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)).