cathode of PEMFC via magnetron sputtering

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Optimization of ionomer-free ultra-low loading Pt catalyst for anode/cathode of PEMFC via magnetron sputtering  nek a,*, Martin Dubau a, Peter Ku´s a, Anna Ostroverkh a, Viktor Joha a  ´d a, Roman Fiala a,b, Michal Va  clav u a, Ivan Khalakhan , Bretislav Smı Yevhenii Ostroverkh a, Vladimı´r Matolı´n a a

Department of Surface and Plasma Science, Charles University, Prague, 18000, Czech Republic Central European Research Infrastructure Consortium, S.S. 14 - Km 163,5 in AREA Science Park, Basovizza, 34149, Trieste, Italy

b

article info

abstract

Article history:

In this study, thin-film Pt catalysts with ultra-low metal loadings (ranging from 1 to

Received 14 November 2017

200 mg cm
2) were prepared by magnetron sputtering onto various carbon-based sub-

Received in revised form

strates. Performance of these catalysts acting as anode, cathode, or both electrodes in a

24 December 2018

proton exchange membrane fuel cell (PEMFC) was investigated in H2/O2 and H2/air mode.

Accepted 27 December 2018

As base substrates we used standard microporous layers comprising carbon nanoparticles

Available online 23 January 2019

with polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP) supported on a gas diffusion layer. Some substrates were further modified by magnetron sputtering of

Keywords:

carbon in N2 atmosphere (leading to CNx) followed by simultaneous plasma etching and

Ionomer-free catalyst

cerium oxide deposition. The CNx structure exhibits higher resistance to electrochemical

Ultra-low platinum membrane

etching as compared to pure carbon as was determined by mass spectrometry analysis of

electrode assembly (MEA)

PEMFC exhaust at different cell potentials for both sides of PEMFC. The role of platinum

Proton exchange membrane fuel cell

content and membrane thickness was investigated with the above four different combi-

(PEMFC)

nations of ionomer-free carbon-based substrates. The results were compared with a series

Magnetron sputtering

of benchmark electrodes made from commercially available state-of-the-art Pt/C catalysts.

Nitrogenated carbon

It was demonstrated that the platinum utilization in PEMFC with magnetron sputtered

Hydrogen

thin-film Pt electrodes can be up to 2 orders of magnitude higher than with the standard Pt/ C catalysts while keeping the similar power efficiency and long-term stability. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Catalysts for fuel cells (FC) must meet a number of requirements: to provide a large active surface, to possess good

electronic conductivity, to be corrosion resistant under the operation conditions of the type of fuel cell under study (which involves working temperature, pH, phase of fuel etc.), to be resistant to the action of other oxidants and reducing agents present in the working cell, to exhibit low sensitivity to

* Corresponding author.  nek). E-mail address: [email protected] (V. Joha https://doi.org/10.1016/j.ijhydene.2018.12.206 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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the action of catalytic poisons, just to name a few [1]. The high cost of the noble metal catalysts used in most FCs brings one of the key challenges for the fuel cell technology. It is widely accepted that in a typical proton exchange membrane fuel cell (PEMFC) with ionomer-impregnated supported Pt nanoparticles typically only about 40% of the total Pt present is active [1e3]. Since the catalytic reaction proceeding at the FC electrodes is a heterogeneous process, it is necessary that the catalyst offers as large active surface area as possible. Nowadays, the most commonly used material in state-ofthe-art PEMFCs is nanoscale platinum catalyst supported on a carbon substrate (Pt/C). Carbon materials with a highly porous surface and good electronic conductivity (such as powders or carbon black [2,4], nanofibers [5e7], or soot [8]), are typically used as the base support. The size distribution of platinum nanoparticles is the most important characteristics of such systems and one of the key factors determining the catalytic activity of the material. As the degree of dispersion of the active metal component of the catalyst increases so does the fraction of atoms exposed at the surface and thus available for electrochemical reaction. Apart from the larger specific area of the catalyst the higher abundance of lowcoordination surface sites with enhanced reactivity brings an additional advantage to its catalytic activity. Yet in the currently available PEMFCs the price of the platinum-based catalysts (which amounts for about 30e40% of the total cost of a fuel cell [6]) remains one of the major factors limiting their commercialization. Recently a number of authors has announced improved utilization of Pt leading to up to 30e50 kW g
1 Pt of specific generated power. However, a reliable long term performance of PEMFC remains, to a large extent, within a domain of catalysts with high noble metal loadings, hence with low Pt utilization [5,9e13]. Despite the recent development of noble-metal-free electrodes the Ptbased catalysts are still the most efficient known materials for PEMFCs in terms of capability to deliver high power densities [14,15]. Hence, besides different approaches in seeking novel materials to replace platinum, there is still a potential for further improvement of Pt-based FC electrodes via an optimization of metal dispersion and morphology of the catalyst. Catalytic systems with almost negligible amounts of highly dispersed noble metal particles may reach the requirements of standard PEMFC applications and still be called almost free of noble metal [16,17]. However, there exists an optimum “sweet spot” in this matter e with decreasing the size of metal particles below a certain value (typically 2e3 nm), the catalytic activity per unit surface area is usually significantly reduced [4,18]. As a result, the total reaction rate and thus FC current density decreases [4,19]. Smaller metal particles can also be more difficult to stabilize against sintering (induced by temperature and/or chemical environment) which depends on the nature of underlying substrate, alloying with other metals etc. [7,20e23]. In hydrogen-fueled PEMFC the kinetics of hydrogen oxidation reaction (HOR) [24] is much faster compared to oxygen reduction reaction (ORR) [4,19,25] and it was already reported that for HOR the Pt loading can be extremely reduced to about 0.02e0.05 mg cm
2 [24e29]. There were also some attempts in the mg cm
2 range employing electrospraying [30], plasma sputtering [10,31e36] or pulsed layer deposition

