Dramatically increased CO2 concentration from several point sources is perceived to cause severe greenhouse effect towards the serious ongoing global warming with associated climate destabilization, inducing undesirable natural calamities, melting of glaciers, and extreme weather patterns. CO2 capture and utilization (CCU) has received tremendous attention due to its significant role in intensifying global warming. Considering the lack of a timely review on the state-of-the-art progress of promising CCU techniques, developing an appropriate and prompt summary of such advanced techniques with a comprehensive understanding is necessary. Thus, it is imperative to provide a timely review, given the fast growth of sophisticated CO2 capture and utilization materials and their implementation. In this work, we critically summarized and comprehensively reviewed the characteristics and performance of both liquid and solid CO2 adsorbents with possible schemes for the improvement of their CO2 capture ability and advances in CO2 utilization. Their industrial applications in pre- and post-combustion CO2 capture as well as utilization were systematically discussed and compared. With our great effort, this review would be of significant importance for academic researchers for obtaining an overall understanding of the current developments and future trends of CCU. This work is bound to benefit researchers in fields relating to CCU and facilitate the progress of significant breakthroughs in both fundamental research and commercial applications to deliver perspective views for future scientific and industrial advances in CCU.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

The continuously increasing CO2 released from human activities poses a great threat to human survival by fluctuating global climate and disturbing carbon balance among the four reservoirs of the biosphere, earth, air, and water. Converting CO2 to value-added feedstocks via electrocatalysis of the CO2 reduction reaction (CO2RR) has been regarded as one of the most attractive routes to re-balance the carbon cycle, thanks to its multiple advantages of mild operating conditions, easy handling, tunable products and the potential of synergy with the rapidly increasing renewable energy (i.e., solar, wind). Instead of focusing on a special topic of electrocatalysts for the CO2RR that have been extensively reviewed elsewhere, we herein present a rather comprehensive review of the recent research progress, in the view of associated value-added products upon selective electrocatalytic CO2 conversion. We initially provide an overview of the history and the fundamental science regarding the electrocatalytic CO2RR, with a special introduction to the design, preparation, and performance evaluation of electrocatalysts, the factors influencing the CO2RR, and the associated theoretical calculations. Emphasis will then be given to the emerging trends of selective electrocatalytic conversion of CO2 into a variety of value-added products. The structure-performance relationship and mechanism will also be discussed and investigated. The outlooks for CO2 electrocatalysis, including the challenges and opportunities in the development of new electrocatalysts, electrolyzers, the recently rising operando fundamental studies, and the feasibility of industrial applications are finally summarized.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

Promises of Main Group Metal–Based Nanostructured Materials for Electrochemical CO2 Reduction to Formate

Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO2 reduction reaction (CO2RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO2. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO2 is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO2 reduction. Given the reaction barriers associated with multiple electron-proton reduction of CO2 to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O-), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduction occurs. Useful catalysts in this area are typically transition-metal complexes with organic ligands and electron transfer properties that utilize combinations of metal and ligand redox levels. As a way to limit the amount of catalyst, in device-like configurations, catalysts are added to the surfaces of conductive substrates by surface binding, in polymeric films, or on carbon electrode surfaces with molecular structures and electronic configurations related to catalysts in solution. Immobilized, homogeneous catalysts can suffer from performance losses and even decomposition during long-term CO2 reduction cycles, but they are amenable to detailed mechanistic investigations. In parallel efforts, heterogeneous nanocatalysts have been explored in detail with the development of facile synthetic procedures that can offer highly active catalytic surface areas. Their high activity and stability have attracted a significant level of investigation, including possible exploitation for large-scale applications. However, translation of catalytic reactivity to the surface creates a new reactivity environment and complicates the elucidation of mechanistic details and identification of the active site in exploring reaction pathways. Here, the results of previous studies based on transition-metal complex catalysts for CO2 electroreduction are summarized. Early studies showed that transition-metal complexes of Ru, Ir, Rh, and Os, with well-defined structures, are all capable of catalyzing CO2 reduction to CO or formate. Derivatives of the complexes were surface attached to conducting electrodes by chemical bonding, noncovalent bonding, or polymerization. The concept of surface binding has also been extended to the preparation of surface area electrodes by the chemically controlled deposition of nanostructured catalysts such as nano tin, nano copper, and nano carbon, all of which have been shown to have high selectivities and activities toward CO2 reduction. In our presentation, we end this Account with recent advances and a perspective about the application of electrocatalysis in carbon dioxide reduction.

CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis.

