The study of electrocatalysts for water splitting is pivotal to improving the efficiency of fuel production. This is an attractive field since electrocatalysts can be used: (1) as part of a photoelectrode, for direct sunlight to fuel transformation, or (2) can be used in an electrolyser to transform the electricity generated through a solar panel to produce a fuel. In a complete system it is thought that water oxidation is the limiting process. Therefore, a lot of effort is devoted to finding a better catalyst for this reaction in order to enhance the final performance of the system.

Recently, iron and nickel oxyhydroxides have been appointed as good candidates for the water oxidation reaction in basic conditions. In this context, interest for understanding the origin of their behaviour has grown in recent years. In 2014, Boettcher and co-workers published for the first time that the high activity towards water oxidation of NiOOH was due to Fe incorporation from the electrolyte.[1] Since then, there has been a lot of effort to determine which species are involved in the catalytic reaction, with no consensus reached. Some experiments suggest the presence of Fe(IV) during the catalysis,[2] while on the contrary, other works cannot detect such iron species and suggest that nickel centres are the active sites.[3] Despite this debate, some structural differences have been found due to iron incorporation which leads to an improved performance. However, the mechanistic and kinetic analysis of metal oxide based electrocatalysts, in general, is hampered by the non-ideal nature of these materials. Most of these electrocatalysts present different redox states and a non-planar and dense structure. Thus the interpretation of the traditional electrochemical techniques, such as tafel plots, is more complicated and sometimes not possible.[4]

In this work, we used spectroelectrochemical methods to analyse three different samples: Pure FeOOH, a mixed FeOOHNiOOH sample composed of different layers, and finally Ni(Fe)OOH with spontaneous Fe incorporation. From these measurements, we are able to determine the active species and study the kinetics under catalytic conditions to yield a basic mechanistic picture.

References

[1]          L. Trotochaud, et al. JACS, 2014, 136, 6744-6753.

[2]          J. Y. C. Chen, et al. JACS, 2015, 137, 15090-15093.

[3]          M. Görlin, et at, JACS, 2017, 139, 2070-2082.

[4]          E. Pastor, et al, Nat. Commun., 2017, 8, 14280.