Elucidating the Role of Hydroxide Electrolyte on Anion-Exchange-Membrane Water Electrolyzer Performance

Abstract

Many solid-state devices, especially those requiring anion conduction, often add a supporting electrolyte to enable efficient operation. The prototypical case is that of anion-exchange-membrane water electrolyzers (AEMWEs), where addition of an alkali metal solution improves performance. However, the specific mechanism of this performance improvement is currently unknown. This work investigates the functionality of the alkali metal solution in AEMWEs using experiments and mathematical models. The results show that additional hydroxide plays a key role not only in ohmic resistance of the membrane and catalyst layer but also in the reaction kinetics. The modeling suggests that the added liquid electrolyte creates an additional electrochemical interface with the electrocatalyst that provides ion-transport pathways and distributes product gas bubbles; the total effective electrochemical active surface area in the cell with 1 M KOH is 5 times higher than that of the cell with DI water. In the cell with 1 M KOH, more than 80% of the reaction current is associate with the liquid electrolyte. These results indicate the importance of high pH of electrolyte and catalyst/electrolyte interface in AEMWEs. The understanding of the functionality of the alkali metal solution presented in this study should help guide the design and optimization of AEMWEs.

Construction of Integrated Electrodes with Transport Highways for Pure‐Water‐Fed Anion Exchange Membrane Water Electrolysis

“…[ 37 ] In pure water, without the additional transport pathways enabled by liquid electrolytes, the ECSA at the catalyst/liquid electrolyte interface maybe not effectively utilized (Figure S18, Supporting Information), which results in kinetic losses because less ECSA is available to support the reaction. [ 38 ] To further verify this, the impact of KOH concentration on electrochemical performance is investigated using both three‐electrode and single‐cell evaluation methods (Figures S19–S20, Supporting Information). The results indicate that the increased concentration of ammonium hydroxides in the electrode, which is beneficial to OER kinetics.…”

What is Next in Anion‐Exchange Membrane Water Electrolyzers? Bottlenecks, Benefits, and Future

“…The neutral pH of water leads to several drawbacks, such as high ohmic drop through the anode layer, increased OER overpotential compared to high pH conditions, and decreased thermodynamic stability for many metal oxides, and in particular for PGM-free metal oxides. [125,128,130] This view of the anode being composed mainly of an electrocatalyst-liquid interface and with a minor contribution from the catalyst-AEI interface was recently supported by different stability behavior with deionized water Table 3. Examples of highly active HER and OER electrocatalysts.…”

“…[116,[125][126][127] Several studies concluded that a dilute KOH feed (0.01-1.0 m KOH) is critical to reaching a high performance with PGMfree anodes in AEM-WE. [52,125,[127][128][129] Figure 6A is an example of the effect of the anolyte feed nature on the AEM-WE performance, in otherwise fixed conditions and for a same MEA, comprising a CuCoO x anode. The effect is huge and can be explained by increased OER overpotential on CuCoO x at neutral pH compared to high pH, and also increased cell ohmic resistance.…”

“…Several studies concluded that a dilute KOH feed (0.01–1.0 m KOH) is critical to reaching a high performance with PGM‐free anodes in AEM‐WE [52,125,127–129] . Figure 6A is an example of the effect of the anolyte feed nature on the AEM‐WE performance, in otherwise fixed conditions and for a same MEA, comprising a CuCoO x anode.…”

“…Thus, when pure water is fed to the anode, most of the electrocatalytic area comprises an electrocatalyst–water interface (represented by the dark grey outlines in Figure 6B), with a minor fraction of the electrocatalytic area consisting of a true AEI‐electrocatalyst interface (represented by the black arcs in Figure 6B). The neutral pH of water leads to several drawbacks, such as high ohmic drop through the anode layer, increased OER overpotential compared to high pH conditions, and decreased thermodynamic stability for many metal oxides, and in particular for PGM‐free metal oxides [125,128,130] . This view of the anode being composed mainly of an electrocatalyst–liquid interface and with a minor contribution from the catalyst‐AEI interface was recently supported by different stability behavior with deionized water feed than with 0.1 m KOH feed on a CuCoO x anode, showing that the change in initial performance is not only due to a lower electrochemical area with a water feed (as might have been the case, if water leads, e. g., to different wetting properties than dilute KOH), but is due to a different pH experienced by most of the electrocatalytic surface [125] .…”