Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

Omasta, TJ; Wang, Lianqin; Peng, Xiong; Lewis, Christopher Alan; Varcoe, John R.; Mustain, William E.

doi:10.1016/j.jpowsour.2017.05.006

Cited by 254 publications

(255 citation statements)

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“…For example, Mustain and coworkers have recently demonstrated very high power densities after water management aspects were considered in terms of humidification and minimizing dryout and flooding of both the cathode and anode. 24 …”

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Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Shiau

Zenyuk

Weber

2017

J. Electrochem. Soc.

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Water management is a serious concern for alkaline-exchange-membrane fuel cells (AEMFCs) because water is a reactant in the alkaline oxygen-reduction reaction and hydroxide conduction in alkaline-exchange membranes is highly hydration dependent. In this paper, we develop and use a multiphysics, multiphase model to explore water management in AEMFCs. We demonstrate that the low performance is mostly caused by extremely non-uniform distribution of water in the ionomer phase. A sensitivity analysis of design parameters including humidification strategies, membrane properties, and water transport resistance was undertaken to explore possible optimization strategies. Furthermore, the strategy and issues of reducing bicarbonate/carbonate buildup in the membrane-electrode assembly with CO 2 from air is demonstrated based on the model prediction. Overall, mathematical modeling is used to explore trends and strategies to overcome performance bottlenecks and help enable AEMFC commercialization. Among existing fuel-cell types, alkaline-exchange-membrane fuel cells (AEMFCs) or hydroxide-exchange-membrane fuel cells (HEMFCs) have intriguing features as compared to proton-exchangemembrane fuel cells (PEMFCs). Their advantage is mainly the possibility of using non-noble catalysts due to faster oxygenreduction-reaction (ORR) kinetics in alkaline media than in acidic media 1-4 as well as perhaps the use of less expensive hydrocarbon membranes. Disadvantages of AEMFCs include lower hydrogenoxidation-reaction (HOR) kinetics, more complicated water management, and lower intrinsic conductivity for hydroxide compared to proton transport. [5][6][7][8][9][10][11][12][13] In addition, CO 2 in the air reacts with hydroxide ions to form bicarbonate and carbonate ions, possibly hindering the terrestrial development of AEMFCs by removing hydroxide available for HOR and further limiting hydroxide conductivity. 14,15 Compared to conventional alkaline fuel cells (i.e., using a liquid electrolyte of potassium hydroxide), AEMFCs use a polymeric membrane and should have better tolerance for CO 2 because the precipitate K 2 CO 3 is absent in the AEMFC due to the lack of mobile cations in the membrane. 16 However, AEMFCs still suffer a decline in performance due to bicarbonate/carbonate formation in the membrane and catalyst layers (CLs) leading to additional ohmic and kinetic losses. [13][14][15] In an AEMFC, the following electrochemical reactions occur in the CLs as follows:Net :Hydrogen combines with hydroxide ions to produce water in the anode. Oxygen reacts with water to generate hydroxide ions in the cathode. Along with the transport of ions from cathode to anode, electro-osmosis moves water from cathode to anode. As shown in the reactions, AEMFCs are expected to have more complicated water management. Water production by the HOR and electro-osmosis may cause flooding in the anode. Since water is a stoichiometric reactant at the cathode, dehydration may occur and limit ORR kinetics as well as ionic conduction in the membrane and cathode CL, ...

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confidence: 99%

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Shiau

Zenyuk

Weber

2017

J. Electrochem. Soc.

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“…When water management issues are well controlled, there is no significant difference between point by point and linear sweep polarization curves, which was shown in our previous work for multiple high performing cells. 14,15 Additionally, tests were repeated after multiple hours and varying the testing conditions to ensure stability, recoverability, and repeatability.…”

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“…3,14,15 Properties of the catalyst layer such as structure, thickness, porosity, component chemistry, and * Electrochemical Society Student Member. * * Electrochemical Society Member.…”

