Membrane Electrode Assembly Structure Research Articles

N, S-coordinated Ni single-atom catalysts for efficient CO2 reduction in a zero-gap membrane electrode assembly electrolyzer

Nickel single-atom catalysts (Ni SACs) are promising for the electrochemical CO2 reduction reaction (CO2RR) to CO. However, the synthetic strategy of the catalysts with high CO2RR performance in a zero-gap electrolyzer is still lacking. Herein, we demonstrate asymmetrically coordinated Ni SACs and membrane electrode assembly (MEA) structures to achieve outstanding CO2RR performance in a zero-gap electrolyzer. N, S-coordinated Ni SAC with the Ni-N3S1 structure (Ni-NSC-1) showed a higher faradaic efficiency (FECO) and partial current density (jCO) of CO than N-coordinated Ni SACs (Ni-NCs). This was due to the coordination environment of Ni and increased N and S content in the carbon support. The FECO, jCO, and stability of the Ni-NSC-1 were further improved by modulating the MEA structure and operating temperature. As a result, a maximum FECO of 92.85% (at 2.1 V) and jCO of 286.54 mA cm-2 (at 2.3 V) were achieved using a Sustainion X37-50 GT membrane at 343 K. Moreover, the Ni-NSC-1 with Sustainion X37-50 GT exhibited long-term stability for over 60 hours with a high FECO of 95% at 100 mA cm-2.

Influence of compression effect on the MEA structure and performance based on M−N–C electrocatalysts for PEMFC

The single-atom dispersed active sites and the thicker catalytic layer of metal-nitrogen-carbon (M-N-C) electrocatalysts based membrane electrode assembly (MEA) exhibit notable distinctions from Pt-based MEA, necessitating targeted optimization. This study systematically investigates the influence of compression on both structure and performance of the M-N-C electrocatalysts based MEA. Four compression ratios, ranging from 11.5% to 39.3%, were examined across diverse operational conditions. The variations in impedance across different electrochemical regions are observed, resulting from the superposition of distinct impedance components. It is found that the insufficient compression leads to higher porosity and the formation of cracks between catalytic layer and membrane, adversely impacting internal resistance and proton transport. Increasing compression first acts the catalytic layer, and then gradually acts on the interface, membrane and gas diffusion layer, thereby reducing internal resistance, charge transfer resistances, albeit at the expense of heightened mass transfer resistance. Among these factors, the influence of internal resistance affects significance to total performance. Furthermore, compression influenced mass transfer differently in various layers. For the catalyst layer, the effect was uniform and sustained, while for the gas diffusion layer, it initially remained constant and then experienced a sudden deterioration over a certain extent. Optimal MEA performance is achieved when the compression ratio attains 40 %, which 2.1 times higher than the low compression MEA. This finding emphasizes the necessity for higher compression on M-N-C electrocatalysts based MEA compared to Pt-based MEA.

Optimal MEA structure and operating conditions for fuel cell reactors with hydrogen peroxide and power cogeneration

The cogeneration of hydrogen peroxide (H2O2) and power in proton exchange membrane fuel cell (PEMFC) reactors via two-electron oxygen reduction reaction on the cathode is an economical, low-carbon, and green route for the on-site production of H2O2. However, in practice, the H2O2 that cannot be collected timely will accumulate and self-decompose in the catalyst layer (CL), reducing the H2O2 generation efficiency. Thus, accelerating the mass transport of H2O2 within the cathode CL is critical to efficient H2O2 generation in PEMFC. Herein, we investigated the effects of the membrane electrode assembly (MEA) fabrication process, cathode CL thickness, and cathode carrier water flow rate on H2O2 generation and cell performance in a PEMFC reactor. The results show that the catalyst-coated membrane-type MEA exhibits high power output due to its lower proton transport resistance. However, the formed CL with a dense structure significantly limits H2O2 collection efficiency. The catalyst-coated gas diffusion electrode (GDE)-type MEA formed macroporous structures in the cathode CL, facilitating carrier water entry and H2O2 drainage. In particular, carbon cloth GDE with thin CL could construct rich macroscopic liquid channels, thus maximizing the generation of H2O2, but will impede fuel cell performance. These results suggest that the construction of a well-connected interface between CL and proton exchange membrane (PEM) in MEA and the establishment of a macroscopic pore structure of the CL are the keys to improve the cell performance and H2O2 collection efficiency.

