Boron-Doping Engineering in AgCd Bimetallic Catalyst Enabling Efficient CO2 Electroreduction to CO and Aqueous Zn-CO2 Batteries.

The limited adsorption and activation of CO2 on catalyst and the high energy barrier for intermediate formation hinder the development of electrochemical CO2 reduction reactions (CO2RR). Herein, this work reports a boron (B) doping engineering in AgCd bimetals to alleviate the above limitations for efficient CO2 electroreduction to CO and aqueous Zn-CO2 batteries. Specifically, the B-doped AgCd bimetallic catalyst (AgCd-B) is prepared via a simple reduction reaction at room temperature. A combination of in situ experiments and density functional theory (DFT) calculations demonstrates that B-doping simultaneously enhances the adsorption and activation of CO2 and reduces the binding energy of the intermediates by moderating the electronic structure of bimetals. As a result, the AgCd-B catalyst exhibits a high CO Faraday efficiency (FECO) of 99% at -0.8 V versus reversible hydrogen electrode (RHE). Additionally, it maintains a FECO over 92% at a wide potential window of 600 mV (-0.6 to -1.1 V versus RHE). Furthermore, the AgCd-B catalyst coupled with the Zn anode to assemble aqueous Zn-CO2 batteries shows a power density of 20.18 mW cm-2 and a recharge time of 33 h.

Molecular Engineering of Porous Fe-N-C Catalyst with Sulfur Incorporation for Boosting CO2 Reduction and Zn-CO2 Battery.

Transition metal-nitrogen-carbon (M-N-C) catalysts have emerged as promising candidates for electrocatalytic CO2 reduction reaction (CO2RR) due to their uniform active sites and high atomic utilization rate. However, poor efficiency at low overpotentials and unclear reaction mechanisms limit the application of M-N-C catalysts. In this study, Fe-N-C catalysts are developed by incorporating S atoms onto ordered hierarchical porous carbon substrates with a molecular iron thiophenoporphyrin. The well-prepared FeSNC catalyst exhibits superior CO2RR activity and stability, attributes to an optimized electronic environment, and enhances the adsorption of reaction intermediates. It displays the highest CO selectivity of 94.0% at -0.58 V (versus the reversible hydrogen electrode (RHE)) and achieves the highest partial current density of 13.64 mA cm-2 at -0.88 V. Furthermore, when employed as the cathode in a Zn-CO2 battery, FeSNC achieves a high-power density of 1.19 mW cm-2 and stable charge-discharge cycles. Density functional theory calculations demonstrate that the incorporation of S atoms into the hierarchical porous carbon substrate led to the iron center becoming more electron-rich, consequently improving the adsorption of the crucial reaction intermediate *COOH. This study underscores the significance of hierarchical porous structures and heteroatom doping for advancing electrocatalytic CO2RR and energy storage technologies.

Confined Ni nanoparticles in N-doped carbon nanotubes for excellent pH-universal industrial-level electrocatalytic CO2 reduction and Zn-CO2 battery.

Electrocatalytic reduction of CO2 (ECR) offers a promising approach to curbed carbon emissions and complete carbon cycles. However, the inevitable creation of carbonates and limited CO2 utilization efficiency in neutral or alkaline electrolytes result in low energy efficiency, carbon losses and its widespread commercial utilization. The advancement of CO2 reduction under acidic conditions offers a promising approach for their commercial utilization, but the inhibition of hydrogen evolution reaction and the corrosion of catalysts are still challenging. Herein, Ni nanoparticles (NPs) wrapped in N-doped carbon nanotubes (NixNC-a) are successfully prepared by a facile mixed-heating and freeze-drying method. Ni100NC-a achieves a high Faraday efficiency (FE) of near 100 % for CO under pH-universal conditions, coupled with a promising current density of CO (>100 mA cm-2). Especially in acidic conditions, Ni100NC-a exhibits an exceptional ECR performance with the high FECO of 97.4 % at -1.44 V and the turnover frequency (TOF) of 11 k h-1 at -1.74 V with a current density of 288.24 mA cm-2. This excellent performance is attributed to the synergistic effect of Ni NPs and N-doped carbon shells, which protects Ni NPs from etching, promotes CO2 adsorption and regulates local pH. Moreover, Ni100NC-a could drive the reversible Zn-CO2 battery with a high power-density of 4.68 mW cm-2 and a superior stability (98 h). This study presents a promising candidate for efficient pH-universal CO2 electroreduction and Zn-CO2 battery.

Ionic Liquids Modulating Local Microenvironment of Ni-Fe Binary Single Atom Catalyst for Efficient Electrochemical CO2 Reduction.

