Renewable‐electricity‐powered electrocatalytic CO2 reduction reactions (CO2RR) have been identified as an emerging technology to address the issue of rising CO2 emissions in the atmosphere. While the CO2RR has been demonstrated to be technically feasible, further improvements in catalyst performance through active sites engineering are a prerequisite to accelerate its commercial feasibility for utilization in large CO2‐emitting industrial sources. Over the years, the improved understanding of the interaction of CO2 with the active sites has allowed superior catalyst design and subsequent attainment of prominent CO2RR activity in literature. This review tracks the evolution of the understanding of CO2RR active sites on different electrocatalysts such as metals, metal‐oxides, single atoms, metal‐carbon, and subsequently metal‐free carbon‐based catalysts. Despite the tremendous research efforts in the field, many scientific questions on the role of various active sites in governing CO2RR activity, selectivity, stability, and pathways are still unanswered. These gaps in knowledge are highlighted and a discussion is set forth on the merits of utilizing advanced in‐situ and operando characterization techniques and machine learning (ML). Using this technique, the underlying mechanisms can be discerned, and as a result new strategies for designing active sites may be uncovered. Finally, this review advocates an interdisciplinary approach to discover and design CO2RR active sites (rather than focusing merely on catalyst activity) in a bid to stimulate practical research for industrial application.

In this study, we prepared nitrogen-removed mesoporous carbon (NRMC) catalysts by applying various heat treatments to nitrogen-doped mesoporous carbon (NMC), which were applied as novel electrocatalysts for CO 2 reduction reaction (CO 2 RR). With the nitrogen moieties being progressively removed, the NRMC catalysts exhibited enhanced CO generation from CO 2 RR, whereas the competing hydrogen evolution reaction (HER) has been suppressed. Through suitable annealing treatment, the defect-rich NRMC catalyst is able to convert CO 2 to CO with a Faradaic efficiency (FE CO ) of ∼80% and a partial current density for CO (j CO ) of −2.9 mA cm −2 at an applied overpotential of 490 mV. Density functional theory (DFT) calculations further revealed the active sites within NRMC catalysts were the defects generated by N removal, which lowered the energy barriers for CO 2 RR and will not be passivated by hydrogen. These findings provide design guidelines to develop efficient carbon-based catalysts that can display metal-like, and even better, performances for potential scalable CO 2 RR to fuels and chemicals.

A well-ordered mesoporous SnO2 prepared by a simple and inexpensive nanocasting method was used as catalysts for the electrochemical reduction of CO2 to formate. The as-prepared catalyst exhibited high activity toward CO2 reduction, which was capable of reducing CO2 to formate with 38% of Faradaic efficiency (FE) at an applied overpotential as low as 325 mV. The maximum FE for formate generation (75%) was achieved at an applied potential of −1.15 V (vs RHE), accompanied by a high current density of 10.8 mA cm–2. The enhanced catalytic activity obtained with the mesoporous SnO2 electrocatalyst is attributed to its high oxygen vacancy defects (promotes CO2 adsorption and lowers overpotential) and crystallinity that provides sufficient active sites for CO2RR as well as its distinctive structural configurations which reduces impedance to facilitate faster CO2RR reaction kinetics.

Thermal annealing of metal oxides in oxygen‐deficient atmosphere, particularly reducing hydrogen gas, has been demonstrated to induce oxygen vacancy formation for enhanced photoactivity of the materials. Here, it is demonstrated that argon annealing (another prevalently used oxygen‐deficient gas) in the temperature range of 300–700 °C greatly affects the activity of dual‐faceted BiVO4 microcrystals for photocatalytic O2 generation and photocurrent generation. While treatment at 300 °C has little to no effect, higher temperatures of 500 and 700 °C significantly improve the crystallinity, alter the local structure distortion, and reduce the bandgap energy of the treated BiVO4. The higher temperature treatment also favors formation of new subgap states attributed to oxygen vacancies, as supported by surface photovoltage and electron paramagnetic resonance spectroscopies. Despite the most profound improvements in structural, optical, and electronic properties displayed by the 700 °C‐treated BiVO4, the sample annealed at 500 °C exhibits the highest photoactivity. The lower activity of the 700 °C‐treated BiVO4 is ascribed to the creation of bismuth vacancies and the loss of well‐defined crystal facets, contributing to impeded electron transport and poor charge separation.