4.2 C2+ product
By coupling with PV, an EC system consisting of copper electrocatalysts
is also utilized to efficiently reduce CO2 for
C2+ value-added hydrocarbons. Si (series) solar
cells,[108] dye-sensitized solar cell (DSSC) solar
cells,[109] copper-indium-galium-sellenide (CIGS)
solar cells, [110] and perovskite solar cells[111] are some of the solar cell types that are
used to provide insufficient energy to PV-integrated EC systems using
copper-based electrocatalysts as a cathode.[112]However, the conversion efficiency to C2+ using a copper
still faces limitations compared to C1 selectivity of
over 90%. Therefore, to improve C2+ formation, various
methods, including exposed facet,[113-115] size
effect,[116] morphology
change,[117] defects,[118]oxide state manipulation,[119] and grain
boundaries[120] are explored from many
perspectives. In addition, we discuss the ideal PV-EC in combination
with an efficient copper catalyst fabricated by the above strategies.
Chen et al. reported a grain-boundary-rich copper, which was fabricated
by controlling the grain growth of copper via electrodeposition, as an
efficient PV-EC CO2RR electrocatalyst and achieved a
high solar-to-C2+ conversion efficiency
(STC).[121] In electrochemical performance,
grain-boundary-rich copper (GB-Cu) exhibited a remarkable FE of 73% for
C2+ formation (propanol, ethylene, and ethanol) over a
wide range of potentials, in particular, FE of 31.74% for ethanol was
confirmed at a high current density of 45 mA cm–2 at
–1.3 V vs RHE. An assembled PV-EC system, which was composed of GB-Cu
and Se-(NiCo)Sx/(OH)x nanosheets as the
cathode and anode, respectively, using six-series a-Si/c-Si
heterojunction (SHJ) module as the photocathode, showed FE of 68% for
C2+ formation and STC conversion efficiency of 3.88%,
accompanied by well-matched LSV curves of each PV and EC system (Figure
7(A) and (B)). Huan et al. adopted an oxide-derived strategy for both
cathode and anode catalysts, in which dendritic nanostructured copper
oxide (DN-CuO) with efficient mass transfer by lowering mass transport
losses was used to limit the poisoning of the cathode
electrode.[122] As illustrated in Figure 7(C) and
(D), an electrochemical cell using DN-CuO as both electrodes exhibited
low electrolyte resistance at a high current density (25 mA
cm–2) at a cell potential below 3 V from the LSV
curve and yielded a high production rate from FE toward
C2+ formation. Zhang et al. achieved maximum FE of
58.6% for ethylene by fabricating Cu (100)-rich films reducing the
energy barrier of C-C coupling formation using the dynamic
deposition-etch-bombardment method and further applied the Cu (100)-rich
films to the efficient cathode portion of the PV-EC
system.[123] As illustrated in Figure 7(E), a
solar-driven electrochemical CO2RR system was
constructed, where high-power reactively sputtered Cu films (HRS-Cu) was
used as the cathode and a Si photodiode was used as the solar energy
absorber. From the I-V characteristic of the PV-EC device, the
intersection of PV and electrocatalytic was confirmed at an operating
current density of 2.41 V and current density of 41.3 mA under simulated
AM 1.5G illumination, which displayed the maximum power point (MPP)
matching of a solar panel, evaluating the solar-to-electricity
conversion value of a PV-EC device (Figure 7(F)). As shown in Figure
7(G), total FE of ~72 % for C2+,
ethylene of ~45%, and STC efficiency of
~6% with 40 mA of current were confirmed by
chronoamperometry measurements under simulated AM 1.5G illumination for
220 min. In addition, to scale up the PV-EC system, a membrane electrode
assembly system (MEA), which has advantages such as no requirement for
additional catalyst loading steps, no electrode contamination, and
suitability for large-area electrodes, was adopted (Figure 7(H)). When
the cathode electrode was enlarged to 4cm2 and
25cm2, the current density and maximum FE for ethylene
reached 120 mA cm–2 and 58.6%, and 480 mA
cm–2 and 50.9%, respectively. Ideal PV-EC, which was
reported by Cheng et.al, composed of selective electrodeposition of Cu
catalysts on Ag catalyst prisms, covered with an optimal amount (35%)
of surface area, exhibits excellent
stability.[124] As shown in Figure 7(I), a
semitransparent metal prism array (PA) was connected to the top layer of
triple junction (3 J) III–V semiconductors to suppress hydrogen
evolution and achieve efficient light harvesting. The intersections
between photovoltaics, including the Spectrolab stack, which is the
light-limiting current in the middle cell, and FhG-ISE 3 J, which is the
light-limiting current in the bottom cell, and electrocatalysts such as
Ag-PA and Cu/Ag-PA with NiOx as an anode, are displayed
from J-V measurements in the 0.1 M CO2-purged
KHCO3 (Figure 7(J) and (K)). Through the intersection ofJ-V curves between Ag-PA+NiOx and
Ag-PA-Spectrolab 3J, and Ag-PA-ISE 3J, cell voltage
(Ucell) of 2.56 V and current (J) of 2.65 mA
cm–2 for Ag-PA-Spectrolab 3J and
Ucell of 2.85 V and J of 5.13 mA cm–2for Ag-PA-ISE 3J are confirmed with high FE of ~ 80%
for CO at broad cell voltages (2.5–2.9 V) (Figure 7(I) and (L). When Cu
was additionally electrodeposited on Ag-PA, the J-V intersection
displayed a Ucell of 2.56 V and J of 2.60 for Spectrolab
3 J, and Ucell of 2.8 V and J of 5.97 mA
cm–2 for Ag-PA-ISE 3 J (Figure 7K). Cu/Ag-PA
exhibited FE of approximately 30% for
C2H5OH in the voltage range of 2.5–2.9
V as well as the formation of value-added C2+ carbon
compounds (Figure 7(M)).