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“…[8][9][10][11][12][13] Additionally, catalyst layer engineering that allows for improved water management, both on the macro-scale 14 and on the micro-scale, 15 has led to AEMFCs that are able to achieve peak power densities nearing 2 W cm −2 and demonstrate operational stability exceeding 500 hours. These are crucial steps toward the realization of commercially viable AEMFCs; however, there remain additional hurdles to overcome, specifically AEMFCs with low platinum group metal (PGM) loadings that are able to not only achieve high power densities but sustain them over long term operation.…”

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“…24,25 Therefore, it is important for researchers in the field to investigate electrode compositions that allow for reduced catalyst loading -the most active hydrogen evolution catalyst known today is PtRu 26 -while still allowing for the water produced during the hydrogen oxidation to be properly managed, a crucial step in the successful operation of AEMFCs. 3,14,15 Properties of the catalyst layer such as structure, thickness, porosity, component chemistry, and ionomer:carbon:catalyst ratio all have a significant effect on the water content and balance of the overall cell.…”

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Strategies for Reducing the PGM Loading in High Power AEMFC Anodes

Omasta

Zhang²,

Park

et al. 2018

ECS Trans.

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Anion Exchange Membrane Fuel Cells (AEMFCs) have experienced a significant rise in attention in recent years, largely motivated by the potential to overcome the costs that have plateaued for proton exchange membrane fuel cells. However, despite significant advances in power generation, membrane conductivity, membrane stability, and catalyst activity, the vast majority of high performing AEMFCs are fabricated with a high PGM loading (0.4-0.8 mg cm −2 ). This work demonstrates an electrode fabrication method that reduces the anode catalyst loading by 85% while still achieving performance ca. 1 W cm −2 -accomplished by designing a multilayered electrode comprised of an optimized ionomer:carbon:PGM ratio catalyst layer coupled with a hydrophobic microporous layer. If paired with a high-performing PGM-free cathode, this new anode shows the potential to meet existing DOE PGM loading and performance targets. Anion exchange membrane fuel cells (AEMFCs) have received significant attention in recent years as a potentially lower cost electrochemical energy conversion device than proton exchange membrane fuel cells (PEMFCs).1-3 There have been several major advancements in the materials and operational understanding of the AEMFC in the past few years, which have allowed AEMFCs to close the performance gap with PEMFCs. Improvements in membrane stability and conductivity have been at the forefront of the material improvements, with conductivities rivaling Nafion, 4-10 and an increasing number of membranes showing stability in highly alkaline environments (up to 2 M KOH at 80• C) for 100s or even 1000s of hours. [8][9][10][11][12][13] Additionally, catalyst layer engineering that allows for improved water management, both on the macro-scale 14 and on the micro-scale, 15 has led to AEMFCs that are able to achieve peak power densities nearing 2 W cm −2 and demonstrate operational stability exceeding 500 hours. These are crucial steps toward the realization of commercially viable AEMFCs; however, there remain additional hurdles to overcome, specifically AEMFCs with low platinum group metal (PGM) loadings that are able to not only achieve high power densities but sustain them over long term operation. 12In order to reduce the PGM loading in operating AEMFCs, there will need to be at least some development of non-PGM catalysts. Due to differences in the water dissociation behavior in alkaline media, 16 the kinetics for the hydrogen oxidation reaction are hindered in alkaline media compared to acid media. 17 The opposite is true of the oxygen reduction reaction at the cathode; therefore, it is more likely that the AEMFC will see a high performance PGM-free cathode electrode. Indeed, some very promising catalysts have already been identified. 3,[18][19][20][21][22][23] However, at the AEMFC anode, it is very likely (akin to the PEMFC cathode) that it will be difficult to move completely away from PGM-based catalysts, though this is presently an active area for research.24,25 Therefore, it is important for researchers in the field to ...

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2021

Nanotechnology and Nanomaterials for Energy

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Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

Cited by 254 publications

References 39 publications

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Strategies for Reducing the PGM Loading in High Power AEMFC Anodes

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