Understanding the activity and stability of oxygen-evolving anodes is crucial for developing better water splitting electrolysers. Researchers now show the importance of interactions between iron and hydr(oxy)oxide hosts in dynamically-stable electrocatalysts that balance dissolution and deposition of iron present in the electrolyte.
The electrochemical conversion of water into oxygen (that is, the oxygen evolution reaction, OER) is the main anodic reaction in several electrochemical devices. These include electrolysers to convert water and CO2 into fuels and chemical commodities, electrowinning devices to obtain and/or recover metals, and metal–air batteries. Since the sluggish kinetics of the OER significantly limits the performance of these devices, many research efforts have been devoted to developing active and cost-effective materials1. Nickel–iron hydr(oxy)oxide (NiFeOxHy) is among the most active alkaline-based oxygen-evolving anodes to date, however it suffers corrosion under operational conditions that lowers its long-term electrocatalytic activity2,3. It is therefore pivotal to find ways to improve the stability of NiFeOxHy to make it a viable anode for commercial devices. Writing in Nature Energy, Nenad M. Markovic and collaborators at Argonne National Laboratory, Valparaiso University and the University of Belgrade now use the Fe impurities in alkaline electrolytes to prevent the degradation of NiFe-based hydr(oxy)oxide and show how the key to the performance lies in the ability of the catalyst to adsorb Fe from the solution4.
The researchers found that when potentiostatic electrolysis was performed in an Fe-free electrolyte, there was a more than 80% drop in the initial OER activity (measured in terms of current density) of a NiFeOxHy anode over 1 hour (Fig. 1). Using in situ inductively coupled plasma mass spectrometry (ICP–MS), they observe that this coincides with an approximately 80% decrease in the amount of Fe in the catalyst. A decrease in OER activity was also observed for CoFeOxHy and FeOxHy anode materials. Conversely, the amount of Fe in the anode — and the OER activity — remained constant when electrolysis was done in an electrolyte with Fe concentrations as low as 0.1 ppm. A small activity enhancement was even observed for the NiFeOxHy electrocatalysts (illustrated by a negative OER activity loss in Fig. 1).
These results not only demonstrate the stability-boosting effect of Fe in solution over the anode material, but also provide solid evidence to support the hypothesis that Fe is the OER active site in the Fe-based hydr(oxy)oxides5,6. The stability enhancement by Fe dissolution–re-adsorption was further verified by coupling the in situ ICP–MS measurement with isotopic labelling. In this experiment, the researchers prepared NiFeOxHy anodes labelled with 56Fe and ran electrolysis using 57Fe-labelled electrolyte. The results clearly showed that within 10 minutes, approximately 70% of the Fe in the electrode had been replaced by Fe from the electrolyte.
By looking at the in situ ICP–MS experiments, one would wonder whether it is possible to further enhance the Fe adsorption properties of NiOxHy and boost the OER activity by increasing the number of Fe active sites on the hydr(oxy)oxide catalysts. Markovic and collaborators successfully explored this by doping Ni-based hydr(oxy)oxides with 3d transition metals and showing that, in an Fe-containing electrolyte, NiCuOxHy has approximately 1.4 times higher activity than NiOxHy.
The results with NiCuOxHy are remarkable since it is known that doping NiOxHy with Cu has a detrimental effect on the material’s OER activity6. This can be observed by comparing the OER activity of NiOxHy and NiCuOxHy in Fe-free electrolyte as shown in Fig. 1. Therefore, the enhanced OER activity of the ternary hydr(oxy)oxides has significant implications for the further development of Ni-based OER anodes. More specifically, one should focus on boosting the OER activity of NiOxHy by incorporating dopants that promote Fe adsorption on the NiOxHy network to generate materials with higher surface concentration of Fe OER active sites.
The experiments on the ternary hydr(oxy)oxides revealed that the OER activity monotonically increases when going from left to right in the 3d transition metal family (that is, increasing number of electrons in the d shell), following the order NiFeMnOxHy < NiFeCoOxHy < NiFeOxHy < NiFeCuOxHy. The researchers show that the number of Fe sites on the hydr(oxy)oxides follow the same trend and the OER activity is proportional to the number of these sites on the material. Taking this into account, it is interesting to consider what the effect would be of doping NiOxHy with Zn, which is a 3d transition metal with a full d shell. Unfortunately, this dopant was not considered by Markovic and collaborators. Since Zn has been reported to completely deactivate the Ni hydr(oxy)oxide towards the OER6, it would be very interesting to see whether NiFeZnOxHy could further boost Fe adsorption and thus outperform NiFeCuOxHy in terms of OER activity.
Overall, the work from Markovic and colleagues highlights the importance of synergy between electrode and electrolyte to generate active and dynamically-stable electrocatalysts, which should be considered in the holistic design of OER electrocatalysts. The low surface area hydr(oxy)oxide materials deposited on single crystals that Markovic and collaborators employed cannot be directly used as OER anodes in electrochemical devices, but their systematic study provides excellent guidance for rational development of high surface area anode materials with enhanced OER activity and stability to be used in devices.
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