Controllable Cu0 -Cu+ Sites for Electrocatalytic Reduction of Carbon Dioxide.

Cu-based electrocatalysts facilitate CO2 electrochemical reduction (CO2 ER) to produce multi-carbon products. However, the roles of Cu0 and Cu+ and the mechanistic understanding remain elusive. This paper describes the controllable construction of Cu0 -Cu+ sites derived from the well-dispersed cupric oxide particles supported on copper phyllosilicate lamella to enhance CO2 ER performance. 20 % Cu/CuSiO3 shows the superior CO2 ER performance with 51.8 % C2 H4 Faraday efficiency at -1.1 V vs reversible hydrogen electrode during the 6 hour test. In situ attenuated total reflection infrared spectra and density functional theory (DFT) calculations were employed to elucidate the reaction mechanism. The enhancement in CO2 ER activity is mainly attributed to the synergism of Cu0 -Cu+ pairs: Cu0 activates CO2 and facilitates the following electron transfers; Cu+ strengthens *CO adsorption to further boost C-C coupling. We provide a strategy to rationally design Cu-based catalysts with viable valence states to boost CO2 ER.

Coupling of Cu(100) and (110) Facets Promotes Carbon Dioxide Conversion to Hydrocarbons and Alcohols.

Copper can efficiently electro-catalyze carbon dioxide reduction to C2+ products (C2 H4 , C2 H5 OH, n-propanol). However, the correlation between the activity and active sites remains ambiguous, impeding further improvements in their performance. The facet effect of copper crystals to promote CO adsorption and C-C coupling and consequently yield a superior selectivity for C2+ products is described. We achieve a high Faradaic efficiency (FE) of 87 % and a large partial current density of 217 mA cm-2 toward C2+ products on Cu(OH)2 -D at only -0.54 V versus the reversible hydrogen electrode in a flow-cell electrolyzer. With further coupled to a Si solar cell, record-high solar conversion efficiencies of 4.47 % and 6.4 % are achieved for C2 H4 and C2+ products, respectively. This study provides an in-depth understanding of the selective formation of C2+ products on Cu and paves the way for the practical application of electrocatalytic or solar-driven CO2 reduction.

Concentrating and activating carbon dioxide over AuCu aerogel grain boundaries.

Alloys are active in CO2 electroreduction due to their unique electronic and geometric structures. Nevertheless, CO2 reduction selectivity is still low due to the low concentration of CO2 near the catalyst surface and the high energy barrier for CO2 activation. This paper describes an AuCu nanochain aerogel (NC-AuCu) with abundant grain boundaries (GBs) that promote the accumulation and activation of CO2 for further electrochemical reduction, employing in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and density functional theory calculations. GBs can induce a strong local electric field to concentrate the electrolyte cations and thus accumulate CO2 near the catalyst surface. NC-AuCu exhibits a superior Faradaic efficiency of close to 100% for CO2 electroreduction to CO at an extremely low overpotential of 110 mV with a high CO partial current density of 28.6 mA cm-2 in a flow cell. Coupling with a Si solar cell to convert solar energy to CO, a very high conversion efficiency of ∼13.0% is achieved. It potentially provides broad interest for further academic research and industry applications.

Shifting the Oxygen-Evolution Reaction Pathway via Cation Engineering to Activate Lattice Oxygen in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) as promising electrocatalysts have been widely studied, but their performance is limited by conductivity and coordinating saturation. This study proposes a cationic (V) modification strategy and evaluates its effect on the electrocatalytic performance of CoFe-MOF nanosheet arrays. The optimal V-CoFe-MOF/NF electrocatalyst exhibits excellent oxygen-evolution reaction (OER)/hydrogen-evolution reaction (HER) performance (231 mV at 100 mA cm-2/86 mV at 10 mA cm-2) in alkaline conditions, with its OER durability exceeding 400 h without evident degradation. Furthermore, as a bifunctional electrocatalyst for water splitting, a small cell voltage is achieved (1.60 V at 10 mA cm-2). The practicability of the catalyst is further evaluated by membrane electrode assembly (MEA), showing outstanding activity (1.53 V at 10 mA cm-2) and long-term stability (at 300 mA cm-2). Moreover, our results reveal the apparent reconstruction properties of V-CoFe-MOF/NF in alkaline electrolytes, where the partially dissolved V promotes the formation of more active ß-MOOH. The mechanism study shows the OER mechanism shifts to a lattice oxygen oxidation mechanism (LOM) after V doping, which directly avoids complex multistep adsorption mechanism and reduces reaction energy. This study provides a cation mediated strategy for designing efficient electrocatalysts.

Cobalt telluride regulated by nickel for efficient electrooxidation of 5-hydroxymethylfurfural.

Replacing the anodic oxygen evolution reaction (OER) in water splitting with 5-hydroxymethylfurfural oxidation reaction (HMFOR) can not only reduce the energy required for hydrogen production but also yield the valuable chemical 2,5-furandicarboxylic acid (FDCA). Co-based catalysts are known to be efficient for HMFOR, with high-valent Co being recognized as the main active component. However, efficiently promoting the oxidation of Co2+ to produce high-valent reactive species remains a challenge. In this study, Ni-doped CoTe (CoNiTe) nanorods were prepared as efficient catalysts for HMFOR, achieving a high HMFOR current density of 65.3 mA cm-2 at 1.50 V. Even after undergoing five successive electrolysis processes, the Faradaic efficiency (FE) remained at approximately 90.7 %, showing robust electrochemical durability. Mechanistic studies indicated that Ni doping changes the electronic configuration of Co, enhancing its charge transfer rate and facilitating the oxidation of Co2+ to high-valent CoO2 species. This work reveals the effect of Ni doping on the reconfiguration of the active phase during HMFOR.

Easy conversion perovskite fluorides KCo1-xFexF3 for efficient oxygen evolution reaction.

Herein, we report an easily oxidized Co-Fe perovskite fluoride as an efficient catalyst for the oxygen evolution reaction (OER). In situ Raman spectroscopy showed that the presence of F promotes reconstruction to form highly active (Co3+Fe3+)OOH, and the current density of 10 mA cm-2 can be achieved at the overpotential of only 118 mV in 1 M KOH aqueous solution. This work helps to understand the role of fluoride during the OER.

Constructing abundant phase interfaces of the sulfides/metal-organic frameworks p-p heterojunction array for efficient overall water splitting and urea electrolysis.

Designing efficient electrocatalysts to improve the overall water splitting and urea electrolysis efficiency for hydrogen generation can greatly solve the dilemma of energy shortage and environmental pollution. In this work, Co8FeS8@CoFe-MOF/NF heterojunction arrays were fabricated by embedding sulfides into the surface of metal-organic frameworks (MOFs) nanosheets as multifunctional electrocatalyst. The introduction of sulfide on CoFe-MOF/NF can not only adjust the electronic structure (electron-rich state) and change the surface properties (more hydrophilic), but also increase the active area to enhance the catalytic activity. The in situ Raman shows Co8FeS8@CoFe-MOF/NF is more easily to generate active species at low potentials and generates a higher content of active ß-MOOH phase than CoFe-MOF/NF. Therefore, the Co8FeS8@CoFe-MOF/NF exhibits excellent oxygen evolution reaction (OER) performance with an overpotential of 213 mV at 10 mA cm-2. Furthermore, when used as a urea oxidation reaction (UOR), only an ultralow potential of 1.311 V at 10 mA cm-2. More importantly, the assembled two-electrode drives overall water splitting and urea electrolysis with cell voltages of 1.62 V and 1.55 V at 10 mA cm-2, respectively. This work provides insights into the preparation of electrocatalysts with multifunctional heterostructure arrays for hydrogen production.

Oxygen-vacancy-rich Co3O4@Fe-B-O heterostructure for efficient oxygen evolution reaction in alkaline and neutral media.

Developing highly efficient OER catalysts is essential for producing hydrogen from water electrolysis to compensate for conventional fossil fuel shortages. Here, the oxygen-vacancy-rich heterostructure grown on the Ni foam (NF) (Co3O4@Fe-B-O/NF) is fabricated. The synergistic effect between Co3O4 and Fe-B-O has been proven effectively modulate the electronic structure and produce highly active interface sites, ultimately leading to enhanced electrocatalytic activity. Co3O4@Fe-B-O/NFrequiresan overpotential of 237 mV to drive 20 mA cm-2 in 1 M KOH, and 384 mV to drive 10 mA cm-2 in 0.1 M PBS, superior to most catalysts currently used. Moreover, Co3O4@Fe-B-O/NF as an oxygen evolution reaction (OER) electrode shows great potential in overall water splitting and CO2 reduction reaction (CO2RR). This work may provide effective ideas for designing efficient oxide catalysts.

Amorphous CoV Phosphate Nanosheets as Efficient Oxygen Evolution Electrocatalyst.

The oxygen evolution reaction (OER) is crucial for hydrogen production. However, OER with four-electron transfer requires electrocatalysts to speed up its sluggish kinetics in alkaline solutions. Herein, amorphous CoV phosphate (denoted as CoV-Pi) nanosheets synthesized by a straightforward one-step hydrothermal approach is reported, which provide a low overpotential of 320 mV at 10 mA cm-2 , a small Tafel slope down to 48.8 mV dec-1 and long-term durability over 80 h. The efficient activity is ascribed to the amorphous nanosheets structure, high electrochemically active surface area, enhanced surface wettability and the synergistic effect of the active metal atoms. This study significantly indicates that CoV-Pi is a promising alternative to replace expensive noble metal-based catalysts for electrochemical water splitting.

Roughness Effect of Cu on Electrocatalytic CO2 Reduction towards C2 H4.

Electrochemical reduction of CO2 to produce valuable multi-carbon products is a promising avenue for promoting CO2 conversion and achieving renewable energy storage, and it has also attracted considerable attention recently. However, the synthesis of Cu electrode with a controllable electrochemical active surface area (ECSA) to understand its role in CO2 reduction to C2 H4 remains challenging. Herein, a series of Cu electrodes with different ECSA is synthesized through a simple oxidation-reduction approach. We reveal that the improved selectivity of C2 H4 is proportional to the ECSA of Cu in the low ECSA range, and a further increase in ECSA has a negligible effect on its selectivity. The enlarged surface area could strengthen the local pH effect near the surface of Cu electrode and suppress the generation of C1 products as well as H2 . The study provides a feasible strategy to rationally design electrocatalysts with high electrochemical CO2 reduction performances.

Three-dimensional self-supporting catalyst with NiFe alloy/oxyhydroxide supported on high-surface cobalt hydroxide nanosheet array for overall water splitting.

The development of available dual-function electrocatalysts is of great significance to the effective storage of excess electricity. Here, we obtained a three-dimensional Co(OH)2 nanosheet with high surface area on nickel foam (Co(OH)2/NF) via conventional hydrothermal. NiFe-coated Co(OH)2 nanosheet array (NiFe@Co(OH)2 NSAs/NF) was further constructed by electrodeposition for water splitting. By optimizing and regulating the deposition time, NiFe@Co(OH)2 NSAs/NF with a deposition time of 500 s (NiFe-500@Co(OH)2 NSAs/NF) only needs 98 mV of overpotential and can be stabilized for 100 h for hydrogen evolution at 10 mA cm-2 due to the rich density active components for NiFe alloy/oxyhydroxide layer and interaction with Co(OH)2 nanosheets. Thanks to the excellent 3D nanosheet array structure and the close integration between Co(OH)2 and the upper layer NiFe, NiFe@Co(OH)2 NSAs/NF with a deposition time of 200 s (NiFe-200@Co(OH)2 NSAs/NF) can provide 10 mA cm-2 with only 204 mV and maintain constant catalysis within 100 h. Therefore, the constructed NiFe@Co(OH)2 NSAs/NF (500||200) double-electrode cell for water splitting requires only 1.58 V drive potential and can maintain 24 h durability at 10 mA cm-2. The design of the catalyst opens up new ideas for the large-scale application of transition metals in water splitting.

Back-illuminated photoelectrochemical flow cell for efficient CO2 reduction.

Photoelectrochemical CO2 reduction reaction flow cells are promising devices to meet the requirements to produce solar fuels at the industrial scale. Photoelectrodes with wide bandgaps do not allow for efficient CO2 reduction at high current densities, while the integration of opaque photoelectrodes with narrow bandgaps in flow cell configurations still remains a challenge. This paper describes the design and fabrication of a back-illuminated Si photoanode promoted PEC flow cell for CO2 reduction reaction. The illumination area and catalytic sites of the Si photoelectrode are decoupled, owing to the effective passivation of defect states that allows for the long minority carrier diffusion length, that surpasses the thickness of the Si substrate. Hence, a solar-to-fuel conversion efficiency of CO of 2.42% and a Faradaic efficiency of 90% using Ag catalysts are achieved. For CO2 to C2+ products, the Faradaic efficiency of 53% and solar-to-fuel of 0.29% are achieved using Cu catalyst in flow cell.

Preparation of a Dual-MOF Heterostructure (ZIF@MIL) for Enhanced Oxygen Evolution Reaction Activity.

Although metal-organic frameworks have proven to be excellent electrocatalytic materials, their application as electrode materials remains limited. The preparation of heterostructures is considered an effective method to improve catalytic activity. Herein, we describe the design and assembly of a dual-MOF heterostructure (CoNi-ZIF-67@Fe-MIL-100, denoted ZIF@MIL). Specifically, we grew a layer of MIL-100 in situ on a bimetallic ZIF-67 surface using a solvothermal method. We demonstrate that the ZIF@MIL has remarkable OER electrocatalytic performance, requiring a low overpotential and showing a small Tafel slope, compared to pure ZIF-67 and MIL-100 in 1.0 m KOH. More importantly, it has excellent operational durability for 50 h at 100 mA cm-2 . The high catalytic activity of ZIF@MIL can be attributed to the heterostructure that can expose more active sites, the synergistic effect between ZIF-67 and MIL-100, and improvement of electron transfer ability. Our work provides a new way to design and prepare dual-MOF crystals with different structures as electrocatalysts.

Loading FeOOH on Ni(OH)2 hollow nanorods to obtain a three-dimensional sandwich catalyst with strong electron interactions for an efficient oxygen evolution reaction.

Sustainable production of hydrogen by water splitting requires the exploration of highly efficient electrocatalysts from abundant non-precious metals on Earth. Ni(OH)2 hollow nanorod arrays were obtained on Ni foam by simple alkali etching, and FeOOH was electrodeposited on the walls of hollow nanorods to construct FeOOH@Ni(OH)2 sandwich hollow nanorod arrays, which help overcome the drawbacks of the poor conductivity and poor stability of FeOOH and boost the catalytic performance of the oxygen evolution reaction (OER) in comparison with the individual components. A fully contacted three-dimensional nanorod array structure provides many exposed catalytically active sites and promotes charge transfer during the electrochemical OER process. The presence of FeOOH can promote the formation of a more conductive catalytically active component, NiOOH, which improves the catalytic performance of Ni(OH)2. The electronic interaction and synergistic catalysis between nickel and iron enhances the electrochemical performance of the catalyst significantly. The optimized FeOOH@Ni(OH)2 sandwich hollow nanorod arrays show an outstanding OER activity with a small overpotential of 245 mV at 50 mA cm-2 and a low Tafel slope of 45 mV dec-1. The catalyst can maintain a substantially constant voltage over 40 h in 1.0 M KOH solution. Our work provides a new strategy to prepare Ni-Fe bimetallic materials as OER electrocatalysts.