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PhD Defense Pieter Westerik

microfabrication for solar-to-hydrogen devices 

Pieter Westerik is a PhD student in the research group Mesoscale Chemical Systems. His supervisors are prof.dr. J.G.E. Gardeniers and prof.dr.ir. J. Huskens from the faculty Faculty of Science and Technology.

This thesis has described the usefulness of some microfabrication techniques for the development of solar-to-hydrogen devices. Solar-to-hydrogen devices are being developed as a way to deal with the intermittent nature of solar energy, to generate a fuel which can be stored, instead of electricity which has to be used directly when it is generated. The voltage and current produced by these solar-to-hydrogen devices is used to electrochemically split water into oxygen and the fuel hydrogen. However the combined management of photons, electrons, ions and bubbles in such devices is challenging and often leads to trade-offs and sub-optimal efficiencies. The challenge is to find device architectures that can circumvent these trade-offs as much as possible, especially by finding the right combination and arrangement of semiconductor materials, making sure that catalysts and gas separators do not interfere with light absorption, and taking care of good ion conduction without reducing the illuminated semiconductor area or creating explosive product streams.

Architectures with micrometer-sized features can help to overcome some of these trade-offs. Microfabrication specializes in the parallel fabrication of numerous micro and nanostructures on silicon wafers, and silicon has proven its value for photovoltaics and is also investigated for solar water splitting devices. However the control that these microfabrication techniques give over silicon structures in the out-of-plane direction is limited. Therefore some new techniques were developed for patterning the sidewalls of silicon structures. A special inclined ion beam etching technique was developed for selective etching of horizontal surfaces, and a special Bosch etching recipe for creating multiple semicircular features along vertical silicon structures. Combined with Retraction Edge Lithography and Corner Lithography, selective opening of the top or bottom or one or several bands along the sidewall of such structures was achieved. The technology is accurate and can produce wafer-scale arrays of the same 3D monocrystaline structures in parallel.

The selective opening of the top part of a silicon structure was applied to a silicon microwire photocathode with a radial n+/p-junction in the wires, to achieve spatioselective deposition of a nickel-molybdenum alloy, an opaque catalyst made of only earth-abundant elements. In this way the amount of catalyst-covered and illuminated surface area could be optimized separately, to avoid both the catalysis limited and light absorption-limited regimes in the parameter space, and reach an efficiency of 10.8 %, a record for silicon wire-based photocathodes.

First steps towards a fully integrated solar-to-hydrogen device were made. Based on band gaps, available catalysts and material stabilities, it was investigated whether a photoabsorber tandem configuration with silicon and hematite could be made fully integrated and used for providing all the necessary current and voltage for splitting water. A Ti99Nb1 connection layer was developed, and tandem operation was shown for the combination of a monocrystaline silicon buried junction solar cell with a reactively sputtered hematite photoanode. However the performance of the tandem device was poor, with the onset of water oxidation around 0.5 V vs RHE and a photocurrent around 40 μA cm−2 at 1.23 V vs RHE. There are certainly promising possibilities for improving the performance, but it remains uncertain if such tandem devices can ever compete with silicon-based triple cells as the driving force for water electrolysis.

Another step towards a fully integrated solar-to-hydrogen device was the investigation of an innovative device concept based on a silicon membrane structure with micropores. The membrane has integrated electrodes on both sides, and can split water with ion transport losses below 100 mV at a current density of 10 mA cm−2 in a 1 M NaOH electrolyte. It can do this while around 99 % of the surface area of the device remains available for light absorption. Without use of an ion-selective membrane, hydrogen crossover to the oxygen generating compartment was around 1 %. This shows the promise of the device architecture, as it circumvents the trade-offs between ion transport, light absorption and gas separation.

Several device architectures were considered, and it was argued that all devices that use photoelectrodes made out of earth-abundant elements need some form of microstructuring to circumvent certain trade-offs. These materials can often be processed with cheap, solution-based methods, and if their performance can be improved they hold great promise for solar hydrogen with a reasonable price. However state-of-the-art silicon-based triple cells can currently drive the water splitting reaction more efficiently, and require less integration with the ‘electrolyzer’ part of the solar-to-hydrogen device because they do not depend on a semiconductor-liquid junction. Therefore trade-offs can be more easily avoided (without microstructuring) and manufac-

turing could be easier. Economically it is probably even more favorable to completely separate the solar cells and the electrolyzer, and only connect them with an electrical wire (and potentially power electronics). This would allow separate optimization of both components. If the installation is additionally connected to the power grid, the electricity could also be used directly when the demand is high enough, thereby avoiding the losses associated with both the water splitting and recombination reactions.

At least for the near future, the implementation of such ‘PV-Electrolysis’ seems more likely than the use of the more integrated photoelectrochemical devices. Because of the low price of fossil fuel-based energy and hydrogen, first applications will probably only be feasible for small, grid-independent installations or perhaps for the generation of hydrogen as a chemical feedstock. However advancements in photoelectrode materials, catalysts and device architectures (including the necessary microstructuring), as well as the depletion of fossil fuels and the implementation of taxes and subsidies that favor the use of renewable energy, could make solar hydrogen interesting for the global energy market in the long term.