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.

A facile nonpolar organic solution approach has been developed for a high performance hematite photoanode at a low sintering temperature (550 °C).

A facile and low-cost dip-coating process for the deposition of silicon doped hematite films (Si:a-Fe 2 O 3) for hydrogen production by solar water splitting in photo-electrochemical cells (PEC) is presented. The precursors include iron nitrate, oleic acid, tetraethyl orthosilicate (TEOS) and tetrahydrofuran as dispersion agent. Sequential dip coating on transparent conducting oxides glass substrates with heat treatment steps at 500 C and 760 C yields mesoporous Si:a-Fe 2 O 3 with a roughness factor of 17 and photocurrent densities >1 mA/cm 2 at 1.23 V vs. reversible hydrogen electrode with SiO x underlayer and surface modification. A PEC demonstrator with 80 cm 2 active area in 1 M potassium hydroxide yields a photocurrent of 35 mA at 1.

Cobalt‐based electrolytes are highly tunable and have pushed the limits of dye‐sensitized solar cells, enabling higher open‐circuit voltages and new record efficiencies. However, the performance of these electrolytes and a range of other electrolytes suffer from slow electron transfer at platinum counter electrodes. High surface area platinum would enhance catalysis, but pure platinum structures are too expensive in practice. Here, a material‐efficient host‐guest architecture is developed that uses an ultrathin layer of platinum deposited upon an electrically conductive scaffold, niobium‐doped tin oxide (NTO). This nanostructured composite enhances the counter electrode performance of dye‐sensitized solar cells (DSCs) using a Co(II/III)BPY3 electrolyte with an increased fill factor and power conversion efficiency (11.26%), compared to analogous flat films. The modular strategy is elaborated by integrating a light scattering layer onto the counter electrode to reflect unabsorbed light back to the photoanode to improve the short‐circuit current density and power conversion efficiency.

Energy research is to a large extent materials research, encompassing the physics and chemistry of materials, including their synthesis, processing towards components and design towards architectures, allowing for their functionality as energy devices, extending towards their operation parameters and environment, including also their degradation, limited life, ultimate failure and potential recycling. In all these stages, xray and electron spectroscopy are helpful methods for analysis, characterization and diagnostics for the engineer and for the researcher working in basic science. This paper gives a short overview of experiments with x-ray and electron spectroscopy for solar energy and water splitting materials and addresses also the issue of solar fuel, a relatively new topic in energy research. The featured systems are iron oxide and tungsten oxide as photoanodes, and hydrogenases as molecular systems. We present surface and sub-surface studies with ambient pressure XPS and hard x-ray XPS, resonant photoemission, light induced effects in resonant photoemission experiments and a photo-electrochemical in-situ/operando experiment, and nuclear resonant vibrational spectroscopy (NRVS).

The tin oxide buffer layer between the transparent conducting oxide current collector and the hematite photoelectrode causes considerable water oxidation enhancement of that electrode. The water oxidation onset potential is lowered by 180 mV. The lifetime of photogenerated charge carriers is increased by a factor of 10. For the investigation of structure and function of the buffer layer, we designed a wedge-shaped multilayer film assembly. Oxygen 1s X-ray photoemission spectra suggest a decrease of oxygen vacancy concentration near the interface of α-Fe 2 O 3 and FTO−SnO 2 , when the SnO 2 buffer layer is introduced. This SnO 2 buffer layer increases the crystallinity of the hematite layer. The oxygen 1s near-edge X-ray absorption fine structure shows that the buffer layer increases the Fe 3d−O 2p hybridization and affects the quasi-Fermi level of electrons in α-Fe 2 O 3 . There is some indication that the α-Fe 2 O 3 layer contains an adverse hole state in the valence band which disappears when the α-Fe 2 O 3 layer is grown on the SnO 2 layer. This layer induces improved orbital overlap with subsequent improved charge transfer between the absorber α-Fe 2 O 3 and the current collector FTO. Our experiments indicate that performance enhancement by this buffer layer is of electronic structure origin.