Highly Active Iridium Doped Titanium-Spinels for the Acidic Oxygen Evolution Reaction in PEM-Water Electrolysers

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Introduction: In proton exchange membrane water electrolysers (PEMWE) the oxygen evolution reaction (OER) is the rate determining step and currently catalysed by scarce and expensive iridium oxides. Because of the high iridium price and limited iridium availability, electrolysers using pure IrO2 will struggle to fulfil the growing demand of affordable green hydrogen.[1] However, in the harsh environment inside a PEMWE anode, no non-noble metal catalyst with sufficient activity and stability is known.[2] A promising approach to reduce the iridium needed for the OER is to embed iridium into different titanium containing non-noble host oxides (e.g., Mg2TiO4) as we show for spinels in this work. In this way, the iridium atoms are ultimately dispersed in the host matrix. In line, the iridium mass specific activity increases and the iridium sites are stabilized through the presence of titanium inside the oxide.[3] This leads to catalysts that maintain high activity and stability with significantly lower iridium content than pure IrO2, even at thermodynamically favoured elevated temperatures.[4]Materials and methods: Metal-oxides with different iridium molar fractions are synthesized via a modified sol-gel approach. The successful inclusion of iridium in the non-noble oxide crystal structure is proven by XRD and ICP measurements. After further analysis (e.g., SEM, conductivity, surface area (N2-physisorption and ECSA)), the OER-activity is determined by half-cell experiments on a rotating-disc-electrode (RDE). Additionally, the catalysts are aged for one to several days at elevated temperature (> 150 °C) in a wet atmosphere to get an idea of the catalyst’s structural behaviour for the subsequent application. First attempts to test the catalyst in a full PEMWE-cell are performed as well. AlfaAesar® IrO2 (AA-IrO2) is used as a reference catalyst.Results and discussion: The different investigated iridium containing spinels show an enhanced iridium mass specific activity compared to the commercially available iridium oxide. Our best performing material Mg2Ti0.7Ir0.3O4 has around triple the iridium mass specific activity at 1.575 V vs. RHE than the IrO2 reference (figure 1a and b). For the activity normalized to the electrocatalytic surface area the performance increase of our catalyst is even more pronounced, indicating highly active Ir-sites. The iridium to titanium ratio as well as the calcination conditions show a strong influence on the crystallite size, ECSA and consequently the electrocatalytic activity of the system. As can be seen in the XRDs in figure 1c and d, lower calcination hold times (tcalc_hold, figure 1c) and temperatures (Tcalc, figure 1d) favour the insertion of iridium in the spinel leading to highly active catalysts. Once a second phase, assigned to rutile IrO2 forms at slightly harsher calcination conditions the OER performance drops. This depicts the importance of the right synthesis parameters. The observed highly active Ir-sites can, in line with literature, be attributed to either the restructuring of the catalyst surface[5], an activation of previously inert titanium[1] or to the interactions of the IrO6-octahedra with neighbouring metals inside the crystal structure.[6]Conclusion: Our approach significantly enhances the iridium mass specific activity of the single Ir-sites. Embedding iridium into a non-noble titanium containing host matrix therefore is a promising approach to achieve high electrocatalytic activity. Therefore, the iridium demand for the acidic OER in PEMWE decreases.