Researchers around the world are working on the development of solar fuel technology. This involves generating sustainable fuels using only sunlight, CO₂ and water. A group of researchers from the University of Twente’s MESA+ research institute are working on a solar-to-fuel device that produces hydrogen.

Large-scale conversion and storage of solar energy via the production of hydrogen would likely rely upon Earth-abundant materials. However, the UT team notes, earth-abundant catalysts for the hydrogen evolution reaction (HER)—for example nickel–molybdenum (Ni–Mo)—are generally opaque and require high mass loading to obtain high catalytic activity, which in turn leads to parasitic light absorption for the underlying photoabsorber (for example silicon), thus limiting production of hydrogen in this type of application.

A good solar-to-fuel device is efficient both in light harvesting and in transforming the photogenerated electricity into chemical bonds. Silicon (Si) is a popular, high-performing photon absorber. However, Si as a photocathode has poor kinetics for the hydrogen evolution reaction (HER) and therefore requires a catalyst to achieve efficient solar-to-hydrogen conversion. Unfortunately, however, for state-of-the-art Earth-abundant catalysts (such as Ni–Mo), the high mass loading that is needed for sufficient catalytic activity also results in considerable parasitic light absorption upon frontside illumination, which reduces the efficiency of such photocathodes.

… Silicon microwire arrays provide an approach to overcome the negative correlation between the catalytic activity, which is directly related to the fill factor, FF, in terms of photovoltaic (PV) cells, and light absorption, J_ph. … Here we describe the spatial and functional decoupling of light absorption and catalytic activity of frontside-illuminated Si microwire array photocathodes.

— Vijselaar et al.

The UT system consists of silicon microwires less than one tenth of a millimeter long, the tops of which are coated with a catalyst. The photons are collected between the microwires. The chemical reaction in which hydrogen is formed takes place on the catalyst at the tips of the microwires.

Top: a–c, Fabrication process for completely exposed microwires (a), microwires passivated by SiO2 with exposed tops (b), or samples with a defined exposed area in the range of 2–36 μm from the top (c). Bottom: HR-SEM image of Si microwire array with Ni-Mo solely on the tops of the microwires. Vijselaar et al. Click to enlarge.

By varying the density and length of the microwires, the researchers ultimately achieved a maximum efficiency of 10.8%. They managed to achieve this by decoupling the site where the photons are collected from the site where the conversion reaction takes place. This is necessary because catalysts usually reflect light.

Although 10.8% is the highest ever efficiency for a silicon-based design, said co-corresponding author Prof. Jurriaan Huskens, a further increase in efficiency to 15% is needed to make the technology economically viable.

Resources

• Wouter Vijselaar, Pieter Westerik, Janneke Veerbeek, Roald M. Tiggelaar, Erwin Berenschot, Niels R. Tas, Han Gardeniers & Jurriaan Huskens (20180 “Spatial decoupling of light absorption and catalytic activity of Ni–Mo-loaded high-aspect-ratio silicon microwire photocathodes” Nature Energy doi: 10.1038/s41560-017-0068-x