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[37,38], but with relatively low output power results or insufficient stability (see Ref. [29] and references therein). Moreover and quite importantly, although most of the current FC electrocatalysts are impregnated by an ionomer which acts as the binder and facilitates proton transport, for properly designed ultrathin electrodes the presence of an ionomer within the catalytic layer (CL) does not seem to be necessary as shown in this paper. So far, two major transport mechanisms have been proposed to explain ion transport in ionomer-free regions of FC anode: 1) adsorbed H-adatom diffusion along the electrode surface, and 2) water-mediated conduction of protons [3,39]. In the case the latter mechanism is involved (specifically for cathode) the proper water management may become a key issue for the PEMFC stability, especially because of the higher sensitivity of thin-film structures to flooding [3,40]. In this work, we present the experimental results of our effort to optimize Pt content on both sides of hydrogen-fueled PEMFC via tuning the properties of ionomer-free ultrathin catalytic layers without compromising the catalyst long-term stability. Numerous works on CL optimization dealing with catalyst loading, ionomer content, reactant diffusivity and ionic and electrical conductivity, have already been reported [41]. Most current CL fabrication methods for PEMFC applications [42] are based on wet deposition of a colloidal dispersion, i.e., catalyst ink, onto either a membrane [26,42] or a diffusion medium [27,43,44]. Such ink typically consists of highly dispersed carbon supported platinum catalyst mixed with ionomer and a dispersion medium such as methanol, ethanol, isopropanol, ethyl acetate, etc. [43,45]. The deposition of CL ink is usually realized by inkjet printing [13,27,42] or some variant of the spray coating technique [30,42,46,47]. The interplay between different compounds of the CL ink, dispersion medium type, ionomer concentration, homogeneity of ink deposition and other parameters make the wet deposition methods quite challenging. A more straightforward and better-defined approach of CL preparation is employing “dry” physical deposition (vapor based) methods performed in vacuum or in well-defined atmosphere directly onto a gas diffusion media which does not incorporate an ionomer compound. In our study we use magnetron sputtering as a preparation technique for very low Pt loadings (starting from less than 1 mg Pt per cm2). This method allows deposition of thin films with variable metal content on virtually any substrate material of choice and ensures relatively simple and well-defined catalyst preparation. As was already mentioned, PEMFCs with catalysts in this range of metal loading has been scrutinized by other groups as well, but no performance similar to the current state-of-the-art catalytic materials has been achieved.

Experimental Magnetron sputtering The carbon (or nitrogenated carbon, CNx) base layers were deposited via DC magnetron sputtering of a carbon target (Goodfellow, 2 inch diameter, 1 mm thickness) on gas diffusion layer (GDL) substrates. For this purpose, a commercial

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coating system MED020 (BAL-TEC) was used. The process gas was either pure argon (0.8 Pa) or nitrogen (4 Pa) in order to prepare pristine carbon or nitrogenated carbon coating, respectively. The platinum films were prepared via DC magnetron sputtering in the same sputtering system using a platinum target (Goodfellow, 2 inch diameter, 1 mm thickness). The deposition was carried out in pure argon (1 Pa) onto different substrates, described below. In all cases the DC discharge current was 20 mA at a discharge voltage of approximately 500 V leading to a discharge power of 10 W. For the deposition of cerium oxide (CeOy), magnetron sputtering with RF discharge (power 17 W) was applied, using a ceria target (Goodfellow, 2 inch diameter, 2 mm thick). The process gas was a mixture of argon and oxygen at 0.4 Pa total pressure and 1 mPa oxygen partial pressure. Deposition rates for all the above materials were determined individually from thicknesses of reference thin films deposited on a flat silicon substrate measured by Atomic Force Microscopy (AFM).

Mass spectrometry For the corrosion test we used the quadrupole mass spectrometer (QMS) Pfeiffer Prisma 200 with a precise dosing valve. The on-line mass spectrometry setup for FC chemical analysis has been described in more detail in Ref. [48].

SEM For the morphological investigation a scanning electron microscope MIRA3 (Tescan) was used. The measurements were carried out in secondary electron (SE) and back-scattered electron (BSE) modes using primary electrons of 30 keV energy.

Catalyst preparation In order to study the morphology effect of the thin film Pt electrodes we utilized four different substrates. The first pair of substrates was prepared by covering a standard gas diffusion layer (GDL) with carbon microporous layer (MPL) purchased from Alfa Aesar (Toray carbon paper 060, TGP-H-60) teflonated by a hydrophobic compound, either polytetrafluoroethylene (PTFE or Teflon™, Alfa Aesar), denoted here as MPL1A or fluorinated ethylene propylene (FEP or Teflon FEP™, Fuel Cell Store), denoted here as MPL2A. In order to further increase the active surface area a second pair of substrates was made by modification of the above substrates by their etching via reactive sputtering of cerium oxide layer onto a previously deposited nitrogenated carbon (CNx) interlayer [49], as will be described in more detail below (see also Fig. 1b). Such treatment of MPL1A and MPL2A type of substrates resulted in layers denoted as MPL1B and MPL2B respectively. Different amounts of Pt were deposited on all four substrates, spanning the range from 0.5 to 125 nm in terms of thin film average thickness (as determined by AFM from deposition on a reference flat Si surface). The active metal content in FC catalysts is commonly given in specific weight units which can be converted from thickness provided the density of the material is known. Theoretical maximal content of platinum in a compact 1 nm layer calculated from bulk Pt density is 2.145 mg Pt per cm2 of surface area. In our case, the actual value was determined by combining thickness determination by AFM and weight measurement using an analytical balance (Kern ABT 120-5DM). Standard commercial Pt/C catalysts with 30, 200, and 300 mgPt cm
2 (purchased from Fuel Cell Store), and 400 mgPt cm
2 (Alfa Aesar) were used as reference electrodes.

XPS

Results and discussion

Concentration and oxidation state of Pt were evaluated by Xray photoelectron spectroscopy (XPS), using Al-Ka excitation source. The fits of the XPS spectra were carried out using a Shirley background and energy separation between the Pt 4f doublet components fixed at 3.35 eV. Besides Pt 4f core level spectra, Ce 3d, C 1s, O 1s, and N 1s electronic levels were also monitored by XPS for all prepared samples.

Catalyst characterization

Fuel cell testing For the PEMFC operation test we used semi-automated stations LeanCat FCS-4M
100 W with an active area of the cell 4 cm2. Temperature of the cell was set to 70  C, temperatures of both humidifiers (for hydrogen and oxygen) were held at 75  C. We used pure hydrogen (5.0 purity, from Linde Gas) and oxygen (4.5 purity, from Linde Gas) with flow rates set to base values of 40 sccm and 30 sccm, respectively; the flow rates were increased as needed under higher power load conditions. In some experiments (indicated in the text below), ambient air was supplied to PEMFC cathode instead of pure oxygen (50e55% relative humidity, air temperature 26e28  C). Back-pressure at the fuel cell exhaust line was stabilized at 1.5 bar.

Top view SEM images (SE imaging mode) of all four catalysts are shown in Fig. 1a. The apparent difference between nonetched (MPL-A) and etched (MPL-B) layers reveals higher porosity of the latter and much smaller size of its surface features. The process of production of MPL-B from MPL-A type of substrate, published previously in more detail in Ref. [49], is illustrated step-by-step in Fig. 1b: Carbon sputter deposition in nitrogen atmosphere onto a fresh MPL2A substrate leads to the formation of a continuous adlayer of CNx (average thickness 200 nm). This adlayer is then subjected to reactive cerium deposition in oxygen atmosphere which results in simultaneous etching of the CNx layer and co-deposition of cerium oxide (generally non-stoichiometric, CeOy; average thickness 10 nm). The final deposition of Pt in pure argon forms the MPL2B catalyst. The measurement of the platinum content described in the previous section yielded (1.7 ± 0.1) mg Pt per cm2 surface area and 1 nm thickness (averaged over a set of 8 measurements of 25 nm and 125 nm thick layers for each substrate). The calculated density was found, within the experimental

