Modeling and optimization of Scaffold-like macroporous electrodes for highly efficient direct methanol fuel cells

Applied Energy 221 (2018) 239–248

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Modeling and optimization of Scaffold-like macroporous electrodes for highly efficient direct methanol fuel cells ⁎

Zhangxun Xiaa, Ruili Suna,b, Fenning Jinga, Suli Wanga, , Hai Suna, Gongquan Suna, a b

T

⁎

Fuel Cell and Battery Division, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China

H I GH L IG H T S

electrode design with ordered macro-porous structure is proposed. • Advanced transport is greatly enhanced by theoretical and experimental analysis. • Mass • Energy production and efficiency of DMFC stack is remarkably enhanced.

A R T I C LE I N FO

A B S T R A C T

Keywords: Direct methanol fuel cells Electrode Freeze-drying Macroporous structure Mass transport

Construction of advanced electrode architecture, and understanding the electrochemical and mass transport phenomena within its structures are core issues that determine the development of fuel cells and other electrochemical energy technologies. Here in this work, we propose a new scaffold-like electrode with controllable porous volume and size via facile freeze-drying process. Deriving from the delicate surface unevenness and the well-defined macro-pores constructed by the ice template, the electrochemical surfaces and mass transport for methanol and oxygen are greatly enhanced by adapting this electrode structure as anode and cathode for direct methanol fuel cells, respectively. Computational fluid dynamics simulation and mathematic model is adopted to elucidate and predict the intrinsic improvement of mass transport within the newly designed electrode structure. Further practical application of such design is validated in a 10-cell short stack of direct methanol fuel cell systems equipped with this novel electrode.

1. Introduction As one of the most promising substitutes for the energy conversion devices, such as portable batteries, combustion engines, and stationary electric power supplies, direct methanol fuel cells (DMFCs) have attracted intensive interest in the past decades for the advanced composite materials and efficient manufacturing techniques [1–6]. To achieve the crucial requirements of commercialization in wide public, reinforcing electrochemical and mass transport processes within the electrodes is a determinative approach for the development of DMFCs. Membrane electrode assembly (MEA) plays a core role as the micro-chemical reactors with multiple electrochemical reactions confined in their micro/ nano-scaled spaces. These spatial confined structures, namely porous electrode, involving catalytically active surfaces, mass transport channels, and conductive pathways for charges, would be critical to the performance, durability, and cost of a fuel cell system [7–9]. However, the complexity of the electrochemical nature renders the electrode of

⁎

fuel cells a complicated composite structure. Firstly, the migration of ions and electrons should occur within the effective conductors, while the boundaries formed by the two kinds of conductors should be evenly distributed through the electrode [10–12]. Secondly, mass (including gas and liquid) transport should take place in suitable channels with macro-porous structure, and hydrophobic and hydrophilic pathways for gas and liquid phase respectively [13–16]. Hence, to construct well defined electrode architecture to meet these requirements is a promising strategy but still remains in challenges. The related electrochemical processes within these structures also lack further discussion. Delicately structural design of catalyst in meso/micro-scale to improve mass transport and reaction kinetics has led to great development in chemical engineering and catalysis [17–19]. For fuel cell and battery techniques, significant progresses in the advanced electrode architecture have been made to boost the mass transport and electrochemical processes based on new fabrication methods or new materials [20]. Ordered nano-arrays with effective mass transport channels were

Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (G. Sun).

https://doi.org/10.1016/j.apenergy.2018.03.100 Received 16 December 2017; Received in revised form 7 March 2018; Accepted 26 March 2018 0306-2619/ © 2018 Published by Elsevier Ltd.

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quickly transferred to the liquid nitrogen bath to form the solidified ice layer. The drying process was then performed in a freeze-drier under the temperature of −51 °C and the pressure of 7–10 Pa for 24 h. The dried electrode samples were denoted as SMPE-1 (scaffold-like macroporous electrode), SMPE-2, and SMPE-3, corresponding to the different DI-water (or ice) content of 40, 65, and 90 wt%, respectively. The control electrode sample with the traditional drying process of ambient evaporation was prepared with the same catalyst loading and Nafion content.

