Due to the ever-rising need for clean energy solutions, efficient utilizing of renewable sources is one of the most important research tasks. However, power generation from renewable sources is inevitably intermittent, which lowers the efficiency and obstructs practical use. The water electrolyzer has attracted much attention as a promising energy storage system in grid scale to solve the intermittency issue. The electrolyzer is a catalytic energy converter that converts electric energy into chemical energy in the form of hydrogen gas. Hydrogen gas production is also important because it is a clean energy carrier with zero emissions. Proton exchange membrane (PEM) based electrolyzer has better efficiency and produces hydrogen gas in higher pressure with a compact design compared to its alkaline counterpart. PEM water electrolyzer can produce pure hydrogen gas without CO impurity which is catalyst poison of Pt electrocatalyst in PEM fuel cells. Major efficiency loss of the electrolyzer comes from high overpotential at the anode reaction, oxygen evolution reaction (OER). A suitable electrocatalyst is needed to reduce the high overpotential. Ir is the only metal that can withstand the highly corrosive environment of the anode in acidic conditions with fine activity. However, Ir is even scarcer than Pt, thus development of Ir based efficient electrocatalysts is an urgent issue for the commercialization. Herein, we report our experimental results about tuning electrochemical property of Ir catalysts to enhance the performance of PEM water electrolyzer device.

Recently, it has been realized that the low cost of 3d metals such as Ni, or Cu can boost the OER activity of Ir oxide. To exploit the synergy, we made several shaped Ir-Ni bimetallic nanoparticles, Ir-Ni TL, Ir-Ni SC, and Ir-Ni LP.^[1] The bimetallic nanoparticles exhibited enhanced OER performance in half-cell experiments; especially, Ir-Ni TL which greatly improved activity. However, they could not be applied to the PEM electrolyzer. Although, not only from our group, many other groups have also reported fancy Ir based alloy electrocatalysts with enhanced OER performance, their application to full electrolyzer has not been reported yet. Severe leaching of the secondary metal, Ni or Cu, from particles produce corresponding metal ions in the system. The leached metal ions contaminate PEM, and lower ion conductivity, which is fatal to cell performance.

We paid attention to adjusting the morphology of Ir oxide particles itself, considering its potential application to PEM water electrolyzers. As a result, we successfully synthesized one-dimensional ultrathin IrO₂ nanoneedles in gram scale.^[2] It is known that one-dimensional structured electrocatalysts possess enhanced performance in various electrochemical reactions. The drawback of conventional one-dimensional electrocatalysts is its low surface area where the reaction would take place. By making ultrathin nanoneedles, sufficient surface area was exposed. The diameter of the nanoneedles was about 2 nm, which consists of 6~8 layers of (110) IrO₂ atomic planes. Molten salt method was applied to synthesize the nanoneedles, because it was hard to control the heterogeneous nucleation on the Ir surface and the homogeneous nucleation of the Ir nuclei in a solution using a conventional colloidal synthesis method. Moreover, the molten salt method does not require toxic chemicals and is readily scalable to gram scale. At higher temperatures above the melting point of the salt, NaNO₃, Ir oxide particles were obtained in the liquid salt. When cysteamine was added together as an organic shaping agent, one-dimensional ultrathin IrO₂ nanoneedles were synthesized. NaNO₃ salt was used as an oxygen donor to produce oxide nanoparticles as well as a solvent. The aspect ratio of the nanoneedles was controlled by the concentration of the shaping agent. When larger amounts of cysteamine were used, thinner and longer IrO₂ nanoneedles were obtained.

Obtained ultrathin IrO₂ nanoneedles exhibited enhanced OER performance. The longer and thinner the particles, the higher electric conductivity and OER activity were observed. The conductivity was directly measured by the 4-point probe method. Also, the stability was enhanced compared to unshaped IrO₂ nanoparticles. Typically, there was an inverse relation between activity and stability for the OER electrocatalysts. The nanoneedles overcame the relation by its unique shape. When the nanoneedles were applied to PEM water electrolyzers, the efficiency and durability were enhanced compared to conventional unshaped counterparts.

We studied how morphology control of Ir based nanoparticles could affect OER property and PEM water electrolyzer performance. We believe our experimental findings will be valuable to researchers who are working on the development of OER electrocatalysts or PEM water electrolyzers.

[1] J. Lim et al., Chem. Commun. 2016, 52, 5641-5644.
[2] J. Lim et al., Adv. Funct. Mater. 2017, 1704796.