Geometric and Electronic Structural Engineering of Isolated Ni Single Atoms for a Highly Efficient CO2 Electroreduction

Natural biomass derived single-atom catalysts for energy and environmental applications

Single atom catalysts (SACs) excel in various chemical processes, including electrocatalysis and industrial chemistry, due to their efficiency. In contrast to chemically synthesized precursors, biomass offers a greener and more cost-effective approach for SACs fabrication. To date, over forty types of SACs have been synthesized using natural sources like starch, cellulose, lignin, hemicellulose, proteins, and chitin. These catalysts incorporate metals such as Fe, Co, Ni, Cu, Zn, Mn, and Pt. This review concentrates on the preparation of SACs from biomass, exploring innovative techniques and their extensive applications in energy conversion and environmental conservation, including but not limited to reactions involving oxygen reduction, oxygen evolution, and hydrogen evolution. It also discusses current challenges and prospective advancements in this domain. This paper updates and expands on the knowledge of SACs derived from biomass, aiming to foster the development of more effective, low-cost catalyst materials from natural sources.

Fabrication of Nickel Single Atoms with Ionic Liquids by Only One-Step Pyrolysis for CO2 Electroreduction

Single-atom catalysts (SACs) are appealing for carbon dioxide (CO2) electroreduction with the utmost advantages; however, their preparation is still challenging because of the complicated procedure. Here, a novel Ni-based single-atom catalyst (Ni-BB-BD) is constructed from raw materials, [BMIM]BF4, [BMIM]DCN, and NiCl2·6H2O, directly without any precursor by only one-step pyrolysis. Ni-BB-BD achieves a maximum carbon monoxide Faradaic efficiency (FECO) of 96.5% at -0.8 V vs RHE, as well as long-term stability over 16 h. High current density up to -170.6 mA cm-2 at -1.0 V vs RHE is achieved in the flow cell along with a CO selectivity of 97.7%. It is identified that [BMIM]BF4 is the nitrogen source, while [BMIM]DCN is mainly taken as the carbon source. Theoretical studies have revealed that the rich nitrogen content, especially for the uncoordinated nitrogen, plays a critical role in lowering rate-limiting barrier height. This work develops a facile and effective strategy to prepare the SACs.

Hybridization State Transition under Working Conditions: Activity Origin of Single-Atom Catalysts

Single-atom catalysts (SACs) have been widely investigated and have emerged as a transformative approach in electrocatalysis. Despite their clear structure, the origin of their exceptional activity remains elusive. Herein, we elucidate a common phenomenon of the hybridization state transition of metal centers, which is responsible for the activity origin across various SACs for different reactions. Focusing on N-doped carbon-supported Ni SAC (NiN4 SAC) for CO2 reduction reaction (CO2RR), our comprehensive computations successfully clarify the hybridization state transition under working conditions and its relation with the activity. This transition, triggered by the reaction intermediates and applied potential, converts the Ni center from the inert dsp2 hybridization state to the active d2sp3 hybridization state. Importantly, the calculated activity and selectivity of the CO2RR over the d2sp3-hybridized Ni center are consistent with the experimental results, offering strong support for the proposed hypothesis. This work suggests a universal principle of electronic structure evolution in SACs that could revolutionize catalyst design, which also introduces a new paradigm for manipulating electronic states to enhance catalytic performance, with implications for various reactions and catalyst platforms.

Enhancing Electrocatalytic Kinetics via Synergy of Co Nanoparticles and Co/Ni-N4-C- and N-Doped Porous Carbon

Transition-metal species embedded in carbon have sparked intense interest in the fields of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, improvement of the electrocatalytic kinetics remains a challenge caused by the synergistic assembly. Here, we propose a biochemical strategy to fabricate the Co nanoparticles (NPs) and Co/Ni-N4-C co-embedded N-doped porous carbon (CoNPs&Co/Ni-N4-C@NC) catalysts via constructing the zeolitic imidazolate framework (ZIF)@yeast precursor. The rich amino groups provide the possibility for the anchorage of Co2+/Ni2+ ions as well as the construction of Co/Ni-ZIF@yeast through the yeast cell biomineralization effect. The functional design induces the formation of CoNPs and Co/Ni-N4-C sites in N-doped carbon as well as regulates the porosity for exposing such sites. Synergy of CoNPs, Co/Ni-N4-C, and porous N-doped carbon delivered excellent electrocatalytic kinetics (the ORR Tafel slope of 76.3 mV dec-1 and the OER Tafel slope of 80.4 mV dec-1) and a high voltage of 1.15 V at 10 mA cm-2 for the discharge process in zinc air batteries. It provides an effective strategy to fabricate high-performance catalysts.

