Electro- and photo-electrochemical water splitting as studied by In-Operando X-Rays Absorption Spectroscopy

In this work we show our most recent results obtained by in-operando X-Ray absorption spectroscopy on hydrous/amorphous [1] and on crystalline/dry [2] iridium oxide electrodes as electrocatalysts for the oxygen evolution reaction (OER). In all cases, XAS evidenced the role of Ir active sites, and the relevant oxidation states assumed during the catalytic cycle. Moreover, the local structure is not significantly influenced by the applied potential, thus suggesting a negligible reorganization energy of the catalyst.On the bases of these results, we were able to directly observe, by means of spectro-photoelectrochemical experiments, the charge transfer between a semiconductor (α-Fe2O3) and hydrous IrOx, the latter used as overlayer for generating a high performance photoanode architecture in photoelectrochemical water splitting[3]. The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of the reaction rate but to a decrease of the electron density in the hematite[4] or to the storage of photogenerates holes[5]. These effects are likely more important when hydrous overlayer, that can act as adapting catalysts[6], are considered. All these hypotheses can explain the observed improved hole lifetime and reduce recombination with electrons. The experimental approach is similar to the one adopted to study Ir oxide particles electrocatalysts[1,2]. In the present case, FEXRAV [7] and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with a 410nm diode. Thanks to this setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination. References [1] A. Minguzzi, O. Lugaresi, E. Achilli, C. Locatelli, A. Vertova, P. Ghigna, Rondinini S., Chem. Sci., 2014, 5, 3591-3597 [2] A. Minguzzi, C. Locatelli, O. Lugaresi, E. Achilli, G. Cappelletti, M. Scavini, M. Coduri, P. Masala, B. Sacchi, A. Vertova, P. Ghigna, S. Rondinini, submitted [3] A. Minguzzi, O. Lugaresi, E. Achilli, F. D'Acapito, A. Naldoni, F. Malara, C. Locatelli, A. Vertova, S. Rondinini, P. Ghigna, In preparation [4] M. Barroso, C.A. Mesa, S.R. Pendlebury, A.J. Cowana, T. Hisatomi, K. Sivula, M. Grätzel, D.R. Klug, J.R. Durrant PNAS, 2012, 109, 15640–15645 [5] L. Badia-Bou, E. Mas-Marza, P. Rodenas, E M. Barea., F. Fabregat-Santiago, S. Gimenez, E. Peris, J. Bisquert, J. Phys. Chem. C, 2013, 117, 3826−3833 [6] F. Lin, S.W. Boettcher Nature Materials, 2014, 13, 81-86 [7] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. d'Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009-7013.