proved as a kind of excellent electrode structures for fuel cell applications. Carbon nano-tubes, conductive polymers, and metal materials could be used as bricks to construct the ordered electrodes by adapting chemical vapor deposition, electrochemical polymerization, or hydrothermal approaches [21–27]. Furthermore, pores in micro- and nanoscales with ordered structures have also achieved enhanced electrochemical and mass transport properties [28–30]. However, due to the mature fabrication techniques and good catalytic properties, carbon supported platinum group metal (PGM) catalysts are still recognized as the most promising electrode materials in the commercialization of fuel cells [31–35]. Based on the carbon supported PGM catalysts, electrode fabrication techniques are crucial issue to achieve well-defined electrode design. Ink-coating process, including brush painting, silk-screen printing, spraying, and blade coating, etc., is the major preparation method for fuel cell electrodes. Some advanced fabrication routes have been proposed in recent years, such as electrospinning, magnetic assisted coating, electrohydrodynamic deposition [36–38]. However, the active sites for electrochemical reactions and pores for mass transport within the electrode are usually randomly distributed [39]. As a result, the blind spots and pores constructed in the electrode could dramatically reduce the utilization of the catalyst and increase the voltage loss caused by mass transport. Theoretical studies of mass transport in electrochemical systems could further validate the phenomena in the alternative electrode designs [13,32,40]. Mathematic models and simulations are powerful approaches to analyze and predict the transportand reaction processes, especially for the new designs of electrode structures [20,41]. To demonstrate the fuel cells based on polymer electrolyte membranes, empirical and semi-empirical models have been developed based on the Butler-Volmer expression [42,43]. However, the intrinsic relationships between mass transport and electrochemical behaviors, and the electrode structures could be hardly illustrated by such models. One-dimensional and multi-dimensional models have been further proposed to understand the effects brought by the micro-structures within the MEAs [44–47]. Besides, single-phase models have been widely adopted for their simplicity, whereas two-phase models could further describe the details of gas phase (carbon dioxide) behaviors in anodes [48,49]. Herein this work, we propose the new electrode architecture with scaffold-like macroporous structure based on commercialized carbon supported catalysts for fuel cells by adapting a simple freeze-drying approach. The tailored channels for gas/liquid transport and the delicate surface structures could lead to the enhanced performance in the application of direct methanol fuel cells (DMFCs). A one-dimensional single-phase model, associating with computational fluid dynamic (CFD) approach, is further developed to validate, predict, and optimize such improvement brought by the new electrode architecture.

2.2. Physical characterizations The as-prepared samples were characterized by a field emission scanning electron microscope (FESEM, JSM-6360LV, JOEL). The wetting behavior was measured by a contact-angle meter (JC2000C1, Shanghai Powereach). The pressure differences of gas pass through the samples were measured by a homemade instrument. 2.3. Fabrication of DMFCs and electrochemical characterizations The MEA (membrane electrode assembly) used for DMFC single cell tests was equipped with the as-prepared electrode samples as cathodes (Pt/C electrodes) or anodes (PtRu/C electrodes) by sandwiching a piece of Nafion 115 membrane with another piece ditional anodes or cathodes, respectively. The MEA with an active area of 4 cm2 (2 × 2 cm2) was inserted into steel end plates with serpentine gas flow channels to assemble single cell units. The single cell tests on DMFC were carried out by using a fuel cell test system (FCTS, Arbin Co.) and an electrochemical workstation (SI1287 and SI1260, Solartron Co.). The methanol stripping tests were carried out following these procedures: 1, the anode was fed with 1 M methanol solution at the flow rate of 1 mL min−1 for 1800 s under the constant polarization potential of 0.1 V vs. DHE; 2, then the methanol solution was replaced by DI water for another 1800 s under the same potential; 3, CV test with a scan rate of 50 mV s−1was then applied to the anode for 5 cycles. The anode polarization curves were obtained by linear scanning applied to anodes with a scan rate of 1 mV s−1. Short stacks with 10 cells were fabricated by using graphite serpentine end plates. The active area of a single MEA is 25 cm2 (5 × 5 cm2). The lifetime tests of the DMFC short stacks were carried out by a fuel cell test system (G20, Green Light Co.). 3. Mathematic model and simulation 3.1. Cathode simulation The mass transport behavior of oxygen through the catalyst layer is simulated by Autodesk CFD 2016. The catalyst layer models for different SMPE samples are constructed based on the structural parameters acquired from the physical and electrochemical characterizations. To simplify the simulation processes from the complicated real electrode environment, we construct interconnected cubic pores with 1–3 layers as illustrated in Fig. 6a. The chamber with tens of times in size below the porous layer is set as the relatively huge flow area of gas diffusion layer and bipolar plate channels. The initial and boundary conditions of simulation are set based on the experimental details listed in Table 1. The mass transport of oxygen in the catalyst layer obeys Fick’s Law of diffusion:

2. Material and methods 2.1. Fabrication of the electrodes The scaffold-like macroporous electrode was fabricated based on the freeze-drying method [50,51]. Firstly, the commercial catalyst of amorphous carbon loaded platinum (Pt/C, 60 wt%, Johnson Matthey Co.) or platinum/ruthenium alloy (PtRu/C, 75 wt%, Johnson Matthey Co.) was mixed with PFSA ionomer (Nafion, water dispersion, 10 wt%, DuPont Co.) and a certain amount of de-ionized water. The weight ratio of catalyst to ionomer is 4:1, and to DI water is 1:10. The mixture was then constantly blended under the protection of nitrogen gas flow to form the homogenous slurry. After that, the as-prepared slurry is kept at 60 °C with constant stirring to evaporate the water until the water content is 40, 65, and 90%. The condensed catalyst slurry was then painted on the substrate of a gas diffusion layer (GDL), and the catalyst loading was fixed as 3.05 mg cm−2 (1.80 mgPt cm−2 for cathodes, 2.25 mgPtRu cm−2 for anodes). The as-prepared slurry-coated GDL was

JO,cc = ρO DO,cc ∇yO,cc

(1)

For the case of oxygen transport within the cathode catalyst layer under mass transport dominated situation, the boundary condition of oxygen mass fraction at the boundary of catalyst layer and gas diffusion layer could be set as yO, cc = 1, whereas at the inner surface of the catalyst, the oxygen mass fraction could be set as yO, cc = 0. The consumption rate of oxygen via ORR on the catalyst surface could be calculated by: 240

