Bernhard Kaiser

Multijunction Si photocathodes with tunable photovoltages from 2.0 V to 2.8 V for light induced water splitting

We report on the development of high performance triple and quadruple junction solar cells made of amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) for the application as photocathodes in integrated photovoltaic–electrosynthetic devices for solar water splitting. We show that the electronic properties of the individual sub cells can be adjusted such that the photovoltages of multijunction devices cover a wide range of photovoltages from 2.0 V up to 2.8 V with photovoltaic efficiencies of 13.6% for triple and 13.2% for quadruple cells. The ability to provide self-contained solar water splitting is demonstrated in a PV-biased electrosynthetic (PV-EC) cell. With the developed triple junction photocathode in the a-Si:H/a-Si:H/μc-Si:H configuration we achieved an operation photocurrent density of 7.7 mA cm−2 at 0 V applied bias using a Ag/Pt layer stack as photocathode/electrolyte contact and ruthenium oxide as counter electrode. Assuming a faradaic efficiency of 100%, this corresponds to a solar-to-hydrogen efficiency of 9.5%. The quadruple junction device provides enough excess voltage to substitute precious metal catalyst, such as Pt by more earth-abundant materials, such as Ni without impairing the solar-to-hydrogen efficiency.

Solar Hydrogen Generation with Wide‐Band‐Gap Semiconductors: GaP(100) Photoelectrodes and Surface Modification

GaP, with its large band gap of 2.26 eV (indirect) and 2.78 eV (direct), is a very promising candidate for direct photoelectrochemical water splitting. Herein, p-GaP(100) is investigated as a photocathode for hydrogen generation. The samples are characterized after each preparation step regarding how their photoelectrochemical behavior is influenced by surface composition and structure using a combination of electrochemical and surface-science preparation and characterization techniques. The formation of an Ohmic back contact employing an annealed gold layer and the removal of the native oxides using various etchants are studied. It turns out that the latter has a pronounced effect on the surface composition and structure and therefore also on the electronic properties of the interface. The formation of a thin Ga(2)O(3) buffer layer on the p-GaP(100) surface does not lead to a clear improvement in the photoelectrochemical efficiency, neither do Pt nanocatalyst particles deposited on top of the buffer layer. This behavior can be understood by the electronic structure of these layers, which is not well suited for an efficient charge transfer from the absorber to the electrolyte. First experiments show that the efficiency can be considerably improved by employing a thin GaN layer as a buffer layer on top of the p-GaP(100) surface.

Development of Thin Film Amorphous Silicon Tandem Junction Based Photocathodes Providing High Open-Circuit Voltages for Hydrogen Production

Hydrogenated amorphous silicon thin film tandem solar cells (a-Si:H/a-Si:H) have been developed with focus on high open-circuit voltages for the direct application as photocathodes in photoelectrochemical water splitting devices. By temperature variation during deposition of the intrinsic a-Si:H absorber layers the band gap energy of a-Si:H absorber layers, correlating with the hydrogen content of the material, can be adjusted and combined in a way that a-Si:H/a-Si:H tandem solar cells provide open-circuit voltages up to 1.87u2009V. The applicability of the tandem solar cells as photocathodes was investigated in a photoelectrochemical cell (PEC) measurement set-up. With platinum as a catalyst, the a-Si:H/a-Si:H based photocathodes exhibit a high photocurrent onset potential of 1.76u2009V versus the reversible hydrogen electrode (RHE) and a photocurrent of 5.3u2009mA/cm2 at 0u2009V versus RHE (under halogen lamp illumination). Our results provide evidence that a direct application of thin film silicon based photocathodes fulfills the main thermodynamic requirements to generate hydrogen. Furthermore, the presented approach may provide an efficient and low-cost route to solar hydrogen production.

XPS characterization and photoelectrochemical behaviour of p-type 3C-SiC films on p-Si substrates for solar water splitting

The electrochemical (EC) properties of single-crystalline p-type 3C-SiC films on p-Si substrates were investigated as electrodes in H2SO4 aqueous solutions in dark and under white light illumination. Before EC tests, the SiC films were etched by HF solution and aqua-regia–HF solution, respectively, and then investigated by x-ray photoelectron spectroscopy (XPS) including one untreated SiC sample. After EC tests, XPS was also applied to investigate the surface chemical state changes. The EC measurements indicate that the p-type 3C-SiC films on p-Si substrates can generate a cathodic photocurrent as the photocathode, which corresponds to hydrogen production, and generate an anodic photocurrent as the photoanode, which corresponds to oxygen evolution. XPS shows the surface of all the SiC films was oxidized due to anodic oxidation applied by a positive bias during the EC test, which indicates the formation of silicon oxides, CO2 or CO and carbonates.

Photoelectrochemical and photovoltaic characteristics of amorphous-silicon-based tandem cells as photocathodes for water splitting.

