3.2 Non-noble metal cocatalysts
Due to the high structural stability of CO2 molecules, many semiconductors face challenges in selectively facilitating CO2 reduction. In such scenarios, cocatalysts play a crucial role in minimizing the overpotential required for CO2 reduction and enhancing the CO2reduction kinetics to improve overall selectivity. Therefore, it is essential to integrate semiconductor materials with appropriate cocatalysts to accelerate surface catalytic conversion and enhance both PEC activity and selectivity. The electrical conductivity of the cocatalyst, along with its adsorption and desorption capabilities based on binding energies with relevant reaction species—especially those with moderate binding affinities for key intermediates—can lead to higher intrinsic CO2 reduction activity.[43-45] This, in turn, facilitates the promotion of CO2 reduction at lower overpotentials, thereby enhancing onset potentials.
Noble metals, such as Ag and Au, remain efficient catalysts for converting PEC energy into CO2. However, the use of noble metal-based catalysts faces significant limitations due to their high cost and limited availability, which constrain commercial implementation.[27,46] Consequently, extensive research efforts have been directed toward developing alternative catalysts for CO2 reduction based on non-noble metal materials, including transition metals. These materials, which encompass metal oxides, metal NPs, metal-organic frameworks (MOFs), and metal molecular catalysts, aim to provide low-cost, high-activity, and long-term stability for CO2 reduction, addressing the challenges associated with noble metal cocatalysts.[27, 47-49]
Among these, cobalt molecular catalysts are widely used as cocatalysts for the photocathode in PEC CO2RR. For instance, Shang et al. prepared a p-type silicon photocathode incorporating a cobalt phthalocyanine molecular catalyst immobilized on graphene oxide (GO/CoPc).[50] Initially, CoPc molecules were immobilized on GO through ultrasonication in a DMF solution, taking advantage of GO’s conductive properties and capability to facilitate electron transfer, as illustrated in Figure 3A. Subsequently, p-Si wafers, coated with a protective TiO2 layer (Si-TiO2), were treated with a solution of (3-aminopropyl)triethoxysilane (APTES), as depicted in Figure 3B. Following this treatment, the Si-TiO2-APTES (STA) substrate was immersed in a GO/CoPc aqueous dispersion, forming a monolayer-like coating on the surface through electrostatic and hydrogen bonding interactions between the amine groups of APTES and the carboxylic acid groups of GO. The STA-GO/CoPc achieved a photocurrent density of 0.7 mA cm−2, while simultaneously attaining a maximum FECO of 86% at −0.28 V vs. RHE, as observed in Figure 3C. Moreover, STA-GO/CoPc exhibited a remarkably low onset potential of −0.36 V vs. RHE, displaying a FECH3OH of 8% at −0.62 V vs. RHE. Roy et al. constructed a hybrid photocathode structure comprising a cobalt phthalocyanine catalyst with four phosphonic acid anchoring groups (CoPcP) immobilized on mesoTiO2, which coated a p-Si photocathode (Si|mesoTiO2|CoPcP).[51]Incorporating the four phosphonic acid anchoring groups facilitated the immobilization of the CoPcP catalyst on meso TiO2. To assess the impact of the CoPcP catalyst on mesoTiO2, they synthesized a mesoTiO2|CoPcP hybrid electrode. The resulting mesoTiO2|CoPcP photocathode demonstrated highly selective CO2 to CO conversion, achieving a turnover number for CO (TONCO) of 1949 ± 5 after 2 h of controlled-potential electrolysis at a 550 mV overpotential in a 0.5M KHCO3 aqueous electrolyte. In addition, when combined with a p-Si photocathode, the Si|mesoTiO2|CoPcP exhibited a TONCO of 939 ± 132 with 66% CO selectivity