrolyte/ionomer is necessary, because a solid form is able to conduct protons and bind powders to form the catalyst layer structures. Such an interface is quite different from the aforementioned two-phase wet chemistry interface. Understanding the interface between Pt 73

based catalysts and solid thin ionomers will better illuminate the underlying principles behind the electrochemical processes within PEM fuel cells, leading to more efficient strategies in materials and structure design. Using neutron reflectometry, smooth idealized layers of Nafion on glassy carbon (GC) and Pt surfaces were used to mimic interactions affecting the polymer electrolyte fuel cell (PEFC) materials that comprise the triple-phase interface.215 Multilayer hydrophobic and hydrophilic domains formed within the Nafion layer and a hydrophobic Nafion region formed adjacent to the Pt film. However, when Nafion was in contact with a PtO surface, the Nafion at the Pt interface became hydrophilic. Next, the electrochemistry of the Pt-Nafion interface was examined using cyclic voltammetry measurements on Nafion-free and Nafion-covered Pt(100), (110), and (111) single crystal surfaces in 0.1 M HClO4 and 0.05 M H2SO4.215 Considering the adsorbing species from Nafion and the effect of counter cations on the sulfonate anion adsorption, a three-phase “spring model” was introduced to describe the interface,216 as shown in Figure 35. There are three interactions governing sulfonate adsorption on the metal electrode including Pt-anion interaction through either electrostatic or covalent action, non-covalent cation-anion interactions within Nafion, and covalent interactions from sulfonate side chains. Using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, sulfonate anions, water molecules and hydrated protons were identified as the adsorbed species at the interfaces of Nafion−Pt/C and Nafion−Pt 3Co/C.216 With increasing potential, the sulfonate anions electrochemically adsorb on the catalyst surface and expel the H+(H2O)n and H2Oad originally adsorbed at less positive potentials.217 The three phases influence not only the interface but also the metal electrode itself. By surface X-ray scattering measurements on the Nafion-Pt (111) interface, it was found that once Pt(111) is positively biased to form Pt oxide, the Nafion thin film will detach from the Pt surface and oxygen atoms will penetrate the Pt lattice.218

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Figure 35. Schematic to explain the “spring model” for the three phase Pt-ionomer interface. Reproduced with permission.216 Copyright 2010, American Chemical Society.

As discussed previously, the confinement effect occurs when the ionomer film becomes very thin. The strong interaction between sulfonate anions and Pt may change the morphology of the ionomer thin film and thus change the properties at the interface where proton conduction, water transport, and O2 transport occur. Atomic force microscopy coupled with an electrochemical (e-AFM) technique reveals that proton conduction decreases with film thickness as well as that Pt substrates exhibit higher proton conduction levels comparable to that of GCs. Using Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS), it was discovered that the presence of Pt improves proton conductivity when compared to that on the SiO2 substrate.219 Vibrational sum frequency generation (VSFG) spectroscopy shows that sulfonate anions cover the Pt surface within a 5 nm region in the thin film. Above this region however, the sulfonate anions demonstrate a random orientation.220 Investigating molecular dynamics indicates that there is an ultra-dense adhesive ionomer layer with a thickness of 0.5 nm close to the interface. Water molecules in the thinner films are found to possess little to no mobility as a result of weak hydrophilic channel connectivity.221 Another finding that resulted from the 75

molecular dynamics study is that on hydrophilic substrates like Pt, surface water will flood their surfaces and the sulfonate anions will accumulate on top of the water, with the hydrophobic backbone facing upward. Conversely, on the hydrophobic substrates Nafion backbones contact the substrate with the sulfonate anions toward the water domain.222 Interfacial water There are actually two important interfaces at which the water can interact with the ionomer thin film. The first is at the Pt-ionomer interface, which involves the hydration process for the ionomer thin film under the confinement effect as well as the role of interfacial water in the three-phase electrochemical interface. The second is at the ionomer-gas interface, which deals with how the water initially contacts the ionomer thin film as well as hydration equilibrium processes. Considering the structure of the catalyst layer, it is highly possible for the ionomer thin film deposited on agglomerates to contact exposed Pt particles on adjacent agglomerates. To elaborate, this means there is no strong interaction or confinement effect from these Pt particles. However, if water exists on the top of the ionomer thin film it will be involved in the electrochemical interface for the Pt particles from the adjacent agglomerates rather than the Pt particles under the ionomer thin film. So far there are very few reports in the literature on this topic. Thin films (∼25−250 nm) of sulfonated Radel (S-Radel) were investigated with the intent of understanding thickness and hydration effects on the properties of the ionomer thin film. It was found that when the thin sample is hydrated, the film density and interfacial water volume fraction increased significantly,223 as shown in Figure 36. The samples with lower thickness anti plasticized, indicating poor water−polymer mobility inside the film. Regarding the water contact with the Nafion, density functional theory (DFT) calculations demonstrate that hydrogen atoms at the water−ionomer interface hydrogen bond to the hydrophilic sulfonate group as well as to the hydrophobic fluorinated backbone.224 Also, in the hydrophilic region, the water molecules orient one O−H group such that it interacts strongly with the sulfonate anions and other groups toward other surrounding water molecules through hydrogen bonding.225 Regarding the 76

interfacial water on top the ionomer thin film, experiments showed that the addition of 130 mg of Nafion into 10 ml of water changed its pH from 6 to 3.7 within 7 minutes, thus protonating the water.226

