Reply to Comment on “Commercially Available WO₃ Nanopowders for Photoelectrochemical Water Splitting: Photocurrent versus Oxygen Evolution”

Abstract

We thank Prof. Augustynski and Dr.R enata Solarska for their comments regarding our paper published in ChemPlusChem. [1] As am atter of fact, their commentse xactly underline one of the big problems in photoelectrochemical water splitting, namely,t hat materials with basically the same compositions perform completely differently.I ft his wasn't the case, then there wouldn'tb es om any research groups still working on well-studied materials like titania, hematitea nd tungsten oxide.In addition to the composition,t he nano-and microstructure of the photoanodes, includingc rystal size, crystal orientation, and porosity,b ut also film thickness and defect chemistry, have as trong influence on the photoelectrochemicalp erformance. If both the microstructure and composition are different, then it becomes nearly impossible to comparep hotoanodes. Therefore, we are not at all surprised that our results are in disagreement with their results published in Adv.E nergy Mater. [2] and in J. Am. Chem. Soc., [3] because in both papers, Augustynski et al. workedw ith Keggin-type polyoxometalate electrocatalysts and/or(Na)WO 3 films, while we only used commerciallya vailable WO 3 powders.If we compare now the results from the ChemPlusChem paper with our previously published paper in J. Mater.C hem. A, [4] it is obvious that in both cases the photoanodes consisted of solelyt ungsten oxide;h owever,t he microstructures differed completely.I no ne case, [4] the photoanode was composedo f crystallographically aligned, smalln anoplatelets, whereas in the other case, [1] the particles were significantly larger with a more spherical shape and also the size distribution was significantly broader.F urthermore, the two powders used for film deposition in these two papers were made by completely different synthesis procedures. To underline the sensitivity of the films to slight microstructural changes,i ti sw orth mentioning that all three commercially tested powders presented in our paper in ChemPlusChem showedd ifferent photocurrent densities, although the films had the same compositiona nd were processedi ni dentical ways. [1] As ac onclusion, we can say that the films used in our ChemPlusChem paper have either ac ompletely different microstructure (compared to those reported in reference [4]) or even different compositions (compared to those reportedi nr eferences [2] and [3]), and therefore ad ifferent behavior duringp hotoelectrochemicalt ests is, in our opinion, not unexpected.On the other hand, we completely agree with the comment of Prof. Augustynski and Dr.S olarska that the fate of the methanesulfonic acid electrolyte is indeed an important question. However,t his question was not addressed in our ChemPlus-Chem paper.T he primary focus was on the processing of three commercially availablet ungsten oxide nanopowdersi nto films of such high quality that they can be used as anodes for photoelectrochemical water splitting. Indeed, all three photoanodes showed excellent photocurrent densities. We then selected one photoanode and compa...