WATER SPLITTING

Improving alkaline ionomers

Low-cost, efficient hydrogen production via water electrolysis is expected to be an important part of a future hydrogen economy. Towards this end, new polymers with high stability are demonstrated and paired with low-cost, earth-abundant metal catalysts in alkaline membrane electrolyzers.

If hydrogen is to be the energy carrier of the future, it will have to be accessible to consumers in distributed locations, as gasoline is today. It will also need to be economical to produce at small and medium scales to avoid expensive and inefficient collection and distribution networks. Electrochemical water electrolysis is expected to play a critical role in enabling widespread hydrogen generation — and to be able to do so with less environmental impact than traditional methane steam reforming. The most commercially mature technology is alkaline water electrolysis, which uses a concentrated (20–30 wt%) NaOH/KOH electrolyte. Although this technology has been successfully scaled to large volume, it operates at a relatively low current density (typically 200–400 mA cm–2) due to high ohmic resistance losses. Moreover, alkaline water electrolysers have difficulty in responding to transient load, which presents problems when pairing with renewable electricity. They also experience high materials corrosion rates.

Electrolysers using a proton exchange membrane (PEM) have been developed as an alternative. They have much lower ohmic resistance losses, operate at lower temperature and higher current density, and are more compact because the recirculated liquid electrolyte is replaced by a solid polymer electrolyte. However, PEM electrolysers are generally higher cost due to the expense of the PEM, which is a perfluorinated polymer, and the need for expensive platinum and iridium catalysts.

Anion exchange membrane (AEM) electrolysis processes combine the simplicity and operational ease of a solid polymer electrolyte with the low cost of alkaline-compatible electrodes and hydrocarbon membranes. However, AEM electrolysers still require research and development. Most work to date has used the same expensive catalysts found in PEM electrolysers and continued to circulate fairly high concentration (0.1–1.0 M) NaOH/KOH electrolytes in order to avoid high operating voltages. Now, writing in Nature Energy, Yuehe Lin, Yu Seung Kim and their co-workers describe an AEM electrolyser that operates without the supporting NaOH/KOH electrolyte and uses inexpensive catalysts1.

The researchers’ work presents new developments in the ionic conducting polymer (also known as ionomer) needed to form the three-dimensional electrodes of AEM electrolysers. The ionomer conducts hydroxide ions to and from the catalyst surface and acts as a binder to mechanically anchor the catalyst particles in the electrode. As such, ionomers must operate effectively for thousands of hours. However, due to the high potential at the anode, where oxygen is produced from water, one of the most common degradation mechanisms for the ionomer is oxidation. Many previously reported ionomers have phenyl groups, which adsorb onto the catalyst surface and interfere with electron transfer. When they are oxidized, phenyl groups also form phenolic compounds that are relatively acidic. These phenolic compounds can be very detrimental to AEM electrolyser performance because they neutralize the alkaline charge carriers in the polymer.

To address this issue, Lin, Kim and colleagues remove the phenyl groups from the polymer backbone (Fig. 1). Ex situ rotating disk electrode experiments were used to screen and compare several ionomers. The team shows that high ionic conductivity and high local pH in the ionomer are desirable to drive efficient AEM electrolysis. Both conductivity and pH were simultaneously increased by increasing the ion exchange capacity of the ionomer, which is the number of positively charged ion-conducting functional groups in the polymer per polymer mass.

Fig. 1: Evolution of electrolysis and future work.
figure1

Schematic showing how combining the best aspects of alkaline electrolysis and proton exchange membrane (PEM) electrolysis can lead to highly efficient and low-cost anion exchange membrane (AEM) electrolysis. The chemical structure shown is one of the polymeric materials used as an ionomer by Lin, Kim and colleagues. To further improve performance and drive commercialization, researchers are presently developing new polymers and platinum group metal (PGM)-free catalysts as well as advancing both cell and electrode design. GDL, gas-diffusion layer. Figure adapted with permission from ref. 1, Springer Nature Ltd.

The researchers report a high current density of 2.7 A cm–2 at 1.8 V using pure water as the electrolyte. Using pure water is very favourable because a key drawback of today’s AEM electrolysers is the need to add a conducting salt to the water feed. Although the salt content does not hurt the cell, it does require that the water be recirculated to conserve the amount of salt used in electrolysis. Salt recirculation adds cost and complexity to the system. Therefore, if the performance reported above were achieved in a practical system it could drastically lower both equipment and operational costs, displacing both PEM electrolysers and replacing existing alkaline electrolysis plants.

However, it should be noted that this performance was obtained in a non-steady state linear sweep voltammetry experiment where mass transport is not typically a concern. In fact, the shape of the polarization curve for the reported cell suggests that mass transport effects are important in these cells. This is further confirmed when electrolysers were exposed to long term, constant current operation. When less than one tenth of the current density above was applied (0.2 A cm–2), the cell voltage steadily increased over the first 40 hours of operation to well over 2.0 V, which suggests that further work to reduce the mass transport resistance of the system will be necessary in the future.

Importantly, however, the work by Lin, Kim and colleagues demonstrates that advances in ionomer design and properties can lower the operating voltage for low temperature electrolysis using solid polymer electrolytes. Moreover, the researchers use low-cost catalysts at the anode and also demonstrate an AEM electrolyser without platinum or iridium catalysts, though further improvements in performance are needed.

In terms of future progress, there is still much for the community to address. For example, steady-state performance at low voltages and high stability still has to be achieved. This requires understanding mass transport effects within the individual components of the electrode comprising the ionomers, catalysts and gas diffusion layers. Mass transport across the membrane, where water is supplied to the hydrogen-evolving electrode by diffusion and ionic drag from the oxygen-evolving electrode, will also need to be explored.

This may require additional advances in ion-conducting polymers that do not have any aromatic groups (including the styrene side groups used by Lin, Kim and colleagues) that further enhance water and ion transport. Future designs will also have to consider the electrode architecture and its role in facilitating the transport of water, ions and product gases; in essence, very little true electrode engineering has yet been done for AEM electrolysers and could go a long way in producing high performance and high stability cells. Finally, researchers and cell designers must continue to push for non-precious metal — or low precious metal content — catalysts that have very high activity. These advances would allow AEM electrolysers to realize their promise to be highly efficient, low-cost enablers for a green energy future.

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Correspondence to William E. Mustain or Paul A. Kohl.

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Mustain, W.E., Kohl, P.A. Improving alkaline ionomers. Nat Energy 5, 359–360 (2020). https://doi.org/10.1038/s41560-020-0619-4

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