A team of researchers in Israel has developed a two-step electrochemical-chemical cycle for decoupled water splitting with high efficiency. The method is described in a paper in the journal Nature Energy.
In a purely electrolytic scheme, the water oxidation and reduction reactions are tightly coupled in both time and space, as they occur simultaneously at two electrodes—an anode and a cathode—placed together in the same cell. This coupling introduces operational challenges, such as H2/O2 crossover at low current densities, which hampers operation under variable renewable energy sources such as solar and wind, and sets strict constraints on material selection and process conditions.
Following on previous work exploring different paths to decouple the water oxidation and reduction reactions, here we propose a method of decoupled water splitting that overcomes a substantial barrier to implementation; namely, the energy conversion efficiency. In doing so, we also provide more degrees of freedom in our scheme, enabling optimization of process parameters beyond conventional electrolysers. We achieve this by dividing the water oxidation reaction into two steps: an electrochemical step that oxidizes the anode, followed by a spontaneous chemical step that reduces the anode back to its initial state by oxidizing water.—Dotan et al.
In the two-step electrochemical–thermally activated chemical (E-TAC) cycle process, water is reduced to hydrogen gas at the cathode, liberating OH– ions. The four-electron oxygen evolution reaction (OER), which takes place at the anode in conventional electrolysis is divided into two consecutive steps comprising four one-electron oxidation reactions of a nickel hydroxide (Ni(OH)2) anode, followed by spontaneous oxygen evolution and anode regeneration in a thermally activated chemical step .
Schematic of alkaline water electrolysis and the E-TAC water-splitting process. a, In alkaline water electrolysis, which typically takes place at elevated temperatures (50–80 °C), the OER and HER are coupled in both time and space, as they occur simultaneously at an anode and a cathode, which are placed together in the same cell. A diaphragm or anion exchange membrane separates the anode and cathode compartments and prevents O2/H2 crossover. b, E-TAC water splitting proceeds in two consecutive steps. An electrochemical step (left) reduces water by the conventional HER at
the cathode, liberating hydroxide ions (OH–) that oxidize a nickel hydroxide (Ni(OH)2) anode into nickel oxyhydroxide (NiOOH). This step is followed by
a chemical step (right), wherein the NiOOH anode reacts with water to spontaneously produce oxygen.
The first (electrochemical) reaction occurs at ambient temperature (~25 °C), whereas the second (chemical) reaction proceeds at elevated temperatures (~95 °C) for the optimum rate of reaction. The first and second reactions sum up to the overall water-splitting reaction, 2H2O → 2H2 + O2. Dotan et al.
The decoupled method enables overall water splitting at average cell voltages of 1.44–1.60 V with nominal current densities of 10–200 mA cm−2 in a membrane-free, two-electrode cell.
This allows the production of hydrogen at low voltages in a simple, cyclic process with high efficiency, robustness, safety and scale-up potential.
Hen Dotan, Avigail Landman, Stafford W. Sheehan, Kirtiman Deo Malviya, Gennady E. Shter, Daniel A. Grave, Ziv Arzi, Nachshon Yehudai, Manar Halabi, Netta Gal, Noam Hadari, Coral Cohen, Avner Rothschild & Gideon S. Grader (2019) “Decoupled hydrogen and oxygen evolution by a two-step electrochemical–chemical cycle for efficient overall water splitting” Nature Energy volume 4, pages 786–795 doi: 10.1038/s41560-019-0462-7