Electrocatalyst For Electrochemical Reduction Of CO2 Research Articles

Me-N-C Electrocatalysts for Electrochemical CO2 Reduction to High-Value Products

The development of sustainable carbon-neutral energy technologies to mitigate greenhouse gas emissions has become imperative and urgent. Of special interest and importance in this context is the use of electricity from intermittent renewable energy sources (wind, solar) for electrochemical conversion of carbon dioxide (CO2) to easily storable and transportable value-added products: fuels and feedstock chemicals. Nanoparticles of non-precious metals, e.g., Cu, Sn, Co, show high activity for electrochemical CO2 reduction reaction (CO2RR) but suffer from poor selectivity, resulting in a mixture of products that require tedious and costly separation. Recently, transition metal- and nitrogen-doped carbon (Me-N-C) materials have been emerged as promising CO2RR catalysts thanks to their well-defined structures and good activity. However, their selectivity, while respectable for the generation of carbon monoxide (CO), is low for high energy-content products. A limited understanding of reaction pathways and degradation mechanism of Me-N-C catalysts for CO2RR has additionally stemmed a rational design of these materials.In this presentation, we summarize our study of the activity, selectivity, and stability of Me-N-C (Me = Fe, Co, Ni, or Cu, etc.) catalysts for the CO2RR, focusing on the role of the local coordination environment at metal centers and metal-carbon substrate interactions. We also report the obtained bimetallic M1M2-N-C catalysts, designed to enable the formation of multi-carbon products through CO2RR and utilizing high surface-area three-dimensional carbon matrix as support for metal sites with improved CO2RR activity. The main objective of this work is to use Me-N-C catalysts to produce high energy-density chemicals, enhance mechanistic understanding, and bring CO2RR closer to practical applications.

Sulfur-Decorated Ni-N-C Catalyst for Electrocatalytic CO2 Reduction with Near 100 % CO Selectivity.

Developing highly efficient electrocatalysts for electrochemical CO2 reduction (ECR) to value-added products is important for CO2 conversion and utilization technologies. In this work, a sulfur-doped Ni-N-C catalyst is fabricated through a facile ion-adsorption and pyrolysis treatment. The resulting Ni-NS-C catalyst exhibits higher activity in ECR to CO than S-free Ni-N-C, yielding a current density of 20.5 mA cm-2 under -0.80 V versus a reversible hydrogen electrode (vs. RHE) and a maximum CO faradaic efficiency of nearly 100 %. It also displays excellent stability with negligible activity decay after electrocatalysis for 19 h. A combination of experimental investigations and DFT calculations demonstrates that the high activity and selectivity of ECR to CO is due to a synergistic effect of the S and Ni-NX moieties. This work provides insights for the design and synthesis of nonmetal atom-decorated M-N-C-based ECR electrocatalysts.

Surface restructuring in AgCu single-atom alloy catalyst and self-enhanced selectivity toward CO2 reduction

Single atom alloy catalysts, consisting of trace amount of isolated solute atoms alloyed into the host matrix, provide a promising approach to tune the electronic structure and performance of bimetallic catalysts for breaking the limits of scaling relationships based on the unique electronic structures. However, the precisely as-synthesized surface usually suffers from surface evolution leading to activity and durability decay. Herein, the understanding of surface restructuring under reaction conditions is of great importance in optimizing catalytic performance of bimetallic catalysts. In this work, on account of the surface evolution of Cu in electrochemical reactions and its limited miscibility in Ag, a nanoporous AgCu single atom alloy catalyst with atomic ratio Cu:Ag = 1:110 is fabricated by chemical dealloying process. And a CO2-reduction-induced surface restructuring is achieved upon the well-designed catalyst. Noteworthily, nanoporous AgCu exhibits self-reinforcing selectivity toward CO2 reduction and suppressed hydrogen evolution in parallel with migration of Cu to the surface, although the charge transfer from Ag to Cu results in a strong tendency toward hydrogen evolution at the initial catalytic stage. By combining in situ electrochemical hypersensitive response to surface state with ex situ characterizations, the surface reconstruction is verified to be driven by electrochemical reduction of CO2 rather than hydrogen evolution reaction, and can be modulated by the applied potential. Well-reconstructed nanoporous AgCu facilitates the adsorption of CO2. Electronic effect of the restructuring of Cu enhances CO desorption ability on the surface. Consequently, electronic and geometric effects synergically lead to excellent performances with CO Faradaic efficiency of 97.5%. The insights gained from the reconstruction-induced self-reinforcing behavior of bimetallic electrocatalyst shall offer an avenue to rationally design bimetallic electrocatalysts for electrochemical CO2 reduction.

