Enhanced electrochemical conversion of CO2 to CO at bimetallic Ag-Zn catalysts formed on polypyrrole-coated electrode

Atomic Zn-Doping Induced Sabatier Optimum in NiZn0.03 Catalyst for CO2 Electroreduction at Industrial-Level Current Densities

The Sabatier principle defines the essential criteria for an ideal catalyst in heterogeneous catalysis, while reaching the Sabatier optimum is still challenging in catalyst design. Herein, an elegant strategy is described to reach the Sabatier optimum of Ni electrocatalyst in CO2 reduction reaction (CO2 RR) by atomically Zn doping. The incorporation of 3% Zn single atom into Ni lattice leads to the moderate degrade of d-band center via Ni-Zn electronic coupling, which balances the bonding strengths of *COOH and *CO, resulting in a relative low energy barrier for CO2 activation while not being substantially poisoned by CO. Consequently, NiZn0.03 /C exhibits unique catalytic activity (jCO >100 mA cm-2 at -0.6 V), wide potential range for selective CO production (FECO >90% from -0.65 to -1.15 V), and outstanding long-term stability (FECO >90% during 85 h electrolysis at -0.85 V). The results provide valuable insights for the rational fabrication of superior non-noble bimetallic electrocatalysts in CO2 electroreduction.

ALD-Engineered CuxO Overlayers Transform ZnO Nanorods for Selective Production of CO in Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) holds tremendous promise as a strategy for lowering atmospheric CO2 levels and creating new clean energy sources. The conversion of CO2RR to CO, in particular, has garnered significant scientific interest due to its industrial feasibility. Within this context, the CuZn-based electrocatalyst presents an attractive alternative to conventional CO-selective electrocatalysts, which are often costly and scarce. Nevertheless, the wide-range utilization of CuZn electrocatalysts requires a more comprehensive understanding of their performance and characteristics. In this study, we synthesized ZnO nanorods through electrodeposition and subsequently coated them with CuxO overlayers prepared by atomic layer deposition (ALD). CuxO significantly enhanced CO selectivity, and 88% CO selectivity at a relatively low potential of -0.8 V was obtained on an optimized CuxO overlayer thickness (CuxO-250/ZnO). The addition of CuxO on ZnO was found to dramatically increase the electrochemical surface area (ESCA), lower the charge-transfer resistance (Rct), and introduce new active sites in the ε-CuZn4 phase. Furthermore, electrochemical Raman spectroscopy results showed that the CuxO-250/ALD electrode developed a ZnO layer on the surface during the CO2RR, while the bare ZnO electrode showed no evidence of ZnO during the reaction. These results suggest that the addition of CuxO by ALD played a crucial role in stabilizing ZnO on the surface. The initial amount of CuxO was shown to further affect the redeposition of the ZnO layer and hence affect the final composition of the surface. We attribute the improvement in CO selectivity to the introduction of both ε-CuZn4 and ZnO that developed during the CO2RR. Overall, our study provides new insights into the dynamic behavior and surface composition of CuZn electrocatalysts during CO2RR.

Elaborate tree-like Cu-Ag clusters from green electrodeposition for efficiently electrocatalyzing CO2 conversion into syngas

The electrocatalytic carbon dioxide reduction (CO2RR) is one of the emerging technologies that can effectively transform carbon dioxide (CO2) into valuable products. Electrocatalysts deriving from green synthesis methods will significantly help to establish a new green carbon cycle. Herein, a green electrodeposition method without additional reducing agents was used to synthesize Cu-Ag bimetallic catalysts, and it is shown that the combination of Cu and Ag obviously affects the morphology of the Cu-Ag catalysts, resulting in the formation of elaborate tree-like Cu-Ag clusters. An as-deposited Cu-Ag/carbon fiber (Cu-Ag/CF) catalyst exhibits high activity, selectivity and stability toward the CO2RR; in particular, the elaborate dendritic Cu-Ag/CF can efficiently reduce CO2 to syngas with high selectivity (Faradaic efficiency (FE) > 95%) at a low onset potential (-0.5 V). This work provides a rational strategy to overcome the significantly different reaction capacities during the reduction of Ag+ and Cu2+, leading to the formation of a controlled morphology of Cu-Ag, which is favourable for the design and development of highly efficient Cu or Ag catalysts via green methods for electrocatalyzing the CO2RR.

Pressure dependence in aqueous-based electrochemical CO2 reduction

Electrochemical CO₂ reduction (CO₂R) is an approach to closing the carbon cycle for chemical synthesis. To date, the field has focused on the electrolysis of ambient pressure CO₂. However, industrial CO₂ is pressurized-in capture, transport and storage-and is often in dissolved form. Here, we find that pressurization to 50 bar steers CO₂R pathways toward formate, something seen across widely-employed CO₂R catalysts. By developing operando methods compatible with high pressures, including quantitative operando Raman spectroscopy, we link the high formate selectivity to increased CO₂ coverage on the cathode surface. The interplay of theory and experiments validates the mechanism, and guides us to functionalize the surface of a Cu cathode with a proton-resistant layer to further the pressure-mediated selectivity effect. This work illustrates the value of industrial CO₂ sources as the starting feedstock for sustainable chemical synthesis.

