Ni single-atom sites supported on carbon aerogel for highly efficient electroreduction of carbon dioxide with industrial current densities

Currently, more than 86% of global energy consumption is still mainly dependent on traditional fossil fuels, which causes resource scarcity and even emission of high amounts of carbon dioxide (CO2), resulting in a severe “Greenhouse effect.” Considering this situation, the concept of “carbon neutrality” has been put forward by 125 countries one after another. To achieve the goals of “carbon neutrality,” two main strategies to reduce CO2 emissions and develop sustainable clean energy can be adopted. Notably, these are crucial for the synthesis of advanced single-atom catalysts (SACs) for energy-related applications. In this review, we highlight unique SACs for conversion of CO2 into high-efficiency carbon energy, for example, through photocatalytic, electrocatalytic, and thermal catalytic hydrogenation technologies, to convert CO2 into hydrocarbon fuels (CO, CH4, HCOOH, CH3OH, and multicarbon [C2+] products). In addition, we introduce advanced energy conversion technologies and devices to replace traditional polluting fossil fuels, such as photocatalytic and electrocatalytic water splitting to produce hydrogen energy and a high-efficiency oxygen reduction reaction (ORR) for fuel cells. Impressively, several representative examples of SACs (including d-, ds-, p-, and f-blocks) for CO2 conversion, water splitting to H2, and ORR are discussed to describe synthesis methods, characterization, and corresponding catalytic activity. Finally, this review concludes with a description of the challenges and outlooks for future applications of SACs in contributing toward carbon neutrality. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cey2.194

Single‐atom catalysis for carbon neutrality

Reticular frameworks including metal−organic frameworks (MOFs) and covalent organic frameworks (COFs), and their derived materials have drawn global attention in the capture and conversion of CO2 as a cheap feedstock into fine chemicals and fuels due to their facile synthesis and programmable highly porous structures. This review comprehensively summarizes the progress in thermo−catalysis of CO2 conversion by reticular framework−based catalysts to afford chemicals such as cyclic carbonates, cyclic carbamates, formamides, carboxylic acid, carbon monoxide, formate, methanol, methane, and light olefins. Firstly, the characteristics and advantages of MOF−based materials for CO2 conversion are introduced. Secondly, the characteristics and advantages of COF−based materials for CO2 conversion are presented. Subsequently, the CO2 conversion reactions are briefly classified and discussed. Particularly, MOF or COF−based catalysts for each reaction are summarized in terms of catalyst design, catalytic performance and catalytic mechanism. Finally, the perspectives for further development of reticular framework−based catalysts for efficient CO2 conversion are discussed. We hope this review can provide an inspiration for the rational design of porous crystalline materials for thermal catalytic CO2 conversion.

Reticular frameworks and their derived materials for CO2 conversion by thermo−catalysis

Synthesis of functional 3D COFs with irreversible bond is challenging. Herein, 3D imide-bonded COFs were constructed via the imidization reaction between phthalocyanine-based tetraanhydride and 1,3,5,7-tetra(4-aminophenyl)adamantine. These two 3D COFs are made up of interpenetrated pts networks according to powder X-ray diffraction and gas adsorption analyses. CoPc-PI-COF-3 doped with carbon black has been employed to fabricate the electrocatalytic cathode towards CO2 reduction reaction within KHCO3 aqueous solution, displaying the Faradaic efficiency of 88-96 % for the CO2 -to-CO conversion at the voltage range of ca. -0.60 to -1.00 V (vs. RHE). In particular, the 3D porous structure of CoPc-PI-COF-3 enables the active electrocatalytic centers occupying 32.7 % of total cobalt-phthalocyanine subunits, thus giving a large current density (jCO ) of -31.7 mA cm-2 at -0.90 V. These two parameters are significantly improved than the excellent 2D COF analogue (CoPc-PI-COF-1, 5.1 % and -21.2 mA cm-2 ).

Maximizing Electroactive Sites in a Three-Dimensional Covalent Organic Framework for Significantly Improved Carbon Dioxide Reduction Electrocatalysis.

Covalent organic frameworks (COFs) are an ideal template for photocatalytic H2O2 synthesis because of the tunable chemical structures and semiconductor properties. However, the photoactivity for COFs is still under-improved due to the inefficient intrinsic charge generation, fast recombination of photogenerated charges, and limited electron transport along the frameworks. Herein, spatially separated and synergistic triazine and acetylene units are first integrated into COFs (EBA-COF and BTEA-COF) for photocatalytic H2O2 production. The spatial separation of triazine and acetylene cores leads to efficient charge separation and suppressed charge recombination, and C═C linkage facilitates electrons transport over the skeletons. Both experimental and computational results suggested that triazine and acetylene units synergistically promote H2O2 synthesis in a two-electron pathway. The EBA-COF showed an attractive activity with a H2O2 production rate of 1830 μmol h–1 gcat–1, superior to that of most other COF-based catalysts. This study provides a method for designing photocatalysts with synergistic photocatalytic active sites based on vinylene-linked COFs. 

Constructing Synergistic Triazine and Acetylene Cores in Fully Conjugated Covalent Organic Frameworks for Cascade Photocatalytic H<sub>2</sub>O<sub>2</sub> Production

Herein, an effective tandem catalysis strategy is developed to improve the selectivity of the CO2 RR towards C2 H4 by multiple distinct catalytic sites in local vicinity. An earth-abundant elements-based tandem electrocatalyst PTF(Ni)/Cu is constructed by uniformly dispersing Cu nanoparticles (NPs) on the porphyrinic triazine framework anchored with atomically isolated nickel-nitrogen sites (PTF(Ni)) for the enhanced CO2 RR to produce C2 H4 . The Faradaic efficiency of C2 H4 reaches 57.3 % at -1.1 V versus the reversible hydrogen electrode (RHE), which is about 6 times higher than the non-tandem catalyst PTF/Cu, which produces CH4 as the major carbon product. The operando infrared spectroscopy and theoretic density functional theory (DFT) calculations reveal that the local high concentration of CO generated by PTF(Ni) sites can facilitate the C-C coupling to form C2 H4 on the nearby Cu NP sites. The work offers an effective avenue to design electrocatalysts for the highly selective CO2 RR to produce multicarbon products via a tandem route.

Rong Cao

Papers

Highly Selective Tandem Electroreduction of CO2 to Ethylene over Atomically Isolated Nickel–Nitrogen Site/Copper Nanoparticle Catalysts