Investigating Water Splitting with CaFe₂O₄ Photocathodes by Electrochemical Impedance Spectroscopy

Abstract

Artificial photosynthesis constitutes one of the most promising alternatives for harvesting solar energy in the form of fuels, such as hydrogen. Among the different devices that could be developed to achieve efficient water photosplitting, tandem photoelectrochemical cells show more flexibility and offer high theoretical conversion efficiency. The development of these cells depends on finding efficient and stable photoanodes and, particularly, photocathodes, which requires having reliable information on the mechanism of charge transfer at the semiconductor/solution interface. In this context, this work deals with the preparation of thin film calcium ferrite electrodes and their photoelectrochemical characterization for hydrogen generation by means of electrochemical impedance spectroscopy (EIS). A fully theoretical model that includes elementary steps for electron transfer to the electrolyte and surface recombination with photogenerated holes is presented. The model also takes into account the complexity of the semiconductor/solution interface by including the capacitances of the space charge region, the surface states and the Helmholtz layer (as a constant phase element). After illustrating the predicted Nyquist plots in a general manner, the experimental results for calcium ferrite electrodes at different applied potentials and under different illumination intensities are fitted to the model. The excellent agreement between the model and the experimental results is illustrated by the simultaneous fit of both Nyquist and Bode plots. The concordance between both theory and experiments allows us to conclude that a direct transfer of electrons from the conduction band to water prevails for hydrogen photogeneration on calcium ferrite electrodes and that most of the carrier recombination occurs in the material bulk. In more general vein, this study illustrates how the use of EIS may provide important clues about the behavior of photoelectrodes and the main strategies for their improvement.

Metal Doping to Enhance the Photoelectrochemical Behavior of LaFeO3 Photocathodes

“…A generalized decrease in the impedance values (semicircle radius) is noticeable with the increment of the Zn and Mg content. The decrease in impedance is correlated with the increment in the stationary photocurrent with x in Figure B and it can also be associated with a higher photogenerated electron flow toward the surface as advanced in models developed to understand the electrochemical impedance spectroscopy (EIS) response of photoelectrodes . Peng et al .…”

“…Many metal oxides, which include those with a complex stoichiometry (i.e., ternary and quaternary oxides), behave as semiconductors and present a good stability in aqueous environments, which makes them attractive candidates for water splitting devices. Among ternary oxides, p‐type behavior has been uncovered for some perovskites, spinels, delafossites, and scheelites . Perovskite‐type oxides may be a good alternative because most of them exhibit a p‐type semiconductor nature and high stability under illumination in aqueous solutions.…”

“…Amongt ernary oxides, p-type behavior has been uncovered for some perovskites, spinels, delafossites,a nd scheelites. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Perovskite-type oxidesm ay be ag ood alternative because most of them exhibit ap -type semiconductor nature and high stabilityu nder illumination in aqueous solutions. For instance, LaFeO 3 is ap romising candidate, and severals tudies in which this material is investigated in the context of photoelectrochemistry have been reported recently.…”

Tunable Spectrum Selectivity for Multiphoton Absorption with Enhanced Visible Light Trapping in ZnO Nanorods

“…As the ZnO NR c ‐axis length increases, more visible light is able to be trapped and reflected in the free space between adjacent ZnO NRs, as shown in Figure c. This multiple reflections reduce the reflectivity loss and increase the optical propagation path length of light, which provides more interactions sites for light and ZnO NRs . As a result, multiphoton absorption process has occurred.…”

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

“…Their subsequent work improved the photoactivity of CaFe 2 O 4 ,b yd oping it with Na + and Mg 2 + to improvet he conductivity,a nd explained the degradation mechanism. [126] The establishmentofasolid-state junction for efficient charge separation could furtheri mprove charge transfer,a nd, as ar esult, the performance. [125] An oteworthy outcomeo f this study was the fact that the carrierm obility of the semiconductor was improved by Ag doping due to ar elaxed distorted crystal lattice,w hich resultedi na23-fold j ph enhancement.…”