Volume 234, Issue 6 - Special Issue: DFG priority program SPP 1613 “Fuels Produced Regeneratively Through Light-Driven Water Splitting” / Guest Editors: Bernhard Kaiser and Wolfram Jaegermann

Thin film silicon based multi-junction solar cells were developed for application in combined photovoltaic electrochemical systems for hydrogen production from water splitting. Going from single, tandem, triple up to quadruple junctions, we cover a range of open circuit voltages from 0.5 V to 2.8 V at photovoltaic cell (PV) efficiencies above 13%. The solar cells were combined with electrochemical (EC) cells in integrated devices from 0.5 cm 2 to 64 cm 2 . Various combinations of catalyst pairs for the oxygen and hydrogen evolution reaction side (OER and HER) were investigated with respect to electrochemical activity, stability, cost and – important for the integrated device – optical quality of the metal catalyst on the HER side as back reflector of the attached solar cell. The combined PV-EC systems were further investigated under varied operation temperatures and illumination conditions for estimation of outdoor performance and annual fuel production yield. For 0.5 cm 2 size combined systems a maximum solar-to-hydrogen efficiency η STH = 9.5% was achieved under standard test conditions. For device upscaling to 64 cm 2 various concepts of contact interconnects for reduced current and fill factor loss when using large size solar cells were investigated. To replace high performance noble metal based catalyst pairs (Pt/RuO 2 or Pt/IrO x ), more abundant and cheaper NiMo (HER) and NiFeO x (OER) compounds were prepared via electrodeposition. With the NiMo/NiFeO x catalyst pair we obtained η STH = 5.1% for a 64 cm 2 size solar cell which was even better than the performance of the Pt/IrO 2 system (η STH = 4.8%). In simulated day-night cycle operation the NiMo/NiFeO x catalyst pair showed excellent stability over several days. The experimental studies were successfully accompanied by simulation of the entire PV-EC device using a series connection model which allowed studies and pre-estimations of device performance by varying individual components such as catalysts, electrolytes, or solar cells. Based on these results we discuss the prospects and challenges of integrated PV-EC devices on large area for hydrogen and solar fuel production in general.

A fabrication route is presented by a novel metal-organic electrochemical approach, allowing facile preparation of electrocatalytic metal and metal oxide thin films from solid metals at room temperature: divalent transition metals such as iron, nickel and cobalt can be deposited as amorphous oxyhydroxide films. Among them, nickel- and cobalt-prepared samples are showing high activity as water oxidation catalysts in alkaline electrolytes. The applicability to waste metal material is exemplified using one of the most abundant waste metal alloys, i.e. steel: with a suitable multi-layer architecture, comprising, a large surface-area iron oxyhydroxide core-geometry and a steel-derived catalytic overlayer, the overpotential for oxygen evolution (at 10 mA cm −2 ) could be improved to only 370 mV. Chemical analysis is provided to elucidate the reaction pathway, encompassing metal halogenation, formation of meta-stable metal-organic intermediates or direct electrochemical reduction, respectively. Structural peculiarities of the derived films are finally demonstrated upon development of a photoactive nickel oxyhydroxide/silicon junction realizing a photocurrent of 1 mA cm −2 at the thermodynamic potential for oxygen evolution and short-term stabilization in the range of several hours.

The electrochemical CO 2 reduction reaction (CO 2 RR) towards CO allows to turn CO 2 and renewable energy into feedstock for the chemical industry. Previously shown electrolyzers are capable of continuous operation for more than 1000 h at high faradaic efficiencies and industrially relevant current densities. However, the crossover of educt CO 2 into the anode gas has not been investigated in current cell designs: Carbonates (HCO 3 − and CO 3 2− ) are formed at the cathode during CO 2 RR and are subsequently neutralized at the anode. Thus, CO 2 mixes into the anodically evolved O 2 , which is undesired from commercial perspectives. In this work this chemical transport was suppressed by using a carbonate-free electrolyte. However, a second transport mechanism via physically dissolved gases became apparent. A transport model based on chemical and physical absorption of CO 2 and O 2 will be proposed and two solutions were experimentally investigated: the use of an anode GDL (A-GDL) and degassing the anolyte with a membrane contactor (MC). Both solutions further reduce the CO 2 crossover to the anode below 0.1 CO 2 for each cathodically formed CO while still operating at industrially relevant current densities of 200 mA/cm 2 .

