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.