The Ni and Fe dual-atom catalysts still undergo strikingly attenuation under high current density and high overpotential. To ameliorate the issue, the ionic liquids with different cations or anions are used in this work to regulate the micro-surface of nitrogen-doped carbon supported Ni and Fe dual-atom sites catalyst (NiFe-N-C) by an impregnation method. The experimental data reveals the dual function of ionic liquids, which enhances CO2 adsorption ability and modulates electronic structure, facilitating CO2 anion radical (CO2 •¯) stabilization and decreasing onset potential. The theoretical calculation results prove that the attachment of ionic liquids modulates electronic structure, reduces energy barrier of CO2 •¯ formation, and enhances overall ECR performance. Based on these merits, BMImPF6 modified NiFe-N-C (NiFe-N-C/BMImPF6) achieves the high CO faradaic efficiency of 91.9% with a CO partial current density of -120 mA cm-2 at -1.0 V. When the NiFe-N-C/BMImPF6 is assembled as cathode of Zn-CO2 battery, it delivers the highest power density of 2.61 mW cm-2 at 2.57 mA cm-2 and superior cycling stability. This work will afford a direction to modify the microenvironment of other dual-atom catalysts for high-performance CO2 electroreduction.

Dual-Anion Regulation for Reversible and Energetic Aqueous Zn-CO2 Batteries.

Aqueous Zn-CO2 batteries can not only convert CO2 into high-value chemicals but also store/output electric energy for external use. However, their performance is limited by sluggish and complicated CO2 electroreduction at the cathode. Herein, a dual-anion regulated Bi electrocatalyst is developed to selectively reduce CO2 to formate with a Faradaic efficiency of up to 97% at a large current density of 250 mA cm-2 . With O and/or F, the rate-determine step of CO2 electroreduction has been manipulated (from the first hydrogenation to *HCOOH desorption step) with a reduced energy barrier. Significantly, the fabricated Zn-CO2 battery exhibits a high discharge voltage of 1.2 V, optimal power density of 4.51 mW cm-2 , remarkable energy density of 802 Wh kg-1 , and energy-conversion efficiency of 70.74%, stability up to 200 cycles and 68 h. This study provides possible strategies to fabricate reversible and energetic aqueous Zn-CO2 batteries by addressing cathodic problems.

Regulating the Electronic Structure of Bismuth Nanosheets by Titanium Doping to Boost CO2 Electroreduction and Zn-CO2 Batteries.

The electrochemical carbon dioxide reduction reaction (E-CO2 RR) to formate is a promising strategy for mitigating greenhouse gas emissions and addressing the global energy crisis. Developing low-cost and environmentally friendly electrocatalysts with high selectivity and industrial current densities for formate production is an ideal but challenging goal in the field of electrocatalysis. Herein, novel titanium-doped bismuth nanosheets (TiBi NSs) with enhanced E-CO2 RR performance are synthesized through one-step electrochemical reduction of bismuth titanate (Bi4 Ti3 O12 ). We comprehensively evaluated TiBi NSs using in situ Raman spectra, finite element method, and density functional theory. The results indicate that the ultrathin nanosheet structure of TiBi NSs can accelerate mass transfer, while the electron-rich properties can accelerate the production of *CO2 - and enhance the adsorption strength of *OCHO intermediate. The TiBi NSs deliver a high formate Faradaic efficiency (FEformate ) of 96.3% and a formate production rate of 4032 µmol h-1  cm-2 at -1.01 V versus RHE. An ultra-high current density of -338.3 mA cm-2 is achieved at -1.25 versus RHE, and simultaneously FEformate still reaches more than 90%. Furthermore, the rechargeable Zn-CO2 battery using TiBi NSs as a cathode catalyst achieves a maximum power density of 1.05 mW cm-2 and excellent charging/discharging stability of 27 h.

Promoting CO2 Dynamic Activation via Micro-Engineering Technology for Enhancing Electrochemical CO2 Reduction.

Optimizing the coordination structure and microscopic reaction environment of isolated metal sites is promising for boosting catalytic activity for electrocatalytic CO2 reduction reaction (CO2 RR) but is still challenging to achieve. Herein, a newly electrostatic induced self-assembly strategy for encapsulating isolated Ni-C3 N1 moiety into hollow nano-reactor as I-Ni SA/NHCRs is developed, which achieves FECO  of 94.91% at -0.80 V, the CO partial current density of ≈-15.35 mA cm-2 , superior to that with outer Ni-C2 N2 moiety (94.47%, ≈-12.06 mA cm-2 ), or without hollow structure (92.30%, ≈-5.39 mA cm-2 ), and high FECO of ≈98.41% at 100 mA cm-2 in flow cell. COMSOL multiphysics finite-element method and density functional theory (DFT) calculation illustrate that the excellent activity for I-Ni SA/NHCRs should be attributed to the structure-enhanced kinetics process caused by its hollow nano-reactor structure and unique Ni-C3 N1 moiety, which can enrich electron on Ni sites and positively shift d-band center to the Fermi level to accelerate the adsorption and activation of CO2 molecule and *COOH formation. Meanwhile, this strategy also successfully steers the design of encapsulating isolated iron and cobalt sites into nano-reactor, while I-Ni SA/NHCRs-based zinc-CO2 battery assembled with a peak power density of 2.54 mW cm--2 is achieved.

Ultrathin Nitrogen-Doped Carbon Encapsulated Ni Nanoparticles for Highly Efficient Electrochemical CO2 Reduction and Aqueous Zn-CO2 Batteries.