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di α F α F = aac i 0 ⎡exp ⎛ o ηs ⎞−exp ⎛− r ηs ⎞ ⎤ dx ⎝ RT ⎠ ⎝ RT ⎠ ⎦ ⎣

Table 1 Parameter values for DMFCs. Symbol/Parameter/Unit

Value

αo/transfer coefficient of oxidation process αr/transfer coefficient of reduction process cM, 0/concentration of fed methanol in flow channel/mol m−3 cH, 0/concentration of fed water in flow channel/mol m−3 DO, cc/Diffusion coefficient of oxygen in cathode catalyst layer [16]/m2 s−1 DH, n/Diffusion coefficient of water in Nafion phase [42]/m2 s−1 DM, n/Diffusion coefficient of methanol in Nafion phase [42]/m2 s−1 DM, l/Diffusion coefficient of methanol in liquid phase [59]/m2 s−1 DC, n/Diffusion coefficient of carbon dioxide in Nafion phase/m2 s−1 DC, l/Diffusion coefficient of carbon dioxide in liquid phase/m2 s−1 δac, st/standard thickness of anode catalyst layer/m δmem/thickness of membrane/m δad/thickness of gas diffusion layer/m Eθ/standard potential of methanol oxidation/V εac, st/standard porosity of anode catalyst layer F/Faraday’s constant/C mol−1 i0/exchange current density for methanol oxidation/A m−2 kH, eff/effective mass transfer coefficient of water/m s−1 kM, eff/effective mass transfer coefficient of methanol [60]/m s−1 kC, eff/effective mass transfer coefficient of carbon dioxide/m s−1 KH/partition coefficient of water KM/partition coefficient of methanol KC/partition coefficient of carbon dioxide σ/conductivity of Nafion/S m−1 loac/mass loading of anode catalyst layer/g m−2 ξH/electro-osmotic coefficient of water ξM/electro-osmotic coefficient of methanol [59] ρO/density of oxygen/g m−3 ρi/density of ice/g m−3 ρac, st/standard density of anode catalyst layer/g m−3 R/gas constant/J mol−1 K−1 T/temperature/K

0.5 0.5 1.00e3 5.38e5 5.22e−6

The standard reference exchange current density for methanol oxidation could be found in previous studies. The surface over-potential on the catalyst and Nafion layer boundaries could be written as: θ ηs = ϕc−ϕn−Eref

JO,cc

(4)

The reference standard potential could be calculated from the standard potential of methanol oxidation by Nernst equation:

7.30e−10* 6.08e−9*

θ Eref = E θ−

2.30e−9* 1.00e−11

RT cC ,n lg nF cM ,n

(5)

Material balance equations for methanol, water, and dissolved carbon dioxide should be further considered. The movement occurred in the liquid phase and the Nafion layers could be ascribed to diffusion and electro-osmotic drag. The flux equation is written as:

1.00e−10 8.00e−5 1.27e−4 3.00e−4 0.029 0.3 96,487 1100 1.00e−5 1.42e−5* 6.13e−4 1.00 0.80 6.30 14.19* 37.50 2.50 2.48e−2 1.00e6 0.90e6 0.50e6 8.3143 353.15

Ji = Di,ac

dci i + ξi dx F

(6)

The movement of the non-polar molecules of carbon dioxide is described by diffusion:

Ji = Di,ac

dci dx

(7)

The effective mass transfer of species from the liquid phase to the Nafion layers can be figured out by adopting this equation:

ji,eff = ki,eff (Ki ci,l−ci,n )

(8)

where the effective mass transfer coefficient (ki, eff) and the partition coefficient (Ki) for different species from the liquid phase to the Nafion layers could be found in other reports. The electrochemical reaction of anode would be consumed methanol and water, whereas carbon dioxide is generated at a rate given by:

* These values are obtained at the temperature of 353.15 K.

i = nFacc

(3)

ji = (2)

−si din nF dx

(9)

Hence, the material balance could be figured out:

dJi = ji −aac ji,eff dx 3.2. Anode mathematic model

(10)

In this work, porosity derived from the construction of ice crystals and the thicknesses of the catalyst layer determined by the ice content are the key variables, which can remarkably affect the anodic performance of DMFCs. The porosity induced by the ice template can be calculated as follow equation:

3.2.1. Model development Even though multi-phase models have been widely studied in the past decade [52,53], a classic single-phase model is still adopted here to validate and predict the relationship between DMFC performance and the electrode structure for its simplicity [42,54–56]. The porous structure of anode catalyst layer with homogenous pores could be used to develop modeling analysis. As many literatures reported, the catalyst particles and their agglomerates would be covered by nano-scaled thin layers of Nafion ionomers, which would provide sufficient protons during the electrochemical reactions. Some assumptions applied in the model are: (i) homogenous porosity; (ii) isothermal conditions; (iii) electrochemical reactions are governed by Bulter-Volmer expression; (iv) dilute solution theory is applied to the mass transport in the Nafion layer and the liquid phase; (v) carbon dioxide remains dissolved in liquid phase; (vi) pressure changes in anode could be neglected; (vii) the potential loss within the Nafion layers could be neglected. The above assumptions are adopted to simplify the anode model and to focus on the critical factors related to the micro-structure of the electrode. Hence some phenomena occurred in DMFCs, such as electrode flood and pressure drop along the flow channels, are neglected in this work [57,58]. Governing equations could be written by applying such assumptions above. Firstly, the electrochemical kinetics could be demonstrated based on Bulter-Volmer expression:

