Nickel Doping in Atomically Thin Tin Disulfide Nanosheets Enables Highly Efficient CO2 Reduction

Atomic bismuth induced ensemble sites with indium towards highly efficient and stable electrocatalytic reduction of carbon dioxide

Structural reconstruction is commonly observed during electrocatalytic CO₂ reduction (CO₂RR) process. However, the proper modulation of interface and defect sites remains challenging with the mechanism understanding to realize the favorable electrocatalysis. Herein, the atomic bridging of bismuth with indium atoms is elaborately designed for improving electrocatalysis of CO₂RR via electrochemical reduction and in situ anchoring strategy. As revealed by in situ structure analysis and theoretical studies, the ensemble sites supported on carbon matrix enable the charge density gradient to significantly promote the adsorption of *OCHO intermediate by the regulation of σ bonding and π* back-donation. Consequently, such unique electrocatalyst achieves the high formate faradaic efficiency of 95.1% over the entire potential range tested and the long-lived stability for 9 d. With coupling of CO₂RR, the solar-driven full cell demonstrates the spontaneous production of formate and 2,5-furandicarboxylic acid via the efficient oxidation of 5-hydroxymethylfurfural with an outstanding yield of 88.2%, highlighting the impressive solar-to-fuel conversion selectivity. Monitoring and understanding the intrinsic active sites of biatomic bridge are crucial to elucidate the synergic electrocatalysis for rationally designing high-performance electrocatalysts.

Electronic Perturbation of Copper Single-Atom CO2 Reduction Catalysts in a Molecular Way

Fine-tuning electronic structures of single-atom catalysts (SACs) plays a crucial role in harnessing their catalytic activities, yet challenges remain at a molecular scale in a controlled fashion. By tailoring the structure of graphdiyne (GDY) with electron-withdrawing/-donating groups, we show herein the electronic perturbation of Cu single-atom CO₂ reduction catalysts in a molecular way. The elaborately introduced functional groups (-F, -H and -OMe) can regulate the valance state of Cu^δ+ , which is found to be directly scaled with the selectivity of the electrochemical CO₂ -to-CH₄ conversion. An optimum CH₄ Faradaic efficiency of 72.3 % was achieved over the Cu SAC on the F-substituted GDY. In situ spectroscopic studies and theoretical calculations revealed that the positive Cu^δ+ centers adjusted by the electron-withdrawing group decrease the pK_a of adsorbed H₂ O, promoting the hydrogenation of intermediates toward the CH₄ production. Our strategy paves the way for precise electronic perturbation of SACs toward efficient electrocatalysis.

Challenges and Opportunities in Electrocatalytic CO2 Reduction to Chemicals and Fuels

The global temperature increase must be limited to below 1.5 °C to alleviate the worst effects of climate change. Electrocatalytic CO2 reduction (ECO2 R) to generate chemicals and feedstocks is considered one of the most promising technologies to cut CO2 emission at an industrial level. However, despite decades of studies, advances at the laboratory scale have not yet led to high industrial deployment rates. This Review discusses practical challenges in the industrial chain that hamper the scaling-up deployment of the ECO2 R technology. Faradaic efficiencies (FEs) of about 100 % and current densities above 200 mA cm-2 have been achieved for the ECO2 R to CO/HCOOH, and the stability of the electrolysis system has been prolonged to 2000 h. For ECO2 R to C2 H4 , the maximum FE is over 80 %, and the highest current density has reached the A cm-2 level. Thus, it is believed that ECO2 R may have reached the stage for scale-up. We aim to provide insights that can accelerate the development of the ECO2 R technology.