Figure 36. Water accumulates at substrate interface for the ionomer thin film. Reproduced with permission.223 Copyright 2018, American Chemical Society.

Sulfonate poisoning It was demonstrated that the sulfonate anions of Nafion are specifically adsorbed on Pt based electrocatalysts, and the strength of the Pt–sulfonate interactions follows the trend Pt(111) > Pt(110) > Pt(100).216, 227 This adsorption on the catalysts is very important because the ORR is always inhibited by the presence of ionomers at the electrode surface.216, 227 Further evidence of the adsorption was acquired using a solid-state cell with a Pt single crystal electrode and a dehydrated ionomer, which caused the anion-adsorption/desorption peaks to shift to lower 77

potentials.228 It is also important to note that, under dry conditions, sulfur poisoning is possible. In situ S-K XANES techniques and DFT calculations show that dry conditions are accompanied by the irreversible decomposition of a small amount of the sulfonic acid group into atomic sulfur, which in turn could be adsorbed onto the Pt surface.229

■ Conclusions and perspectives Current cutting edge electrode structures in PEM fuel cells consist of carbon supported Pt based catalysts as well as ionomers. Although the catalyst layer is already very thin, it still requires the ionomer to enforce the 3D structure and provide proton pathways. To accomplish this 3D structuring, while also fulfilling the electron, proton and gas transport pathways and achieving high performance of the catalyst layer, which is significantly critical to PEM fuel cell performance and durability, fuel cell catalysis from catalyst design to electrode structure optimization relies on three key topics that are comprehensively discussed in this review. Pt-based catalysts have attracted great interest in the research community and currently, our understanding of them seems quite sophisticated. The Pt based particles have to shrink to nanoscale, with the optimum size of 2-4 nm giving the highest mass specific activity. To further decrease catalyst cost, the core-shell, porous structures, polyhedron-facets and novel nano architectures are the main focuses for research. Composition effects including ligand and strain effects may require further exploration and could be applied to designs such as binary or triple Pt based catalysts. It is widely accepted that the performance with respect to mass- and specificactivities, the cost which can be manifested in mass activity, and overall catalyst durability are the three main considerations for developing novel fuel cell catalysts. For future work, perhaps more consideration could be given to durability, especially in a real PEM fuel cell environment under practical working conditions. More fundamental concepts of Pt-based catalysts might be awaiting further exploration regarding the nucleation and growth of such nanoparticles, the 78

interaction of Pt based nanoparticles with their supports, and the control strategies of Pt particle size distribution. Carbon supports which are commonly used in industry, do not seem to show very significant synergistic effects for ORR. However, it is undeniable that the carbon surface and structure properties play a significant role in determining the arrangement and distribution of Pt catalyst nanoparticles. There is still a lack of fundamental understanding of nucleation and growth mechanisms for the catalyst particles that deposit on the carbon surface and as a result, further investigation on how the surface functional groups, defects, and graphite edges influence the distribution of catalyst particles is necessary. Additionally, the carbon particle structure itself, the surface area, pore size distribution, and pore volume, make the system even more complicated. Figuring out how to design a carbon supported catalyst that offers more available active sites in solid ionomer thin film rather than a liquid electrolyte is tremendously important for PEM fuel cell performance and durability, not to mention its cost. Furthermore, the carbon particles or carbon supported catalysts will form agglomerations and aggregates either at in a dry state, or in solution during the catalyst deposition process or ink preparation. Therefore, the material structure and processing add another level of difficulty in controlling and tracking the relationship between the material properties of carbon supported catalysts and the performance of the catalyst layer. It would be exceptionally interesting to find a correlation between each processing step and a corresponding material morphology change reflective of the component properties. Ionomer in the catalyst layer received a little attention regarding the specific morphology and distribution of such a thin film, though it has been accepted that the Nafion thin film is roughly 7nm thick. For this topic, most findings are still based on Nafion, a material discovered five decades ago whose utility continues to surprise researchers. However, the morphology of the ionomer suspension, ionomer-ionomer interactions, ionomer-carbon interactions, and ionomer-Pt interactions are still unclear. The morphology of these interactions and ionomer suspension are 79