Correction: Understanding the water molecule effect in metal-free B-based electrocatalysts for electrochemical CO2 reduction

Correction for ‘Understanding the water molecule effect in metal-free B-based electrocatalysts for electrochemical CO2 reduction’ by Yuxiao Meng et al., J. Mater. Chem. A, 2022, DOI: 10.1039/d1ta10127g.

Thiophene Functionalized Porphyrin for Electrochemical Carbon Dioxide Reduction

The production of catalysts that display strong efficiencies in aqueous media for the electrochemical carbon dioxide reduction reaction (CO2RR) is essential both for a healthy world and for realistic application of energy waste to generate value-added fuels. In this study, thiophene functionalized metal-free (poly-H2Por) and cobalt porphyrin-based (poly-CoPor) organometallic catalysts were easily attached on the pencil graphite electrode surface via electrochemical polymerization method and these, porphyrin coated, pencil graphite electrodes (PGE) were used as electrocatalysts for electrochemical CO2 reduction for the first time in the literature. To reveal the electrochemical activity of CO2RR, the electropolymerized catalysts were investigated with linear sweep voltammetry in 0.1 M KHCO3 solution. The results showed that, the electrode which is modified with poly-CoPor decreased the overpotential of CO2RR, according to bare pencil graphite electrode, from −1.35 V to −0.63 V.

A promising single-atom Co-N-C catalyst for efficient CO2 electroreduction and high-current solar conversion of CO2 to CO

Excavating highly efficient M-N-C electrocatalysts for electrochemical CO2 reduction (ECR) is of paramount importance. Herein, we report a single-atom Co-N-C catalyst (CoN4-CB) with a high CO Faradaic efficiency (FECO, 98.7%) in ECR comparable to that of a similarly prepared NiN4-CB catalyst. Impressively, CoN4-CB demonstrates a CO turnover frequency of 27173 h−1 and a CO current density of −33.6 mA∙cm−2 at −0.76 V, 20.2 and 6.8 times higher than that of NiN4-CB, respectively. In a solar-driven ECR system composed of a Si solar cell and a flow cell, CoN4-CB shows a remarkably large current density of 98.3 mA∙cm−2 with an average FECO of 92.1%. Theoretical calculations suggest that the energy barrier for *COOH formation largely decreases on CoN4 sites compared with NiN4 sites, leading to a low onset potential and high activity for CO production. This work will boost the development of efficient M-N-C electrocatalysts and further practical application of solar-driven ECR system.

Oxide-derived Tin Nanospheres for Efficient CO2 Electroreduction to Formate

Rational design and engineering of electrocatalysts for electrochemical CO2 reduction (CO2RR) is crucial to effectively, selectively converting CO2 into carbon-neutral chemicals and fuels. In this work, we have developed hierarchical tin oxide nanosphere electrocatalysts to boost the production of formate. Three-dimensional (3D) SnO2 spheres constructed from small crystalline nanoparticles exhibited 71-81% Faradaic efficiencies (FE) and energy efficiency (40-50%) toward formate production at partial current densities of 22-72.4 mA/cmgeo 2 from -0.9 V to -1.3 V vs. the reversible hydrogen electrode (RHE). Such hierarchical sphere-like catalysts outperformed non-hierarchical SnO2 nanoparticles and commercially benchmarking catalysts, achieving a peak 81% FE at superior formate current density of ~50 mA/cmgeo 2 at -1.2 V vs. RHE. The spherical oxide-derived Sn catalysts were highly stable up to 36 hours, preserving long-term partial current density of ~ 45 mA/cmgeo 2 and 68% FE for formate. Such an outstanding performance can be credited to the 3D hierarchical structure, which offers enlarged surface area, larger number of electrochemically active sites, and strongly suppressed H2 evolution. Our results may provide more guidelines for the introduction of hierarchical porosity into CO2RR electrocatalysts for large-scale applications.

