CO2 Reduction under Realistic Ambient Conditions with Directly coupled PV-EC Device

Long-term storage in molecules like fuels or other industrially useful chemicals is most relevant for counterbalancing natural yearly oscillations in photovoltaic power generation. We address this challenge via ‘artificial leaf’ approach – conversion of carbon dioxide (CO2), water (H2O) and sunlight into valuable chemical products. This approach facilitates further deployment of photovoltaics and simultaneously utilizes exhaust or atmospheric CO2 with potential to entirely close CO2 cycle. Conversion of CO2 into chemical fuels such as syngas (mixture of CO and H2 products) is demonstrated with photoelectrochemical (PEC) devices, direct electrolysis, and direct photovoltaic-driven electrolysis (PV-EC). The PV-EC approach in direct connection is attractive as a good compromise between the design flexibility and high solar to chemical efficiencies. In this work we design, and test ‘A-leaves’ with emulated PV devices reproducing IV characteristics of real PV modules in the field at most relevant irradiance/temperature combinations. The characteristic operating point were obtained using the NREL public database for SHJ module installed in Eugine USA. With our newly developed PV emulator routine any PV IV can be replicated at high accuracy and at the same PV part of the PV-EC system can be adjusted to required size matching the size of the lab scale CO2 reduction EC cell. In our work PV module of emulated area of 51.7 cm² drives flow-type stack EC cell (area 8.8 cm²) with silver/gas diffusion layer (GDL) cathode [1] and iridium oxide anode. The EC cell consists of two electrode chambers separated by a Nafion membrane (N117, thickness 0.007 inch) and a 1 mol/l KHCO₃ electrolyte saturated with CO₂ (20 cm³min^-1) flowing in both chambers.<br/>We have chosen five most probable irradiance and temperature combinations out of variety of ambient conditions with the highest PV energy generation, i.e., the PV-EC device is optimized for the region of major PV energy generation. In our case this is 1 Sun irradiance and 45°C PV temperature. Performance, power coupling and CO and H₂ products evaluation have been investigated in the context of most relevant PV irradiance (1 – 0.2 Sun) and temperature (25 – 45 °C) obtained from real PV data. Power coupling, or power matching between the photovoltaic and electrochemical components have been quantified with ‘coupling factor’ C – coefficient indicating proximity of the working point (WP) of the PV-EC device to the maximum power point (MPP) of the PV module.<br/>High degree of coupling, more than 0.9 and solar-to-chemical efficiency in the range from 7.5 % to 10.8 % is observed at the most relevant PV irradiance (1 – 0.2 Sun) and temperature (25 – 45 °C). Successful production of CO with by-product of H2 in the ration of 3:1 with Faradaic efficiency (F_E) of 70 - 100% was measured via chromatographic analytics. Over the experiment, operating voltages of 2.6 - 3.1 V and current densities 9 - 44.9 mA/cm² were observed. Energy loss analysis of PV-EC device at 1 Sun 45 °C was performed. SHJ module converts 22.5 % of solar power into electric power and due to relatively low PV-EC coupling loss, 21.3 % of the initial sun power reaches the EC cell. At the working voltage (V_WP) of 3.1 V and working current density of 44.9 mA/cm², EC operates at 70 % Faradaic efficiency and 13.8% overpotential loss that translates into solar-to-chemical efficiency (STC) of 7.5 % estimated by using the equation:<br/>STC= PVeff.×C×(Ecell/V_WP)×F_E,where Ecell is the energy stored in product CO or H2 in voltage.<br/>[1] Liu, G., et al., Chemical Engineering Journal 2023, 40, 141757.