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Fig. 1 e a) Top view SEM images of the substrates used in this study: MPL1A, MPL2A, MPL1B, MPL2B. b) Sequence of SEM images taken at different stages of the preparation of MPL2B substrate from MPL2A; Left to right: Fresh MPL2A substrate, after deposition of approx. 200 nm of CNx, during the deposition of cerium oxide and simultaneous oxygen plasma etching, after the deposition of approx, 10 nm CeOy (constituting MPL2B substrate).

precision, independent of the substrate morphology. This value, 20% below the bulk density, also provides a certain quantitative measure of the intrinsic porosity of the sputterdeposited Pt thin film. A second level of porosity (in the order of tens to hundreds nanometers) is determined by the structure of the underlying substrate. The schematic illustration in Fig. 2 describes Pt distribution within MPL for various loadings of Pt on both nonetched (top row) and etched (bottom row) substrates, determined from SEM images owing to the contrast between the metal and the substrate. The initial formation of separated Pt

nanoparticles (further evidenced by XPS as will be shown below) is followed by their coalescence into the shape of metal whiskers with high specific surface area. Upon FC assembly these metal structures presumably become partially embedded into the membrane, contributing to good MEA conductivity. The higher corrugation of the etched surface is reflected in the more complex structure of platinum. With increasing Pt coverage the microchannels in the substrate begin to fill up, leading to formation of larger metal conglomerates with reduced roughness. This process is shown at the bottom of Fig. 2 for the example of MPL2B substrate as

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Fig. 2 e Top panel: An illustrative depiction of platinum deposition on MPL-A (top row) and MPL-B (bottom row) substrates for average thickness of Pt between 1 and 100 nm. Left images are tilted SEM views of MPL2A and MPL2B substrates, respectively. Black lines represent selected surface contours of the respective substrate, yellow structures represent platinum. Bottom: Top view SEM images of Pt deposited on MPL2B with 3 different amounts of metal: 8.5 mg equivalent to 5 nm Pt, 34 mg (20 nm Pt), 170 mg (100 nm Pt). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

captured by SEM for 3 different Pt doses. From a certain quantity (around 200e250 nm) the Pt overlayer becomes so thick that it blocks most of the mass transport through MEA. Porous or highly dispersed metal structures are thermodynamically unstable so the platinum has natural tendency towards ordering and particle coalescence upon thermal or electrochemical activation [12,20,21,23]. This is something to be avoided if the low coordinated surface sites are responsible for high activity of the catalyst. It is known, for instance, that nanoparticles of platinum alloyed with other transition metals [50] are less prone to surface diffusion compared to pure Pt. The use of the alloying component can change the nature of the distribution of nanoparticles on a substrate. The presence of oxides (such as ceria [22]) and amorphous phases of the transition metal between the alloy particles [7,21] can also prevent their agglomeration. In our case such Pt dispersion stabilization role is adopted by the highly corrugated

magnetron sputtered carbon layer or even more efficiently by the addition of cerium oxide, owing to the strong metalsupport interaction between the two components [51,52]. Cerium oxide seems to provide a dual function here e it acts as the aforementioned stabilizer [22,51] and contributes to higher carbon porosity via etching process during preparation [53]. In addition, the electronic interaction between cerium oxide and platinum [54,55] may have a positive impact on the catalyst reactivity as well. Under oxidizing conditions (such as at FC cathode), cerium oxide is also capable of inhibiting Pt oxide formation, leading to an enhancement of the ORR rate [56]. Chemical analysis of as-prepared thin film Pt-doped MPLs was done by the means of X-ray photoelectron spectroscopy. Apart from platinum and carbon, some oxygen and traces of nitrogen were also present, presumably as a result of sample transfer through the air. As can be seen in Fig. 3, in all types of

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Fig. 3 e XPS spectra of the Pt 4f level of (from bottom up): 1 nm (1.7 mg) Pt on MPL2A substrate and Pt on MPL2B in the amounts of 1 nm (1.7 mg), 2 nm (3.4 mg), and 5 nm (8.5 mg). The black dots are raw data, fitted with 4 individual components (doublets denoted as Pt0, PtNP, Pt2þ, and Pt4þ). See the text for more details.

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metal doses the overall Pt 4f spectra intensity is lower on the etched layer. We assume it is a result of the higher roughness of the MPL2B substrate in which the platinum penetrates more into the material through its pores, and become effectively more shielded for the surface sensitive XPS method. In principle, it could also be explained by a different growth modes of Pt on either substrate, however, it would probably result in an opposite trend since the metal-substrate interaction is much stronger in the case of the cerium oxide substrate [22,52,57]. The omnipresent occurrence of Pt2þ component at 72.5e72.6 eV (and in some samples with very low Pt content also a trace amounts of Pt4þ around 73.7 eV) can be attributed to the above mentioned oxidation during sample transfer. Formation of surface platinum oxide which is confirmed by the appearance of an oxygen (O 1s) peak at 531.7 eV corresponding to PtO or Pt (OH)2 [58e60], clearly distinguishable from the lattice oxygen of cerium oxide at 530.0 eV (spectra not shown). However, noticeably higher abundance of Pt in ionic state is observed on the MPL2B samples, i.e. those containing CeOy (see Fig. 3), presumably due to the enhanced oxidation via oxygen transfer from cerium oxide [22,55].