We present an alternative synthesis strategy for developing nanocrystalline (Ga 1−x Zn x )(N 1−x O x ) semiconductors known to be very efficient photoabsorbers. In a first step we produce mixtures of highly crystalline β-Ga 2 O 3 and wurtzite-type ZnO nanoparticles by chemical vapor synthesis. (Ga 1−x Zn x )(N 1−x O x ) nanoparticles of wurtzite structure are then formed by reaction of these precursor materials with ammonia. Microstructure as well as composition (zinc loss) changes with nitridation time: band gap energy, crystallite size and crystallinity increase, while defect density decreases with increasing nitridation time. Crystallite growth results in a corresponding decrease in specific surface area. In the UV regime photocatalytic activity for overall water splitting can be monitored for samples both before and after nitridation. We find a significantly lower photocatalytic activity in the nitrided samples, even though the crystallinity is significantly higher and the defect density is significantly lower after nitridation. Both properties should have led to a lower probability for charge carrier recombination, and, consequently, to a higher photocatalytic activity.

In this study, we present different silicon based integrated devices for photoelectrochemical water splitting, which provide enough photovoltage to drive the reaction without an external bias. Thin films of titanium dioxide, prepared by atomic layer deposition (ALD), are applied as a surface passivation and corrosion protection. The interfaces between the multi-junction cells and the protective coating were optimized individually by etching techniques and finding optimal parameters for the ALD process. The energy band alignment of the systems was studied by X-ray photoelectron spectroscopy (XPS). Electrochemically deposited platinum particles were used to reduce the HER overpotential. The prepared systems were tested in a three-electrode arrangement under AM 1.5 illumination in 0.1 M KOH. In final tests the efficiency and stability of the prepared devices were tested in a two-electrode arrangement in dependence of the pH value with a ruthenium-iridium oxide counter electrode. For the tandem-junction device solar to hydrogen efficiencies (STH) up to 1.8% were reached, and the triple-junction device showed a maximum efficiency of 4.4%.

Using transmission and scanning electron microscopy, we study mechanisms which determine the stability of Silicon photocathodes for solar driven water splitting. Such tandem or triple devices can show a promising stability as photocathodes if the semiconductor surface is protected by an ultrathin TiO 2 protection layer. Using atomic layer deposition (ALD) with Cl-precursors, 4–7 nm thick TiO 2 layers can be grown with high structural perfection. The layer can be electrochemically covered by Pt nanoparticels serving as electro-catalysts. However, Cl-remnants which are typically present in such layers due to incomplete oxidation, are the origin of an electrochemical degradation process. After 1 h AM1.5G illumination in alkaline media, circular shaped corrosion craters appear in the topmost Si layer, although the TiO 2 layer is intact in most parts of the crater. The crater development is stopped at local inhomogenities with a higher Pt coverage. The observations suggests that reduced Titanium species due to Cl − /O 2− substitution are nucleation sites of the initial corrosion steps due to enhanced solubility of reduced Ti in the electrolyte. This process is followed by electrochemical dissolution of Si, after direct contact between the electrolyte and the top Si layer surface. To increase the stability of TiO 2 protected photocathodes, formation of reduced Ti species must be avoided.

The synthesis conditions of the semiconductor BiVO 4 have marked influence on its physicochemical and photocatalytic properties. In this work, a pH-controlled co-precipitation route was systematically investigated. Special attention was paid to the pH value during co-precipitation and co-precipitate post-treatment. In a sacrificial photocatalytic water oxidation test, the highest activity was observed for a sample precipitated at pH = 1 and 70 °C, which was post treated at 600 °C in air for 3 h after washing and drying. The activity was further improved by another 90% by the addition of the Co 3 O 4 -based co-catalyst. Good synergy between semiconductor and co-catalyst was obtained for the deposition of pre-formed nanoparticles at a loading of 0.1 w% (physical impregnation). The effects of different synthesis conditions of the semiconductor and impregnation method for the co-catalyst on the structure, morphology and optical properties of the catalysts were investigated by PXRD, SEM, UV-vis spectroscopy, and TEM, while the water oxidation activity was compared in the dark and with the aid of visible light using cerium (IV) or silver (I) as sacrificial agents, respectively, with the aim of establishing structure-activity correlations. The roles of semiconductor particle anisotropy and co-catalyst particle distribution for optimal photo-activity in the oxygen evolution reaction are discussed.

The performance of free-standing parallel-aligned nanowire arrays and interconnected networks of single-crystalline cuprous oxide (Cu 2 O) coated with titanium oxide (TiO 2 ) as photocathodes for solar energy harvesting was analyzed. The nanostructures were synthesized by electrodeposition in polymer membranes prepared by ion-track technology. To enhance the photoelectrochemical stability of the nanowires in aqueous solution, they were conformally coated with a 10 nm thick TiO 2 layer by atomic layer deposition. The diameter, size, geometry and number density of the parallel nanowires were systematically varied. The generated photocurrents show a clear increase as a function of wire diameter and wire number. In turn, the photocurrent does not get larger with increasing wire length. Highly interconnected networks of nanowires under 45° from various directions enabled further increase of wire density number and exhibited higher photocurrent densities compared to parallel arrays.