Matthias H. Richter

Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell.

Efficient solar hydrogen generation in microgravity environment

Long-term space missions require extra-terrestrial production of storable, renewable energy. Hydrogen is ascribed a crucial role for transportation, electrical power and oxygen generation. We demonstrate in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel. The photoelectrochemical cell consists of an integrated catalyst-functionalized semiconductor system that generates hydrogen with current densities >15 mA/cm2 in the absence of buoyancy. Conditions are described adverting the resulting formation of ion transport blocking froth layers on the photoelectrodes. The current limiting factors were overcome by controlling the micro- and nanotopography of the Rh electrocatalyst using shadow nanosphere lithography. The behaviour of the applied system in terrestrial and microgravity environment is simulated using a kinetic transport model. Differences observed for varied catalyst topography are elucidated, enabling future photoelectrode designs for use in reduced gravity environments.While renewable energy production is a terrestrial concern, far less attention is devoted to solar-to-fuel conversion for long-term space missions. Here, the authors explore photoelectrochemical hydrogen generation in microgravity and overcome microgravity’s limitations by electrode nanostructuring.

Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H2 evolution

Similar to natural photosynthetic systems, artificial photosynthetic systems require synergistic cooperation between light harvesting, charge separation and redox catalysis. Herein, a three-dimensional (3D) hierarchical photocatalyst is designed with a novel Z-scheme two-photon excitation, defined by the complementary absorption of higher energy and lower energy photons by cadmium sulfide nanowires (CdS NWs) and cobalt–benzimidazole (Co-bIm) coordination polymers (CBPs), respectively. Without any noble-metal co-catalyst, the microscopically integrated CdS–CBP photocatalysts demonstrated dramatically enhanced photocatalytic activities of H2 evolution, which were up to 10.6 folds higher than those of pristine CdS NWs. Structurally, the intimate interfacial contact between the 3D CdS NW scaffold and the discrete CBP microstructures benefits their strong electronic interaction and efficient charge separation. Upon simultaneous light excitation, a tandem solid-state electron flow from CdS to CBP and then from metal (Co) to ligand (bIm) precisely catalyzes the reduction of pre-activated H atoms on the bIm ligands for efficient H2 evolution.

Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency

Efficient unassisted solar water splitting, a pathway to storable renewable energy in the form of chemical bonds, requires optimization of a photoelectrochemical device based on photovoltaic tandem heterojunctions. We report a monolithic photocathode device architecture that exhibits significantly reduced surface reflectivity, minimizing parasitic light absorption and reflection losses. A tailored multifunctional crystalline titania interphase layer acts as a corrosion protection layer, with favorable band alignment between the semiconductor conduction band and the energy level for water reduction, facilitating electron transport at the cathode–electrolyte interface. It also provides a favorable substrate for adhesion of high-activity Rh catalyst nanoparticles. Under simulated AM 1.5G irradiation, solar-to-hydrogen efficiencies of 19.3 and 18.5% are obtained in acidic and neutral electrolytes, respectively. The system reaches a value of 0.85 of the theoretical limit for photoelectrochemical water splittin...