Jinkyu Lim1 2 3 Hyunjoo Lee1

1, Korea Advanced Institute of Science and Technology, Daejeon, , Korea (the Republic of)
2, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, Lawrence Berkeley National Laboratory, Berkeley, California, United States

To utilize intermittent renewable sources, much attention has been devoted to proton exchange membrane (PEM) electrolyzer as a promising grid-scale energy storage solution. It can convert electric energy into chemical energy in the form of H2 which is a clean energy carrier with zero emission. Ir is the only element that has both high electrocatalytic activity towards the oxygen evolution reaction (OER) and good stability at highly corrosive environment of the anode. However, Ir is even scarcer than Pt on the earth, which hinders its development and commercialization. Although a couple of fancy Ir-Ni or Ir-Cu nanoparticles were reported for Ir minimization recently, they could not be applied to the PEM electrolyzer due to severe leaching of the secondary metal.[1] Leached 3d metal ions contaminate PEM, which has deteriorating effect on ion conductivity and cell performance.
Although the control of Ir oxide nanostructure has been difficult, we have concentrated on tuning Ir oxide property itself for better OER performance considering its potential application. Herein, we report one-dimensional structure of ultrathin IrO2 nanoneedles with enhanced OER electrocatalytic activity and durability. [2] More specifically, IrO2 nanoneedles have a diameter of 2 nm consisting of 6~8 atomic layers and a length of approximately 30 nm. The nanoneedles were synthesized through scalable molten salt method on a gram scale. At higher temperatures than the melting point of the salt, the liquid salt served as a solvent and an oxygen donor. Aspect ratios of the particles were controlled by the amount of used shaping agent. Ultrathin structure gave sufficient electrochemical surface area as well as BET specific surface area.
Prepared one-dimensional ultrathin IrO2 nanoneedles exhibited enhanced OER performance. The higher the aspect ratio, the better the OER activity was. Thanks to one-dimensional structure, electrical conductivity and stability increased, which made them overcome the inverse relation between activity and stability of OER electrocatalysts. Typically, higher activity causes poorer durability in the OER electrocatalysts. Our IrO2 nanoneedles could enhance both activity and durability in the OER by the unique shape. When the nanoneedles were introduced into PEM water electrolyzer, they showed better efficiency and durability compared to the unshaped IrO2 particles case. We believe this finding would stimuli the development of Ir based OER electrocatalysts and PEM water electrolyzer.

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