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

Reply to Comment on “Commercially Available WO3 Nanopowders for Photoelectrochemical Water... We thank Prof. Augustynski and Dr. Renata Solarska for their comments regarding our paper published in ChemPlusChem. As a matter of fact, their comments exactly underline one of the big problems in photoelectrochemical water splitting, namely, that materials with basically the same compositions perform completely differently. If this wasn't the case, then there wouldn't be so many research groups still working on well‐studied materials like titania, hematite and tungsten oxide.In addition to the composition, the nano‐ and microstructure of the photoanodes, including crystal size, crystal orientation, and porosity, but also film thickness and defect chemistry, have a strong influence on the photoelectrochemical performance. If both the microstructure and composition are different, then it becomes nearly impossible to compare photoanodes. Therefore, we are not at all surprised that our results are in disagreement with their results published in Adv. Energy Mater. and in J. Am. Chem. Soc., because in both papers, Augustynski et al. worked with Keggin‐type polyoxometalate electrocatalysts and/or (Na)WO3 films, while we only used commercially available WO3 powders.If we compare now the results from the ChemPlusChem paper with our previously published paper in J. Mater. Chem. A, it is obvious that in both cases the photoanodes consisted of solely tungsten oxide; however, the microstructures differed completely. In one case, the photoanode was composed of crystallographically aligned, small nanoplatelets, whereas in the other case, the particles were significantly larger with a more spherical shape and also the size distribution was significantly broader. Furthermore, 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, it is worth mentioning that all three commercially tested powders presented in our paper in ChemPlusChem showed different photocurrent densities, although the films had the same composition and were processed in identical ways. As a conclusion, we can say that the films used in our ChemPlusChem paper have either a completely different microstructure (compared to those reported in reference ) or even different compositions (compared to those reported in references  and ), and therefore a different behavior during photoelectrochemical tests is, in our opinion, not unexpected.On the other hand, we completely agree with the comment of Prof. Augustynski and Dr. Solarska that the fate of the methanesulfonic acid electrolyte is indeed an important question. However, this question was not addressed in our ChemPlusChem paper. The primary focus was on the processing of three commercially available tungsten oxide nanopowders into 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 compared its photocurrent values in different electrolytes with the amount of measured oxygen, and we found that the CH3SO3H electrolyte led to the highest photocurrent, but the lowest oxygen evolution. We fully agree that the behavior of that particular photoanode was in contrast to other results reported in the literature. But as mentioned above, different photoanodes behave differently.Finally, we don't know why Prof. Augustynski and Dr. Solarska interpret our conclusions, which clearly refer to the particular photoanode studied in the ChemPlusChem paper, as a general statement that all WO3 photoanodes are not able to split water when CH3SO3H is used as the electrolyte. Of course, this was never our claim. However, there is a generally valid conclusion in our paper, which we believe is important for the community, namely that “a good photocurrent value might not be directly correlated with high oxygen evolution”.S. Reinhard, F. Rechberger, M. Niederberger, ChemPlusChem 2016, 81, 935–940.M. Sarnowska, K. Bienkowski, P. Barczuk, R. Solarska, J. Augustynski, Adv. Energy Mater. 2016, 6, 1600526.L. G. Bloor, R. Solarska, K. Bienkowski, P. J. Kulesza, J. Augustynski, M. D. Symes, L. Cronin, J. Am. Chem. Soc. 2016, 138, 6707–6710.S. Hilaire, M. J. Suess, N. Kranzlin, K. Bienkowski, R. Solarska, J. Augustynski, M. Niederberger, J. Mater. Chem. A 2014, 2, 20530–20537. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ChemPlusChem Wiley

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

Niederberger, Markus
ChemPlusChem , Volume 82 (9) – Sep 1, 2017
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References (4)

Publisher
Wiley
Copyright
© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim
ISSN
2192-6506
eISSN
2192-6506
DOI
10.1002/cplu.201700316
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Abstract

We thank Prof. Augustynski and Dr. Renata Solarska for their comments regarding our paper published in ChemPlusChem. As a matter of fact, their comments exactly underline one of the big problems in photoelectrochemical water splitting, namely, that materials with basically the same compositions perform completely differently. If this wasn't the case, then there wouldn't be so many research groups still working on well‐studied materials like titania, hematite and tungsten oxide.In addition to the composition, the nano‐ and microstructure of the photoanodes, including crystal size, crystal orientation, and porosity, but also film thickness and defect chemistry, have a strong influence on the photoelectrochemical performance. If both the microstructure and composition are different, then it becomes nearly impossible to compare photoanodes. Therefore, we are not at all surprised that our results are in disagreement with their results published in Adv. Energy Mater. and in J. Am. Chem. Soc., because in both papers, Augustynski et al. worked with Keggin‐type polyoxometalate electrocatalysts and/or (Na)WO3 films, while we only used commercially available WO3 powders.If we compare now the results from the ChemPlusChem paper with our previously published paper in J. Mater. Chem. A, it is obvious that in both cases the photoanodes consisted of solely tungsten oxide; however, the microstructures differed completely. In one case, the photoanode was composed of crystallographically aligned, small nanoplatelets, whereas in the other case, the particles were significantly larger with a more spherical shape and also the size distribution was significantly broader. Furthermore, 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, it is worth mentioning that all three commercially tested powders presented in our paper in ChemPlusChem showed different photocurrent densities, although the films had the same composition and were processed in identical ways. As a conclusion, we can say that the films used in our ChemPlusChem paper have either a completely different microstructure (compared to those reported in reference ) or even different compositions (compared to those reported in references  and ), and therefore a different behavior during photoelectrochemical tests is, in our opinion, not unexpected.On the other hand, we completely agree with the comment of Prof. Augustynski and Dr. Solarska that the fate of the methanesulfonic acid electrolyte is indeed an important question. However, this question was not addressed in our ChemPlusChem paper. The primary focus was on the processing of three commercially available tungsten oxide nanopowders into 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 compared its photocurrent values in different electrolytes with the amount of measured oxygen, and we found that the CH3SO3H electrolyte led to the highest photocurrent, but the lowest oxygen evolution. We fully agree that the behavior of that particular photoanode was in contrast to other results reported in the literature. But as mentioned above, different photoanodes behave differently.Finally, we don't know why Prof. Augustynski and Dr. Solarska interpret our conclusions, which clearly refer to the particular photoanode studied in the ChemPlusChem paper, as a general statement that all WO3 photoanodes are not able to split water when CH3SO3H is used as the electrolyte. Of course, this was never our claim. However, there is a generally valid conclusion in our paper, which we believe is important for the community, namely that “a good photocurrent value might not be directly correlated with high oxygen evolution”.S. Reinhard, F. Rechberger, M. Niederberger, ChemPlusChem 2016, 81, 935–940.M. Sarnowska, K. Bienkowski, P. Barczuk, R. Solarska, J. Augustynski, Adv. Energy Mater. 2016, 6, 1600526.L. G. Bloor, R. Solarska, K. Bienkowski, P. J. Kulesza, J. Augustynski, M. D. Symes, L. Cronin, J. Am. Chem. Soc. 2016, 138, 6707–6710.S. Hilaire, M. J. Suess, N. Kranzlin, K. Bienkowski, R. Solarska, J. Augustynski, M. Niederberger, J. Mater. Chem. A 2014, 2, 20530–20537.

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ChemPlusChem – Wiley

Published: Sep 1, 2017

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