Modulating p-Orbital of Bismuth Nanosheet by Nickel Doping for Electrocatalytic Carbon Dioxide Reduction Reaction

Electrochemical reduction of CO₂ (CO₂ RR) to value-added chemicals is an effective way to harvest renewable energy and utilize carbon dioxide. However, the electrocatalysts for CO₂ RR suffer from insufficient activity and selectivity due to the limitation of CO₂ activation. In this work, a Ni-doped Bi nanosheet (Ni@Bi-NS) electrocatalyst is synthesized for the electrochemical reduction of CO₂ to HCOOH. Physicochemical characterization methods are extensively used to investigate the composition and structure of the materials. Electrochemical results reveal that for the production of HCOOH, the obtained Ni@Bi-NS exhibits an equivalent current density of 51.12 mA cm^-2 at -1.10 V, which is much higher than the pure Bi-NS (18.00 mA cm^-2 at -1.10 V). A high Faradaic efficiency over 92.0 % for HCOOH is achieved in a wide potential range from -0.80 to -1.10 V, and particularly, the highest efficiency of 98.4 % is achieved at -0.90 V. Both experimental and theoretical results reveal that the superior activity and selectivity are attributed to the doping effect of Ni on the Bi nanosheet. The density functional theory calculation reveals that upon doping, the charge is transferred from Ni to the adjacent Bi atoms, which shifts the p-orbital electronic density states towards the Fermi level. The resultant strong orbital hybridization between Bi and the π* orbitals of CO₂ facilitates the formation of *OCHO intermediates and favors its activation. This work provides an effective strategy to develop active and selective electrocatalysts for CO₂ RR by modulating the electronic density state.

Interfacial Electron Regulation of Rh Atomic Layer-Decorated SnO2 Heterostructures for Enhancing Electrocatalytic Nitrogen Reduction

Ammonia (NH₃), which serves as a fertilizer supply, is struggling to satisfy the ever-growing population requirements over the world. The electrocatalytic nitrogen reduction to NH₃ production is highly desired but shows the extremely poor activity and selectivity of reported electrocatalysts. In this work, we rationally design a novel Rh atomic layer-decorated SnO₂ heterostructure catalyst through the interfacial engineering strategy, simultaneously achieving the highest NH₃ yield rate (149 μg h^-1 mg_cat^-1) and Faradaic efficiency (11.69%) at -0.35 V vs the reversible hydrogen electrode. This result is superior to the optimum response of previously reported SnO₂- or Rh-based catalysts for electrochemical nitrogen reduction. Both X-ray absorption spectra characterization and density functional theory calculations reveal the strong electron interaction between the Rh atomic layer and the SnO₂ heterostructure, which effectively regulated the interfacial electron transfer and d-band center. The downshift of the d-band center results in the greatly reduced H adsorption energy and the highly accelerated reaction kinetics for nitrogen reduction. This work endows a new insight into the interfacial electron regulation for weakening H adsorption and further enhancing the electrocatalytic N₂ reduction.

Hybrid Metal-Boron Diatomic Site Embedded in C2 N Monolayer Promotes C-C Coupling in CO2 Electroreduction

Double-atom catalyst (DAC) has gained much interest for its versatile tuning and synergistic effect of dual-atom active sites. Metal (M)-metal (M) diatomic sites, either homo- or heteronuclear, are typically researched. Hybrid metal-non-metal combined sites have rarely been studied and even the viability of such active sites are unknown. Herein, CO₂ electroreduction (CO₂ RR) is explored on M@X-C₂ N (M = Fe, Co, Ni, and Cu; X = S, P, and B) which renders naturally generated M-X diatomic site. Using spin-polarized density functional theory coupled with computational hydrogen electrode model, it is demonstrated that the functionality of hybrid M-B dual-atom center is superior over that of a single- or double-M center in driving CO₂ RR especially C-C coupling. Among metal-boron DACs studies, Fe@B-C₂ N (μ = 2μ_B ) exhibits the lowest free energy barrier of 0.17 eV in C-C coupling whereas Ni@B-C₂ N (μ = 0μ_B ) mainly produces CH₄ with the lowest barrier of 0.42 eV. Hence, the electronic spin state of M can be particularly important in modulating selectivity and C-C coupling barrier in CO₂ RR. Fe@B-C₂ N is predicted as the promising catalyst for CO₂ RR towards C₂₊ products owing partially to its enhanced spin state. The findings can enrich the design strategy of electrocatalysts normally running at ambient conditions.

