Improving CO₂ photoconversion with ionic liquid and Co single atoms

The global drive towards carbon neutrality necessitates the development of efficient CO₂ conversion technologies. Photocatalytic CO₂ conversion using pure water is a promising approach for producing valuable chemicals, but constructing efficient and stable photocatalytic systems remains a significant challenge. Graphitic carbon nitride (g-C₃N₄, CN), particularly in the form of two-dimensional nanosheets, is a promising photocatalyst due to its low cost, visible-light response, and robust nature. The negatively positioned conduction band (CB) of CN provides sufficient thermodynamic energy for CO₂ reduction. However, limitations such as rapid charge carrier recombination and a lack of catalytic sites hinder its performance. Therefore, designing CN-based photocatalysts that prolong electron lifetime and improve CO₂ reduction selectivity is crucial. Loading suitable cocatalysts onto CN is a promising strategy to address these limitations simultaneously. Ionic liquids (ILs), a class of molten salts, possess unique properties like high ion conductivity, a wide electrochemical window, and favorable CO₂ adsorption/dissolving capabilities, making them suitable cocatalysts. The imidazolium cation in ILs, with its delocalized electron structure, can potentially extract and stabilize electrons from the excited semiconductor. Simultaneously introducing another cocatalyst to capture holes and catalyze water oxidation can further enhance electron lifetime. Transition metal oxides, such as Co oxide, are known hole trappers and catalysts for water photooxidation. Employing single-atom catalysts, like O-coordinated Co single atoms, can further enhance this process. This research investigates a CN nanosheet photocatalyst co-loaded with IL [emim][BF₄] and borate-anchored Co single atoms (IL/Co-bCN) to achieve efficient and selective CO₂ reduction. The individual and synergistic effects of these cocatalysts on the electron kinetics of CO₂ photoreduction are also investigated using transient absorption spectroscopy (TAS).

The literature extensively covers the use of graphitic carbon nitride (g-C3N4) as a photocatalyst for CO2 reduction, highlighting its advantages and limitations. Several studies explore strategies for improving g-C3N4's performance, including modifications with cocatalysts and the utilization of two-dimensional nanosheets to enhance surface area and charge transport. The incorporation of ionic liquids (ILs) as cocatalysts in photocatalysis has shown promise, particularly in improving selectivity and reducing overpotentials in electrochemical CO2 reduction. Research has demonstrated the ability of imidazolium-based ILs to extract electrons and facilitate CO2 activation. Similarly, transition metal oxides, particularly cobalt oxides, are widely investigated as cocatalysts for hole trapping and water oxidation. The single-atom catalyst (SAC) concept has emerged as a powerful approach for enhancing catalytic efficiency, with SACs exhibiting unique electronic and geometric properties. The synthesis of borate-mediated O-coordinated metal single atoms on g-C3N4 has been previously reported, demonstrating their potential as efficient cocatalysts for photocatalytic reactions. However, the combined effect of ILs and metal single-atom sites in a photocatalytic system for CO2 reduction remains largely unexplored, offering a unique opportunity for innovation in this field.

The study involved synthesizing a dual-cocatalyst modified g-C₃N₄ photocatalyst (IL/Co-bCN). First, ultrathin porous CN nanosheets were prepared from melamine and cyanuric acid through a self-assembling process followed by calcination and acid treatment. Boric acid was then used to modify the CN nanosheets (BCN) via dative B-N coordination during a low-temperature hydrothermal process. Subsequently, O-coordinated Co single atoms were constructed on the BCN (Co-bCN) via an ion exchange process using an aqueous Co(NO₃)₂ solution. Finally, the ionic liquid [emim][BF₄] was assembled onto the Co-bCN surface, creating the IL/Co-bCN photocatalyst. The photocatalytic activity of IL/Co-bCN and its constituent materials was evaluated for gas-phase CO₂ photoreduction using a 300 W Xenon lamp. Product analysis was conducted using gas chromatography and mass spectrometry. The roles of IL and Co single atoms were investigated using various characterization techniques: Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray absorption near-edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, steady-state photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), atmosphere-controlled surface photovoltage spectroscopy (AC-SPS), and electrochemical reduction/oxidation measurements. In-situ µs-transient absorption spectroscopy (TAS) was employed to study the charge dynamics and quantify the electron transfer efficiency (ETE) during CO₂ photoreduction. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations were performed to understand the interactions between the components and the reaction mechanism. In-situ FTIR measurements were used to detect intermediates involved in the CO₂ reduction process.

