Abstract
Among the different strategies that are being developed to solve the current energy challenge, harvesting energy directly from sunlight through a tandem photoelectrochemical cell (water splitting) is most attractive. Its implementation requires the development of stable and efficient photocathodes, NdFeO3 being a suitable candidate among ternary oxides. In this study, transparent NdFeO3 thin-film photocathodes have been successfully prepared by a citric acid-based sol–gel procedure, followed by thermal treatment in air at 640 °C. These electrodes show photocurrents for both the hydrogen evolution and oxygen reduction reactions. Doping with Mg2+ and Zn2+ has been observed to significantly enhance the photoelectrocatalytic performance of NdFeO3 toward oxygen reduction. Magnesium is slightly more efficient as a dopant than Zn, leading to a multiplication of the photocurrent by a factor of 4–5 for a doping level of 5 at % (with respect to iron atoms). This same trend is observed for hydrogen evolution. The beneficial effect of doping is primarily attributed to an increase in the density and a change in the nature of the majority charge carriers. DFT calculations help to rationalize the behavior of NdFeO3 by pointing to the importance of nanostructuring and doping. All in all, NdFeO3 has the potential to be used as a photocathode in photoelectrochemical applications, although efforts should be directed to limit surface recombination.
Highlights
In the current context of growing global energy demand and depletion of fossil fuels, photoelectrochemical (PEC) water electrolysis driven by sunlight on semiconductor electrodes could be a suitable avenue for the sustainable generation of H2.1−5 hydrogen is identified as a technically viable and carbon-free energy vector for applications ranging from smallscale power supply to large-scale energy storage and transportation.[6−8]Since the seminal demonstration of photoelectrochemical hydrogen production with a cell comprising TiO2 and Pt electrodes by Fujishima and Honda,[9] revolutionary advances made in the field of PEC water-splitting systems have led to H2 production with solar-to-hydrogen efficiencies higher than 10%10 but using relatively complex device architectures and/or electrodes made of rare and/or unstable materials
There are no important changes in the X-ray diffraction (XRD) patterns upon magnesium and zinc doping, there is a small shift of less than 0.1° in the (112) reflection position toward smaller 2θ values upon doping, in the case of Mg
We have shown that NdFeO3 thin-film electrodes prepared by a layer-by-layer coating of conducting glass substrates by a citrate-route sol−gel method, followed by heat treatment, behave as photocathodes for oxygen and water reduction in alkaline media
Summary
In the current context of growing global energy demand and depletion of fossil fuels, photoelectrochemical (PEC) water electrolysis driven by sunlight on semiconductor electrodes could be a suitable avenue for the sustainable generation of H2.1−5 hydrogen is identified as a technically viable and carbon-free energy vector for applications ranging from smallscale power supply to large-scale energy storage and transportation.[6−8]. None of them meets the criteria to be considered as practical photocathodes in water-splitting devices (chemical photostability in aqueous environments, efficient light absorption, and low cost) These facts, along with the low values for charge carrier mobility and charge carrier lifetime that usually characterize oxide photocathodes, make critical further research on this topic. In the field of water splitting on semiconductor electrodes, theoretical and computational studies based on DFT calculations have gained importance.[35] they provide valuable information regarding key aspects, such as band-gap values, band structures, charge transport and transfer, as well as the kinetics of oxygen and hydrogen evolution reactions at the electrode/electrolyte interface.[36] DFT has become a useful tool that contributes substantially to the understanding of the photoactive materials’ response, and it has oriented the optimization of the PEC performance through proper modeling and simulation strategies.[37−39].
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