Dynamic Restructuring of Cu-Doped SnS2 Nanoflowers for Highly Selective Electrochemical CO2 Reduction to Formate

With ever-increasing energy consumption and continuous rise in atmospheric CO₂ concentration, electrochemical reduction of CO₂ into chemicals/fuels is becoming a promising yet challenging solution. Sn-based materials are identified as attractive electrocatalysts for the CO₂ reduction reaction (CO₂ RR) to formate but suffer from insufficient selectivity and activity, especially at large cathodic current densities. Herein, we demonstrate that Cu-doped SnS₂ nanoflowers can undergo in situ dynamic restructuring to generate catalytically active S-doped Cu/Sn alloy for highly selective electrochemical CO₂ RR to formate over a wide potential window. Theoretical thermodynamic analysis of reaction energetics indicates that the optimal electronic structure of the Sn active site can be regulated by both S-doping and Cu-alloying to favor formate formation, while the CO and H₂ pathways will be suppressed. Our findings provide a rational strategy for electronic modulation of metal active site(s) for the design of active and selective electrocatalysts towards CO₂ RR.

Surface Oxygen Injection in Tin Disulfide Nanosheets for Efficient CO2 Electroreduction to Formate and Syngas

Surface chemistry modification represents a promising strategy to tailor the adsorption and activation of reaction intermediates for enhancing activity. Herein, we designed a surface oxygen-injection strategy to tune the electronic structure of SnS₂ nanosheets, which showed effectively enhanced electrocatalytic activity and selectivity of CO₂ reduction to formate and syngas (CO and H₂). The oxygen-injection SnS₂ nanosheets exhibit a remarkable Faradaic efficiency of 91.6% for carbonaceous products with a current density of 24.1 mA cm^-2 at -0.9 V vs RHE, including 83.2% for formate production and 16.5% for syngas with the CO/H₂ ratio of 1:1. By operando X-ray absorption spectroscopy, we unravel the in situ surface oxygen doping into the matrix during reaction, thereby optimizing the Sn local electronic states. Operando synchrotron radiation infrared spectroscopy along with theoretical calculations further reveals that the surface oxygen doping facilitated the CO₂ activation and enhanced the affinity for HCOO* species. This result demonstrates the potential strategy of surface oxygen injection for the rational design of advanced catalysts for CO₂ electroreduction.

Boosting Formate Production from CO2 at High Current Densities Over a Wide Electrochemical Potential Window on a SnS Catalyst

The flow-cell design offers prospect for transition to commercial-relevant high current density CO2 electrolysis. However, it remains to understand the fundamental interplay between the catalyst, and the electrolyte in such configuration toward CO2 reduction performance. Herein, the dramatic influence of electrolyte alkalinity in widening potential window for CO2 electroreduction in a flow-cell system based on SnS nanosheets is reported. The optimized SnS catalyst operated in 1 m KOH achieves a maximum formate Faradaic efficiency of 88 ± 2% at -1.3 V vs reversible hydrogen electrode (RHE) with the current density of ≈120 mA cm-2 . Alkaline electrolyte is found suppressing the hydrogen evolution across all potentials which is particularly dominant at the less negative potentials, as well as CO evolution at more negative potentials. This in turn widens the potential window for formate conversion (>70% across -0.5 to -1.5 V vs RHE). A comparative study to SnOx counterpart indicates sulfur also acts to suppress hydrogen evolution, although electrolyte alkalinity resulting in a greater suppression. The boosting of the electrochemical potential window, along with high current densities in SnS derived catalytic system offers a highly attractive and promising route toward industrial-relevant electrocatalytic production of formate from CO2 .