Integrated Capture and Electrochemical Conversion of CO2 into CO

The capture and electrochemical conversion of CO2, powered by renewable electricity, is an attractive method of sustainably producing valuable chemicals and fuels (e.g. carbon monoxide (CO)), reducing atmospheric CO2, and storing intermittent renewable energy. Integrated capture and conversion (reactive capture) of CO2 presents a CO2-to-CO electrolysis pathway that eliminates most of the upstream capital and energy costs by releasing CO2 directly inside the electrolyzer using an internal pH-swing. The reactive capture system readily allows for the collection of produced gas products via phase separation, thus minimizing downstream separation costs.Industrial-scale integration of reactive capture systems with upgrading processes require a pure and consistent product stream. Previous studies using bicarbonate electrolytes have demonstrated high selectivity towards CO. However, the limited CO2 capture capacity of bicarbonate electrolytes dilute the cathode product gas stream with excess CO2. This mandates a secondary CO2 capture unit and increases the cost of downstream separation. Other studies using carbonate or carbamate electrolyte as the inlet feed did not simultaneously achieve high CO selectivity and long-term stability.This study sought to improve the Faradaic efficiency (FE) toward CO in our carbonate electrolysis system by engineering a novel membrane electrode assembly structure. We designed a composite CO2 diffusion layer (CDL) between the cathode and the membrane that attains high CO selectivity by simultaneously achieving high alkalinity and sufficient CO2 availability at the cathode. We determined that the thickness, wettability, and permeability of the CDL affected species transport and were important optimization parameters. Applying this strategy, we produced syngas, a mixture of CO and hydrogen (H2), with an industrial H2/CO ratio of 1.16 at 200 mA cm-2. This corresponded to a CO Faradaic efficiency (FE) of 46% and energy intensity of 52 GJ tsyngas-1. The syngas produced in this system was not diluted by CO2 and contained sufficient CO content to meet industrial standards. We further increased the FE towards CO by exploring different capture solutions and designing selective catalysts for energy efficient CO production. System parameters such as temperature and pressure effects were also investigated to improve the CO2 concentration at the cathode. This study illustrated the potential for the industrial implementation of an energy efficient and capital cost effective CO2-to-CO pathway via reactive capture.

Localized Study of the CCM with Graded Ionomer and Catalyst Loading across the X-Y and Z Directions Using Current Scan Shunt

In project CAMELOT, it is aimed to diagnose the fundamental transport properties that limit performance in state-of-the-art (SoA) and prototype beyond-SoA Membrane Electrode Assembly (MEA)s and materials. To accomplish this goal, a series of in-situ characterization methods are used to create correlations between the observed performance, voltage loss breakdowns at the cell level, and the fundamental material property characteristics of the MEA and MEA materials. To accomplish this goal, thin Catalyst Coated Membrane (CCM) with low Pt loading is used as target study material; and the logic for using thinner layers in each of the MEA components is clear: thinner constituent layers (membrane, catalyst layers, microporous layers (MPL), gas diffusion layers (GDL)) means shorter transport distances for protons, oxygen, hydrogen and electrons and should thus reduce the associated transport losses. Thin‐layer X‐Y and Z direction graded catalyst layers are developed in project CAMELOT to optimize porosity, ionomer content and catalyst loading across the X‐Y and Z directions of the CCM. For the X‐Y direction CAMELOT matches the catalyst layer composition to the conditions at different locations within the flow field. For example, the porosity and ionomer content of the catalyst layer near the outlet of the flow field should be tuned for significantly wetter conditions than at the flow field inlet. For Z direction CAMELOT matches the catalyst layer composition to the conditions of the membrane and the GDL layer. The goal is to ensure that at each point in the active area, the catalysts can operate as close to their maximum (kinetically limited) performance as possible by minimizing mass transport limitations under the conditions that prevail at the different locations. For example, the porosity and ionomer content of the catalyst layer near the outlet of the flow field should be tuned for significantly wetter conditions than at the flow field inlet. Current Scan Shunt (CSS) is used as a convenient tool to measure real-time current density and temperature distribution within the fuel cell in situ operando, to analyze the cell behavior depending on the components and the direction of the grading catalyst layer on the developed CCMs. The experimental work will later be supplemented by a theoretical analysis carried out in the modelling work and, together, the modelling and experimental analysis can be used to construct a complete picture of correlated relationships between the MEA structure and properties and the associated voltage loss causes of the performance limitations for current SoA materials.

