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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

Comments

Engineer-Poet

Clever!

I'd have to do a bunch of digging into heats of formation to learn whether the OER is endothermic or exothermic.  If it's the former (which I suspect it is, given the temperature dependence) then it can be driven by low-grade heat.  This could be taken from the un-converted solar energy of PV panels or low-pressure steam from e.g. nuclear power plants.

One possibility here is for the two reactions to be separated both in time and space, such as a Ni(OH)2 slurry on a graphite substrate being oxidized and then circulated to a hot chamber where the OER occurs.  This would allow near-continuous operation of the electrolysis section, pausing only to exchange the oxidized electrode slurry for fresh.

Davemart

As it says in the article above, the oxygen release happens at just 95C, and confirmed here:
https://www.h2pro.co/our-technology

Fairly low grade heat should be fine.

Davemart

A bit more here:
https://fuelcellsworks.com/news/fuel-of-the-future-technion-researchers-have-developed-an-inexpensive-environmentally-friendly-safe-hydrogen-technology/

Roger Brown

The possibility of two stage alkaline electrolysis has been discussed previously (https://www.nature.com/articles/ncomms11741). Alkaline electrolyzers are cheaper than PEM electrolyzers because they do not require platinum group metals for catalysts, and they use porous separators between the two half cells rather than expensive and relatively short-lived PEM membranes. However, the use of porous separators requires that the half cell pressure should be equalized at all times to prevent gas crossover between the half cells. This pressure equalization requirement means that alkaline electrolyzers cannot be ramped up and down quickly, thus making it difficult to couple them to variable electrical input from renewable energy sources.

I believe that this new two stage electrolysis scheme without a separator frees alkaline electrolyzers from this limitation. Of course there is any performance degradation in terms of efficiency or current density then these changes would effect the overall economic evaluation of the new design scheme.

Keith D. Patch

Two stage alkaline electrolysis might eventually prove to be a useful method of producing hydrogen, but I am as yet unimpressed.

1) The E-TAC offers nominal current densities of 10–200 mA-cm−2. These are an order of magnitued below what is currently offered commercially.

2) Heating and cooling sources for somehow magically heating large quantities of liquid reactants from ambient temperature (~25 °C), to then allow the second (chemical) reaction to proceed at elevated temperature (~95 °C), and then somehow cyclically cool-off back to ambient temperatures has not been supported by calculations nor reality as being favorable nor reasonable. Given that cooling tower outlet water design supply temperatures are often ~30 °C, and there is a 10 °C delta between cooling water temperatures and process liquid temperatures, I only see a ~40 °C (not ~25 °C) temperature for the E-TAC first (electrochemical) water splitting reaction, and not ~25 °C.

3) Many electrolysis processes can operate at 1.44V at 10–200 mA-cm−2. The issue is operating at commercially-viable current densities and voltages.

4) "Robustness" is claimed. Without data showing replicable data at 1-2 microvolts of voltage degradation per hour, this is only a low-TRL laboratory curosity.

--Keith
@KeithDPatch

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