Highly Efficient and Selective CO2 Electro-Reduction to HCOOH on Sn Particle-Decorated Polymeric Carbon Nitride

Electrochemical conversion of CO₂ into liquid fuels by efficient and earth-abundant catalysts is of broad interest but remains a great challenge in renewable energy production and environmental remediation. Herein, a Sn particle-decorated polymeric carbon nitride (CN) electrocatalyst was successfully developed for efficient, durable, and highly selective CO₂ reduction to formic acid. High-resolution X-ray photoelectron spectroscopy confirmed that the metallic Sn particles and CN matrix are bound by strong chemical interaction, rendering the composite catalyst a stable structure. More notably, the electronic structure of Sn was well tuned to be highly electron-rich due to the electron transfer from N atoms of CN to Sn atoms via metal-support interactions, which favored the adsorption and activation of CO₂ molecules, promoted charge transport, and thus enhanced the electrochemical conversion of CO₂ . The composite electrocatalyst demonstrated an excellent Faradaic efficiency of formic acid (FE_HCOOH ) up to 96±2 % at the potential of -0.9 V vs. reversible hydrogen electrode, which remained at above 92 % during the electrochemical reaction of 10 h, indicating that the present Sn particle-decorated polymeric carbon nitride electrocatalyst is among the best in comparison with reported Sn-based electrocatalysts.

Size-Dependent Nickel-Based Electrocatalysts for Selective CO2 Reduction

Closing the anthropogenic carbon cycle by converting CO₂ into reusable chemicals is an attractive solution to mitigate rising concentrations of CO₂ in the atmosphere. Herein, we prepared Ni metal catalysts ranging in size from single atoms to over 100 nm and distributed them across N-doped carbon substrates which were obtained from converted zeolitic imidazolate frameworks (ZIF). The results show variance in CO₂ reduction performance with variance in Ni metal size. Ni single atoms demonstrate a superior Faradaic efficiency (FE) for CO selectivity (ca. 97 % at -0.8 V vs. RHE), while results for 4.1 nm Ni nanoparticles are slightly lower (ca. 93 %). Further increase the Ni particle size to 37.2 nm allows the H₂ evolution reaction (HER) to compete with the CO₂ reduction reaction (CO₂ RR). The FE towards CO production decreases to under 30 % and HER efficiency increase to over 70 %. These results show a size-dependent CO₂ reduction for various sizes of Ni metal catalysts.

Surface-Regulated Rhodium-Antimony Nanorods for Nitrogen Fixation

Surface regulation is an effective strategy to improve the performance of catalysts, but it has been rarely demonstrated for nitrogen reduction reaction (NRR) to date. Now, surface-rough Rh₂ Sb nanorod (RNR) and surface-smooth Rh₂ Sb NR (SNR) were selectively created, and their performance for NRR was investigated. The high-index-facet bounded Rh₂ Sb RNRs/C exhibit a high NH₃ yield rate of 228.85±12.96 μg h^-1  mg^-1 _Rh at -0.45 V versus reversible hydrogen electrode (RHE), outperforming the Rh₂ Sb SNRs/C (63.07±4.45 μg h^-1  mg^-1 _Rh ) and Rh nanoparticles/C (22.82±1.49 μg h^-1  mg^-1 _Rh ), owing to the enhanced adsorption and activation of N₂ on high-index facets. Rh₂ Sb RNRs/C also show durable stability with negligible activity decay after 10 h of successive electrolysis. The present work demonstrates that surface regulation plays an important role in promoting NRR activity and provides a new strategy for creating efficient NRR electrocatalysts.

Electroreduction of CO2 in Ionic Liquid-Based Electrolytes

Electroreduction of carbon dioxide (CO₂) to value-added chemicals and fuels is a promising approach for sustainable energy conversion and storage. Many electrocatalysts have been designed for this purpose and studied extensively. The role of the electrolyte is particularly interesting and is pivotal for designing electrochemical devices by taking advantage of the synergy between electrolyte and catalyst. Recently, ionic liquids as electrolytes have received much attention due to their high CO₂ adsorption capacity, high selectivity, and low energy consumption. In this review, we present a comprehensive overview of the recent progress in CO₂ electroreduction in ionic liquid-based electrolytes, especially in the performance of different catalysts, the electrolyte effect, as well as mechanism studies to understand the reaction pathway. Perspectives on this interesting area are also discussed for the construction of novel electrochemical systems.