Decay behaviour of ultrathin reinforced membranes in PEMFCs subjected to the combination of mechanical/chemical accelerated stress testing

To improve the performance of proton exchange membrane fuel cells (PEMFCs), proton exchange membranes (PEMs) have been made much thinner. However, the thinner the PEM, the more challenging its durability. In this paper, the degradation of ultrathin reinforced PEMs with two thicknesses, 8 μm and 12 μm, under the combination of mechanical/chemical accelerated stress testing (AST) is researched. The results show that the cycle numbers of the two ultrathin reinforced PEMs are equivalent and meet the target proposed by the United States Department of Energy (DOE). They present very different trends from those of changes in the durability of traditional homogeneous PEMs with thickness. The growth of the hydrogen crossover current follows an exponential function, which is the same as that of traditional homogeneous PEMs but with an order of magnitude smaller characteristic parameter k, which might be the key reason that the cycle numbers of ultrathin reinforced PEMs far exceed those of other PEMs. The failure analysis shows that the fracture of the two ultrathin reinforced PEMs is located at the edge of the membrane electrode assembly (MEA) frame. These failures could be attributed to the stress concentration at the edge of the frame and contribute to the similar lifetime and decay regulations. More details are observed that the edge failure points of the two PEMs at the gas inlet or outlet region. In addition, the effects of thinner PEMs at the gas inlet and outlet are more obvious than that at other parts. It is concluded that the durability of ultrathin reinforced PEMs is almost independent of thickness, while the stress concentration caused by the MEA structure and the condition control at the location of the gas inlet and outlet are mainly responsible for the failure of the PEM. The results in this work will provide a new perspective for the development of ultrathin reinforced PEMs.

Cathode diffusion layer and current collector with slotted foam stainless steel for a micro direct methanol fuel cell.

In order to reduce the contact and mass transfer impedance of the diffusion layer and current collector of a Micro Direct Methanol Fuel Cell (μDMFC), a novel Membrane Electrode Assembly (MEA) structure is designed by using Foam Stainless Steel (FSS) with a slotting rate of 38.47% for both the cathode diffusion layer and the current collector. Electrochemical tests are performed on the Foam Stainless Steel Membrane Electrode Assembly (FSS-MEA) and the Conventional Carbon Paper Membrane Electrode Assembly (CCP-MEA) μDMFCs. The experimental results show that the maximum power density of FSS-MEA μDMFC is 46.55 mW cm-2 at 343 K, which is 42.88% higher than that of CCP-MEA μDMFC, and the optimum working concentration of FSS-MEA μDMFC is 2.5 mol L-1, which is 1 mol L-1 higher than that of CCP-MEA μDMFC. Electrochemical Impedance Spectroscopy (EIS) test results show that the contact impedance of FSS-MEA μDMFC is 0.55 Ω cm-2, which is 15.38% lower than that of CCP-MEA μDMFC. The mass transfer impedance of FSS-MEA μDMFC is 0.99 Ω cm-2, which is 25.56% lower than that of CCP-MEA μDMFC. This implies that the novel slotted FSS-MEA structure alleviates the methanol crossover and reduces the contact and mass transfer impedance, thus improving μDMFC power density.

Effect of Metallic Bipolar Plates Fillet Radii on Fuel Cell Performance

This paper studies the effects of compression, deformation, and the contact area in the membrane electrode assembly (MEA). The electrical impedance of fuel cell stacks due to the assembly of the metallic bipolar plates is also considered. According to decades-long fuel cell (FC) assembly experience, an increase in compression force can result in adequate contact resistance, but excessive compression may cause extra contact resistance and damage to the MEA structure. The study suggests a design for improving the performance of the FC stack by proposing different fillet radii metallic bipolar plates. It is found that the appropriate fillet radius reduces contact resistance by 13% and avoids the accumulation of compression, thereby maintaining contact resistance at adequate levels. The current design proposes a simple and effective method to minimize the dimension tolerance of single fuel cell units and support sufficient compression.

