Ternary tungstates with general composition AWO_x are promising semiconductor materials for both electrochemical and photoelectrochemical water splitting, due to the ability to tune the properties through selection of the A-site cation element, as well as other characteristics such as good stability and earth abundance. In order to fully exploit the tunability of the properties, it is necessary to have a good understanding of how the properties change depending on the A-site cation and the fundamental origins of these changes. Such an understanding can be provided through ab initio computational methods.

In this work, we have used Density Functional Theory (DFT) to calculate the properties of various tungstate compositions. These compounds are found to have different band gaps and band energies, which vary depending on the orbitals forming the valence and conduction bands and structural factors, such as the distance between adjacent layers of WO₆ polyhedra [1]. The tunability of the band structures allows optimization of properties for different applications, such as selection of a compound with a high conduction band energy that is suitable for overall water splitting, or a low conduction band energy hence low band gap for optimization of solar absorption and use as a photoanode for tandem photoelectrochemical water splitting.

In addition to calculating bulk properties, we have also analyzed the ability of these tungstates to catalyze the oxygen evolution half-reaction of water splitting by calculating the adsorption energy of each intermediate species in the reaction (H, OH, OOH). It is found that the adsorption energies can be related to the capacity of the different A-site cations to exchange charge with the adsorbed species. Transition metal A-site cations tend to underbind intermediate species while Sn tends to overbind. Understanding these trends highlights a potential avenue for optimization, i.e. incorporation of Sn atoms into the surface of a transition metal tungstate (or vice versa) is found to allow tuning of the adsorption strength and hence give catalytic performance close to ideal.

[1] B. Huang and J. N. Hart, Physical Chemistry Chemical Physics, 2020, 22, 1727.