εac =

w ρi

+ w ρi

1−w ε ρac,st ac,st

+

1−w ρac,st

(11)

where w is the weight content of ice in the fabrication of the electrodes. The thickness of the anode catalyst layer for different samples with varied ice content can be figured out as follow:

δac = δac,st +

w loac 1−w ρi

(12)

Hence, the diffusion coefficient of methanol in anode can be demonstrated as follow: 1.5 Di,ac = Di,n × εac

(13)

According to the operation conditions of DMFCs, the boundary conditions for different variables could be described as fallows. As the one-dimensional model proposed here, the boundary of GDL and catalyst layer is set as x = 0, whereas the boundary of catalyst layer and membrane is set as x = δac. The potential of the Nafion layers could be described as: 241

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dϕ RT dcH ,n RT dcM ,n ⎞ i= −σ ⎜⎛ n + ξH + ξM ⎟ d F d F x c x cM ,n dx ⎠ H n , ⎝

of water ice crystals would be influenced by the intertangled nanoparticles suspended in the solvent, and tends to follow a fixed orientation to trap the solute in platelet-like spaces with multiple little dendrites on the surface. After the drying procedure in the vacuum chamber, the ice crystal would sublimate to remain the scaffold-like structure of the catalyst and ionomer hybrid with rough surfaces. As illustrated in Fig. 2b–d, compared to the traditionally prepared electrode with a dense catalyst layer (Fig. 2a), the novel electrode exhibits well organized porous structure in micrometer scale, and the walls of these porous connect with each other to form the matrix of micro-scaffold. With this scaffold-like construction in micro scale, the thickness of the catalyst layer is observed significantly increased to 133 μm compared to that of the traditional electrode without the porous structure, indicating an enhancement of porosity (the porosities attributed to the different scaffold structures are calculated as listed in Table 2). To further explore the morphological details of this electrode architecture, we could observe that the walls constructed by the hybrid of catalyst and ionomers possess an average thickness of about 600 nm, with highly rough surfaces, as shown in Fig. 2d. This roughness formed by the nano-dendrites of ice would attribute to the potential enhancement of the electrochemical properties. In order to study the structural controllability of the scaffolds, we have adjusted the ice content (as shown in Table 2) in the mixture before the drying process, and found a significant increase of the average diameter of the scaffold pores. With an ice content of 40 wt%, dispersive pores (φ = 0.5–1.0 μm) could be constructed with a scaffold-porosity of 24.2%. As the contents of the sublimated component increased to 65 wt% and 90 wt%, the average pore-sizes of the scaffold are also expanded to 2.2 and 3.6 μm, respectively, with the consistent enhancement of the porosity as 39.1 and 74.6% respectively. Interestingly, the growth of porosity is found much smaller than the increase of ice content, which could be explained by that the water initially occupies the second pores formed by the clusters of carbon-supported catalyst. Compared to the conventional electrode with the simple porous structure, the enhancement of the surface area for the scaffold-like electrode might greatly boost the electrochemical capability for the related reactions, while the mass transportation could also be facilitated for the construction of gas/liquid channels. To investigate the electrode characteristics related to the fuel cell performance, the water wetting behaviors and the gas penetration properties are measured, as shown in Fig. 3. Obviously, after the formation of the scaffold structure in the electrode, the surface hydrophobicity is enhanced as the water contact angle of the electrode surface increases from 120.2° to 142.6° (Fig. 3a and b). This result could be rationally explained by the Cassie–Baxter model [64,65], in which the droplet is energetically favorable to bridge across the tops of the pores to form air gaps, and could form greater contact angles compared to the flat surfaces. This enhanced hydrophobicity might help boosting the transport of water within the electrode. Additionally, the pressure loss when gas penetrates through porous materials could be an indicator for gas transport performance of the electrode. As shown in Fig. 3c, compared to the dense structure of the traditional electrode, the samples with scaffold morphologies exhibit much lower pressure drop of nitrogen flow penetrating through. Rationally, the electrode with higher porosity also presents better performance of gas penetration.

(14)

where at x = δac, ϕn = 0. The current density governed by ButlerVolmer expression is described as Eq. (3) (where at x = 0, ϕn = 0). 3.2.2. Solution method The flux balance of water, methanol, and carbon dioxide in the Nafion layers could be described via the followed equations respectively:

d ⎛ dc i s di DH ,ac H + ξH ⎞ = −aac kH ,eff (KH cH ,l−cH ,n )− H dx ⎝ dx F⎠ nF dx

(15)

d ⎛ dc i s di DM ,ac M + ξM ⎞ = −aac kM ,eff (KM cM ,l−cM ,n )− M dx ⎝ dx F⎠ nF dx

(16)

d ⎛ dc s di DC,ac C ⎞ = −aac kC,eff (K C cC ,l−cC ,n )− C dx ⎝ dx ⎠ nF dx

(17)

where at x = 0, Ji, n = 0. At x = δac, the flux of different species could be described as:

JH ,n = −DH ,ac

cH ,c−cH ,n i + ξH −cH ,l νH ,l δmem F

(18)

JM ,n = −DM ,ac

cM ,c−cM ,n i + ξM −JM ,l δmem F

(19)

