The Influence of Tin Addition on the Electrochemical Performance of IrO2 and Ir Black Oxygen Evolution Catalysts in Polymer Electrolyte Membrane Electrolysis

Abstract

Polymer electrolyte membrane (PEM) electrolysis is widely considered a promising technology for energy storage and clean hydrogen production, due to the advantages it has over state-of-the-art alkaline water electrolysis.1 Typically, PEM electrolysers can operate at higher current densities than their alkaline counterparts, have a more robust design, and due to the zero-gap design, exhibit smaller Ohmic losses. Furthermore, PEM electrolysers can yield hydrogen at elevated pressures, eliminating the need for compression. Nevertheless, a series of challenges hinder the large-scale commercial implementation of PEM electrolysers. One of the most critical drawbacks is the need for noble metal electrocatalysts, especially at the anode of the cell, such as iridium or ruthenium, to catalyze the oxygen evolution reaction (OER) (1).2H2O → O2 + 4H+ + 4e- E° = 1.23 V vs NHE (1)These metals are used due to their high catalytic activity and long-term stability in the corrosive acidic environment of the PEM cells and their replacement by more available materials is difficult. Over the years, the main efforts of the scientific community towards solving this challenge were focused on increasing the noble metal dispersion, by using various high surface area catalyst supports, using core-shell catalysts, with the aim to minimize the amount of noble metal, modifying the morphology of the electrode, to improve mass transport limitations and electric contact between components and maximize noble metal utilization.2-4 Antimony-doped tin oxide has been reported as a stable OER catalyst support, offering sufficient electronic conductivity at the same, in order to minimize Ohmic losses.5 Furthermore, recent reports indicate that SnO2 does not only act as a catalyst support but takes part in catalysis.6 Building upon literature reports, we prepared a series of Sn/Ir OER catalysts by a two-step sodium borohydride reduction method, in which Sn nanoparticles were deposited over the surface of iridium black particles, decorating it. Catalysts with varying Sn content were incorporated in membrane electrode assemblies (MEAs) and their cell performance was evaluated in a single cell PEM electrolyser. We found that catalysts with up to 20 wt% tin content exhibited an improved cell performance compared to MEAs with similar iridium black loading. We modeled Ohmic, kinetic and mass transport overpotential contributions to experimentally acquired polarization curves and identified that the better cell performance arises mainly from the higher intrinsic catalytic activity, reflected by a higher apparent exchange current density of Sn/Ir OER catalysts compared to Ir black catalysts. The Sn/Ir MEAs also displayed lower mass transport overpotentials, which indicate improved transport in the catalyst layer. This can be due to a more porous structure and less agglomeration of iridium particles induced by the presence of tin over their surface.Interestingly, this performance improvement was only observed for iridium black catalysts. We prepared a series of IrO2-SnO2 catalysts by solution combustion synthesis method, which is known to yield high surface area disperse oxide materials. Comparing the performance of these catalysts to commercial iridium oxide catalysts in the PEM electrolysis cell, the MEAs with SnO2-containing catalysts displayed worse cell performance than MEAs with commercial iridium oxide. Modelling the overpotential contributions showed a lower apparent exchange current density, indicating a lower intrinsic catalytic activity of IrO2-SnO2 catalysts, as well as a higher Ohmic resistance, due to SnO2 addition. These observations indicate the importance of iridium oxidation state for harnessing the promotion effect of tin addition and give important insights into designing more efficient OER catalysts for PEM electrolysis cells.