High Throughput Preparation of Ag-Zn Alloy Thin Films for the Electrocatalytic Reduction of CO2 to CO

Ag-Zn alloys are identified as highly active and selective electrocatalysts for CO₂ reduction reaction (CO₂RR), while how the phase composition of the alloy affects the catalytic performances has not been systematically studied yet. In this study, we fabricated a series of Ag-Zn alloy catalysts by magnetron co-sputtering and further explored their activity and selectivity towards CO₂ electroreduction in an aqueous KHCO₃ electrolyte. The different Ag-Zn alloys involve one or more phases of Ag, AgZn, Ag₅Zn₈, AgZn_3, and Zn. For all the catalysts, CO is the main product, likely due to the weak CO binding energy on the catalyst surface. The Ag₅Zn₈ and AgZn₃ catalysts show a higher CO selectivity than that of pure Zn due to the synergistic effect of Ag and Zn, while the pure Ag catalyst exhibits the highest CO selectivity. Zn alloying improves the catalytic activity and reaction kinetics of CO₂RR, and the AgZn₃ catalyst shows the highest apparent electrocatalytic activity. This work found that the activity and selectivity of CO₂RR are highly dependent on the element concentrations and phase compositions, which is inspiring to explore Ag-Zn alloy catalysts with promising CO₂RR properties.

Ordered Ag Nanoneedle Arrays with Enhanced Electrocatalytic CO2 Reduction via Structure-Induced Inhibition of Hydrogen Evolution

Silver is an attractive catalyst for converting CO₂ into CO. However, the high CO₂ activation barrier and the hydrogen evolution side reaction seriously limit its practical application and industrial perspective. Here, an ordered Ag nanoneedle array (Ag-NNAs) was prepared by template-assisted vacuum thermal-evaporation for CO₂ electroreduction into CO. The nanoneedle array structure induces a strong local electric field at the tips, which not only reduces the activation barrier for CO₂ electroreduction but also increases the energy barrier for the hydrogen evolution reaction (HER). Moreover, the array structure endows a high surface hydrophobicity, which can regulate the adsorption of water molecules at the interface and thus dynamically inhibit the competitive HER. As a result, the optimal Ag-NNAs exhibits 91.4% Faradaic efficiency (FE) of CO for over 700 min at -1.0 V vs RHE. This work provides a new concept for the application of nanoneedle array structures in electrocatalytic CO₂ reduction reactions.

Hydrophobic Electrode Design for CO2 Electroreduction in a Microchannel Reactor

Microchannel reactor is a novel electrochemical device to intensify CO₂ mass transfer with large interfacial areas. However, if the catalyst inserted in the microchannel is wetted, CO₂ is required to diffuse across the liquid film to get access to reaction sites. In this paper, a hydrophobic polytetrafluoroethylene (PTFE)-doped Ag nanocatalyst on a Zn rod was synthesized through a facile galvanic replacement of 2Ag⁺+Zn → 2Ag + Zn²⁺. The catalyst layer, which was PTFE incorporated into the Ag matrix, was detected to distribute uniformly on the Zn substrate with a thickness of 77 μm. Consequently, the PTFE-doped electrode demonstrated enhanced activity with an optimal 96.19% CO faradaic efficiency (FE_CO) in the microchannel reactor. Typically, the catalyst could maintain over 90% FE_CO even at the current density of 24 mA cm^-2, which was nearly 30% higher than that of a similar catalyst without PTFE. In addition, influences of the concentration of PTFE and deposition time were also investigated, determining that 1 vol % of PTFE and 30 min of coating yielded best electrocatalytic efficiency. To achieve a further breakthrough of CO₂ mass transfer limitations, reactions were applied under relatively high pressures (3-15 bar) in a single-compartment high-pressure cell. The maximum CO partial current density (j_CO) can reach 106.76 mA cm^-2 at 9 bar.

Highly Efficient SO2 Sensing by Light-Assisted Ag/PANI/SnO2 at Room Temperature and the Sensing Mechanism

Sulfur dioxide (SO₂) is one of the most hazardous and common environmental pollutants. However, the development of room-temperature SO₂ sensors is seriously lagging behind that of other toxic gas sensors due to their poor recovery properties. In this study, a light-assisted SO₂ gas sensor based on polyaniline (PANI) and Ag nanoparticle-comodified tin dioxide nanostructures (Ag/PANI/SnO₂) was developed and exhibited remarkable SO₂ sensitivity and excellent recovery properties. The response of the Ag/PANI/SnO₂ sensor (20.1) to 50 ppm SO₂ under 365 nm ultraviolet (UV) light illumination at 20 °C was almost 10 times higher than that of the pure SnO₂ sensor. Significantly, the UV-assisted Ag/PANI/SnO₂ sensor had a rapid response time (110 s) and recovery time (100 s) to 50 ppm SO₂, but in the absence of light, the sensors exhibited poor recovery performance or were even severely and irreversibly deactivated by SO₂. The UV-assisted Ag/PANI/SnO₂ sensor also exhibited excellent selectivity, superior reproducibility, and satisfactory long-term stability at room temperature. The increased charge carrier density, improved charge-transfer capability, and the higher active surface of the Ag/PANI/SnO₂ sensor were revealed by electrochemical measurements and endowed with high SO₂ sensitivity. Moreover, the light-induced formation of hot electrons in a high-energy state in Ag/PANI/SnO₂ significantly facilitated the recovery of SO₂ by the gas sensor.