In this study amorphous silicon tandem solar cells are successfully utilized as photoelectrodes in a photoelectrochemical cell for water electrolysis. The tandem cells are modified with various amounts of platinum and are combined with a ruthenium oxide counter electrode. In a two-electrode arrangement this system is capable of splitting water without external bias with a short-circuit current of 4.50 mAu2009cm(-2). On the assumption that no faradaic losses occur, a solar-to-hydrogen efficiency of 5.54% is achieved. In order to identify the relevant loss processes, additional three-electrode measurements were performed for each involved half-cell.

A Molecular Approach to Manganese Nitride Acting as a High Performance Electrocatalyst in the Oxygen Evolution Reaction

The scalable synthesis of phase-pure crystalline manganese nitride (Mn3 N2 ) from a molecular precursor is reported. It acts as a superiorly active and durable electrocatalyst in the oxygen evolution reaction (OER) from water under alkaline conditions. While electrophoretically deposited Mn3 N2 on fluorine tin oxide (FTO) requires an overpotential of 390u2005mV, the latter is substantially decreased to merely 270u2005mV on nickel foam (NF) at a current density of 10u2005mAu2009cm-2 with a durability of weeks. The high performance of this material is due to the rapid transformation of manganese sites at the surface of Mn3 N2 into an amorphous active MnOx overlayer under operation conditions intimately connected with metallic Mn3 N2 , which increases the charge transfer from the active catalyst surface to the electrode substrates and thus outperforms the electrocatalytic activity in comparison to solely MnOx -based OER catalysts.

Silicon based tandem cells: novel photocathodes for hydrogen production

A photovoltaic tandem cell made of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) was investigated as a photocathode for hydrogen evolution in a photoelectrochemical device. The electronic and electrochemical properties of the samples were characterized using X-ray photoemission spectroscopy (XPS) and cyclic voltammetry (CV), whereas the morphology of the surface in contact with the electrolyte was investigated by scanning electron microscopy (SEM). The electric efficiency of the tandem cell was determined to be 5.2% in a photoelectrochemical (PEC) setup in acidic solution which is only about half of the photovoltaic efficiency of the tandem cell. A significant improvement in efficiency was achieved with platinum as a catalyst which was deposited by physical vapour deposition (PVD) under ultra-high vacuum (UHV) conditions.

The Electronic Interaction of Pt-Clusters with ITO and HOPG Surfaces upon Water Adsorption

Abstract In order to investigate the catalytic properties of supported platinum clusters, their interaction with water was monitored using photoelectron spectroscopy. The clusters were exposed to up to five Langmuir of water at cryogenic temperatures. Additionally, the influence of the substrate was studied by employing HOPG and ITO as complementary support materials. In contrast to bulk platinum a distinct chemical shift is observable in the Pt4f binding energies for Pt clusters deposited on ITO. The same clusters on HOPG show no changes in binding energy. We propose that this trend is due to a change in the surface Fermi level in ITO, hence highlighting the strong interaction between the platinum cluster and the substrate material. Therefore it is reasonable to assume, that the catalytic efficiency of these clusters in general can not solely be described by the electronic structure of the cluster alone, but that also the electronic changes induced in the substrate may have a major impact on the catalytic performance as well.

Electrocatalytic Performance of High‐Surface‐Area Platinum Catalysts Synthesized by Chemical Vapor Deposition for Water Splitting

We present the deposition of Pt particles and films by chemical vapor deposition from platinum(II) acetylacetonate for electrochemical water splitting. High‐surface‐area particles can be obtained at substrate temperatures as low as 150u2009°C by this technique. Clear differences in morphology were identified, which depended on the type of reactive gas introduced in the process. Although oxygen leads to a crystalline film growth, hydrogen leads to an open structure of stacked Pt spheres. The electrochemically active surface area of these deposits were determined by hydrogen adsorption–desorption measurements in alkaline solution. With respect to water splitting and other catalytic processes, we present a way to synthesize and characterize high‐surface‐area Pt samples easily. To evaluate their electrocatalytic performance, the catalysts were analyzed in terms of the overvoltage for the hydrogen evolution reaction.

Interface Engineering of Semiconductor Electrodes for Photoelectrochemical Water Splitting: Application of Surface Characterization with Photoelectron Spectroscopy

In this chapter we discuss at first bulk and interface related requirements of efficient photoelectrochemical device structures for water splitting. Maximized conversion efficiencies need photovoltages produced in the photovoltaic component of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. The photocurrents must approach quantum efficiencies of one for all absorbed photons above the band gap, which will only be possible for adjusted minority carrier diffusion lengths. We emphasize that to our expectations only multi-junction devices will provide photovoltages high enough for water splitting without any additional bias. Appropriate interface engineering layers must be developed for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction, must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials