Grain-Boundary-Rich Copper for Efficient Solar-Driven Electrochemical CO2 Reduction to Ethylene and Ethanol.

The grain boundary in copper-based electrocatalysts has been demonstrated to improve the selectivity of solar-driven electrochemical CO2 reduction toward multicarbon products. However, the approach to form grain boundaries in copper is still limited. This paper describes a controllable grain growth of copper electrodeposition via poly(vinylpyrrolidone) used as an additive. A grain-boundary-rich metallic copper could be obtained to convert CO2 into ethylene and ethanol with a high selectivity of 70% over a wide potential range. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the existence of grain boundaries enhances the adsorption of the key intermediate (*CO) on the copper surface to boost the further CO2 reduction. When coupling with a commercially available Si solar cell, the device achieves a remarkable solar-to-C2-products conversion efficiency of 3.88% at a large current density of 52 mA·cm-2. This low-cost and efficient device is promising for large-scale application of solar-driven CO2 reduction.

Enriched Surface Oxygen Vacancies of Photoanodes by Photoetching with Enhanced Charge Separation.

A facile photoetching approach is described that alleviates the negative effects from bulk defects by confining the oxygen vacancy (Ovac ) at the surface of BiVO4 photoanode, by 10-minute photoetching. This strategy could induce enriched Ovac at the surface of BiVO4 , which avoids the formation of excessive bulk defects. A mechanism is proposed to explain the enhanced charge separation at the BiVO4  /electrolyte interface, which is supported by density functional theory (DFT) calculations. The optimized BiVO4 with enriched surface Ovac presents the highest photocurrent among undoped BiVO4 photoanodes. Upon loading FeOOH/NiOOH cocatalysts, photoetched BiVO4 photoanode reaches a considerable water oxidation photocurrent of 3.0 mA cm-2 at 0.6 V vs. reversible hydrogen electrode. An unbiased solar-to-hydrogen conversion efficiency of 3.5 % is realized by this BiVO4 photoanode and a Si photocathode under 1 sun illumination.

Enhanced CO2 Electroreduction on Neighboring Zn/Co Monomers by Electronic Effect.

It is of great significance to reveal the detailed mechanism of neighboring effects between monomers, as they could not only affect the intermediate bonding but also change the reaction pathway. This paper describes the electronic effect between neighboring Zn/Co monomers effectively promoting CO2 electroreduction to CO. Zn and Co atoms coordinated on N doped carbon (ZnCoNC) show a CO faradaic efficiency of 93.2 % at -0.5 V versus RHE during a 30-hours test. Extended X-ray absorption fine structure measurements (EXAFS) indicated no direct metal-metal bonding and X-ray absorption near-edge structure (XANES) showed the electronic effect between Zn/Co monomers. In situ attenuated total reflection-infrared spectroscopy (ATR-IR) and density functional theory (DFT) calculations further revealed that the electronic effect between Zn/Co enhanced the *COOH intermediate bonding on Zn sites and thus promoted CO production. This work could act as a promising way to reveal the mechanism of neighboring monomers and to influence catalysis.

Crucial Role of Surface Hydroxyls on the Activity and Stability in Electrochemical CO2 Reduction.

The role of surface hydroxyls is significant for understanding catalytic performance of metallic oxides for CO2 electroreduction reaction (CO2ER). This Communication describes, employing SnO x as a model system, how to moderate coverage of hydroxyl to derive a stable Sn branches catalyst for CO2ER with a 93.1% Faradaic efficiency (FE) of carbonaceous products. With use of in situ attenuated total reflection surface enhanced infrared adsorption spectroscopy (ATR-SEIRAS) and density functional theory (DFT) calculations, we found that a proper amount of surface hydroxyls offered effective sites to boost CO2 adsorption via hydrogen bond. However, a higher surface coverage of hydroxyls leads to self-reduction of Sn-OH. We also explained the competition between self-reduction and CO2 reduction over Sn-based catalysts. The findings revealed the quantitative correlation between surface coverage of hydroxyl and CO2ER activity and suggested a logical extension to other metal oxide catalysts for CO2ER.

Ultrathin Pd-Au Shells with Controllable Alloying Degree on Pd Nanocubes toward Carbon Dioxide Reduction.

Electrocatalytic reduction of carbon dioxide (CO2ER) to reusable carbon resources is a significant step to balance the carbon cycle. This Communication describes a seed-mediated growth method to synthesize ultrathin Pd-Au alloy nanoshells with controllable alloying degree on Pd nanocubes. Specifically, Pd@Pd3Au7 nanocrystals (NCs) show superior CO2ER performance, with a 94% CO faraday efficiency (FE) at -0.5 V vs reversible hydrogen electrode and approaching 100% CO FE from -0.6 to -0.9 V. The enhancement primarily originates from ensemble and ligand effects, i.e., appropriately proportional Pd-Au sites and electronic back-donation from Au to Pd. In situ attenuated total reflection infrared spectra and density functional theory calculations clarify the reaction mechanism. This work may offer a general strategy for the synthesis of bimetallic NCs to explore the structure-activity relationship in catalytic reactions.

Abundant Ce3+ Ions in Au-CeOx Nanosheets to Enhance CO2 Electroreduction Performance.

The electroreduction of CO2 to CO provides a potential way to solve the environmental problems caused by excess fossil fuel utilization. Loading transition metals on metal oxides is an efficient strategy for CO2 electroreduction as well as for reducing metal usage. However, it needs a great potential to overcome the energy barrier to increase CO selectivity. This paper describes how 8.7 wt% gold nanoparticles (NPs) loaded on CeOx nanosheets (NSs) with high Ce3+ concentration effectively decrease the overpotential for CO2 electroreduction. The 3.6 nm gold NPs on CeOx NSs containing 47.3% Ce3+ achieve CO faradaic efficiency of 90.1% at -0.5 V in 0.1 m KHCO3 solution. Furthermore, the CO2 electroreduction activity shows a strong relationship with the fractions of Ce3+ on Au-CeOx NSs, which has never been reported. In situ surface-enhanced infrared absorption spectroscopy shows that Au-CeOx NSs with high Ce3+ concentration promote CO2 activation and *COOH formation. Theoretical calculations also indicate that the improved performance is attributed to the enhanced *COOH formation on Au-CeOx NSs with high Ce3+ fraction.

Morphological and Compositional Design of Pd-Cu Bimetallic Nanocatalysts with Controllable Product Selectivity toward CO2 Electroreduction.

Electrochemical conversion of carbon dioxide (electrochemical reduction of carbon dioxide) to value-added products is a promising way to solve CO2 emission problems. This paper describes a facile one-pot approach to synthesize palladium-copper (Pd-Cu) bimetallic catalysts with different structures. Highly efficient performance and tunable product distributions are achieved due to a coordinative function of both enriched low-coordinated sites and composition effects. The concave rhombic dodecahedral Cu3 Pd (CRD-Cu3 Pd) decreases the onset potential for methane (CH4 ) by 200 mV and shows a sevenfold CH4 current density at -1.2 V (vs reversible hydrogen electrode) compared to Cu foil. The flower-like Pd3 Cu (FL-Pd3 Cu) exhibits high faradaic efficiency toward CO in a wide potential range from -0.7 to -1.3 V, and reaches a fourfold CO current density at -1.3 V compared to commercial Pd black. Tafel plots and density functional theory calculations suggest that both the introduction of high-index facets and alloying contribute to the enhanced CH4 current of CRD-Cu3 Pd, while the alloy effect is responsible for high CO selectivity of FL-Pd3 Cu.

Low-Coordinated Edge Sites on Ultrathin Palladium Nanosheets Boost Carbon Dioxide Electroreduction Performance.

Electrochemical conversion of carbon dioxide (CO2 ) to value-added products is a possible way to decrease the problems resulting from CO2 emission. Thanks to the eminent conductivity and proper adsorption to intermediates, Pd has become a promising candidate for CO2 electroreduction (CO2 ER). However, Pd-based nanocatalysts generally need a large overpotential. Herein we describe that ultrathin Pd nanosheets effectively reduce the onset potential for CO by exposing abundant atoms with comparatively low generalized coordination number. Hexagonal Pd nanosheets with 5 atomic thickness and 5.1 nm edge length reached CO faradaic efficiency of 94 % at -0.5 V, without any decay after a stability test of 8 h. It appears to be the most efficient among all of Pd-based catalysts toward CO2 ER. Uniform hexagonal morphology made it reasonable to build models and take DFT calculations. The enhanced activity originates from mainly edge sites on palladium nanosheets.

Selective Electroreduction of Carbon Dioxide over SnO2 -Nanodot Catalysts.

The development of electrochemical CO2 conversion allows green carbon utilization. Formate and syngas are two typical products of electrochemical CO2 reduction, and the coproduction of these two products will maximize the energy efficiency of CO2 conversion. However, few works have successfully achieved the cogeneration of formate and syngas. This paper describes a novel strategy to maximize the efficiency of CO2 conversion through coproduction of formate and syngas on ultrasmall SnO2 nanodots (NDs) homogeneously anchored on carbon nanotubes (CNT#SnO2 NDs) electrodes. The CNT#SnO2 NDs not only decreased the adsorption energy of *OCHO but also reduced the adsorption energy difference of *COOH and *H. High energy efficiency toward formate and adjustable H2 /CO ratio were obtained over a broad potential window with long-term stability. In addition, CNT#SnO2 NDs and Ir foil were coupled together to construct an electrolyzer for electrochemical CO2 reduction reaction and oxygen evolution reaction (CO2 ERR-OER), which also produced formate and syngas with 24 h stability. A promising approach is presented for the electrochemical CO2 conversion in fuel production.

Synergism of Geometric Construction and Electronic Regulation: 3D Se-(NiCo)Sx /(OH)x Nanosheets for Highly Efficient Overall Water Splitting.

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt-based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru-based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large-scale applications. This paper describes the construction of the ultrathin defect-enriched 3D Se-(NiCo)Sx /(OH)x nanosheets for overall water splitting through a facile Se-induced hydrothermal treatment. Via Se-induced fabrication, highly efficient Se-(NiCo)Sx /(OH)x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as-prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm-2 for OER and HER, respectively. Moreover, an overall water-splitting device delivers a current density of 10 mA cm-2 for ≈66 h without obvious degradation.

Edge Sites with Unsaturated Coordination on Core-Shell Mn3 O4 @Mnx Co3-x O4 Nanostructures for Electrocatalytic Water Oxidation.

Transition-metal oxides are extensively investigated as efficient electrocatalysts for the oxygen evolution reaction (OER). However, large-scale applications remain challenging due to their moderate catalytic activity. Optimized regulation of surface states can lead to improvement of catalytic properties. Here, the design of Mn@Cox Mn3-x O4 nanoparticles with abundant edge sites via a simple seed-mediated growth strategy is described. The unsaturated coordination generated on the edge sites of Cox Mn3-x O4 shells makes a positive contribution to the surface-structure tailoring. Density functional theory calculations indicate that the edge sites with unsaturated coordination exhibit intense affinity for OH- in the alkaline electrolyte, which greatly enhances the electrochemical OER performance of the catalysts. The resulting Mn@Cox Mn3-x O4 catalysts yield a current density of 10 mA cm-2 at an overpotential of 246 mV and a relatively low Tafel slope of 46 mV dec-1 . The successful synthesis of these metal oxides nanoparticles with edge sites may pave a new path for rationally fabricating efficient OER catalysts.