We reassess experimental soft X-ray absorption spectra of the oxygen K-shell which we recorded operando from iron oxide during photoelectrochemical water splitting in KOH electrolyte. In particular, we refer to recently reported transitional electron hole states which originate within the charge carrier depletion layer of the iron oxide and on the iron oxide surface. For the latter we find that an intermediate oxy-peroxo species is formed on the iron oxide with increasing bias potential, which disappears upon further polarization of the electrode, concomitantly with the evolution and disappearance of the aforementioned surface state. The oxygen spectra contain also the spectroscopic signatures of the electrolyte water, the position of which changes with increasing bias potential towards lower x-ray energies, revealing the breaking and formation of hydrogen bonds in the water during the experiment. Combined with potential dependent impedance spectroscopy data we are able to sketch the molecular structure of chemical intermediates and their charge carrier dynamics.
Cobalt-based electrolytes have significantly advanced the tunability and performance of dye-sensitized solar cells. The typically used platinum cathodes are expensive and non-optimal for cobalt complexes, motivating the search for replacements. Graphene nanopowders are a viable alternative but they are mechanically unstable as catalysts due to their poor substrate adhesion. Here we report a new type of carbon-graphene nanocomposite that maintains the catalytic performance of graphene with enhanced adhesion via a conductive carbon matrix. These nanocomposites were synthesized by carbonizing mixtures of graphene nanoplatelets with a carbon-source, poly(acrylonitrile). The resulting materials had tunable performance with a low charge transfer resistance of $1 U cm 2 using as little as 20% graphene.Dye-sensitized solar cells fabricated with these carbon-graphene nanocomposites had enhanced fill factors and enhanced power conversion efficiencies as compared to platinum cathodes. Accelerated mechanical aging led to the complete detachment of graphene-only electrodes whereas carbongraphene nanocomposites were stable.
The
origin, the nature, and the electronic structure of surface defects
causing surface states on metal oxides and their role in solar water
splitting have been under scrutiny for several decades. In the present
study, the surface of hematite films is treated with an oxygen plasma
and then subject to a detailed investigation with electroanalytical
methods and element orbital specific X-ray spectroscopy. We observe
a systemic variation of photoelectrochemical properties with oxygen
treatment time. Fe 2p and O 1s core level X-ray photoelectron spectra
and resonant valence band photoemission at the Fe 3p edge reveal the
filling of prevalent oxygen vacancies with concomitant oxidation of
Fe2+ to Fe3+ upon the oxygen treatment. The
dc bias dependent impedance spectra confirm how a prevalent capacitive
surface state, which evolves parallel with the photocurrent onset
potential, becomes diminished upon oxygen treatment. Surface states
of iron induce higher reactivity toward water oxidation than oxygen
surface states. The correlation between oxygen vacancy filling, concentration
of surface states, and photocurrent density in the course of treatment
confirms that the surface defects are of a capacitive nature and that
the onset of water splitting can be considered as a result of dielectric
breakdown in an interfacial hydroxide layer between photoanode and
water.
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