Experimental validation for enhancement of PEMFC cold start performance: Based on the optimization of micro porous layer

The cold start is one of the key research fields in PEMFC studies. The improvement of PEMFC’s (proton exchange membrane fuel cell) unassisted cold start performance has played a positive role in promoting the industrialization and commercialization of FCV (fuel cell vehicle). In this work, as the key point of MEA (membrane electrode assembly) structure optimization, MPL (micro porous layer) aims to make the generated water can be retained in the CLs (catalyst layer) and membrane in the form of membrane water at the initial period of cold start. And the cold start performance of the optimized MEA single cell and stack are observed and analyzed by the segmented cell technology. After comparing with the cold start performance of the commercial MEA single cell and stack under the same conditions, the improvement of the cold start performance is verified. The single cell of MEA1 could achieve a quick unassisted cold start at a start-up temperature of −15 °C, and the cold start time under 0.2 V in CV (constant voltage) mode is 153 s, while the commercial MEA single cell can only achieve a successful cold start under the minimum start-up temperature of −10 °C. The optimized MEA stack can realize a quick unassisted cold start at a startup temperature of −20 °C, and the cold start time under 0.1 V in CV mode is 63 s, while the commercial MEA stack can only achieve a successful cold start under the minimum startup temperature of −15 °C.

A Trilaminar-Catalytic Layered MEA Structure for a Passive Micro-Direct Methanol Fuel Cell.

A membrane electrode assembly (MEA) with a novel trilaminar-catalytic layered structure was designed and fabricated for a micro-direct methanol fuel cell (μ-DMFC). The trilaminar-catalytic layer comprises three porous layers. The medial layer has a lower porosity than the inner and outer layers. The simulation results predicted a lower water content and a higher oxygen concentration in the trilaminar-catalytic layer. The novel trilaminar-catalytic layer enhanced the back diffusion of water from the cathode to the anode, which reduces methanol crossover and improves oxygen mass transportation. The electrochemical results of the half-cell test indicate that the novel MEA has a greatly increased cathode polarization and a slightly increased anode polarization. Thus, this novel μ-DMFC structure has a higher power density and a longer discharging time, and hence may be used in portable systems.

(Invited) Benchmarking Carbon-Supported Pt-Based Oxygen Reduction Reaction Electrocatalysts for PEMFC Cathodes

The present performance of state-of-the-art proton exchange membrane fuel cells (PEMFC) enable their commercialization, as demonstrated by the recent release of fuel-cell powered vehicles (forklifts by Plug Power, FCEV Miraï by Toyota, etc.). Now, all PEMFC manufacturers need to reduce the PEMFC systems initial cost and to enhance their durability and reliability in operation to reach the market requirements and industrial sustainability. To this goal, one must develop highly-efficient oxygen reduction reaction (ORR) electrocatalysts, which was claimed to be obtained by numerous groups in a recent past [1-6]. Unfortunately, the scientific community still struggles to obtain the best performance of these materials in operating PEFMCs: their practical performance in membrane electrode assembly (MEA) do not approach those expected from an analytical approach performed using the rotating disk electrode setup, RDE (the most widely-used setup to assess a material’s ORR performance).There are several reasons that may explain why RDE data are not relevant to account for MEA data. Firstly, in RDE, the O2 reactant is fed as gas dissolved in the electrolyte (slow diffusivity and solubility), hence the mass-transfer rate and related limiting current density are ca. 3 orders of magnitude lower than in real PEMFC conditions. Secondly, the activity data is only extracted at high potential values in RDE (E > 0.85 V vs RHE), although PEMFC optimal performance is reached in the 0.6 – 0.8 V vs RHE cathode potential range. Thirdly, the proton conduction can be limiting in the thin layer of ionomer covering the electrocatalyst nanoparticles in MEA, which could limit their operation, especially at the high current densities at stake in MEA. In essence, one is not sure that the electrocatalysts are used in optimized conditions in MEA, as emphasized mass-transport of reactants (both H+ and O2) to the catalytic sites is not granted for present ionomers and MEA structure [7, 8]. Recent results obtained in newly-developed electrochemical characterization setups like the floating electrode (FE) [9], the gas-diffusion electrode (GDE) [10] or differential cell (DC), could enable to reach much higher ORR current densities at the lab scale and therefore better match MEA data, a prerequisite for relevant benchmarking.In the present contribution, these setups will be used to characterize three state-of-the-art ORR electrocatalysts (50 wt% Pt/Vulcan XC72, 50 wt% Pt3Co/Vulcan XC72 and 30 wt% Pt/graphitized carbon black), and comparison will be made with large single-cell PEMFC data in automotive conditions. We will show whether the “RDE promises” of alloyed electrocatalysts (measured in RDE configuration) do maintain in FE, GDE and DC, and whether the “advanced electrocatalysts” still exhibit their plain potential in real PEMFC catalytic layer configuration.

