(Invited) High Productivity Electrocatalytic Water Splitting Using Diverse Water Sources

Abstract

Water electrolysis is a commercially established process. However, significant cost reductions are desired for the large scale applications anticipated in the future. Electrolysis is typically performed under extreme pH conditions due to the high reaction rate of the electrocatalyst and low solution resistance. On the other hand, energy conversion using electricity generated from renewable energy requires a water splitting electrocatalyst that is versatile, safe, and durable. Recent water splitting research has focused on the development of catalyst materials and elucidation of their efficient action under non-extreme pH conditions. However, there are several problems associated with water splitting in the mild pH range. The first problem is the lack of protons and hydroxide ions at mild pH, which leads to large concentration overpotentials. To solve these problems, concentrated buffered electrolytes are used to suppress local pH shifts and to compensate for the steady-state proton or hydroxyide ion. To maximize the buffering effect, concentrated buffer electrolyte solutions should be applied near the pKa values where protonated and deprotonated species coexist. A second problem at near-neutral pH levels is the inherently lower conductivity of the electrolyte solution compared to strongly acidic or strongly alkaline solutions. The ohmic potential (iR) drop between the anode and cathode is critical to the cell voltage of water splitting devices, especially at the high current densities that are industrially important. The conductivity at pH 7.0 of phosphate buffer can be maximized by varying the molar ratio and selecting the appropriate counter cation (K among Li, Na, K, and Cs). The choice of conditions essentially reduces viscosity and maximizes solubility, which strongly affects the conductivity of the solution. The conductivity of saturated K phosphate reaches a maximum of ~20 S m−1, but this value is still not as high as the alkaline one (~130 S m−1). Viscosity and solution resistance can be adjusted by mixing different electrolytes and fine-tuning the position of pKa within the target pH range. In water containing Mg2+ and Ca2+, precipitation of these ionic salts can be a problem. Keeping the pH between 8 and 10 prevents such precipitation. We are also attempting to intentionally use chloride ions, which have the highest concentration anion in seawater. In this way, solution resistance is further improved. Thus, it is emphasized that electrolyte engineering is an important technology for cost-effective water electrolysis by finding new reaction environments to efficiently electrolyze water under more moderate conditions.