Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Experimental Approach for Determining Semiconductor/liquid Junction Energetics by <em>Operando</em> Ambient-Pressure X-ray Photoelectron Spectroscopy

Operando Ambient Pressure X-ray photoelectron spectroscopy (operando AP-XPS) investigation of semiconductor/liquid junctions provides quantitative ...

California Institute of Technology

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2% under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten-bismuth vanadate (W:BiVO4 ) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO4 and the nc-Si:H cell.

Efficient water-splitting device based on a bismuth vanadate photoanode and thin-film silicon solar cells.

Photosynthesis is nature's route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators. 

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure.

Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community's focus on detailed balance. However, due to their multicomponent nature, photoelectrochemical devices do not have an equivalent analogue to detailed balance, and reported theoretical efficiency limits vary depending on the assumptions made. Here we introduce a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood and contextualized. Ideal and experimentally realistic limiting efficiencies are presented, and then generalized using five representative parameters-semiconductor absorption fraction, external radiative efficiency, series resistance, shunt resistance and catalytic exchange current density-to account for imperfect light absorption, charge transport and catalysis. Finally, we discuss the origin of deviations between the limits discussed herein and reported water-splitting efficiencies. This analysis provides insight into the primary factors that determine device performance and a powerful handle to improve device efficiency.

Efficiency limits for photoelectrochemical water-splitting.

The interfacial properties of electrolessly deposited Pt nanoparticles (Pt-NPs) on p-Si and p -Si electrodes were investigated on the nanometer scale using a combination of scanning probe methods. Atomic force microscopy (AFM) showed highly dispersed Pt-NPs with diameters of 20-150 nm on the Si surface. Conductive AFM measurements showed that only approximately half of the particles exhibited measurable contact currents, with a factor of 10 difference in current observed between particles at a given bias. Local current-voltage measurements revealed a rectifying junction with a resistance ≥10 MΩ at the Pt-NP/p-Si interface, whereas the Pt-NP/p -Si samples formed an ohmic junction with a local resistance ≥1 MΩ. The particles were strongly attached to the sample surface in air. However, in an electrolyte, the adhesion of the particles to the surface was substantially lower, and most of the particles had tip-contact currents that varied by a factor of approximately 10. Scanning electrochemical microscopy (SECM) showed smaller but more uniform electrochemical currents for the particles relative to the currents observed by conductive AFM. In accord with the conductive AFM measurements, the SECM measurements showed conductance through the substrate for only a minority of the particles. These results suggest that the electrochemical performance of the electrolessly deposited Pt nanoparticles on Si can be ascribed to: 1) The high resistance of the contact between the particles and the substrate, 2) the low (<50 %) fraction of particles that support high currents, and 3) the low adhesion of the particles to the surface when in contact with the electrolyte.

Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p -Si Electrodes.

Photoelectrochemical (PEC) cells offer the possibility of carbon-neutral solar fuel production through artificial photosynthesis. The pursued design involves technologically advanced III-V semiconductor absorbers coupled via an interfacial film to an electrocatalyst layer. These systems have been prepared by in situ surface transformations in electrochemical environments. High activity nanostructured electrocatalysts are required for an efficiently operating cell, optimized in their optical and electrical properties. We demonstrate that shadow nanosphere lithography (SNL) is an auspicious tool to systematically create three-dimensional electrocatalyst nanostructures on the semiconductor photoelectrode through controlling their morphology and optical properties. First results are demonstrated by means of the photoelectrochemical production of hydrogen on p-type InP photocathodes where hitherto applied photoelectrodeposition and SNL-deposited Rh electrocatalysts are compared based on their J-V and spectroscopic behavior. We show that smaller polystyrene particle masks achieve higher defect nanostructures of rhodium on the photoelectrode which leads to a higher catalytic activity and larger short circuit currents. Structural analyses including HRSEM and the analysis of the photoelectrode surface composition by using photoelectron spectroscopy support and complement the photoelectrochemical observations. The optical performance is further compared to theoretical models of the nanostructured photoelectrodes on light scattering and propagation.

Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography.

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/cm 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.

Hans-Joachim Lewerenz

Joint Center for Artificial Photosynthesis,
Division of Engineering and Applied Science and Joint Center for Artificial Photosynthesis

California Institute of Technology

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Hans-Joachim Lewerenz

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Joint Center for Artificial Photosynthesis,Division of Engineering and Applied Science and Joint Center for Artificial Photosynthesis

California Institute of Technology

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Joint Center for Artificial Photosynthesis,
Division of Engineering and Applied Science and Joint Center for Artificial Photosynthesis