Impact of Electrode Thick Spot Irregularities on Polymer Electrolyte Membrane Fuel Cell Performance

Polymer electrolyte membrane fuel cells (PEMFC) have great potential for power generation in many applications, but to implement them on a larger scale, understanding of quality requirements, tolerances, and controls during manufacturing must improve. As part of this broad challenge, irregularities resulting from the membrane electrode assembly (MEA) production process must be studied to determine if they impact the performance of the cell.1,2 One potential irregularity is a locally increased thickness of the coated or deposited electrode. In order to understand and provide insight into the impact of such irregularities, we intentionally created thick spots in the center of cathode electrodes, either by liquid casting or ultrasonic spraying of the additional catalyst ink. The thick spots fabricated by these two different approaches exhibited different sizes, morphologies, and Pt loading distributions, which were carefully characterized by optical microscopy, X-ray fluorescence spectroscopy (XRF), and scanning electron microscopy (SEM). Edge-protected MEAs were fabricated using both pristine electrodes and electrodes with thick spot irregularities. The effect of these irregularities on MEA structure were investigated by cross-sectional SEM. Electrochemical performance, including electrochemically active surface area (ECSA), H2/O2 polarization curves, mass activity and H2/air polarization curves, was measured to study the impact of the electrode irregularities on catalyst utilization and PEMFC initial performance. NREL’s segmented cell was utilized to investigate the resulting spatial performance of a representative thick spot MEA.The results indicate that thick spot irregularities that are present in the center of the cathode electrode impact initial cell performance although the impact of the thick spots on ECSA and mass activity was minimal. The latter suggests that the number of active Pt sites and the kinetics were not significantly altered. However, the H2/air polarization curves indicated that liquid-cast thick spots and large sprayed thick spots decrease cell performance dramatically at current densities above 0.8 A/cm2. Based on cross-sectional SEM analysis, the likely causes of this performance drop may originate from (i) delamination between the GDL and CL, (ii) the presence of thin spots in the membrane due to deformation associated with the thick spot, (iii) additional compression of the GDL and/or (iv) thick catalyst layer sections inhibiting local mass transport. Spatial performance diagnostics confirmed that the performance loss of MEAs containing liquid-cast thick spots is more pronounced at high current density than at low current density. Moreover, the performance of the area surrounding the liquid-cast thick spot is impacted as well, likely due to the various morphological changes resulting from the irregularity. We expect that the morphological changes resulting from these irregularities could serve as seed points for premature cell failure in long-term PEMFC operation. Future work will focus on the impact of electrode thick spot irregularities on PEMFC performance-over-time and lifetime, for both reinforced and non-reinforced membranes.

Research progress of catalyst layer and interlayer interface structures in membrane electrode assembly (MEA) for proton exchange membrane fuel cell (PEMFC) system

Stricter requirements for MEA structure have to be met in order to realize the rapid commercialization and marketization of polymer electrolyte membrane fuel cell (PEMFC). Optimization and design of catalyst layer and interlayer interface structures is helpful to increase the utilization of Pt, reduce mass transfer loss and realize the high-performance output of membrane electrode assembly (MEA) with Low-Pt loading. Herein, this paper reviews recent developments of catalyst layer and GDL/PEM/CL interlayer interface structures in MEA. The evolution process from conventional catalyst layer to gradient and ordered catalyst layer is presented. The interactions among Pt-based catalysts, carbon supports, ionomers and solvent in catalyst ink are comprehensively discussed, and its influence on the structure of conventional porous catalyst layer are analyzed. Moreover, this paper discusses the gradient distribution law of ionomer, Pt loading and hydrophilicity/hydrophobicity in the catalyst layer, and introduces the design and preparation strategies of ordered electrode based on nanoarray structure. In addition, novel approaches to engineer the PEM/CL interface are schematically summarized as engineering surface-patterned membranes, direct deposited membrane technology, and porous membrane design. GDL/CL interface gap lead to liquid water pooling in the interfacial void space and reduces contact area, and the impact of GDL/CL interface characteristics on contact resistance and internal mass transfer in MEA are also taken into account. This review highlights the importance of combination design strategy of catalyst layer bulk structure and interlayer structure, and gives a brief discussion about future development prospects. • Interactions among catalysts, carbon supports, ionomers and solvent are discussed. • Novel approaches to engineer the PEM/CL interface are schematically summarized. • GDL/CL interface characteristics influence the contact resistance and internal mass transfer. • Combination design strategy of catalyst layer bulk structure and GDL/PEM/CL interlayer structure.