Electrochemical CO2 reduction reaction (CO2 RR), powered by renewable electricity, has attracted great attention for producing high value-added fuels and chemicals, as well as feasibly mitigating CO2 emission problem. Here, this work reports a facile hard template strategy to prepare the Ni@N-C catalyst with core-shell structure, where nickel nanoparticles (Ni NPs) are encapsulated by thin nitrogen-doped carbon shells (N-C shells). The Ni@N-C catalyst has demonstrated a promising industrial current density of 236.7 mA cm-2 with the superb FECO of 97% at -1.1 V versus RHE. Moreover, Ni@N-C can drive the reversible Zn-CO2 battery with the largest power density of 1.64 mW cm-2 , and endure a tough cycling durability. These excellent performances are ascribed to the synergistic effect of Ni@N-C that Ni NPs can regulate the electronic microenvironment of N-doped carbon shells, which favor to enhance the CO2 adsorption capacity and the electron transfer capacity. Density functional theory calculations prove that the binding configuration of N-C located on the top of Ni slabs (Top-Ni@N-C) is the most thermodynamically stable and possess a lowest thermodynamic barrier for the formation of COOH* and the desorption of CO. This work may pioneer a new method on seeking high-efficiency and worthwhile electrocatalysts for CO2 RR and Zn-CO2 battery.

Metal-Free Bifunctional Ordered Mesoporous Carbon for Reversible Zn-CO2 Batteries.

The fabrication of Zn-CO2 batteries is a promising technique for CO2 fixation and energy storage. Herein, nitrogen-doped ordered mesoporous carbon (NOMC) is adopted as a bifunctional metal-free electrocatalyst for CO2 reduction and oxygen evolution reaction in the near-neutral electrolyte. The ordered mesoporous structures and abundant N-dopings of NOMC facilitate the accessibility and utilization of the active sites, which endow NOMC with excellent electrocatalysis performance and outstanding stability. Especially, a nearly 100% CO Faradaic efficiency is achieved at an ultralow overpotential of 360 mV for CO2 reduction. When constructed as an aqueous rechargeable Zn-CO2 battery using NOMC as the cathode, it yields a high peak power density of 0.71 mW cm-2 , a good cyclability of 300 cycles, and excellent energy efficiency of 52.8% at 1.0 mA cm-2 .

Elucidation of the Synergistic Effect of Dopants and Vacancies on Promoted Selectivity for CO2 Electroreduction to Formate.

Sn-based materials are identified as promising catalysts for the CO2 electroreduction (CO2RR) to formate (HCOO- ). However, their insufficient selectivity and activity remain grand challenges. A new type of SnO2 nanosheet with simultaneous N dopants and oxygen vacancies (VO -rich N-SnO2 NS) for promoting CO2 conversion to HCOO- is reported. Due to the likely synergistic effect of N dopant and VO , the VO -rich N-SnO2 NS exhibits high catalytic selectivity featured by an HCOO- Faradaic efficiency (FE) of 83% at -0.9 V and an FE of > 90% for all C1 products (HCOO- and CO) at a wide potential range from -0.9 to -1.2 V. Low coordination Sn-N moieties are the active sites with optimal electronic and geometric structures regulated by VO and N dopants. Theoretical calculations elucidate that the reaction free energy of HCOO* protonation is decreased on the VO -rich N-SnO2 NS, thus enhancing HCOO- selectivity. The weakened H* adsorption energy also inhibits the hydrogen evolution reaction, a dominant side reaction during the CO2RR. Furthermore, using the catalyst as the cathode, a spontaneous Galvanic Zn-CO2 cell and a solar-powered electrolysis process successfully demonstrated the efficient HCOO- generation through CO2 conversion and storage.

Gas Diffusion Strategy for Inserting Atomic Iron Sites into Graphitized Carbon Supports for Unusually High-Efficient CO2 Electroreduction and High-Performance Zn-CO2 Batteries.

Emerging single-atom catalysts (SACs) hold great promise for CO2 electroreduction (CO2 ER), but the design of highly active and cost-efficient SACs is still challenging. Herein, a gas diffusion strategy, along with one-step thermal activation, for fabricating N-doped porous carbon polyhedrons with trace isolated Fe atoms (Fe1 NC) is developed. The optimized Fe1 NC/S1 -1000 with atomic Fe-N3 sites supported by N-doped graphitic carbons exhibits superior CO2 ER performance with the CO Faradaic efficiency up to 96% at -0.5 V, turnover frequency of 2225 h-1 , and outstanding stability, outperforming almost all previously reported SACs based on N-doped carbon supported nonprecious metals. The observed excellent CO2 ER performance is attributed to the greatly enhanced accessibility and intrinsic activity of active centers due to the increased electrochemical surface area through size modulation and the redistribution of doped N species by thermal activation. Experimental observations and theoretical calculations reveal that the Fe-N3 sites possess balanced adsorption energies of *COOH and *CO intermediates, facilitating CO formation. A universal gas diffusion strategy is used to exclusively yield a series of dimension-controlled carbon-supported SACs with single Fe atoms while a rechargeable Zn-CO2 battery with Fe1 NC/S1 -1000 as cathode is developed to deliver a maximal power density of 0.6 mW cm-2 .