Rational Design for Selective Oxygen Evolution Reaction over CoFeOxHy at High Current Density in the Presence of Chloride Ion
Authors: Komiya Hiroki , Obata Keisuke , Takanabe Kazuhiro 
Published: September 2023

Seawater electrolysis is expected to play an important role in the circular economy by reducing the hydrogen production cost due to the simplified process lines[1] and by saving scarce sources of safe and fresh water.[2] Seawater impurities especially chloride ions can cause the corrosion of the catalyst and be oxidized at high potential, competing with the oxygen evolution reaction (OER). Produced chlorine and hypochlorite are toxic to humans and the environment,[3] thus selective OER is desirable. Furthermore, impurity cations, such as Ca and Mg, may form hydroxide precipitates in a highly alkaline condition which is commonly used in the commercial electrolyzer. Therefore, selective OER in non-extreme pH (pH < 10) is highly desired, which requires additional challenges on the electrocatalyst and the electrolyte. In this study, we have effectively utilized the thermodynamic potential gap between chloride oxidation reaction (COR) and OER, and engineering of non-noble metal hydroxide electrocatalysts and buffer electrolyte successfully achieved the selective OER in the presence of chloride ion at commercially relevant operation conditions in non-extreme pH within the thermodynamic window.

Firstly, the stability of non-noble metal materials in 1.0 mol kg−
1 potassium borate (K-borate, pH 9.2) buffer solution with 0.5 mol kg−
1 KCl was investigated. Ni foam and Ni hydroxide, which are commonly used in water splitting at extreme pH levels, were unstable, and Co, Fe, and Ti-based materials were found to be the candidates for seawater splitting. Among various combinations of these elements, the CoFeOxHy/TF (Ti felt) electrode showed better performance than CoOxHy/TF and FeOxHy/TF electrodes. The CoFeOxHy electrode possessed a three-hold larger double layer capacitance (C
dl) which is the electrochemically active surface area than its counterparts. Our kinetic study revealed that Tafel slopes and apparent activation energies of CoFeOxHy resembled the control CoOxHy and FeOxHy electrodes, indicating that the performance difference was originated from the enlarged surface area caused by mixed Co and Fe.

In addition to the development of electrocatalysts, electrolyte engineering plays a key role. By employing the CoFeOxHy/TF electrode, the OER performance was investigated in varied molality and counter cation borate electrolytes at pH 9.2. The molarity of K-borate above 1.0 mol kg−
1 was found to be essential to minimize the concentration overpotential, which helps to maintain the electrode potential below the thermodynamic window of COR. The OER performance was insensitive to cation identity except for the Na cation counterpart having a low solubility product. Furthermore, Cl−-containing electrolyte had a higher conductivity than that without Cl−, which can be an advantage to decrease the ohmic resistance in two electrode configurations for future application.

The influence of the potential window on the stability and selectivity was investigated by using the developed electrode and 1.0 mol kg−1 K-borate (pH 9.2) with 0.5 mol kg−1 KCl. Below the redox potential of ClO− formation at 1.72 VRHE (V vs. Reversible hydrogen electrode), current density-potential relationships were insensitive to the Cl−. At 1.72 VRHE in the presence of Cl−, the faradaic efficiency (FE) of O2 (FEO2) reached 98±1%, whereas the FE of hypochlorite (FEHC) was merely 1±1%. Above this threshold potential, under chronopotentiometry (CP) testing at 50 mA cm−2 corresponding to approximately 1.82 VRHE, FEO2 decreased to 93±2% and FEHC increased to 7±2%. Critically, the potential of 1.82 VRHE was larger than 1.78 VRHE observed without Cl−, likely due to the blockage of the OER active site by Cl− or related species. Finally, aiming for the practical application, the OER performance over CoFeOxHy was examined at 353 K. The FEO2 reached 99±2% at 500 mA cm−
2 and 1.67 VRHE (Figure 1). Our calculations on thermodynamics revealed that the redox potential of OER decreases to 1.18 VRHE while that of ClO− formation reaction is 1.71 VRHE at 353 K, leaving a potential gap of 530 mV. High selectivity toward the OER was attributed to the consequence of fine engineering of electrocatalysts and electrolytes that successfully maintained the potential below the 1.71 VRHE where the OER is thermodynamically dominant. In addition, the electrode was stable for multiple on-off accelerated stability testing for 10 h at 500 mA cm−
2 and 353 K.

Reference

[1] S. Dresp, F. Dionigi, M. Klingenhof, P. Strasser, ACS Energy Lett.
2019, 4, 933–942.

[2] C. J. Vörösmarty, P. B. McIntyre, M. O. Gessner, D. Dudgeon, A. Prusevich, P. Green, S. Glidden, S. E. Bunn, C. A. Sullivan, C. R. Liermann, Nature
2010, 467, 555–561.

[3] J. G. Vos, M. T. M. Koper, J. Electroanal. Chem.
2018, 819, 260–268.

Figure 1