Electronic Delocalization of Bismuth Oxide Induced by Sulfur Doping for Efficient CO2 Electroreduction to Formate

Developing efficient electrocatalysts for electrochemical CO2 reduction (ECR) to fuels and chemicals with high product faradaic efficiency (FE) and current density is desirable but remains challeng...

P-block metal-based (Sn, In, Bi, Pb) electrocatalysts for selective reduction of CO2 to formate

Electrochemical reduction of CO2 to fuels and chemical feedstocks using renewable electricity provides a promising approach toward artificial carbon recycling to address the global challenges in energy and sustainability. The most crucial step for this technique is to develop efficient electrocatalysts capable of reducing CO2 to valuable hydrocarbon products at a low overpotential with high selectivity and stability. In this article, we present a review on the recent developments and understanding of p-block post-transition metal (e.g., Sn, In, Pb, and Bi) based electrocatalysts for electrochemical CO2 reduction. This group of electrocatalysts shows particularly high selectivity for reduction of CO2 to formate or formic acid. Our main focus will be on the fundamental understanding of surface chemistry, active sites, reaction mechanism, and structure–activity relationships. Strategies to enhance the activity including morphology control, nanostructuring, defect engineering, doping, and alloying to modulate the electronic structure will also be briefly discussed. Finally, we summarize the existing challenges and present perspectives for the future development of this exciting field.

Low‐Energy CO2 Reduction on a Metal‐Free Carbon Material

AbstractElectrochemical CO2 reduction to value‐added chemicals on carbon‐based catalysts is a potential approach for low‐cost value‐added CO2 recycling. However, high overpotential and limited understanding of the mechanism remain challenging. Herein, a nitrogen and phosphorus co‐doped carbon (NPC) material, which is synthesized through a simple pyrolysis, promoted low‐energy electrochemical CO2 reduction. The NPC electrocatalyst combines the advantages of active sites co‐optimized by N and P, faster electrokinetics with bicarbonate providing proton, and more accessible active sites exposed by soft‐template precursors. The NPC electrocatalyst exhibited more than ten‐ and fourfold enhancement of the partial current density of CO production compared to N‐doped and P‐doped carbon materials, respectively, with high selectivity (86 % faradaic efficiency) at only −0.45 V. Experimental data coupled with density function theory calculations revealed that N and P dopants cooperated to enhance CO2 adsorption, first electron transfer to CO2 and then *COOH intermediate stabilization, leading to active and selective CO2‐to‐CO conversion. This work extends the design and understanding of metal‐free carbon‐based electrocatalysts for electrochemical CO2 reduction.

The Additional Effect of Transition Metals (Cu, Zn, Sn and Ag) in Palladium Electrocatalyst for CO2 Reduction Reaction

It is important to investigate the electrochemical reduction reaction of CO2 on the catalytic selectivity dependence the materials because it might be guideline to create a co-catalyst of photocatalytic CO2 reduction. Widely studied electrocatalysts for electrochemical CO2 reduction are precious metals, such as Pd, Ag, and Au. It is known that precious metals can efficiently reduce the CO2 to carbon monoxide (CO) or formate (HCOO-). In particular, Pd nano-particles (Pd NPs) was proved as a unique electrocatalyst for CO2 reduction. HCOO- and CO generation for electrochemical reduction of CO2 on Pd NPs can be observed at low and high potential region, respectively. It should be believed that high selectivity towards CO formation can be realized by a formation of bimetallic compounds with transition metals in Pd structure.

(Invited) Bimetallic Electrocatalysts for Electrochemical CO2 Reduction

Bimetallic nanoparticles (NPs) offer great flexibility to tailor the activity and selectivity in electrocatalytic conversion of carbon dioxide (CO2) to valuable chemicals. Here, we report on the electrocatalytic behavior of Au-Sn, Au-Ag and Au-Pb NPs, to highlight the effect of the intermetallic phase formation and the bimetallic composition in CO2 electroreduction.Firstly, a series of Au–Sn electrocatalysts with different intermetallic phases were obtained via tin incorporation into the gold lattice through gradual diffusion. Stoichiometric intermetallic phases were formed, and under optimal conditions, two high-value products formed with reasonable current density: formic acid in the liquid phase and syngas in the gas phase. Notably, the phase composition had a massive influence on both the activity and the product distribution. In situ Raman spectroelectrochemistry studies demonstrated that catalytic performance of the AuSn phase was superior to that its parent metals and other bimetallic counterparts.Secondly, the correlation between the electrocatalytic activity and the composition of Au-Ag alloy NPs was studied. The CO Faradaic efficiency (and thus the forming CO:H2 ratio) was changed linearly by controlling the Au:Ag ratio in the alloy. This trend was attributed to the changes in the electronic properties, as there was a monotonous shift in the Fermi level away from the vacuum level with increasing Au content. This shift correspondingly reduces the binding energy of the intermediates (*COOH and *CO), resulting in the gradually enhanced CO2-reduction activity.Finally, I will show results gained with Au-Pb electrodes, where the formation of methane (CH4) was demonstrated. Factors underpinning this unusual behavior were studied by both experimental and theoretical means.

Selective Conversion to Formic Acid from CO2 By Electrochemical Reduction over Cuxsny Intermetallic Compounds

Electrochemical reduction of CO2 is of high value because it can reduce the amount of greenhouse gas in the atmosphere. However, since CO2 is a very stable compound due to the strength of C=O bonds, it is extremely hard to turn CO2 into other compounds. In addition, potassium bicarbonate, which is commonly used as electrolyte for electrochemical CO2 reduction, decomposes on most catalysts to form hydrogen (H2) by reduction of proton (H+) at the potentials required to reduce CO2. Therefore, it is important to use a catalyst that prevents the H2 evolution reaction. Widely studied electrocatalysts for electrochemical CO2 reduction are novel metals, such as Pd, Ag and Au, because they can efficiently reduce the CO2 to CO in high selectivity with low overpotentials. Although non-precious metals such as Zn, Sn and/or Bi are promising as the electrocatalysis for the CO2 reduction, using them require high overpotentials and also result in the formation of H2. Sn electrode is a viable candidate for CO2 reduction catalyst because it shows relatively high selectivity towards formation of formate. However, Sn demonstrates a function towards H2 evolution reaction. In order to overcome this problem, we have attempted alloying of Sn with Cu which can modify the electronic state and crystal structure of active site of Sn. In this study, we successfully prepared CuxSny intermetallic nanoparticles supported on carbon, which not only significantly improved the electrocatalytic activity of CO2 reduction, but also showed a highly selective conversion to formic acid. The structural difference of surface between Sn and SnCu, which corresponds to the high activity and selectivity towards CO2 reduction reaction, is also discussed in this poster session.

High Index Non-Noble Metal Electrocatalysts for Electrochemical CO2 Reduction to C1 Products

