Performance, stability, and transport studies of hydroxide exchange membrane electrolyzers

Abstract

With the global transition to renewable energy, some sectors like the chemical industry, heavy-duty transportation sector, and long-term energy storage are difficult to decarbonize by electricity alone. Green hydrogen production by water electrolysis can play a critical role to meet these needs, but it must reach economic viability before widespread commercial implementation is possible. The current electrolysis technologies include alkaline water electrolyzers (AELs) and proton exchange membrane electrolyzers (PEMELs). AELs use a concentrated liquid alkaline electrolyte and are very durable but have limited efficiencies for hydrogen production. PEMELs use a zero-gap solid polymer electrolyte to achieve high efficiencies and hydrogen production rates, but they generate an acidic environment that is corrosive to all but expensive titanium and platinum-group metal (PGM) materials. Hydroxide exchange membrane electrolyzers (HEMELs) are a promising new technology that uses a solid hydroxide-conducting polymer electrolyte. This balances the efficiency of PEMELs with the alkaline environment of AELs, giving HEMELs the opportunity to produce green hydrogen at high efficiencies with low capital cost. Currently, HEMELs are a very immature technology which requires improvements in performance and stability before it is commercially viable. In this work, we strive to understand the operating mechanisms of HEMELs and how they contribute to performance and stability. We then propose new designs of the membrane electrode assembly (MEA) and operating conditions to improve performance and stability. ☐ The HEMEL anode is one of the greatest sources of overpotential due to its limited kinetic activity and poor stability. Through a novel corrosion-based synthesis of a self-supported NiFeOOH-20F oxygen evolution electrocatalyst, we eliminate the wash out of catalyst. We investigate synthesis techniques to increase the catalyst loading and alter the microstructure of the electrode to further improve performance. In a careful investigation of three different microstructural parameters of the self-supported anode, we determine that anodes with small pores and high porosity can yield improved HEMEL performance. ☐ With these improved electrodes, we study the water transport in HEMELs operating at high current densities, where the cathode can rapidly lose water and demonstrate increases in overpotential. We isolate the sources of this overpotential and determine that the dry out of polymer lowers the conductivity of the electrolyte, particularly within the cathode catalyst layer and at the interface between catalyst layer and membrane. By increasing the ionomer content in the catalyst layer and introducing an interfacial layer of ionomer at the membrane, we reduce the overpotential at high currents by more than 500 mV and determine that maintaining a conductive interface is most important to reduce the efficiency losses associated with cathode dry out. ☐ Lastly, we explore the tolerance of HEMELs to potential contaminants that might be present under commercial operation. We find that CO2 carbonation can poison the MEA with carbonate and bicarbonate ions, similar to carbonation in HEM fuel cells. These unfavorable ions can reduce the conductivity of the membrane and ionomer and cause additional overpotentials through the formation of a pH gradient across the cell. We determine that (bi)carbonates are not a serious detriment to cell performance and durability because they can be purged to the anode at high current densities, where they are decomposed back into CO2 and removed, at the cost of up to 200 mV. Operation with dilute KOH can remedy this problem by reducing the pH gradient. Other anion contaminants, however, such as those found in tap water, can cause detrimental performance and stability losses and corrode the NiFe anode catalyst. Chloride and bromide at tap water concentrations are likely responsible for the corrosion, suggesting that removing halide contaminants or using grades of water in between DI and tap water are worth pursuing in future research. These findings can help increase HEMEL efficiencies to make them competitive with existing water electrolysis technologies and act as guidelines for commercial operation.