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カーボン・エネルギーコントロール社会協議会 (CanApple)
事務局：民秋均
発行責任者：石谷治
編集責任者：八木政行

Towards Solar Fuel Devices:
Challenges and Scopes
Niigata University Debraj Chandra
For the sustainable development of humansociety, the supply of secure and clean energy isarguably the most important scientific andtechnical challenge in the 21st century.1 Ever since,
the French scientist Edmond Becquerel pioneered
the photoelectric effect in 1839, researchers andengineers have been impassioned with the idea of
converting light into electricity or chemical fuels.
Water splitting by solar light energy usingsemiconductors, to produce clean hydrogen (H2)fuel in an economically viable way could become anew industrial photosynthesis whose only waste
product upon utilization is water. Back in 2010,
when I started to research on solar water splittingby extending my expertise from nanomaterials
development, my dedication to research remainunchanged in this field. In this newsletter, I would
like to cover the basics of solar water splitting andthen discuss the present challenges in this filed andprerequisite for materials development.
Production of H2 and O2 from water is an uphillreaction. The standard Gibbs free energy change
for the conversion of one H2O molecule into H2
and 1/2 O2 is ΔG° = 237.2 kJ/mol, which,according to the Nernst equation, corresponds toΔE° = 1.23 eV per electron transferred. Therefore,
for solar water splitting, the band gap energy (Eg)of the semiconductor should be 1.23 eV (photonswavelength of 1000 nm). However, to utilize
visible light, Eg should be 3.0 eV (wavelength400 nm). In semiconductors, solar lightabsorption induces charge separation to generateelectron-hole pairs when the energy of the incident
photons matches or exceeds the band gap. Thevalence-band (VB) holes exhibit an oxidative
potential of +1.0 to +3.5 V versus the normalhydrogen electrode (NHE); hence can drive theoxygen evolution reaction (OER; 2H2O → O2 +4H+ + 4e-). The conduction-band (CB) electrons
have a chemical potential of +0.5 to –1.5 V versus
NHE; hence can act as reductants and drive thehydrogen evolution reaction (HER; 4H+ + 4e- →2H2). However, the photoinduced free charge
carriers(electronsandholes),without
recombination
musttravel
tothe

semiconductor/aqueous interface and react withsolution species to carry out OER and/or HER.
Any other processes that consume free charge
carriers should be evaded in order to maximize theefficiency of H2 production.
To date, three kinds of solar water splitting
systems have been well established: photocatalytic(PC),photoelectrochemical(PEC),
andphotovoltaic-electrolyzer (PVE) systems. At
present scenario, the major challenges forcommercialization of the water splitting devices
are centralized on the development of materials
that able to support efficient separation of charge
carriers, provide rapid charge transfer at a
semiconductor/aqueous interface, have fastsurface-reaction kinetics, exhibit long-term
stability, and can efficiently harvest a large portionof the solar spectrum.
Thus, factors such as chemical composition,nanostructures, crystallinity and electronic
properties all of which determine the PC and PEC
activity of materials, need to be further elucidated.
Nanomaterials can provide advantages of large
surface areas, abundant active sites, diverse
morphologies, and easy device modeling. A small
mesopore system of crystalline WO3 photoanodehas been attained by ligand assisted fabrication
approach, which provided high surface area and ashorter carrier diffusion length than conventional
photoanodes and highly improved the visible-lightdriven PEC water splitting.2 Band gap engineeringof semiconductors by ion-doping, semiconductor
sensitization etc. can expand the opportunities ofvisible light response. N2-intercaleted WO3photoanodes through band gap engineering withimproved visible-light absorption has been utilized
for efficient PEC water oxidation.3 Crystal-facet
design is another way that can offer highly active
atomic configuration on the surface ofphotoelectrodes.4 Computational DFT calculations
can provide useful insights to identify newmaterials. It is thus of great importance torecognize and design new semiconductor materials
for solar water splitting that are efficient, stableand abundant. I believe, after commercialization ofsolar fuel devices in the future, our descendant will
wonder at the “Fossil Fuel Wars” in this century.
[1] Nature 2012, 488, 294[2] Angew. Chem. Int. Ed. 2013 52, 12606.
[3] ACS Sustainable Chem. Eng. 2019, 7, 17896.
[4] ACS Appl. Mater. Interfaces 2023, 15, 20885.