JC ,n = −

DC ,ac / δmem DC ,ac cC ,n ⎜⎛ −1⎟⎞−JC ,l / k D δ δmem + C , eff C , ac mem ⎝ ⎠

(20)

The flux balance of methanol in liquid phase could be described as:

dcM ,l d ⎛ DM ,ac −cM ,l νH ,l⎞ = −aac kM ,eff (KM cM ,l−cM ,n ) dx ⎝ dx ⎠ ⎜

⎟

(21)

where at x = 0, cM, l = cM, 0. At x = δac,

JM ,l = εac kM ,eff (KM cM ,l−cM ,n )

(22)

The flux balance of carbon dioxide in liquid phase could be described as:

dcC,l d ⎛ DC,ac −cC,l νH ,l⎞ = −aac kC ,eff (K C cC,l−cC,n ) dx ⎝ dx ⎠ ⎜

⎟

(23)

where at x = 0, cC, l = 0. At x = δac,

JC,l = εac kC,eff (K C cC,l−cC,n )

(24)

The velocity of water movement could be described as:

aac kH ,eff (KH cH ,l−cH ,n ) dνH ,l dJH ,l/ dx = = dx cH ,l cH ,l

(25)

At x = δac,

νH ,l =

aac kH ,eff (KH cH ,l−cH ,n ) cH ,l

(26)

The governing equations developed above could be solved by commercial software Matlab. The related parameters and their values used for the solution are listed in Table 1.

4.2. Electrochemical properties and DMFC performance

4. Results and discussion

The applied performance of this novel electrode architecture is further measured as the anode and cathode of the direct methanol fuel cells (DMFCs) respectively. The DMFCs equipped with the different samples as anodes to catalyze methanol oxidation are tested as shown in Fig. 4. The electrochemical surface area (ECSA) could be estimated based on the peak area of methanol stripping (Fig. 4a) by assuming that the electric charge of the single layer adsorption of methanol is 0.42 mC cm−2. As listed in Table 2, the anodes equipped with different scaffold structures possess similar ECSA values of 57–63 m2 g−1 PtRu, with about

4.1. Morphological details and structural properties The scaffold-like electrode is constructed based on the rheological behavior of water during the freezing process as illustrated in Fig. 1 [61–63]. The nano-particles of catalyst and the ionomers of Nafion are well combined and distributed in the solvent of water after continuing stirring under ultra-sonication. During the freezing process, the growth 242

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Fig. 1. Schematic of the freeze-drying method and the construction of the scaffold-like macroporous electrode.

Fig. 2. SEM images of different samples. a, e, cross-section (a) and top (e) views for the traditional electrode; b–d, top (b–d) and cross-section (b insertion) views for a typical scaffold-like electrode (SMPE-2) in different magnifications; f–h, details for the porous structures of SMPE-1, SMPE-2, and SMPE-3, respectively. Table 2 Structural properties and electrochemical performances for different samples. Samples

Traditional SMPE-1 SMPE-2 SMPE-3 1 2

Ice content1/wt%

– 40 65 90

Average scaffold-porous diameter/μm

– 0.7 2.2 3.6

Scaffold-porous fraction2/%

0 24.2 39.1 64.6

ECSA/m2 g−1 Pt (PtRu)

Peak power density/mW cm−2

Anode

Cathode

Anode

Cathode

48.2 59.4 63.0 57.2

49.8 71.7 68.4 68.3

66.0 90.6 85.8 94.2

58.7 74.5 89.2 104.2

Based on the weight loss after freeze-drying process. Calculated by the thickness differences of the catalyst layers.

outperform the traditional one in both electrochemical (low current density) and mass transport (high current density) polarization regions according to the polarization curves in Fig. 4c. Compared to that of the traditional one, the enhancements in the peak power densities for SMPE-1, SMPE-2, SMPE-3 are found to be 37, 30, and 43%, respectively, as shown in Table 2. The similar enhancements in cathodes have also been demonstrated as resulted in Fig. 5. The ECSAs obtained from the hydrogen desorption peaks (0.04–0.4 V vs. DHE, Fig. 5a) are found to possess similar increase of 44, 37, and 37% for cathodes using samples of SMPE-1, SMPE-2, and

20–30% enhancement compared to that of the traditional sample. The extra surface area could be generated from the roughness surfaces constructed by the nano-dendrites of ice, as illustrated in Fig. 2. On the aspect of methanol transport, the limited current densities attributed to methanol diffusion in the anodic polarization curves (Fig. 4b) is increasing with the growing porosities and average pore-sizes, indicating the process of methanol transport should be enhanced within the scaffold structures. As a result of the reinforcements in electrochemical and mass transport properties, the DMFC performances of the single cells equipped with the scaffold-like electrodes are rationally observed 243

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Fig. 3. a, b, Water contact behaviors for the traditional (a) and the scaffold-like (b) electrodes; c, gas transmission performance of different samples.

to that of the conventional one. It is also noticeable that the performance differences between the scaffold cathodes are remarkable for the samples with higher porosities possess better IV performances, especially compared to the cases of anodes. Such phenomenon could be generated from the different structural requirements of DMFC anodes and cathodes for mass transport and electrochemical processes. Firstly, the enhanced porosity and mass transport of methanol could render the methanol crossover severer for the increased concentration of methanol on the boundary of anode and membrane, which is evidenced by the polarization curves of methanol crossover as shown in Fig. 4d. Hence, the mixed potential produced from the penetrated methanol oxidation on cathode could reduce the cell voltage. As illustrated in Fig. 4c, the DMFC with SMPE-A3 as anode exhibits the relatively lowest voltages in the low current density range compared to the two other samples with lower porosity and poorer mass transport capability. Additionally, the gas transport in the cathode should be much more sensitive towards the structures of electrode components with small pores, such as microporous layers and catalyst layers, while the liquid transport in the anode could be more crucially dominated by the large scale channels,

