CONCLUSIONS AND PERSPECTIVE
In this review, we introduce diverse systems, including electrochemical (EC), photoelectrochemical (PEC), photovoltaic-assisted electrochemical (PV-EC), and photovoltaic-assisted photoelectrochemical (PV-PEC), for efficient CO2 reduction and conversion to achieve carbon neutrality by introducing catalysts, photoabsorbers, and ideal bias-free PV-assisted devices. In the EC system, Cu, the only metal capable of producing high value-added C2+ carbon compounds, has limitations due to the lack of energy supply for the formation of C-C couplings, while electrocatalysts such as noble metals and MOF-based and single atom-based materials show excellent CO2conversion rates of approximately 100% for C1 products. Therefore, as mentioned, designing Cu-based materials that establish various strategies, such as manipulating surfaces, reconfiguring morphology, and inducing synergies from heterogeneous catalysts, is imperative. Despite numerous efforts, the low FE toward C2+ chemicals is a problem to be solved from various perspectives, including not only the EC system but also PEC and PV-PEC devices. Moreover, obstacles to the PEC system, such as complex reaction paths, large photovoltage requirements, low solar-to-fuel efficiency, and poor light-harvesting properties, can be overcome by combining a co-catalyst with a light-absorbing semiconductor. Additionally, as in the EC system, manufacturing ideal PEC catalysts causing synergetic effects from heterogeneous materials and coating photocathodes with MOF materials may be a potential substitute for expensive noble metal catalysts, maintaining high energy conversion efficiency.
To acquire large-scale installations of CO2RR devices, we proposed two novel PV-powered systems: PV-EC and PV-PEC. Both systems have an outstanding ability to lead the techno-economy to achieve high STC efficiency by serving the 2.6 V needed to reduce CO2from photovoltaics. However, large total voltage requirements, which are determined by analyzing intersections including the maximum power point (MPP) and operating point of current/current density – voltage (I/J-V ) characteristics, remain a key hindrance. To overcome these challenges, Si-, perovskite-, DSSC-, CIGS-, and GaAs-based solar absorbers, as well as those of multi-junction or tandem structures, have gained a lot of attention. Nevertheless, Group III-V semiconductors composed of multi-component elements containing GaAs-based absorbers show excellent STC efficiency. However, they are not cost efficient for constructing light harvesting system. On the other hand, perovskite solar cells connected in series, which have tunable band gaps, high STC efficiency, high open-circuit voltages, and cost efficiency, exhibit a lack of long-term stability in the CO2RR system. Therefore, for an ideal approach, the PV-PEC device, which operates by receiving a voltage from both the photocathode and the PV cell, not only requires a lower voltage than the PV-EC, but also generates a higher current density to ensure high STC efficiency. Overall, to realize bias-free PV-EC and PV-PEC devices, high-performance of various combinations of cathodes and anode catalysts, zero-gap electrolysis, and engineering series-connected photovoltaics are required, and further development is essential for industrial applications.