Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Abstract

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, ...

Beyond catalysis and membranes: visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs

“…Among these, water is particularly important in AEMFCs because of its severe intrinsic imbalance. 34,[40][41][42][43] In a PEMFC, water is only generated at the cathode at a rate of 2 water molecules for every 4 electrons transferred. In an AEMFC, there are 4 water molecules generated at the anode in addition to 2 water molecules consumed at the cathode for every 4 electrons transferred (Fig.…”

A high conductivity ultrathin anion-exchange membrane with 500+ h alkali stability for use in alkaline membrane fuel cells that can achieve 2 W cm−2 at 80 °C

“…49 mU cm 2 for the LDPE25-AEM) and the ability to generate higher current densities before the onset of mass-transport limitations: the latter is hypothesised to be due to enhanced, more rapid H 2 O transport from the anode to the cathode. 14,46 The result of this was an impressive 50% increase in peak power density (from 1.35 W cm À2 to 2.02 W cm À2 ), and this fully highlights the advantages of operating AEMFCs with the thinnest AEM possible (as long as this does not compromise mechanical robustness and integrity).…”

Towards membrane-electrode assembly systems for CO2 reduction: a modeling study

“…The CO 2 losses need to be defined and quantified as a function of operating conditions because a portion of the CO 2 is consumed homogeneously at the cathode, transported to and released at the anode (i.e., electrochemically pumped in a similar fashion as that observed in AEM fuel cells 70 ), as expected from reactions (6)- (10) and the transport equations in Table 2. Using the gaseous species fluxes obtained from the model and assuming a 50 sccm CO 2 feed at 100% RH (97 mol% CO 2 ), the CO 2 consumption (the percentage of CO 2 feed that is consumed either electrochemically or by homogeneous reactions) and the CO 2 conversion (the percentage of CO 2 feed that is converted to CO) as a function of the TCD (Fig.…”