Iridium Core/Shell Catalysts for PEM Water Electrolyzer Anodes Synthesized Via Galvanic Exchange
Authors: Thorbjørnsen Kristian Fredrik Klepp , Singh Gurvinder , Manikandan Maidhily , Tolchard Julian Richard , Thomassen Magnus Skinlo , Sunde Svein 
Published: February 2020

The transfer to renewable energy requires stable long-term storage of any surplus energy created by e.g. solar power. Water electrolysis is a simple method for conserving energy in the form of hydrogen gas. Offering several advantages compared to alkaline water electrolysis, the polymer electrolyte membrane water electrolyzers (PEMWEs) can respond more rapidly to fluctuations in the power grid and enables a more compact cell design[1]. However, the components used in PEM cells are more costly. As different cost-reduction measures are being implemented, the anode catalysts fraction of the total cost is increasing. This calls for reducing the amount of scarce and precious iridium catalyst for the oxygen evolution reaction (OER).
Galvanic exchange is a well-known way of producing metal thin-films on different metal substrates, where the substrate acts as both a reducing agent and template for a more noble metal. Applying iridium as a thin film on to a suitable substrate using galvanic exchange is a promising way of drastically reducing the iridium loading in PEMWE units. Although a substantial amount of effort has been made in producing core/shell electrocatalysts by galvanic exchange, not so much has been done on iridium structures[2]. A few groups have used nickel as a template for exchange in aqueous solution, either in the form of a thin metal foil[3, 4] or as nickel electrodeposited on to a glassy carbon electrode[5] or a nickel electrode[6]. Copper has also been used as a template in organic solvents [7, 8]. Although yielding promising results, the use of small-scale synthesis methods using expensive and toxic organic components is not feasible for industrial upscaling. Thus, we aim to use simple, scalable water-based methods for the synthesis of iridium electrocatalysts via galvanic exchange.
In this work, we have exchanged nickel and copper nanoparticles supported on metal oxides (MOx) for iridium by galvanic exchange in an aqueous iridium solution. The Ir:M’@MOx electrocatalysts (M’=Ni,Cu) have been evaluated for stability and activity towards the OER in acidic media. For comparison, Ir:M’ catalysts were also made from a simple organic phase synthesis of M’ nanoparticles[9], which were subjected to galvanic exchange by iridium using different methods. Figure 1 illustrates the first cycle during cyclic voltammetry in acidic media of Ir:Ni electrocatalysts synthesized from galvanic exchange between iridium and nickel in aqueous iridium solutions at different temperatures. As is clearly shown in the figure, the sample synthesized at 95⁰C displays voltammetric signatures consistent with the formation of iridium oxide at the substrate. In addition to outlining the conditions for the formation of iridium oxide, polarization data and other characteristics of iridium oxide made by galvanic displacement will be described. The discussion will emphasize the feasibility of manufacturing core/shell or other overlayer structures of iridium for application in PEM water electrolysis.

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Captions

Figure 1: Cyclic voltammogram (1st cycle) of Ir:Ni electrocatalysts in acidic media. The Ir:Ni catalysts were synthesized by galvanic exchange between iridium and nickel in aqueous iridium solutions at different temperatures. As clearly shown, the temperature of the aqueous iridium solution strongly affects the oxidation state of the iridium in the resulting Ir:Ni catalyst.

Figure 1