Fuel cell performance substrates platinum was found to be predominantly in the metallic state (doublet with 4f7/2 component at 71.1 eV corresponding to bulk Pt, labelled Pt0). The metallic Pt 4f doublet, however, exhibits an asymmetry which cannot be fitted with a mere Pt bulk-like component (featuring asymmetric line shape due to electron-hole excitation) but a second feature at higher binding energy has to be added. This doublet at ~71.8e71.9 eV (labelled PtNP) is typical for nanometer-sized metal particles (electronic size-effect). A comparison of sample with 1 nm Pt on MPL2A and MPL2B substrates and of three different contents of metal deposit on MPL2B is shown in Fig. 3. As could be expected the relative intensity ratio between bulk (Pt0) and nanoparticle (PNP) spectral intensity is noticeably higher for the smallest Pt amount, in agreement with the growth model rendered above. Despite the equal

Catalysts comprising Pt supported on all the above substrates with various noble metal loadings were tested as PEMFC electrode (anode or cathode) with a counterpart electrode made from a commercial state-of-the-art compound e Pt coated carbon nanopowder with 400 mgPt cm
2 (purchased from Alfa Aesar). In all cases Nafion NR-212 was used as a membrane forming a basis for the membrane electrode assembly (MEA) of the fuel cell. The fuel cells with sputter-coated anodes were compared to the standard commercial Pt/C reference anodes with respect to their efficiency for hydrogen oxidation reaction (HOR), using pure H2 and O2. The results for Pt content spanning 2 orders of magnitude are presented in Fig. 4. All the points were obtained from the respective polarization curves of each MEA composition scrutinized. For the thin-film

Fig. 4 e The efficiency of PEMFC as a function of Pt loading at the anode (a) and at the cathode (b) on different MPLs on Nafion NR-212 membrane, measured with pure H2 (40 sccm) and O2 (30 sccm) mixture and the cell temperature of 70  C. The counterpart electrode was in all cases made from a commercial Pt/C nanopowder catalyst with 400 mgPt cm¡2. Green symbols (ref) represent the efficiency of the Pt/C powder reference electrode. The solid lines serve as a guide to the eye only. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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electrodes all the power density maxima were found within 3% of 480 mV and 450 mV cell voltage for anode and cathode, respectively. Great performance up to 1.3 W cm
2 was achieved for anodes containing Pt loading of about 2e10 mg cm
2 on the CeOy/CNx/(C þ FEP) substrate (MPL2B). It even exceeds the performance of the standard commercial Pt/C anodes (green symbols in Fig. 4a), including that with the highest platinum loadings (400 mg cm
2). Above approx. 100 mg cm
2 the power density curves converge to similar values for all types of substrates, followed by a slightly decreasing trend as the amount of Pt increases, eventually reaching the benchmark value of the reference anode near 200 mg cm
2. The identical set of experiments with various substrates and Pt loadings was also performed for oxygen reduction reaction (ORR) with the magnetron sputtered catalyst as

cathode and the commercial Pt/C (400 mgPt cm
2) reference anode. The best performance (>0.4 W cm
2) of PEMFC with magnetron sputtered cathode was measured for MPL2B-based catalyst with Pt loadings in the range of 30e150 mg cm
2, with a shallow maximum of 0.54 W cm
2 at 85 mg cm
2. Regardless of the type of substrate used the Pt amount of about 100 mg cm
2 was found as the efficiency limit value for the magnetron sputtered thin film e a further increase of Pt content leads to an absolute output power reduction of PEMFC. This is likely the result of the combination of 1) mass transport limitation increasing as the substrate micropores are filled with more metal and 2) lower roughness of platinum nanowhiskers at higher coverages (see Fig. 2). When PEMFC is switched from H2/O2 to the H2/air operation, the power decreases 2.0 to 2.2-times when reference Pt/C

Fig. 5 e Schematic structural models (left) and corresponding SEM image views (right) of two types of PEMFC electrodes used in this study. a) Standard commercial reference with high Pt loading (400 mgPt cm¡2), b) Magnetron sputtered Pt ultrathin film (8.5 mgPt cm¡2) on MPL1B. The SEM images show cross sections and the corresponding top views of the two electrodes, imaged in secondary electron (SE) and back-scattered electron (BSE) modes.