Low temperature durability and consistency analysis of proton exchange membrane fuel cell stack based on comprehensive characterizations

The cold start is one of the key research fields in proton exchange membrane fuel cell investigation. In this work, a low temperature durability and consistency analysis is performed on a 5-cells stack. After 20 times freezing and thaw cycles, coefficient of variation of individual cell voltage increase significantly. Through the polarization curve test, it is found that the output performance degradation of each individual cell is inconsistent. The No.1 cell which is the nearest from the gas inlets has the maximum performance degradation. Through electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscope, segmented current density test and synchrotron radiation X-ray scan 3D image reconstruction, the cracks on the interface of membrane and catalyst layer are observed in situ successfully for the first time. Under the condition in this work, the water content of No.1 cell is the most, and a lot amount of water is frozen at the air inlet area. Due to the frost heave stress, the structure of membrane electrode assembly is damaged. This causes the loss and the agglomeration of catalyst, and even the delamination of catalyst layer. The above structure changes cause a decrease in catalyst activity and an increase in cell resistance, and eventually lead to the inconsistent degradation in output performance. The characterization results in this work are helpful for the optimization and application of membrane electrodes assembly and stacks for vehicle fuel cells.

Improving cell performance and alleviating performance degradation by constructing a novel structure of membrane electrode assembly (MEA) of DMFCs

The conventional electrodes of direct methanol fuel cells (DMFCs) usually encounter a problem that the catalysts sink into the diffusion layer after a period of operation, causing a lowered catalyst utilization and degraded cell performance. Aiming to alleviate this problem, in this work a novel anode electrode structure is proposed, in which a microporous layer containing Nafion polymer is added between the catalyst layer and the microporous layer with PTFE. The presence of the Nafion-contained layer can expand the three-phase interface region of the electrochemical reactions and improve the utilization of the catalyst. The single cell test showed that the peak power densities of the novel membrane electrode assembly (MEA) fed with 0.5 M and 2 M methanol solutions reached 38.35 mW cm−2 and 101.82 mW cm−2, which increased by 100.42% and 15.27% compared with those of conventional single microporous layer. Electrochemical impedance spectroscopy (EIS) measurements indicated the charge transfer resistance of the conventional MEA structure was increased by 303.78%, while the new one was decreased by 47.91% after continuously operating for 48 h. The anode electrochemical active surface area (ECSA) values of the novel MEA and the conventional MEA were 52.6 m2 g-1 and 44.3 m2 g-1. These experimental results showed that the performance of the double microporous layer MEA was higher than that of the conventional MEA. This new microporous layer structure is promising to be used in fuel cells to improve cell performance and alleviate performance degradation after long-term operating.

Development of high-performance membrane-electrode assembly in unitized regenerative fuel cells

In this work, we investigated the membrane-electrode assembly (MEA) parameters of an oxygen electrode to develop a high-performance unitized regenerative fuel cell (URFC) that can be operated in fuel cell (FC) and water electrolysis (WE) mode. The MEA parameters including gas diffusion layer, ionomer content, oxygen reduction reaction (ORR) type, oxygen evolution reaction (OER) catalyst, and catalyst loading were optimized by calculating the round-trip efficiency of URFC. The performance in FC mode was largely affected by the MEA parameters compared to that of the WE mode performance. The FC mode performance is crucial for the achievement of high URFC performance. The optimized round-trip efficiency was 49% at 500mAcm−2, which is comparable or superior to that reported in literature. This result can be attributed to the highly efficient MEA structure suitable for bifunctional catalysts to participate in both ORR and OER.