under 0.5M KHCO3 conditions. Wen et al. prepared CoII (BrqPy) (BrqPy=4’,4”-bis(4-bromophenyl)-2,2’ :6’,2” :6”,2”’-quaterpyridine) molecular catalysts with multiwalled carbon nanotubes (CNT) on TiO2-protected p/n-Si photocathodes (Si|TiO2|CNT|CoII(BrqPy)),[52] as illustrated in Figure 3D. First, a p/n-Si wafer was covered with TiO2 through atomic-layer deposition (ALD). Then, a CNT layer was drop-cast onto the Si|TiO2 layer. Finally, a DMF solution containing CoII (BrqPy) molecular catalysts was drop-cast, and the catalysts were immobilized on the CNT layer through π–π stacking interactions. The hybrid photocathodes, benefiting from the highly conductive nature of CNT, achieved a remarkable photocurrent density of up to −1.4 mA cm–2 at −0.11 V vs.RHE, as seen in Figure 3E. Furthermore, due to the exceptional activity and selectivity of the cobalt molecular catalyst for CO2reduction, the total FE and FECO reached 99% and 97%, respectively, as revealed in Figure 3F. These results surpassed other hybrid photocathodes employing more complex configurations. Leung et al. employed cobalt(Ⅱ) bis(terpyridine) molecular catalysts (CotpyP) as cocatalysts to construct a highly efficient photocathode for CO2 reduction.[53] This hybrid photocathode consisted of a p-type silicon photocathode coated with a mesoporous TiO2 layer with anchored CotpyP catalysts (Si|mesoTiO2|CotpyP). The mesoporous TiO2 layer protected the Si, allowing for high loading of CotpyP catalysts and ample surface contact with the electrolyte due to its high surface area. Photoelectrochemical tests indicated CO and formate (HCOO) production in aqueous acetonitrile (MeCN) with 0.1M tetrabutylammonium tetrafluoroborate (TBABF4) and pure CO2-saturated 0.1M KHCO3. The turnover number for CO2reduction reached 381 during a 24 h test in aqueous MeCN.
Furthermore, nanowires (NWs), NPs, metal oxides, and MOFs, which consist of non-noble metals, are also widely used as photocathodes in PEC CO2RR. For example, Dong et al. designed a photocathode comprising CuS-covered GaN NWs on silicon wafers (CuS/GaN/Si) to efficiently convert H2S-containing CO2mixture gas to HCOOH.[54] The fabrication involved thermally evaporating Cu NPs and plasma-assisted molecular beam epitaxy (MBE) of GaN NWs on Si wafers. PEC experiments were conducted in a 0.1M KHCO3 solution purged with CO2 and H2S mixture gas under 1-sun illumination, as shown in Figure 4A. Notably, Cu NPs spontaneously transformed into CuS NPs during the PEC experiment. Compared to other photocathodes of Cu/Si, CuS/Si, and Cu/GaN/Si, the CuS/GaN/Si photocathode revealed an outperforming faradaic efficiency for HCOOH (FEHCOOH) of 70.2% at −1.0 V vs. RHE and achieved a maximum current density for HCOOH of 7.07 mA cm–2, as shown in Figure 4B,C. Zhou et al. fabricated Bi NP cocatalysts supported on GaN NWs photocathodes for PEC CO2RR to HCOOH.[55] Focusing on the electronic interaction between Bi NPs and GaN NWs grown on a planar Si wafer (Bi/GaN/Si), the strong electronic interaction enhanced CO2 conversion due to electron sharing between Bi NPs and GaN NWs. The Bi/GaN/Si photocathode demonstrated outstanding selectivity for HCOOH, achieving a FEHCOOH of ~98% at −0.3 V vs. RHE, a high current density of 10.3 mA cm–2 at −0.6 V vs. RHE, and sustained stable operation for 12 h under 1-sun illumination. This underlines the critical role of electronic interactions between photocathodes and cocatalysts in enhancing PEC performance. Dong et al. showcased a novel nanoarchitecture with a Sn NP/GaN NW/Si photocathode for aqueous PEC reduction of CO2 to HCOOH.