Selective and efficient catalytic conversion of carbon dioxide (CO2) into value-added fuels and feedstocks provides an ideal avenue to high-density renewable energy storage. An impediment to enabling deep CO2 reduction to oxygenates and hydrocarbons (e.g., C2+ compounds) is the difficulty of coupling carbon-carbon bonds efficiently. Copper in the +1 oxidation state has been thought to be active for catalyzing C2+ formation, whereas it is prone to being reduced to Cu0 at cathodic potentials. Here we report that catalysts with nanocavities can confine carbon intermediates formed in situ, which in turn covers the local catalyst surface and thereby stabilizes Cu+ species. Experimental measurements on multihollow cuprous oxide catalyst exhibit a C2+ Faradaic efficiency of 75.2 ± 2.7% at a C2+ partial current density of 267 ± 13 mA cm-2 and a large C2+-to-C1 ratio of ∼7.2. Operando Raman spectra, in conjunction with X-ray absorption studies, confirm that Cu+ species in the as-designed catalyst are well retained during CO2 reduction, which leads to the marked C2+ selectivity at a large conversion rate.

Protecting Copper Oxidation State via Intermediate Confinement for Selective CO2 Electroreduction to C2+ Fuels.

Formic acid (or formate) is suggested to be one of the most economically viable products from electrochemical carbon dioxide reduction. However, its commercial viability hinges on the development of highly active and selective electrocatalysts. Here we report that structural defects have a profound positive impact on the electrocatalytic performance of bismuth. Bismuth oxide double-walled nanotubes with fragmented surface are prepared as a template, and are cathodically converted to defective bismuth nanotubes. This converted electrocatalyst enables carbon dioxide reduction to formate with excellent activity, selectivity and stability. Most significantly, its current density reaches ~288 mA cm−2 at −0.61 V versus reversible hydrogen electrode within a flow cell reactor under ambient conditions. Using density functional theory calculations, the excellent activity and selectivity are rationalized as the outcome of abundant defective bismuth sites that stabilize the *OCHO intermediate. Furthermore, this electrocatalyst is coupled with silicon photocathodes and achieves high-performance photoelectrochemical carbon dioxide reduction. Carbon dioxide can be electrochemically reduced to form valuable chemical feedstocks, but efficiency of electrocatalysts should be improved. Here the authors report nanotube-derived bismuth for electrocatalytic reduction of carbon dioxide to formate, with performance that is enhanced by defects.

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Structural defects on converted bismuth oxide nanotubes enable highly active electrocatalysis of carbon dioxide reduction.

Neutral water splitting is attractive for its use of non-corrosive and environmentally friendly electrolytes. However, catalyst development for hydrogen and oxygen evolution remains a challenge under neutral conditions. Here we report a simple electrodeposition and reductive annealing procedure to produce a highly active Ni-Co-Cr metal/metal oxide heterostructured catalyst directly on Ni foam. The resulting electrocatalyst for hydrogen evolution reaction (HER) requires only 198 mV of overpotential to reach 100 mA/cm2 in 1 M potassium phosphate (pH = 7.4) and can operate for at least two days without significant performance decay. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) imaging reveals a Ni-Co alloy core decorated with blended oxides layers of NiO, CoO and Cr2O3. The metal/metal oxide interfaces are suggested to be responsible for the high HER activity.

An electrodeposition approach to metal/metal oxide heterostructures for active hydrogen evolution catalysts in near-neutral electrolytes

Heterogeneous catalysts are the most important catalysts in industrial reactions. Nanocatalysts, with size ranging from hundreds of nanometers to the atomic scale, possess activities that are closely connected to their structural characteristics such as particle size, surface morphology, and three-dimensional topography. Recently, the development of advanced analytical transmission electron microscopy (TEM) techniques, especially quantitative high-angle annular dark-field (HAADF) imaging and high-energy resolution spectroscopy analysis in scanning transmission electron microscopy (STEM) at the atomic scale, strengthens the power of (S)TEM in analyzing the structural/chemical information of heterogeneous catalysts. Three-dimensional reconstruction from two-dimensional projected images and the real-time recording of structural evolution during catalytic reactions using in-situ (S)TEM methods further broaden the scope of (S)TEM observation. The atomic-scale structural information obtained from high resolution (S)TEM has proven to be of significance for better understanding and designing of new catalysts with enhanced performance.

High resolution electron microscopy for heterogeneous catalysis research

https://iopscience.iop.org/article/10.1088/1674-1056/27/5/056804/pdf

Yong Zhu

Papers

Structural defects on converted bismuth oxide nanotubes enable highly active electrocatalysis of carbon dioxide reduction.

An electrodeposition approach to metal/metal oxide heterostructures for active hydrogen evolution catalysts in near-neutral electrolytes

High resolution electron microscopy for heterogeneous catalysis research

High-resolution electron microscopy for heterogeneous catalysis research*