SMPE-3, respectively, compared to that of the traditional one. These expanded electrochemical surfaces for oxygen reduction could be also derived from the delicate surface structure formed by the ice nanodendrites. Additionally, the differences between the polarization curves under the cathode inlet of oxygen and air, also called the oxygen gain, are obtained, as shown in Fig. 5b. This parameter reflects the ability of oxygen transport through the electrode, which is crucially related to the path structure for gas convection and diffusion. By the construction of the scaffold structure in the cathodes, the single cells equipped with the SMPE samples demonstrate lower oxygen gain compared to the conventional one, suggesting that the more efficient channels for gas transport would be built based on the macro-porous structure. As illustrated in Fig. 5b, the SMPE-3 sample with the highest porosity rationally displays the lowest oxygen gain, while the sample with smallest porosity (SMPE-1) results a relatively high oxygen gain. Hence, by benefiting from the amelioration in electrochemical and mass transport aspect, the polarization curves of the DMFCs assembled with the novel cathodes exhibit superior performance with the enhancements of 27, 42, and 77% for SMPE-1, SMPE-2, and SMPE-3, respectively, compared

Fig. 4. Performance of the fuel cells equipped with the scaffold-like electrodes and the traditional electrode as anodes at the cell temperature of 80 °C. a, Methanol stripping results, scan rate 50 mV s−1; b, Anodic polarization curves, anodes fed with 1 mL min−1 1 M methanol, cathodes fed with 40 mL min−1 H2, scan rate 1 mV s−1; c, Polarization curves, anodes fed with 1 mL min−1 1 M methanol, cathodes fed with 80 mL min−1 O2; d, Methanol crossover curves, anodes fed with 1 mL min−1 1 M methanol, cathodes fed with 80 mL min−1 N2, scan rate 1 mV s−1. 244

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Fig. 5. Performance of the fuel cells equipped with the scaffold-like electrodes and the traditional electrode as cathodes at the cell temperature of 80 °C. a, CV curves, scan rate 50 mV s−1; b, Oxygen gain results, anodes fed with 1 mL min−11 M methanol, cathodes fed with 80 mL min−1 O2 or air; c, Polarization curves, anodes fed with 1 mL min−11 M methanol, cathodes fed with 80 mL min−1 O2.

such as flow fields and backing layers.

is observed in the inner pores with small size (Fig. 6i). Such results could be found consistent with the results from the gas transmission analysis in Fig. 3b. The enhanced oxygen transport is also validated in the better polarization performance and oxygen gain produced from DMFC equipped with SMPE-3 sample. To further understand the structural effects of the SPMEs, mathematic model was adopted to investigate the relationship between the anode structures and the fuel cell performance. Among the many approaches to calculate the mass transport and electrochemical processes of DMFCs, single phase model could be recognized as a simple, effective, and accurate way to analyze the methanol transport through the catalyst layers. Besides, as the macro-scaled structures on the horizontal dimension of the electrode studied in this work are homogenous, whereas the unevenness of the flow distribution through the flow channels could be neglected, the one-dimensional model is applied here. The calculated details and the related parameters could be found in other literature and the Supplementary Materials [42,54]. With the calculated concentration of methanol in the liquid phase of the catalyst layers, profiles of cM, l for different samples are obtained in Fig. 7a.

4.3. Modeling analysis of mass transport for DMFCs In order to investigate the mass transport within the tailored electrodes, fluid dynamic approach could be used for demonstration of the flow behavior. The flow patterns of oxygen within the catalyst layer are calculated via Autodesk CFD 2016, as shown in Fig. 6. Color gradient presents the different absolute velocity and oxygen fraction of the flow. It is noticeable that the distribution of oxygen flow velocity in the largest pores could be more homogenous with a higher average speed of transport, as demonstrate in Fig. 6b. In the case of the catalyst layer with smallest porous size (Fig. 6g), the velocity of oxygen transport in the inner layer can hardly remain a similar value as that of the outer layer, which means there is higher resistance of mass transport within the small-sized porous structure compared to that of the larger ones. The oxygen fraction results also indicate such phenomenon with higher concentration of oxygen distributed through the modeled layer with greater pore size (Fig. 6c), whereas obviously lower fraction of oxygen

Fig. 6. Profiles of velocity and fraction distribution of oxygen via CFD simulation in the catalyst layers with different porous sizes: a–c, 5 μm; d–f, 3 μm; g–i, 1.5 μm. 245

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Fig. 7. Comparison of the experimental and calculated anodic performance. a, Methanol concentration profiles of the liquid phase in the different catalyst layers; b, Typical anodic polarization data obtained by calculation (solid lines) and experiments (squares) with anodes fed with 1 M methanol at a flow rate of 1 mL min−1; c, Anodic polarization data obtained at the same conditions for different anodes; d, Predicted effects of the ice contents on the anodic polarization potential at a current density of 1000 A m−2.