Parametric Study of Fabrication Conditions for High-Performance Gas-Diffusion-Electrode-Based Membrane-Electrode Assemblies

In polymer electrolyte membrane fuel cells (PEMFCs), the membrane electrode assembly (MEA) is the critical component for power production as it is responsible for the electrochemical reactions that convert the fuel to power. The two most common MEA architectures are the catalyst-coated membrane (CCM) and the gas-diffusion electrode (GDE), also referred to as catalyst-coated diffusion media (CCDM). GDEs are of interest for MEA fabrication because under some fabrication and/or operating conditions GDE MEAs have shown improved performance over CCM MEAs.[1] GDEs may also have advantages for roll-to-roll manufacturing due to more robust mechanical properties of the diffusion media and avoidance of membrane swelling during catalyst layer coating. However, the challenge with GDEs is that the catalyst layer/membrane interface is usually inferior to that of CCMs, leading to inferior performance. It has been previously demonstrated that adding a layer of ionomer on top of the GDE catalyst layer can improve the performance of GDE-based MEAs [2], though most studies still show performance inferior to that of CCM-based MEAs. Building on this foundation, our work comprehensively investigated and optimized the fabrication and processing conditions (ionomer overlayer thickness, hot pressing) required to produce high performance GDE-based MEAs with performance comparable to CCM-based MEAs. It was demonstrated that coating a thin ionomer overlayer and then hot pressing the GDEs to the membrane could achieve comparable catalyst activity and air performance to CCM MEAs. Atomic force microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with Energy Dispersive X-Ray spectroscopy( EDS) were employed to investigate the MEA structure including the ionomer overlayer, and interfaces of GDL/catalyst and catalyst/membrane. In situ performance testing was used to assess the impact of MEA fabrication conditions on catalyst utilization and air current-voltage performance. Modeling of electrochemical impedance spectroscopy measurements showed lower catalyst layer resistances in the MEAs with the ionomer overlayer, which was in good agreement with the electrode structure observations. The critical amount of ionomer required to achieve maximized performance was defined. The mechanism for improved performance from the ionomer overlayer and hot pressing were proposed and evidenced: 1) adhering the GDE to the membrane to increase the interfacial contact area of catalyst layer/membrane; and 2) creating a smooth GDE surface with a better contact between GDE and membrane. Moreover, the enhancement of the performance by ionomer overlayer was demonstrated to be highly related to the surface roughness of the gas diffusion media (GDM). The GDM with a smoother surface required less amount of the overlayer ionomer than a GDM with a rougher surface to achieve comparable fuel cell performance.

Effect of Dispersing Solvents of Ionomer Binder Solutions in Catalyst Inks on Performance and Durability of Membrane-Electrode Assembly

Proton exchange membrane fuel cells (PEMFCs) is currently considered as a potential next-generation alternative energy technology because of the high energy density and high abundance of hydrogen in nature. However, many issues still exist to be improved in terms of performance and durability for the wide use of PEMFCs as a clean and efficient power system. One of the most important parts in PEMFCs is membrane-electrode assembly (MEA) which provides the electrochemical reactions at electrode where fuel/oxidant (gas), hydrated ions (liquid) and electron (solid) co-exist (so called triple phase boundary), the permeation of fuel and/or oxidant gas and the drainage of water product. In MEA, water and IPA based Nafion dispersion is the most widely used as the ionomer binder for conduction of hydrated ions, formation of pores and adhesion of solid catalyst. The structure of electrodes in MEA could be significantly determined by characteristics of ionomer binder solutions where polymer dispersion interacts with solvent. Upon the structure of the electrodes formed by agglomerates of catalyst and ionomer binder with different size, it has reported that MEA performance and durability would be highly affected. The former could be achieved by reducing overvoltages, and the latter by reducing electrode cracking (i.e., well adhesion of catalyst by ionomer). Electrode structure of MEA, including the Pt-ionomer interface, is determined when electrodes are solidified from catalyst inks during a drying step. Thus, the properties of the solvent system in catalyst inks play an important role in determining the electrode structure in MEA. A fundamental understanding of the phenomenological outcome from the relationship between morphology and property for perfluorosulfonic acid (PFSA) membranes is crucial in obtaining high power and durable MEA. In this study ion and liquid water behavior in PFSA membranes formed from different dispersing solvents were investigated with ionomer particle sizes, solvent viscosity, solvent dielectric constants and distribution which cause various morphology. In addition, MEAs with different electrodes prepared by different ionomer binder solutions were investigated in terms of I-V polarization, cyclic voltammetry, electrochemically-active surface area (ECSA), and chemical stability. Acknowledgment This work was supported by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20173010032100).

