Establishing Structure-Function Relationship of Iridium-Cobalt Oxide As Oxygen Evolution Reaction (OER) Catalyst in Acidic and Alkaline Media
Authors: Labata Marc Francis , Li Guangfu , Altoe Maria Virginia , Chuang Po-Ya Abel 
Published: July 2021

Owing to its unique electrochemical properties and high energy density, hydrogen is one of the leading alternative energy carriers towards a growing share of decarbonized society. However, current hydrogen production is still heavily relies on fossil fuels, with natural gas reforming as the primary source. Electrochemical water splitting process is an environmentally friendly approach when coupled with renewable energy technologies, such as solar or wind. Despite its high cost, iridium oxide (IrO2) is the most commonly used catalyst for the sluggish oxygen evolution reaction (OER). To further enhance the performance and reduce the cost, it is essential to establish a fundamental understanding of OER mechanism. Combining Ir with other non-noble metals is one strategy to improve Ir utilization that would exhibit the advantageous properties of both materials. Cobalt is one of the metals considered in the literature that can be combined with Ir to enhance its activity. Previous literature reported that incorporating Co into the IrO2 structure can modify the conductivity and the electrochemical properties of the catalyst. In this work, we synthesized binary Ir-Co metal oxide in a 1:1 molar ratio via surfactant-assisted Adam’s fusion method to control its structure. The electrochemical OER performance was evaluated using half-cell tests in acidic and alkaline media. Preliminary results show that incorporating Co with Ir improved the catalytic activity compared to homemade IrO2 and commercially available catalysts of Ir black and IrOx. By employing acid etching process, the performance of homemade Ir-Co oxide was further improved in both acid and alkaline media. In terms of durability, despite the enhanced beginning of life performance, the acid-etched Ir-Co sample show significant activity loss (~ 34%) in alkaline media. In contrast, for acid media, both the non-etched and the etched Ir-Co samples show similar and minor degradation (~8-9%). In our results, catalysts without Co content (homemade and commercial) did not show any degradation in acid media and lower degradation rate than Co-containing catalysts in alkaline media. Full-cell electrolyzer stability tests were done, and the results show similar degradation behavior compared to half-cell experiments. To establish a better understanding of the mechanism, several physicochemical characterizations were performed to establish potential structure-function relationships. The X-ray diffraction results indicate that the non-etched sample formed a biphasic oxide dominated by rutile IrO2 (101) and Co3O4 (311). Meanwhile, the acid-etched sample only shows the IrO2 (101) peak, suggesting that Co atoms are either removed or doped in the IrO2 matrix. Energy dispersive x-ray spectroscopy performed in scanning transmission electron microscope and x-ray photoelectron spectroscopy both confirmed the elemental composition and agreed well with the XRD results. The initial findings from this work propose a scalable synthesis method for Ir-Co oxide electrocatalyst, achieving higher performance with a reduced Ir loading.