Strategies towards high yield syngas production from solar CO2 recycling

The photo-electrochemical recycling of CO2 emerged as alluring way to store intermittent renewable energy whilst converting it back into chemical fuels. In this contribution, we report on a novel prototype reactor device for... [ view full abstract ]

The photo-electrochemical recycling of CO₂ emerged as alluring way to store intermittent renewable energy whilst converting it back into chemical fuels. In this contribution, we report on a novel prototype reactor device for high yield conversion of CO₂ to syngas (H₂ + CO), which by design, is integrated, scalable to large areas, and compatible with state-of-the-art photovoltaics and electrocatalysts. Within this contribution, mainly three aspects will be addressed: adaption and integration of silicon based solar cells as photoanodes, cathode material development, and prototype reactor assembly.

The application of silicon photovoltaic cells as photoanodes requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient oxygen evolution reaction (OER). Under photoelectrolysis conditions, the photovoltage can be adjusted up to 2.4 V by connecting up to four solar cells in series. We demonstrate that this high photovoltage of the photovoltaic structure enables the usage of earth-abundant catalyst materials for the OER, such as Ni.

The CO₂ reduction reaction is performed by large scale three-dimensional metallic foam cathodes, which are decorated with highly active nanosized catalysts for selective syngas production. We investigate the deposition of Zn and Ag catalysts on Cu- and Ni-foams. The performance of the as-produced (gas-diffusive) cathodes is evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. Stable and tunable H₂:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 39.4 mA/cm² are measured (Fig.1).

In the complete integrated reactor assembly we combine the optimized silicon photoanode and the gas diffusive cathode (both with 10 cm² active surface area) and investigate the most efficient membrane configuration, in terms of low overall cell voltage. Additionally, we stepwise optimize the reactor, regarding clever packaging, efficient gas management, and electrolyte flow. Finally, we demonstrate a bias-free operation of the complete reactor device providing a photocurrent density of 5.0 mA/cm² measured under 100 mW/cm² illumination (Fig.2). This corresponds to a solar-to-syngas conversion efficiency of 4.3%.