The intermittent photovoltaic (PV) power generation requires long term storage to stabilize energy system. Conversion of the excess PV power into valuable chemicals (products of CO₂ reduction, for example) represents an attractive solution for the problem. We address this challenge via ‘artificial leaf’ approach – photovoltaic driven conversion of carbon dioxide (CO₂) and water (H₂O) into valuable chemicals. In our experiment an electrochemical cell (EC), responsible for the CO₂ reduction, was directly connected to the highly efficient silicon heterojunction PV module that generates the sunlight-based electricity.

In this work, we study the performance of the directly coupled PV-driven CO₂ reduction cell with emphasis on the effects of realistic temperatures (25-65°C) and irradiances (0.2 to 1 sun) for the PV module. During the experiment we characterized coupling efficiency in the system and product selectivity of the electrolyzer, catalyst stability and finally solar-to-chemical (STC) efficiency. Coupling efficiency in the directly connected PV-EC system is an important performance parameter and is characterized by the coupling factor C_PV-EC, the ratio between the operating power and the maximum power deliverable by the PV device. The current-voltage parameters of both PV and EC system components were preselected to ensure high power coupling factor under standard test conditions of PV (1 sun irradiance A.M 1,5G and 25°C). A 5-cells silicon heterojunction PV module with maximum power point voltage of 3.2V, current of 383.7 mA, aperture area of 51.7 cm², and power conversion efficiency of 23.82% was connected to a commercially available EC cell with an 8.8 cm² CO-targeted silver catalyst [1].

The directly connected PV-EC device withstands realistic temperature and irradiance variation of the PV module maintaining high coupling factor C_PV-EC for the PV-EC device between 0.89 and 1.00. Furthermore, while temperature minimally affects PV-EC working points (V_WP, I_WP), the variations in irradiance between 0.2 and 1 sun significantly influence the voltage at working point (between 2.55 V and 3.44V respectively) and therefore affect electrolyzer products selectivity. The effect of operating voltage on the electrolyzer product selectivity is studied in a separate experiment with constant voltage steps with 1 hour duration using a potentiostat. As the voltage is increased between 2.6 to 3.4V the product selectivity for CO increases from 80% to 95%. In the operation stability study, we have observed high CO selectivity of approximately 90% over 7 hours of experiment at 3.4V and 330 mA. Performance of the EC cell characterized with the potentiostat suggests that under proper coupling, the PV-EC system can operate with solar to chemical efficiency for CO of approximately 10% over the operating voltage range.

In the ongoing experiments we investigate the CO-selectivity, operating stability and solar to CO efficiency with the directly connected PV-EC device in realistic irradiance and temperature conditions. In addition to that, the catalyst degradation rate on microstructure scale is studied using electron microscopy.