[56] Integrating defect-free GaN NWs grown on a planar Si via MBE with electrodeposited Sn NPs, the photocathode’s PEC testing revealed exceptional performance, unprecedented TOF of 107 min–1, a total current density of 17.5 mA cm–2, and a high FEHCOOH of 76.9% at a low potential of −0.53 V vs. RHE under 1-sun illumination, corresponding to a productivity of 201 μmol cm–2h–1. The photocathode also exhibited a high TON for HCOOH production, reaching 64,000 during stable operation over 10 h. DFT calculations suggested that the synergistic effects of covalent Ga-C bonding and ionic-like Sn-O bonding played a crucial role in activating CO2, contributing to the remarkable activity and selectivity for CO2 reduction. Deng et al. developed a Cu2O photocathode coated with MOFs named Cu3(BTC)2 (BTC = benzene-1,3,5-tricarboxylate) for PEC CO2RR to CO.[57] The Cu3(BTC)2 coating played multiple roles in the PEC CO2RR process, including preventing photocorrosion of the Cu2O layer, facilitating electron transfer, and providing catalytic active sites for CO2reduction. PEC performance was measured in a CO2-saturated MeCN containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6). Results indicated that the Cu3(BTC)2/Cu2O/ITO photocathode reached a maximum FECO of about 95% at applied potentials ranging from −1.77 to −1.97 V vs.ferrocene/ferrocenium (Fc/Fc+), as displayed in Figure 4D. Furthermore, the solar-to-CO (STC) efficiency of this photocathode reached 0.83% at −2.07 V vs. Fc/Fc+ under AM 1.5G illumination, as observed in Figure 4E. Long-term chronoamperometry indicated that the photocathode maintained a nearly constant current density under visible light and a steady photocurrent density under chopped visible light, highlighting its stability and performance under varying light conditions.
Wang et al. constructed a hybrid photocathode structure comprising CuO adorned with asymmetric Cu-N sites (CuNx/CuO) for PEC CO2RR.[58] The CuNx/CuO photocathode exhibited a faradaic efficiency towards C2 products (FEC2) of 15.2%, accompanied by a photocurrent density of −1.0 mA cm–2at 0.2 V vs. RHE in CO2-saturated 0.1M KHCO3 under AM 1.5G simulated sunlight. DFT calculations demonstrated that the adsorption of OCCO* and*COCH2 intermediates on Cu-N sites, crucial for the formation of C2 products, was more favorable than on Cu-Cu sites, as presented in Figure 4F,G. Theoretical calculations also indicated that the CuNx/CuO photocathode had higher electron migration efficiency than CuO due to the asymmetric d-p orbitals at Cu-N sites, which lowered the energy barrier for C–C coupling. Roh et al. fabricated Si NWs connected with a Cu NP ensemble to create photocathodes (Cu NPs/Si NWs) for PEC CO2RR to C2H4.[59] Assessed under 100 mW cm−2 air mass (AM) 1.5 simulated sunlight in CO2-purged 0.1M KHCO3, the photocathode achieved a selectivity for CO2 reduction to C2H4 with a faradaic efficiency (FEC2H4) of approximately 25% and demonstrated activity with partial current densities exceeding 2.5 mA cm−2at −0.50 V vs. RHE. Moreover, the Cu NPs/Si NWs photocathode exhibited long-term stability, maintaining PEC CO2RR under 50 h of continuous bias and illumination. Kim et al. constructed a metal-insulator-semiconductor (MIS) structure comprising Cu/TiO2/p-Si photocathodes for PEC CO2RR to multicarbon products.[60] They also investigated the effects of ionomer bilayer coatings, specifically Nafion atop Sustainion, on the Cu surface of the Cu/TiO2/p-Si MIS photocathodes. PEC testing indicated that Cu/TiO2/p-Si photocathodes coated with Nafion on top of Sustainion revealed partial current densities for C2H4 ranging from −0.9 mA cm−2 to −2.3 mA cm–2 in CO2-saturated 0.1M CsHCO3 under wet-side illumination, compared to the condition without ionomer bilayers on the Cu surface. This underscores the enhancement of both activity and selectivity for C2H4 due to the Cu cocatalyst and bilayer coatings.