Prof. Avner Rothschild
In conventional water electrolysis, the water oxidation and reduction reactions are coupled in both time and space, therefore the cell must be divided into hydrogen and oxygen compartments. The Electrochemical – Thermally-Activated Chemical (E-TAC) water splitting cycle decouples these reactions by dividing the process into two stages; an electrochemical (E) stage that reduces water at the cathode and charges a nickel hydroxide anode to nickel oxyhydroxide, followed by a chemical (TAC) stage that reduces the charged anode back to its initial state by oxidizing water. This chemical reaction is accelerated at elevated temperatures (~100 deg. C), providing a handle to control the evolution of oxygen in the cell so as to avoid mixing with hydrogen. The E-TAC cycle enables overall water splitting at an average cell voltage of ~1.5 V in a membraneless cell architecture that offers potential for cost reduction by eliminating membranes and sealing components and supports high-pressure hydrogen production. High electrolytic efficiency of 98.7% is achieved by dividing the four-electron oxygen evolution reaction, with a minimum onset overpotential of 300-400 mV, into four one-electron reactions wherein four Ni(II) atoms are charged to Ni(III), followed by a chemical reaction with water that evolves oxygen spontaneously. The operational challenges that arise from swinging between the cold E and hot TAC stages and the material challenges that arise from swinging between bias and open-circuit conditions and from the finite capacity of the nickel (oxy)hydroxide anode will be presented.