Fig. 8. DMFC stack performance in long term tests for different samples at the temperature of 80 °C. a, Polarization curves, anodes fed with 50 mL min−1 1 M methanol, cathodes fed with 4 L min−1 air; b, Lifetime tests at a constant current density of 100 mA cm−2, anodes fed with 50 mL min−1 1 M methanol, cathodes fed with 4 L min−1 air; c, Comparison of energy outputs based on the lifetime tests.

potential on anode could be calculated as a polynomial curve with the change of the ice contents in the fabrication of the electrodes. The valley-like shape of the curve is reasonable, because the low porosity brought by the decreased ice content and the high catalyst-layer thickness brought by the increased ice content could be both detrimental to the mass transport within the electrode. Such results are wellfitted with the experiments for the SMPE samples. However, the over potential obtained from the traditional sample without the adding of iceis observed to deviate from the predicted curves obviously. This phenomenon could be explained by that the original pores within the traditional electrode would initially filled by the ice, and then the scaffold-like structure would be fabricated, as illustrated in the inset scheme of Fig. 7d. Hence, the predicted curve at the region with the ice content lower than a certain value (corresponding to a porosity of 0.3 from previous reports) will act as a flat line [42], which could fit well with the experimental result of the traditional sample. To testify the practical value of the energy production and utilization aspects for the scaffold-like electrodes, a short DMFC stack assembled with 10 pieces of 5 × 5 cm2 MEAs equipped with SMPE-3 as anodes and cathodes (denoted as SMPE-D3) is fabricated and tested. As

Obviously, greater concentration of methanol is distributed through the catalyst layer with higher porosity, whereas the traditional electrode presents the lowest distribution of methanol concentration. Furthermore, the concentration at the boundary of the catalyst layer and the membrane could be experimentally figured out based on the limit current density of methanol crossover tests (Fig. 4d):

iMC,lim = nFDM ,m

cM ,l δmem

(27)

As a result, there is a well agreement of calculated and experimental value, as shown in Fig. 7a. The calculated anodic polarization curves with different porosity, catalyst layer thickness, and electrochemical surface area, which are obtained from the measurements above for different samples, are also observed fitting well with the experimental data, as shown in Fig. 7c. Greater limited current density and more positive onset potential of methanol oxidation could be also found in the calculated curves for the cases based on the structural parameters of the SMPEs. Additionally, the optimization of the electrode architecture by tuning the porous structures could be further predicted as shown in Fig. 7d. At a current density of 1000 A m−2 (100 mA cm−2), the over 246

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shown in Fig. 8a, polarization curve obtained from the stack equipped with the novel electrode presents higher performance than that of the traditional one, especially at the situation dominated by mass transport. For long term performance, at a constant current density of 100 mA cm−2, a better durability with 6% voltage loss after 300 h lifetime tests for the stack with the scaffold-like electrodes is demonstrated, compared to a 15% voltage loss for a traditional stack (Fig. 8b). Furthermore, by collecting and analyzing the cathode outlet product generated within the 300 h life-time tests, an enhanced fuel utilization is calculated as 94% for the SMPE sample, compared to 91% for that of the traditional sample [66], which could be explained by the enhanced catalyst utilization and mass transport within the anode electrode with the scaffold-like structure. The energy density ascribing to the weight of stack and fuel consumed within the 300 h tests is calculated to be 1.56 kWh kg−1, which is 16% enhanced compared to that of the traditional one. Such increased energy outputs further yield a higher energy efficiency of 46%, compared to 39% for that of the traditional electrode. The enhanced performance and durability on the stack level for this scaffold-like electrode leads to great potential of SMPE as a new electrode architecture and fabrication technique for the commercialization of DMFC.

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5. Conclusion In this study, here we describe a new method to fabricate a scaffoldlike macro-porous electrode based on ice template process. By adopting the freeze-drying route in this method, an oriented micrometer-size porous structure with surface of nanometer-scaled roughness is constructed with commercial Pt/C catalyst mixed with perfluorosulfonic acid ionomer. The well-defined pores could facilitate the process of mass transport, while the enhanced surface roughness greatly boosts the electrochemical property by increase of reaction boundaries. As a result, the direct methanol fuel cells equipped with this new electrode demonstrate noticeable single cell performance surpassing that of the conventional one with a maximum enhancement of 73% for peak power density. The reinforcement of the electrode performance brought by the porous structure is further testified and predicted via calculation based on computational fluid dynamics simulation and a mathematic model. The practical value of this new electrode architecture and its fabrication techniques in energy outputs and durability is also testified on the direct methanol fuel cell stack level. In addition, we believe that this approach would not only promote the development of fuel cell techniques, but also enlighten the new thoughts on other electrochemical devices, micro-fluids sciences, and biological materials. Acknowledgement This work is financially supported by National Natural Science Foundation of China (No. 21503228), and China Scholarship Council (No. 201608210061). References [1] Debe MK. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012;486:43–51. [2] Stamenkovic VR, Strmcnik D, Lopes PP, Markovic NM. Energy and fuels from electrochemical interfaces. Nat Mater. 2017;16:57–69. [3] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345–52. [4] Nai JW, Lu Y, Yu L, Wang X, Lou XW. Formation of Ni-Fe mixed diselenide nanocages as a superior oxygen evolution electrocatalyst. Adv Mater. 2017;29. [5] Achmad F, Kamarudin SK, Daud WRW, Majlan EH. Passive direct methanol fuel cells for portable electronic devices. Appl Energy 2011;88:1681–9. [6] Wang A, Yuan W, Huang S, Tang Y, Chen Y. Structural effects of expanded metal mesh used as a flow field for a passive direct methanol fuel cell. Appl Energy 2017;208:184–94. [7] Litster S, McLean G. PEM fuel cell electrodes. J Power Sour 2004;130:61–76. [8] Xia ZX, Wang SL, Jiang LH, Sun H, Liu S, Fu XD, et al. Bio-inspired construction of advanced fuel cell cathode with Pt anchored in ordered hybrid polymer matrix. Sci