highly related to ink processing, the drying process, and the formation of the catalyst layer. Again, similar to the carbon supports, the material properties and morphology-processing-component structure and performance still desperately require further delving and investigation. The confinement effect for the ionomer thin film has been accepted to cause the water uptake, water sorption kinetics, water transport, proton transport, O2 transport, multi-layered or domain structure, and the thin film mechanical properties et al to be quite different from the bulk material. It seems that the substrate will also influence the ionomer thin film structure. In this area, it is projected that correlation between the electrochemical and transport properties regarding the morphology and structure of the ionomer thin film is still sorely needed. The three-phase spring model simplifies the electrochemical interface of Pt-Nafion but more effort is required to obtain a detailed understanding of the electrochemical interface with different ionomers and Pt based catalyst surfaces. Interfacial water might be an interesting topic not only because the ionomer needs hydration to conduct protons, but also because it might be beneficial in increasing the number of active sites once it is protonated.

■ Acknowledgement This work is supported by the National Natural Science Foundation of China (21503134, 21406220), the National Key Research and Development Program of China (2016YFB0101201), and the Science and Technology Commission of Shanghai Municipality (15YF1406500). G.W. thanks the support from U.S. Department of Energy, Office Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program.

■ Author contributions

80

Junbo Hou and Min Yang wrote the original draft, and contributed equally to this review. Changchun Ke, Guanghua Wei, Cameron Priest, Zhi Qiao, Gang Wu, and Junliang Zhang reviewed and edited the manuscript.

■ Conflict of interest The authors declare no conflict of interest.

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■ AUTHOR BIOGRAPHIES

Junbo Hou received his B.S. from Harbin Institute of Technology in 2003 and Ph.D. from Dalian Institute of Chemical Physics in 2008. After two postdocs in University of Leoben, Austria and Virginia Tech, USA, in 2013 he was invited to join a Caltech start-up SAFCell, Inc as a Fuel cell scientist. In 2017, as a Principle engineer he became a co-founder of a fuel cell start-up, CEMT in China. In 2018, he was appointed as an Associate Professor in Shanghai Jiao Tong University. He is experienced in fuel cell technology, electrochemistry, energy storage and conversion. 102

Min Yang obtained her Ph.D degree from Dalian Institute of Chemical Physics in 2008. After staying in University of Leoben, Austria as a Postdoc researcher for two years, she came to Virginia Tech, USA as a staff. In 2016, she was invited to join CEMT, China as a Vice Chief Engineer. In 2019, she became a Chief engineer in Central Research Institute, Shanghai Electric Group. Both from Academic and Industrial, she holds a lot of know-how in applied science and critical technology of fuel cells like SOFC, PEMFC, SAFC etc and batteries like Li-ion and Lead acid batteries.

Changchun Ke is an Associate Professor in Shanghai Jiao Tong University. He received his B.S. from Harbin Institute of Technology and Ph.D. from Dalian Institute of Chemical Physics in 2006 and 2012, respectively. His research focuses on fuel cell technology, singlet Oxygen, electrochemical regeneration and cycling, electrocatalysts.

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Cameron Priest is currently a master's student in chemical engineering at the University at Buffalo. He obtained his bachelor's degree in chemical engineering at the University at Buffalo in 2018. His primary research interests include electrocatalysis for energy conversion applications and materials electrochemistry for fuel cells.

Gang Wu is an associate professor in the Department of Chemical and Biological Engineering at the University at Buffalo, The State University of New York (SUNY-Buffalo). He completed his Ph.D. studies at the Harbin Institute of Technology in 2004 followed by extensive postdoctoral trainings at Tsinghua University (2004–2006), the University of South Carolina (2006–2008), and Los Alamos National Laboratory (LANL) (2008–2010). Then, he was promoted as a staff scientist at LANL. He joined SUNY-Buffalo as a tenure-track assistant professor in 2014 and was early promoted as a tenured associate professor in 2018. His research focuses on functional materials and catalysts for electrochemical energy technologies. He has published more than 200 papers with citation of 18000 to date. He was ranked as a Highly Cited Researcher by Thomson Reuters, Clarivate Analytics in 2018 and 2019. 104

Junliang Zhang is Zhiyuan Chair Professor, Director of Institute of Fuel Cells, Executive Deputy Dean of Zhiyuan College at Shanghai Jiao Tong University (SJTU). He received his BS and MS from SJTU in 1994 and 1997, respectively, and his PhD from State University of New York at Stony Brook in 2005. He then worked as a Research Associate till 2007 at Brookhaven National Laboratory of US Department of Energy in New York. From 2007-2011, Dr. Zhang worked at General Motors Global Research & Development, Electrochemical Energy Research Laboratory in New York, as a Research Scientist and later a Senior Scientist and Team Leader. In 2011, he joined the Institute of Fuel Cells at SJTU.

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