Tuning the CO2 Reduction Selectivity of an Immobilized Molecular Ag Complex beyond CO

The electrochemical reduction of carbon dioxide (CO2) to produce fuels and chemicals has garnered significant attention. However, achieving control over the selectivity of the resulting products remains a challenging task, particularly within molecular systems. In this study, we employed a molecular silver complex immobilized on graphitized mesoporous carbon (GMC) as a catalyst for converting CO2 into CO, achieving an impressive selectivity of over 90% at -1.05 V vs RHE. Notably, the newly formed silver nanoparticles emerged as the active sites responsible for this high CO selectivity rather than the molecular system. Intriguingly, the introduction of copper ions into the restructured Ag-nanoparticle-decorated carbon altered the product selectivity. At -1.1 V vs RHE in 0.1 M KCl, we achieved a high C2 selectivity of 75%. Furthermore, not only the Ag-Cu bimetallic nanoparticle but also the small-sized Ag-Cu nanocluster decorated over GMC was proposed as active sites during catalytic reactions. Our straightforward approach offers valuable insights for fine-tuning the product selectivity of immobilized molecular systems, extending beyond C1 products.

Coordination Confined Thermolysis Synthesis of the Ni Single Atom Catalyst on the N-Doped Commercial Carbon for the Production of Syngas

The electrochemical conversion of CO2 into controllable syngas (CO/H2) over a wide potential range is challenging. The main electrocatalysts are based on the noble metals Au (Ag) or heavy metal Pb. The development of alternative nonprecious catalysts is of paramount importance for practice. In this work, a simple coordination confined thermal pyrolysis method has been developed for the synthesis of Ni single-atom catalyst loaded onto nitrogen-doped commercial carbon. The catalyst is in the form of NiN3-C, which exhibits a high-performance electrocatalytic reduction of CO2 toward producing syngas with Faraday efficiencies of 62.28% of CO and 36.7% of H2. The Gibbs free energies of COOH* and H* on the NiN3-C structure were estimated by using density functional theory (DFT). The formation of COOH* intermediate is the speed-limiting step in the process, with ΔG COOH* being 0.7 eV, while H* is the speed-limiting step in the hydrogen evolution, respectively. This work provides a feasible method for the achievement of nonprecious catalysts for the resourceful use of CO2.

Facile Synthesis of Hierarchically Porous Ni-N-C for Efficient CO2 Electroreduction to CO

The reasonable design of atomically dispersed Ni-Nx sites in porous carbon nanostructures is an efficient strategy to enhance the electrochemical CO2 reduction reaction (CO2RR) catalytic activity. In this work, atomically dispersed Ni-Nx sites on hierarchically porous carbon catalysts (HP-Ni-NC) were fabricated by a facile NaCl template-assisted pyrolysis method. The catalysts exhibit a large specific surface area and a hierarchical porous structure, facilitating the exposure of numerous active sites and the mass/electron transport during the CO2RR. Consequently, the CO Faradaic efficiency maintained over 90% in a wide potential window on the optimized HP-Ni-NC-2 catalyst. The CO partial current achieved 15.2 mA cm-2 at -0.9 V (vs reversible hydrogen electrode) in a H-cell. Furthermore, the current density can achieve 250 mA cm-2 at a cell voltage of 3.11 V in a membrane electrode assembly electrolyzer, demonstrating great promise for commercial-scale application. This study presents a facile approach to synthesizing hierarchically porous structure single-atom catalysts with superior catalytic performance toward CO2RR.