The direct photo-electrochemical conversion of water and carbon dioxide (CO2) into valuable fuels and chemical commodities using concentrated solar light represents a promising route for sustainable fuel and material production. This thesis presents the design, characterization, operation, and optimization of an integrated photo-electrochemical (P-EC) device for CO2 reduction to carbon monoxide (CO) and ethylene (C2H4) with possible subsequent one-step thermochemical conversion into aviation fuels. The functionality of a water-splitting version of the P-EC device was further extended and characterized in reversible operation.
The P-EC device integrates a concentrated photovoltaic (CPV) cell with a zero-gap electrolyzer (EC) cell, addressing limitations of conventional PV-electrolysis and photoelectrochemical (PEC) systems. While most reported studies operate under 1 sun illumination (i.e., 1000 Wm-2), this work focuses on devices operating with highly concentrated light. Concentrated solar light is used in all presented solar devices, reducing the use of expensive photoactive and catalytic materials and thus making the device more cost-competitive.
This thesis addresses: (i) the design, develop, and characterize a P-EC device for CO2 to CO conversion using concentrated solar light; (ii) generalize the P-EC device concept for CO2 reduction focusing on gaseous products by extending the design to obtain multi-carbon products such as C2H4; and (iii) explore the reversible operation of a water-splitting version of the P-EC device.
The implemented P-EC device for CO2 to CO conversion operated at up to 450 suns, achieving a partial CO current density of 160 mA cm-2, a solar-to-CO conversion efficiency of 17%, and an overall solar-to-fuel efficiency of 19%. The combination of high input power and high solar-to-CO conversion efficiency represented the first reported case of P-EC CO2 reduction into CO at a production rate exceeding 1 NL h-1.
For multi-carbon products, the P-EC device was adapted using a two-cell series-connected CPV cell to provide the required overpotential. The device achieved a solar-to-C2H4 efficiency of 3.2% under 99 suns, with a stable current density of 233 mA cm-2. The device demonstrated operational stability over 8 hours, using ultrapure water as the anolyte. By optimizing CO2 flow rates and solar concentration, solar-driven CO2 utilization was demonstrated with total solar-to-fuel efficiencies of 6.9%.
To demonstrate the versatility of the P-EC device, the reversible operation of the P-EC device was tested, allowing a stand-alone platform for both solar fuel and power generation. In the solar mode, the device produced hydrogen (H2) and oxygen (O2), and in the reverse mode, power. The device achieved a solar-to-H2 efficiency of 18.11% and a current density of 292 mA cm-2 at 518 suns. In fuel cell mode, a stable power density output of 35.7 mW cm-2 at a 50 mA cm-2 load and a voltage efficiency of 48.2% were achieved.
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