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powder electrodes are used and 2.8 to 3.0-times with a thin film Pt cathode. The drop of the FC performance is most likely due to the limitation of oxygen mass transport to the cathode surface [61], which is by expectation more pronounced in the case of the more dense structure of the thin film electrode. An additional reason might potentially be a partial blocking effect of other constituents of ambient air, such as SO2, NOx, NH3, H2S, O3, and CO, which, however, often results in long-term degradation of the CL [62,63], not observed in this case. Although the overlap of the data acquired with reference and custom cathodes is relatively small to make a reliable judgement our measurements suggest that the MPL2B catalyst probably brings a slight efficiency advantage (in terms of metal utilization) over the reference one in the low-Pt region below approx. 30e50 mg cm
2. This is in accord with increased ORR activity of Pt on carbon-supported partially reduced cerium oxide reported in Ref. [22]. On the other hand, none of the magnetron sputtered CLs presented here can cope with the commercial catalyst if the high absolute power density (>0.54 W cm
2) should be required but it takes its toll in the increased consumption of the noble metal (see Fig. 4b). From a certain level of metal loading (about 100 mgPt cm
2) the FC power becomes nearly invariant of MPL morphology, which holds true for both anode and cathode. This phenomena is in a good agreement with the observed filling of pores in the MPL substrates with increasing Pt content as discussed above. As more metal is added by sputtering the importance of the original substrate structure diminishes. A somewhat higher threshold value for HOR (anode) is likely related to more facile diffusion of hydrogen through MPL as compared to oxygen. The basis of the main differences between the deposited catalytic layers and the reference electrodes in a fuel cell is depicted in Fig. 5. One of the advantages of the catalysts prepared by magnetron sputtering deposition is the absence of the hydrophilic side chains of the Nafion ionomer compound [64]. Lower hydrophilicity facilitates the water diffusion through the material of the electrodes and thus places less demands on humidification of the inlet gases (H2, O2) which is often a major factor determining efficiency or stability of the working fuel cell. Ionomer can also partially cover platinum particles preventing them from effective participation in the electrochemical reaction [3]. Last but not least, the accumulation of highly dispersed platinum in the vicinity of MPLePEM interface minimizes effective loss of noble metal due to its partial localization in the distant areas with high electron transport resistivity or even unreachable by electrons due to separation of some Pt/C particles or their aggregates from the rest of CL [65]. The localization of Pt in a narrow region in the vicinity of the membrane surface in contrast to fairly homogeneous distribution within the reference catalyst is best seen by comparing BSE cross-sectional SEM images in Fig. 5 where the metal component appears brighter than the remaining material. The power efficiency of PEMFC can also be improved through membrane thickness reduction leading to lower resistance losses of MEA, as already know from previous studies [66]. In Fig. 6 a quantification of this effect has been provided for the setup with both standard Pt/C electrodes (400 mgPt cm
2) and various Nafion membranes with thickness

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Fig. 6 e I-V polarization (black/grey) and power (green) curves of PEMFCs with a standard state-of-the-art Pt/C anode and cathode for different membrane thicknesses (indicated in the plot) e Nafion XL (28 mm), NR-212 (51 mm), NE1035 (89 mm), and N115 (127 mm); The dashed lines represent I-V and power curves for PEMFC with MEA comprising Nafion XL (28 mm) membrane, standard cathode (400 mgPt cm¡2) and ultrathin film MPL2B anode (5 nm average Pt thickness, equiv. to 8.5 mgPt cm¡2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) between 28 and 127 mm. For the thinnest (Nafion XL) membrane a comparison to MEA having its anode replaced with the sputter-coated ultrathin film MPL2B catalyst containing only 8.5 mgPt cm
2 is also shown in Fig. 6 (dashed lines). PEMFC based on this MEA is capable of generating nearly equal power density as the commercial reference on the same membrane, giving a maximum power density above 1.4 W cm
2. Indeed, the reduction of the membrane thickness has its limitations due to the inverse proportionality between thickness and the intensity of fuel cross-over, long-term stability, and ability to handle higher differential pressures [66,67]. During the operation of the PEMFCs with Pt thin film catalysts (up to 10e20 nm thickness) supported on MPL-A type substrate (MPL1A and MPL2A) we detected a significant irreversible decay of cell efficiency. A steady decrease of output power density from 0.72 to 0.45 W cm
2 was observed for MPL2A anode with 5 nm (8.5 mg cm
2) Pt measured at external cell voltage of 0.5 V over 24-h period (see Fig. 7, black points). This is in contrast to noticeably more durable cells with Pt supported on MPL-B substrates containing CNx interlayer. Although some fluctuations (temporary drops) can be seen in the time evolution of power density produced by MPL2B anode with 2 nm (3.4 mg cm
2) Pt (Fig. 7, red points), the main trend remains nearly constant. Very similar behavior is observed with a reference anode as long as the amount of platinum remains high e compare the power density time dependence for commercial powder catalysts containing 400 mg cm
2 (solid green symbols) and 30 mg cm
2 (open green symbols) of platinum. In the latter case it is not only that the maximal cell performance is decaying quite rapidly but also huge temporary fluctuations are present at the cell output, dropping near zero power at some points.

