Towards the Development of Practical Solar-Fuels Devices: Silicon-Based Solar-Hydrogen Generators with 14.2% Solar-to-Hydrogen Efficiency
Authors: Modestino Miguel A. , Schuettauf Jan-Willem , Chinello Enrico , Rodriguez Claudia A. , Lambelet David , Delfino Antonio , Dominé Didier , Faes Antonin , Despeisse Matthieu , Bailat Julien , Ballif Christophe , Psaltis Demetri , Moser Christophe 
Published: February 2020

The implementation of practical solar-fuel technologies can trigger a significant increase in the use of renewables in our energy ecosystem. These technologies can directly capture and store solar energy in the form of energy rich molecules, which could be used at a later stage as fuels for transportation or electricity generation. Storing solar-energy into fuels can also serve as a mean of seasonal energy storage, so that excess solar energy in the summer can be used during the periods of low solar irradiation in the winter. Also, solar-fuel generators can be operated in centralized facilities without disrupting the operation of the electricity grid. The spatio-temporal decoupling of the energy capture and utilization that solar-fuel technologies provide makes them an attractive renewable energy storage solution. For solar-hydrogen generators to be implemented, they need to be able to operate robustly, stably and continuously for prolonged periods of time – in the order of several years to decades. They also need to produce fuels in a cost-effective way and the energy produced over their lifetime needs to exceed the energy inputted into their manufacturing and operation.
In this presentation, we describe a system agnostic approach to assess promising designs and components of solar-hydrogen generators from a technoeconomic perspective. Using this framework, we evaluate the effects of device design and material selection on the cost of the generated hydrogen. The results presented here provide insights and device engineering directions that would lead to cost-optimized solar-hydrogen generators. Our findings demonstrate that in order to minimize hydrogen production costs, optimized devices will need to be manufactured with water splitting components that are significantly smaller in area than photovoltaic units (by a factor of 10-100’s). Additionally, the analysis points out that devices based on silicon (Si) photovoltaics (either thin-film or crystalline Si cells) could reach hydrogen production costs that approach those of hydrogen from fossil fuels resources. As the cost of hydrogen is primarily driven by the light-absorbing units, further economic gains can be obtained by improving the overall solar-to-hydrogen efficiencies of devices. These efficiency improvements can be achieved by the implementation of Silicon heterojunction (SHJ) solar cells into solar-hydrogen devices. SHJ cells present higher open-circuit voltages (VOC) than crystalline Si cells and can reach levels above 700 mV. Their high VOC values are mainly due to an interfacial passivation with a thin (~5 nm) film of hydrogenated intrinsic amorphous silicon between the c-Si wafer and the oppositely doped emitter, forming the p-n junction. Thanks to the high VOC of SHJ cells, we demonstrate that modules of three cells in series can provide enough potential to drive the water splitting reaction at high current densities in electrolysis units. Devices based on SHJ modules and Nafion-based membrane-electrode assemblies with platinum and iridium oxide electrocatalysts show stable performance over 100 hrs of operation at an unprecedented SHE of 14.2%. The same efficiency level is demonstrated for devices operated under alkaline conditions using microstructured nickel electrodes; representing the highest SHE achieved with earth-abundant components to date. As both the SHJ cells and electrolysis units implemented in this study are commercially viable, easily scalable and have long lifetimes, the devices presented here have the potential to disruptively accelerate the deployment of cost effective solar-hydrogen generators.