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[42]

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[52]

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cM, l: Concentration of methanol in liquid phase, mol m−3 cH, l: Concentration of water in liquid phase, mol m−3 cC, l: Concentration of carbon dioxide in liquid phase, mol m−3 cM, 0: Concentration of fed methanol in flow channel, mol m−3 cH, 0: Concentration of fed water in flow channel, mol m−3 DO, cc: Diffusion coefficient of oxygen in cathode catalyst layer, m2 s−1 Di, ac: Diffusion coefficient of species i in anode catalyst layer, m2 s−1 Di, n: Diffusion coefficient of species i in Nafion phase, m2 s−1 DH, ac: Diffusion coefficient of water in anode catalyst layer, m2 s−1 DM, ac: Diffusion coefficient of methanol in anode catalyst layer, m2 s−1 DC, ac: Diffusion coefficient of carbon dioxide in anode catalyst layer, m2 s−1 δac, st: Standard thickness of anode catalyst layer, m δac: Thickness of anode catalyst layer, m δmem: Thickness of membrane, m δad: Thickness of gas diffusion layer, m Eθref: Reference potential of methanol oxidation, V Eθ: Standard potential of methanol Oxidation, V εac: Porosity of anode catalyst layer εac, st: Standard porosity of anode catalyst layer F: Faraday’s constant, C mol−1 i: Current density, A m−2 i0: Exchange current density for methanol oxidation, A m−2 in: Superficial current density in Nafion phase, A m−2 iMC, lim: Limited current density of methanol crossover in cathode, A m−2 Ji: Mass flux of species i, mol m−2 s−1 JO, cc: Mass flux of oxygen in cathode catalyst layer, mol m−2 s−1 JH, n: Mass flux of water in Nafion phase, mol m−2 s−1 JM, n: Mass flux of methanol in Nafion phase, mol m−2 s−1 JC, n: Mass flux of carbon dioxide in Nafion phase, mol m−2 s−1 JM, l: Mass flux of methanol in liquid phase, mol m−2 s−1 JC, l: Mass flux of carbon dioxide in Liquid phase, mol m−2 s−1 ji: Specific mass flux of species i in x direction, mol m−2 s−1 ji, eff: Effective specific mass flux of species i in x direction, mol m−2 s−1 ki, eff: Effective mass transfer coefficient of species i, m s−1 kH, eff: Effective mass transfer coefficient of water, m s−1 kM, eff: Effective mass transfer coefficient of methanol, m s−1 kC, eff: Effective mass transfer coefficient of carbon dioxide, m s−1 Ki: Partition coefficient of species i KH: Partition coefficient of water KM: Partition coefficient of methanol KC: partition coefficient of carbon dioxide σ: Conductivity of Nafion, S m−1 loac: Mass loading of anode catalyst layer, g m−2 n: Transferred electron number of methanol oxidation ηs: Surface overpotential, V ξi: Electro-osmotic coefficient of species i ξH: Electro-osmotic coefficient of water ξM: Electro-osmotic coefficient of methanol ρO: Density of oxygen, g m−3 ρi: Density of ice, g m−3 ρac, st: Standard density of anode catalyst layer, g m−3 R: Gas constant, J mol−1 K−1 si: Stoichiometric coefficient of species i in the methanol oxidation reaction sH: Stoichiometric coefficient of water sM: Stoichiometric coefficient of methanol sC: Stoichiometric coefficient of carbon dioxide T: Temperature, K ϕc: Electric potential in the catalyst phase, V ϕn: Electric potential in the Nafion phase, V νH, l: Velocity of the liquid phase, m s−1 w: Weight content of ice x: Distance in the catalyst layer from the gas diffusion boundary yO, cc: Oxygen fraction in the cathode catalyst layer

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Glossary

Subscripts

acc: Active surface area of oxygen reduction in cathode, m2 m−2 aac: Active surface area of methanol oxidation in anode, m2 m−2 αo: Transfer coefficient of oxidation process αr: Transfer coefficient of reduction process ci: Concentration of species i, mol m−3 ci, l: Concentration of species i in liquid phase, mol m−3 ci, n: Concentration of species i in Nafion phase, mol m−3 cM, n: Concentration of methanol in Nafion phase, mol m−3 cH, n: Concentration of water in Nafion phase, mol m−3 cC, n: concentration of carbon dioxide in Nafion phase, mol m−3

ac: Anode catalyst layer cc: Cathode catalyst layer mem: Polymer electrolyte membrane l: Liquid phase n: Nafion phase H: Water M: Methanol C: Carbon dioxide MC: Methanol crossover

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