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Fig. 7 e Time evolution of output power density of H2/O2 fed PEMFCs working continuously at a constant cell voltage of 0.5 V with MEA comprising commercial powder Pt cathode (400 mgPt cm¡2) and sputter coated anode of MPL2A type with 8.5 mgPt cm¡2 (black points), MPL2B type with 3.4 mgPt cm¡2 (red points), and reference commercial Pt/C catalyst (green triangles) with 400 mgPt cm¡2 (solid symbols) or 30 mgPt cm¡2 (open symbols). The solid lines serve as a guide to the eye only, rendering an approximate upper envelope curve of the experimental points. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Carbon etching was identified as the main cause of the catalyst degradation. We employed an on-line mass spectrometry technique in parallel to the measurement of electrical quantities [48] in order to better characterize and quantify the carbon depletion in the electrochemical environment of a fuel cell as a function of cell voltage. Etch rates of carbon originating from the catalytic layers were determined from the amount of generated carbon dioxide, giving values

consistent with the measured temporal decays of current density. In these experiments PEMFC was operated at 70  C in electrolysis regime (with external voltage source). The etching of carbon on the hydrogen side of the cell (almost 2 orders of magnitude lower in intensity than on the opposite side) is assumed to happen via reaction with oxygen permeating the membrane (oxygen cross-over) [68]. As can be seen in Fig. 8, on both sides of FC (anode and cathode) nitrogenated carbon provides higher stability against electrochemical carbon etching as compared to pristine carbon. For the purpose of the corrosion resistance measurements the catalysts were deposited directly onto a Nafion NR-212 membrane. 200 nm thick carbon or CNx layer, respectively, was covered by Pt nanoparticles equivalent to 10 nm average thickness (17 mg cm
2 metal loading). In order to further verify the electrochemical stability of the MPL-B type substrate a PEMFCs built on a combination of standard Pt/C electrode (400 mgPt cm
2) and ultrathin Pt on CeOy/CNx/(C þ FEP) (MPL2B) were also subjected to a 48-h durability tests under heavy duty operation simulated by pulsed regime of 30 min open voltage and 30 min under 400 mA cm
2 load (see Fig. 9). The current density of the duty cycle was chosen to fall within the Tafel regions of the IeV polarization curves of the scrutinized cells. The measurements were performed for our thin-film catalysts on both anode and cathode sides of the PEMFC. None of the examined cells showed any substantial loss of power efficiency (i.e., undetectable within the level of small fluctuations present due to water or fuel management, up to about 3e5% in magnitude). For selected MEAs (based on MPL2B supported Pt CLs) long-term 450-h durability tests using the same current pattern as above were taken. The result for the magnetron sputtered anode with Pt loading of 8.5 mg cm
2 is presented in Fig. 9. The thin film catalyst proved to be quite stable with voltage decrease rate <40 mV h
1 for the first 300 h (and ~85 mV h
1 within 450 h) under the above conditions of cycled 400 mA cm
2 load and only 15 mV h
1 in open voltage. A more comprehensive examination of long-term stability of PEMFCs

Fig. 8 e Dependence of carbon etch rate at the anode (“hydrogen side”) (a) and cathode (“oxygen side”) (b) of PEMFC operated in the electrolysis regime at 70  C as a function of the cell voltage for Pt-CeOy or Pt catalyst, respectively, supported on amorphous carbon (black) and nitrogenated carbon (red). In all cases the catalysts contained an equivalent of 10 nm Pt (17 mg cm¡2). The etch rates are expressed in the terms of carbon monolayer equivalents per second, as described in Ref. [48]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 9 3 4 4 e1 9 3 5 6

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Fig. 9 e 450-h accelerated durability test of PEMFC with standard commercial Pt/C (400 mgPt cm¡2) cathode and magnetron sputtered anode comprising 8.5 mg cm¡2 Pt on CeOy/CNx/(C þ FEP) (MPL2B) substrate. Nafion NR-212 was used as PEM. The test was performed by cycling between 30 min at 0.4 A cm¡2 current load and 30 min open voltage; cell back-pressure was 1.5 bar.