Micro-Membrane Electrode Assembly Design to Precisely Measure the in Situ Activity of Oxygen Reduction Reaction Electrocatalysts for PEMFC.

An in situ micro-MEA technique, which could precisely measure the performance of ORR electrocatalyst using Nafion as electrolyte, was designed and compared with regular thin-film rotating-disk electrode (TFRDE) (0.1 M HClO4) and normal in situ membrane electrode assembly (MEA) tests. Compared to the traditional TFRDE method, the micro-MEA technique makes the acquisition of catalysts' behavior at low potential values easily achieved without being limited by the solubility of O2 in water. At the same time, it successfully mimics the structure of regular MEAs and obtains similar results to a regular MEA, thus providing a new technique to simply measure the electrode activity without being bothered by complicated fabrication of regular MEA. In order to further understand the importance of in situ measurement, Fe-N-C as a typical oxygen reduction reaction (ORR) free-Pt catalyst was evaluated by TFRDE and micro-MEA. The results show that the half wave potential of Fe-N-C only shifted negatively by -135 mV in comparison with state-of-the-art Pt/C catalysts from TFRDE tests. However, the active site density, mass transfer of O2, and the proton transfer conductivity are found to strongly influence the catalyst activity in the micro-MEA, thereby resulting in a much lower limiting current density than Pt/C (8.7 times lower). Hence, it is suggested that the micro-MEA is better in evaluating the in situ ORR performance, where the catalysts are characterized more thoroughly in terms of intrinsic activity, active site density, proton transfer, and mass transfer properties.

A New Membrane Electrode Assembly Structure with Novel Flow Fields for Polymer Electrolyte Fuel Cells

A conventional polymer electrolyte fuel cell (PEFC) incorporates a membrane electrode assembly (MEA) which is comprised of the anode catalyst layer-polymer electrolyte -cathode catalyst layer sandwiched between two gas diffusion layers (GDLs), and bipolar plates. Conventional GDL is typically comprised of highly porous carbon paper or cloth to help diffuse the reactant gases onto the electrode, which is coated with a thin micro-porous layer (MPL). At high current densities, mass transport limitations of fuel or oxidizer in the PEFC occur in porous structures of the GDL, particularly at the cathode, which result in a sharp drop in the output voltage. The key to minimizing mass-transport losses is effective water management in the cell. Liquid water in macro-pores of GDL decreases the fuel cell performance at high current density due to the lack of oxygen reaching the catalyst layer. A new MEA structure is recently introduced, where the carbon paper backing layer is eliminated and the entire gas diffusion layer consists of only the MPL [1]. We have further improved on this concept by directly depositing the MPL onto the CCM, resulting in an improvement in the interfacial contact between the MPL and the catalyst layer, as well as a simplified fabrication and assembly process. Spray deposition method was used for depositing this MPL onto a commercial CCM [2]. This concept was proven to work and perform better than a conventional cell with a micro-channel flow field used to provide the desired pathway for the reactant gases throughout the cell substituting for the macro porous carbon paper and the millimeter sized flow field. In this work, a porous foam is utilized as the flow field to distribute the reactant gases over the micro-porous layer instead of the micro-channel flow field. Various pore sizes of the foam, i.e. 60-100 PPI, are used to compare with the conventional micro channel flow field. Contact resistance, permeability, electrochemical impedance spectroscopy, and mechanical properties are used to further characterize the new MEA structure incorporating the porous foam flow field. This method can be an effective way of enabling very high power density operation by reducing the mass transport limitations, and provides an improvement in the interfacial contact between the MEA and the flow field as well as improved mechanical properties. [1] T. Kotaka, Y. Tabuchi, U. Pasaogullari, and C. Y. Wang, Electrochim. Acta, 146, 618 (2014). [2] J. Park, U. Pasaogullari, L. J. Bonville, ECS Transactions, 69, 1355-1362 (2015).