Researchers develop two-step method for efficient decoupled water splitting

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 .


Dotan

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.

Resources

  • 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

Source: FS – Transport 2
Researchers develop two-step method for efficient decoupled water splitting



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