The optimized IL/Co-bCN nanocomposite demonstrated a remarkable 42-fold enhancement in CO₂ conversion rate compared to pristine CN and a 9-fold improvement over Co-bCN alone. The selectivity towards CO₂ reduction was nearly 100%, with CO and CH₄ as the primary products. In-situ µs-TAS revealed a significant 35.3% electron transfer efficiency (ETE) for IL/Co-bCN, representing a 93-fold increase compared to CN. The IL [emim][BF₄] was found to effectively extract electrons and catalyze selective CO₂ reduction, while Co single atoms trapped holes and promoted water oxidation. EPR, PL, and EIS measurements confirmed that both IL and Co single atoms contribute to improved charge separation. AC-SPS experiments indicated that IL acts as an electron acceptor, while Co single atoms function as hole trappers. Electrochemical analysis showed that IL facilitates CO₂ reduction, and Co single atoms catalyze water oxidation. DFT calculations and MD simulations supported the experimental findings, showing that the Co single atoms trap holes, while the IL interacts with CO₂, facilitating its reduction. In-situ FTIR measurements revealed the presence of key intermediates (COOH*, HCO₃⁻, CO₃²⁻, CHO⁻) involved in the CO₂ reduction pathway. The linear relationship between IL loading amount and electron transfer rate (ETR) suggested a first-order reaction kinetics for the IL-catalyzed CO₂ photoreduction, with a rate constant of 2.4 × 10⁷ M⁻¹s⁻¹ for IL/bCN and 3.3 × 10⁷ M⁻¹s⁻¹ for IL/Co-bCN.

The results demonstrate the successful design and implementation of a highly efficient and selective photocatalyst for CO₂ reduction. The synergistic combination of IL [emim][BF₄] and borate-anchored Co single atoms significantly enhances the photocatalytic performance, addressing the key limitations of pristine g-C₃N₄. The high electron transfer efficiency (ETE) achieved through the spatially separated reduction and oxidation domains highlights the effectiveness of this strategy. The observed linear correlation between IL loading and ETR suggests that further optimization of IL loading could potentially lead to even greater enhancements. The detailed mechanistic insights gained from in-situ spectroscopy and theoretical simulations provide valuable guidance for designing advanced photocatalysts for CO₂ reduction. This approach offers a significant advancement in artificial photosynthesis, contributing to the development of sustainable technologies for CO₂ capture and conversion.

This work demonstrates a highly effective approach to improve the photocatalytic conversion of CO₂ into valuable chemicals using a dual-cocatalyst system comprising an ionic liquid and cobalt single atoms. The resultant IL/Co-bCN nanocomposite exhibited significantly enhanced photoactivity and selectivity for CO₂ reduction, exceeding previous performances by a large margin. The comprehensive characterization revealed the synergistic contributions of the ionic liquid and cobalt single atoms, clarifying the underlying reaction mechanisms. This research provides valuable insights into the design of highly efficient photocatalysts for CO₂ reduction, contributing significantly to the advancement of sustainable energy technologies. Future research could focus on further optimizing the catalyst structure, exploring different ionic liquids and transition metals, and investigating the scalability and long-term stability of the system.

While this study provides compelling evidence for the enhanced performance of the IL/Co-bCN photocatalyst, some limitations exist. The experiments were conducted under specific conditions, and the generalizability of the findings to other environments or reaction conditions needs further investigation. Long-term stability tests under continuous operation are also essential for practical applications. The theoretical calculations might not perfectly capture all the complexities of the system, and the precise nature of the interactions between the components could require further study. The impact of specific parameters such as the concentration of reactants and the intensity of light might not have been explored comprehensively.