Abstract
Solar-driven water splitting is considered one of the promising future routes to generate fuel in a sustainable way. A carbon-free solar fuel, molecular hydrogen, can here be produced along two different but intimately related routes, photoelectrochemical (PEC) water splitting or photovoltaic electrolysis (PV-electrolysis), where the latter builds on well-established solar cell and electrolysis materials with high efficiency. The PV-electrolysis approach is also possible to construct from an integrated PEC/PV-system avoiding dc–dc converters and enabling heat exchange between the PV and electrolyzer part, to a conventionally wired PV-electrolysis system. In either case, the operating voltage at a certain current needs to be matched with the catalyst system in the electrolysis part. Here, we investigate ((Ag),Cu)(In,Ga)Se2 ((A)CIGS)-materials with varying Ga-content modules for combination with NiMo–NiO catalysts in alkaline water splitting. The use of (A)CIGS is attractive because of the low cost-to-performance ratio and the possibility to optimize the performance of the system by tuning the band gap of (A)CIGS in contrast to Si technology. The band gap tuning is possible by changing the Ga/(Ga + In) ratio. Optoelectronic properties of the (A)CIGS materials with Ga/(Ga + In) ratios between 0.23 and 0.47 and the voltage and power output from the resulting water splitting modules are reported. Electrolysis is quantified at temperatures between 25 and 60 °C, an interval obtainable by varying the thermal heat exchange form a 1-sun illuminated PV module and an electrolyte system. The band gaps of the (A)CIGS thin films were between 1.08 to 1.25 eV and the three-cell module power conversion efficiencies (PCE) ranged from 16.44% with 1.08 eV band gap and 19.04% with 1.17 eV band gap. The highest solar-to-hydrogen (STH) efficiency was 13.33% for the (A)CIGS–NiMo–NiO system with 17.97% module efficiency and electrolysis at 60 °C compared to a STH efficiency of 12.98% at 25 °C. The increase in STH efficiency with increasing temperature was more notable for lower band gaps as these are closer to the overpotential threshold for performing efficient solar-driven catalysis, while only a modest improvement can be obtained by utilizing thermal exchange for a band gap matched PV-catalysts system. The results show that usage of cost-effective and stable thin film PV materials and earth abundant catalysts can provide STH efficiencies beyond 13% even with PV modules with modest efficiency.
Highlights
The prospects of using hydrogen as an energy carrier has long been considered a promising
Optimal band gap and Ga content of (A)CIGS materials and the subsequently created modules were examined for solar water splitting systems with NiMo and NiO used as a cathode and anode catalysts, respectively. (A)CIGS modules consisting of three sub-cells, and Ga/(Ga + In) ratios were varied between 0.23 and 0.47
Water electrolysis was performed using NiMo–NiO as catalysts at different temperatures between 25 and 60 ◦ C, a temperature range that would be available by thermal exchange between the PV and electrolyzer parts
Summary
The prospects of using hydrogen as an energy carrier has long been considered a promising. For a high generation optimum operation current-potential point of the system, which in turn depend on which catalyst of charge carriers with theFor required theofsemiconductor a band gap energy Large-scale applications devices with multiple-band instead havesolar-driven been utilized for the absorption of infrared photons, of tandem approaches, are limited by the manufacturing complexity and have the same improved photovoltage, and providing highly efficient solar-driven water splitting. Ga content and the subsequent increase in the band gap of an absorber layer is frequently followed by an increase in open circuit voltage (VOC ) and the operating voltage at the maximum power point up to a certain Ga-content It results in a decrease in the generated current due to the lower amount of photons harvested in the solar spectrum. The influence of basic parameters of the (A)CIGS material and charge separation on STH was discussed for different electrolyzer temperatures
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