Accumulation of CO2 in the atmosphere triggers abnormal weather phenomenon through global warming and greenhouse effect. To reduce the CO2 concentration and its emission, extensive researches for CO2 capture, storage, and utilization have been conducted. Among them, electrochemical CO2 conversion is highly promising due to its ambient reaction conditions, high energy efficiency, and facile combination with other renewable energy source. However, because CO2 is chemically stable, the principal obstacle is to develop suitable catalyst having good catalytic activity, selectivity, and stability. Among various metal candidates, transition and post-transition metals have attracted much attention due to its low-cost, low-toxicity, and intrinsic catalytic property for CO2 reduction. Since general electrochemical catalysis is highly correlated with mass diffusion, crystal orientation, surface area, and conductivity, manipulating the catalyst structure is an effective method to improve catalytic performance. Furthermore, collaborative studies integrating experimental and theoretical approaches have been conducted for the investigation of CO2 reduction mechanism and suggestion of a rational design of a CO2 reduction catalyst. Herein, we developed hierarchical nanostructured Sn, Bi, and Zn electrodes as electrocatalysts for CO2 reduction to C1 products (e.g., formic acid/formate and carbon monoxide). Various structured catalysts such as Sn dendrite, Bi dendrite, and hexagonal Zn were fabricated by facile electrodeposition methods. The synthesized catalyst electrodes showed highly efficient CO2 reduction activity in terms of current density, Faradaic efficiency, and more importantly, stable performance during long-term operation. Sn dendrite and Bi dendrite electrodes exhibited a superior formate/formic acid production rates (Sn dendrite: 228.6 mmol h-1 cm-2 at -1.36 VRHE) and high Faradaic efficiency (Bi dendrite: 90% at -0.73 VRHE) without any considerable catalytic degradation during 18 h and 12 h of long-term operations, respectively. Furthermore, the hexagonal Zn catalyst showed a high CO selectivity up to ~95% during unprecedented long-time over 30 h. It is worth noted that their high selectivity towards CO2 reduction is attributed to their local (or chemical) structures. In case of Sn electrode, we found that the native O content on the Sn surface is strongly correlated with the stabilization of reaction intermediate and the formate selectivity. To understand in-depth the factors to affect the CO2 reduction, we further conducted the theoretical studies about the mechanism of CO2 conversion to formic acid on various Bi planes such as close-packed and high-index surfaces using density functional theory calculation (DFT). We demonstrated that the most energetically favorable pathway was a path through the formation of oxygen bidentate intermediate (*OCOH) among the three possible pathways for formic acid formation. In addition, it was also revealed that the high-index Bi surfaces exhibited the lower reduction potential than the closed-packed surface of (003) plane. Similarly, in electrochemical analysis using Zn electrodes, it was figured out that Zn (101) facet was favorable to CO formation whereas Zn (002) facet, most stable surface, favors the H2 evolution during CO2 electrolysis. Indeed, DFT calculations showed that Zn (101) facet lowers the reduction potential for CO2 to CO by more effectively stabilizing a *COOH intermediate than Zn (002) facet. Consequently, the coordinately unsaturated sites derived from the nanostructured non-noble metal catalysts can effectively stabilize the reaction intermediate by lowering the energy barrier for its binding to the site. These results may suggest a design principle for further developments in other advanced catalysts as well as in CO2 reduction. Figure 1

Identification of a new substrate effect that enhances the electrocatalytic activity of dendritic tin in CO2 reduction.

In this study, a selective Sn based catalytic system that electrochemically reduces CO2 to formate has been developed based on a new substrate effect. Dendritic Sn catalysts were synthesised by hydrogen gas bubble assisted electrodeposition on Pt, Cu, Sn or In substrates and applied to electrocatalytic reduction of CO2. The four substrates exhibit a hydrogen evolution activity that follows the order Pt > Cu > Sn > In. However, the Cu supported dendritic Sn catalyst provides the best selectivity towards formate formation (67.3% at -0.95 V vs. RHE). A substrate induced local pH change is proposed as the origin of formate selectivity. This was confirmed by the bulk electrolysis results obtained from two electrolyte solutions with different buffer capacities. The high buffer capacity phosphate buffer electrolyte solution provides minimal local pH change while an electrolyte with a low buffer capacity such as NaHCO3 maximises this effect to enhance the selectivity towards formate. The strategy reported here does not only focus on the catalyst, but also takes into consideration the local chemical environment. Hence, this work provides an optimal approach to improving the catalytic performance of electrocatalysts for electrochemical CO2 reduction.