Study on Nanostructured Transition Metal-based Catalysts for Electrochemical CO2 Reduction

Abstract

Electrochemicacl reduction reaction of CO2 (CO2RR) to chemical fuels is a promising method to mitigate global climate changes and can achieve carbon neutralization. Transition metal-based catalysts have been studied compared to other materials for the CO2RR because it offers several advantages of high catalytic activity, easy to control the selectivity of products, durability, earth-abundance, and cost-effectiveness. Particularly, Cu exhibit unique catalytic properties for producing multi-carbon products that cannot be produced with other transition metals. However, pristine Cu shows a large overpotential for CO2RR and high activity for hydrogen evolution reaction (HER), which competes with CO or formate production. Thus, engineering the Cu surface by nanostructuring, formation of bimetallics, and designing cell reactors could be promising approaches to improve catalytic performances. First, we experimentally developed sharpened Sn/Cu nano-cone catalysts through the fabrication by template-based nanoimprint lithography, electroplating of Cu film, and electroless coating of Sn nanoparticles. The finite-element-based simulation provides evidence that the local electric field intensified as the curvature of catalysts increased. As a result, Sn/Cu cones exhibited much better faradaic efficiency of CO (FECO) = 82.7% and current density of CO (jCO) = 5.43 mA/cm2 than Sn/Cu foil (FECO = 41.3% and jCO = 2.29 mA/cm2) and Sn/Cu rods (FECO = 59.7% and jCO = 3.87 mA/cm2). This work reveals that the local electric field induced by the sharp tip plays a significant role in improving the FECO and lowering the onset potential of CO2 reduction reaction. Second, we present a novel method to create abnormal growth of CuSx nanostructures by immersing the thin Ag layers deposited on Cu foil (Ag/Cu) into a sulfur-containing solution, and investigated their catalytic activity and stability. When the CO2RR was carried out in the sulfur-containing CO2-purged 0.1 M KHCO3 electrolyte, thin Ag (3 nm) on Cu substrate is spontaneously transformed to CuSx due to the dissolution of underlying Cu induced by the galvanic corrosion between two metals. It was found that the synergistic effects among the Cu, Ag, and H2S strongly affect the growth of CuSx nanostructures on the surface; therefore, it might be tailored to promote HCOOH selectivity as well as suppress HER characteristics, by the formation of Cu-S bonds. Due to the boosted charge-transfer activity and increased electrochemical catalytic surface area (ECSA) compared to pristine CuSx which were grown on Cu foil, Ag-mediated CuSx catalysts exhibited remarkably enhanced CO2RR activities (FEHCOOH = 87.37% at -0.6 VRHE) and achieved long-term stability (> 60 h) as well as suppressed HER performance (FEH2 = 10.41% at -0.6 VRHE) in sulfur-containing CO2-purged 0.1 M KHCO3 electrolyte. This work provides insight into the design of catalysts for achieving high efficiency and stability in the conversion of sulfur-containing CO2 to formate. Third, to date, nanostructure-based metal catalysts have been demonstrated to improve the catalytic performances because they have a large catalytic surface area for the CO2RR. But it still suffered from large overpotentials and highly competitive the HER in an aqueous system. Thus, single-atom catalyst (SAC) has emerged as a promising candidate for the CO2RR because it has atomically dispersed active sites and can easily change the local coordination environments. Especially, non-noble and earth-abundant Ni-based SAC coordinated in a carbon matrix with neighboring N configurations (Ni-Nx-C structure) has been demonstrated to be highly selective in CO production. However, despite several kinds of research on Ni- based SAC for the CO2RR, the reason for the enhanced catalytic activities remains unclear due to the complexity of various factors, and more importantly, the role of different types of N configurations on the CO2RR still remains elusive. So, we reported pyrrolic N-stabilized Ni SAC with low-coordinated Ni-Nx sites by thermal activation of Ni ZIF-8, which was conducted in a 3-compartment microfluidic flow cell system at the industrial level. When the pyrolysis temperature increased from 800 °C (Ni SAC-800) to 1000 °C (Ni SAC-1000), the content ratio of pyrrolic N/pyridinic N increased from 0.37 to 1.01 as well as the coordination number of Ni in Ni-Nx sites decreased from 3.14 to 2.63. Theoretical calculations revealed that the synergistic effect between the high content ratio of pyrrolic N and low-coordinated Ni can decrease the energy barrier for the desorption of *CO during the CO2RR. Therefore, the Ni SAC-1000 exhibited superior catalytic performances with high CO selectivity (FECO = 98.24% at -0.8 VRHE) compared to that of Ni SAC-800 (FECO = 40.76% at -0.8 VRHE). Moreover, the Ni SAC- 1000 based on the flow cell system showed a higher current density (~200 mA/cm2) compared to that of the H-type cell system (~20 mA/cm2). As a result, this study experimentally demonstrated that the pyrrolic N-stabilized and low-coordinated Ni SAC-1000 in the microfluidic flow cell reactor provide great chances for scaling up the productivity of CO2RR at the industrial level. Fourth, electrocatalytic nitrogen reduction reaction (ENRR) is a promising strategy to produce ammonia (NH3) under ambient conditions using renewable energy and to achieve a carbon-free society. However, the ENRR is extremely challenging due to the low efficiency of electrocatalysts, competing hydrogen evolution reactions, as well as a lack of understanding of the mechanism for the ENRR. Here, we demonstrated the effect of high-index facets (HIFs) and morphology on the catalytic properties of electrochemically reduced Bi2O2CO3 (R-BOC) petals for the ENRR. The R-BOC petals were fabricated through oblique angle deposition of Bi on Cu foil (Bi NHs/Cu), followed by immersing them into a carbonate solution and subsequent electrochemical reduction reaction (ECR). According to the Pourbaix diagram, the BOC petals spontaneously grew from Bi NHs/Cu by the reaction between BiO+ and CO3 2- ions in an aqueous carbonate solution at room temperature. After the ECR process, the R-BOC petals inherited the porously petal-like morphology consisting of metallic Bi and BOC. We found that the BOC and R-BOC petals derived from the Bi NHs/Cu had multi- and high-index facets, which exhibited much higher catalytic activities for the ENRR than those with low- index facets. Together with the high-index faceted structure and enlarged electrochemical surface area, R-BOC petals on Bi NHs/Cu (R-BOC/Bi NHs/Cu) exhibited FENH3 = 10.1% at - 0.6 VRHE and jNH3 = 623.8 µA/cm2 at -1.0 VRHE in N2-purged 0.5 M K2SO4 electrolyte. This work provides a rational design of low-cost and highly efficient Bi-based nanostructures for NH3 production.