based on magnetron sputtered PteC CLs will be the subject of a follow-up publication. Based on the above findings, the optimal platinum utilization was found for MEA based on CeOy/CNx/(C þ FEP) supported Pt catalysts (MPL2B) with anode containing 1.7 mg Pt per cm2 and cathode with 34 mg Pt per cm2. Such balanced combination yields total power density of 0.44 W cm
2 with as

Fig. 10 e I-V polarization (black/grey) and power (green) curves of PEMFC with MEAs based on magnetron sputtered ultrathin film platinum anode and cathode (MPL2B type) with 3 different combinations of Pt loadings: a) 1.7 mg cm¡2 (anode) þ 8.5 mg cm¡2 (cathode), b) 1.7 mg cm¡2 (anode) þ 34 mg cm¡2 (cathode), and c) 3.4 mg cm¡2 þ 51 mg cm¡2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

little as 35.7 mg cm
2 of total noble metal content, i.e., 12.3 kW per gPt. Because ORR is the rate limiting process in our MEA [4,24,25] any increase of anode activity by adding more platinum would not affect the overall performance of the fuel cell. If, on contrary, the absolute PEMFC power should be the primary target, the optimum Pt loading combination for the sputter-coated thin film catalysts was found to be around 3e5 mg cm
2 (anode) and 50e70 mg cm
2 (cathode), providing slightly better performance than the above MEA, topping at 0.53 W cm
2. Although even higher power densities are achievable with anode Pt loadings around 10 mg cm
2 (see Fig. 4a), such MEA relies on a commercial cathode with much higher Pt content, diminishing the advantage of the low noble metal usage on the anode. On the other hand, under low or moderate power draw requirements with power densities not exceeding 0.45e0.5 A cm
2 the performance of PEMFC with the sputtercoated catalysts on both sides is practically independent of the Pt content within the range of values discussed in this paragraph, thus favoring MEA with the lowest of Pt loading (1.7 þ 34 mgPt cm
2). A comparison of polarization and power curves for 3 different combinations of Pt loadings (including the above two cases) is provided in Fig. 10.

Conclusions Using magnetron sputtered thin-film ionomer-free Pt catalysts in a hydrogen-fueled PEMFC can bring a significant advantage of high noble metal utilization over conventional methods without trading off for FC output power. Four different substrates based on carbon-teflon (PTFE or FEP)

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composite microporous layers covered with various amounts of platinum were examined under real working conditions of H2þO2 fueled PEMFC. On FC anode (HOR) this type of catalyst exhibits its main advantage, while on the cathode side (ORR) it can compete with the state-of-the-art Pt/C material only at moderate power densities not exceeding approx. 0.5 A cm
2. For both FC anode and cathode the best results were obtained with the CeOy/CNx/(C þ FEP) substrate. This catalyst represents one of the highest metal utilization values ever reported for low Pt loading electrodes. Highly dispersed platinum sputter-deposited onto this material provides stable output even under high loads, with great power performance (above 1.4 W cm
2) comparable to that of the current state-ofthe-art Pt/C catalysts containing 1e2 orders of magnitude higher amounts of the noble metal. Maximal power density was achieved with as little as approx. 8e10 mg cm
2 Pt on anode and 50e85 mg cm
2 on cathode. Optimal platinum utilization with only a slight reduction of the absolute output power was obtained using extremely low Pt loadings of 2e8 mg cm
2 (anode) and 30e50 mg cm
2 (cathode), respectively, by far exceeding the U.S. Department of Energy target value of 125 mg cm
2 for 2020 [69]. In addition to the high catalytic activity, very low degradation of the sputter coated thin-film catalysts incorporating nitrogenated carbon interlayer under FC operating conditions was verified by accelerated durability test. During the first 450 h the average FC voltage decay rate remained under 85 mV h
1 (at 0.4 A cm
2 load) or 15 mV h
1 (open voltage) for PEMFC using a sputter coated anode containing a mere 8.5 mg cm
2 Pt, coupled with the standard Pt/C cathode. It can be concluded that magnetron sputtering proved to be a precise, clean, and well-tunable method for the preparation of fairly homogeneous and highly efficient ionomer-free PEMFC Pt catalysts with metal loadings in a wide range spanning from units to hundreds of mgPt cm
2. By employing this technique the content of noble metal in the fuel cell MEA can be significantly reduced as compared to conventional Pt/C powder catalysts without compromising output performance or long-term stability of PEMFC. Moreover, our very recent results indicate [70] that platinum utilization can be additionally improved by co-sputtering of Pt and C, giving an outlook for further optimization.

Acknowledgments This work was supported by The Ministry of Education, Youth and Sports of the Czech Republic under grant LM2015057 in frame of the international consortium CERIC-ERIC and by the Charles University Grant Agency (GAUK No. 1580817). P.K. also acknowledges the financial support provided within the GAUK project No. 1016217.

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