F Doped (Ir,Sn,Nb)O2 oxygen Evolution Reaction Electro-Catalysts for PEM Water Electrolysis

Hydrogen possesses the potential to provide clean, reliable, and affordable energy supply to meet the growing global demand. A promising way to produce hydrogen is by splitting water using electricity. Large capital costs of current electrolyzer technologies are a major barrier to attaining the targeted hydrogen production cost. High capital costs arise due to the use of expensive catalyst materials, relatively small systems, relatively low efficiencies of the system, customized power electronics, and labor intensive fabrication. In the case of proton exchange membrane (PEM) electrolysis cells in particular, use of non-precious metal electro-catalysts for electrodes exhibiting high electrochemical activity and durability would considerably decrease the overall capital costs. Decreasing the noble metal loading as well as improving the electro-catalytic activity and durability could be achieved using novel synthesis techniques to generate high specific surface area (HSA) nanostructured electro-catalysts, or improved low cost support structure (diluents) for electro-catalysts.

The present work is carried out to identify a novel support for electro-catalysts which can decrease the precious metal loading without compromising the electrochemical activity of the noble metal oxide electro-catalyst. F-doped SnO₂ with different F concentration has already been extensively studied in order to improve the electrical conductivity of SnO₂. Accordingly, we have shown in a recent publication that (Ir,Sn,Nb)O₂ is a promising anode electro-catalyst resulting in ~60 mol.% reduction in IrO₂ content without compromising the electrochemical activity¹. Also, F has been shown as a novel dopant for IrO₂ leading to improvement in the electro-catalytic activity of the oxygen evolution reaction in PEM electrolysis². We also showed that F doped SnO₂ as a support with IrO₂ and/or RuO₂ gives similar electrochemical activity with ~80 mol.% reduction in the noble metal oxide content^3,4. In this study, F doped (Sn,Nb)O₂ has been used as a support to be combined with IrO₂as the catalyst for anode oxidation in PEM based water electrolysis.

The present investigation is thus directed at the synthesis and characterization of F doped ternary Ir-Sn-Nb oxide solid solution systems. Consequently, solid solutions of F along with IrO₂, SnO₂ and NbO₂, denoted as (Ir,Sn,Nb)O₂:F, corresponding to composition (Ir_0.30Sn_0.35Nb_0.35)O₂:x wt. % F, where x ranges from 0 to 20% has been synthesized and studied. The catalyst containing 10 wt. % F exhibits electrochemical activity and stability matching that of pure IrO₂. Results of the synthesis, structural, microstructural, and electrochemical activity of these novel catalysts will be presented and discussed. Figure 1 shows the polarization curve of (Ir,Sn,Nb)O₂:F electro-catalysts in 1 N sulfuric acid conducted at a scan rate of 1mV/sec. In addition, to obtain a better understanding of the fundamental electrochemical activity and long term stability of the (Ir,Sn,Nb)O₂:F electro-catalyst, first-principles calculations of the total energies and electronic structures of the model systems with chemical compositions similar to those of the experimentally synthesized materials have been carried out to complement the present experimental study.

As a result, (Ir_0.30Sn_0.35Nb_0.35)O₂:F doped with ~10 wt.% of F is potentially a preferred oxygen evolution reaction (OER) electro-catalyst composition for water electrolysis resulting in almost ~70% reduction in noble metal content. Thus, the composition identified combined with its excellent performance can be considered viable for contributing to significant reduction in the overall capital cost of PEM based water electrolyzers.

References:

1. Kadakia K. et al., International Journal of Hydrogen Energy 2012; 37: 3001-3013.

2. Kadakia K. et al., Journal of Power Sources 2013; 222: 313-317.

3. Datta M.K., Kadakia K. et al., Journal of Materials Chemistry A 2013; 1: 4026-4037.

4. Kadakia K. et al., Journal of Power Sources 2014; 245: 362-370.

Acknowledgements: Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0001531. PNK also acknowledges the Edward R. Weidlein Chair Professorship funds, NSF and the Center for Complex Engineered Multifunctional Materials (CCEMM) for partial support of this research.