Hydrogen production by alkaline water electrolysis using hydroxide anion exchange membranes has seen a renewed interest as an alternative to traditional liquid alkaline water electrolyzers, proton exchange membrane (PEM) electrolyzers, and solid oxide water electrolyzers (1). Anion exchange membrane electrolyzers provide an efficient, modular, and reliable method to produce hydrogen from water and renewable electricity sources. One of the main benefits of anion exchange membrane electrolyzers arises from better oxygen evolution reaction (OER) kinetics in alkaline environments and the possibility of employing non-platinum group catalysts (non-PGM) and PGMs modified with transition metals for enhanced activity without suffering the stability problems common in acid media. However, one challenge is the sluggish kinetics for the HER in alkaline media.

During our comprehensive research into active and stable OER catalysts for alkaline operation, pyrochlores emerged as among the most active and stable OER catalysts reported in the literature. Electrically conducting metal oxides (eliminating the need of a conducting support for the catalyst) with the pyrochlore structure (A₂B₂O_7-y, with A=Pb or Bi and B= Ru, Ir or Os) were synthesized via precipitation/crystallization in alkaline medium and/or via solid-state reaction. The electrocatalytic activity for the oxygen evolution reaction (OER) in 0.1M KOH showed that lead and bismuth ruthenate pyrochlores had significantly lower overpotentials for the OER than the state-of-the-art IrO₂ catalyst. Specific activities (at 1.5 V vs. RHE) of 3.0±0.2 Am^-2, 1.3±0.2 Am^-2 and 0.06±0.01 Am^-2 were obtained for Pb₂Ru₂O_6.5, Bi_2.4Ru_1.6O₇, and IrO₂ respectively. Specific activities for iridate-based pyrochlores (0.3-0.5 Am^-2) were 5-10 times lower than those for ruthenate-based pyrochlores. Lead osmate pyrochlore showed the lowest OER activity among all the pyrochlores evaluated, with a specific activity of 0.10±0.07 Am^-2. The exceptional OER activity and stability of lead ruthenate pyrochlore catalysts were evaluated in an anion exchange membrane water electrolyzer. The overpotentials obtained were 0.1-0.2 V lower than for IrO₂ across the entire current density range and the performance was stable for at least 200 h.

Since operation in alkaline media leads to more sluggish HER kinetics, we have evaluated ways to improve the HER catalytic kinetics of Pt electrocatalysts. Bifunctional electrocatalysts containing Pt clusters to combine the protons to form hydrogen gas, and hydrophilic domains able to help in water dissociation are promising candidates to substantially improve HER kinetics under alkaline conditions and to facilitate the introduction of commercial alkaline membrane water electrolyzers. The hydrophilic moieties that help the water splitting are varied. We have investigated nickel hydroxide and ruthenium oxide as co-catalysts together with Pt. Rotating disk electrode (RDE) measurements were performed in 0.1M KOH at temperatures ranging from 273.15 K to 303.15 K, and the HOR/HER kinetic currents, obtained after IR and mass transport corrections, were fitted using the Butler-Volmer equation to estimate the exchange current densities at each temperature. Arrhenius plots showed very similar activation energies for Pt/C (35±6 kJ/mol) and the bi-functional catalysts (38±6 kJ/mol) –Pt/C/X%Ni(OH)₂. The maximum exchange current density (2.44±0.07 mA cm^-2_Pt at 303.15 K) was obtained with the catalyst containing 10wt% Ni(OH)₂, and was 2.4 times higher than for Pt/C (1.03±0.07 mA cm^-2_Pt at 303.15 K). The bi-functional catalysts were evaluated in an anion-exchange membrane water electrolyzer operated with ultrapure water, and outperformed Pt/C by about 0.15 V across the entire current density range. Similarly, electrolyzer experiments showed that Pt/C/10%Ni(OH)₂ performs as well as Pt/C with only half the Pt loading. Long-term stability experiments in the electrolyzer are ongoing.

We have also found enhanced HER activity in a Pt electrocatalyst deposited onto a mixed-metal-oxide support composed of titanium dioxide (TiO₂) and ruthenium dioxide (RuO₂). The Pt/RuO₂-TiO₂ (Pt/RTO) electrocatalyst outperformed the benchmark Pt/C over the entire range of temperatures evaluated (275-313K). The exchange current density for Pt/RTO was 2.31±0.06mAcm^-2_Pt (at 295 K), which was more than five times the value for Pt/C. In a solid-state alkaline water electrolyzer, MEAs fabricated with Pt/RTO in the cathode and IrO₂ as anode catalyst exhibited a 100-200mV reduction in the cell voltage, when compared to MEAs fabricated with Pt/C in the cathode.

References

1. A. Brisse, J. Schefold and M. Zahid, Int. J. Hydrogen Energy, 33, 5375 (2008).