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Ni-based materials are auspicious electrocatalysts for 5-hydroxymethylfurfural oxidation reaction (HMFOR), including the adsorption and conversion of HMF and OH

Aqueous organic electrosynthesis such as nucleophile oxidation reaction (NOR) is an economical and green approach. However, its development has been hindered by the inadequate understanding of the synergy between the electrochemical and non-electrochemical steps. In this study, we unravel the NOR mechanism for the primary alcohol/vicinal diol electrooxidation on NiO. Thereinto, the electrochemical step is the generation of Ni3+-(OH)ads, and the spontaneous reaction between Ni3+-(OH)ads and nucleophiles is an electrocatalyst-induced non-electrochemical step. We identify that two electrophilic oxygen-mediated mechanisms (EOMs), EOM involving hydrogen atom transfer (HAT) and EOM involving C-C bond cleavage, play pivotal roles in the electrooxidation of primary alcohol to carboxylic acid and the electrooxidation of vicinal diol to carboxylic acid and formic acid, respectively. Based on these findings, we establish a unified NOR mechanism for alcohol electrooxidation and deepen the understanding of the synergy between the electrochemical and non-electrochemical steps during NOR, which can guide the sustainable electrochemical synthesis of organic chemicals.

The electrocatalytic upgrade of biomass-derived platform molecules (alcohols, furans and carboxylic acids) into high-value-added chemicals and fuels is expected to compensate for traditional organic synthesis based on nonrenewable fossil resources.

Electro-oxidation of 5-hydroxymethylfurfural (HMFOR) is a promising green approach to realize the conversion of biomass into value-added chemicals. However, considering the complexity of the molecular structure of HMF, an in-depth understanding of the electrocatalytic behavior of HMFOR has rarely been investigated. Herein, the electrocatalytic mechanism of HMFOR on nickel nitride (Ni3N) is elucidated by operando X-ray absorption spectroscopy (XAS), in situ Raman, quasi in situ X-ray photoelectron spectroscopy (XPS), and operando electrochemical impedance spectroscopy (EIS), respectively. The activity origin is proved to be Ni2+δN(OH)ads generated by the adsorbed hydroxyl group. Moreover, HMFOR on Ni3N relates to a two-step reaction: Initially, the applied potential drives Ni atoms to lose electrons and adsorb OH− after 1.35 VRHE, giving rise to Ni2+δN(OH)ads with the electrophilic oxygen; then Ni2+δN(OH)ads seizes protons and electrons from HMF and leaves as H2O spontaneously. Furthermore, the high electrolyte alkalinity favors the HMFOR process due to the increased active species (Ni2+δN(OH)ads) and the enhanced adsorption of HMF on the Ni3N surface. This work could provide an in-depth understanding of the electrocatalytic mechanism of HMFOR on Ni3N and demonstrate the alkalinity effect of the electrolyte on the electrocatalytic performance of HMFOR.

Nickel hydroxide (Ni(OH)2 ) is a promising electrocatalyst for the 5-hydroxymethylfurfural oxidation reaction (HMFOR) and the dehydronated intermediates Ni(OH)O species are proved to be active sites for HMFOR. In this study, Ni(OH)2 is modified by platinum to adjust the electronic structure and the current density of HMFOR improves 8.2 times at the Pt/Ni(OH)2 electrode compared with that on Ni(OH)2 electrode. Operando methods reveal that the introduction of Pt optimized the redox property of Ni(OH)2 and accelerate the formation of Ni(OH)O during the catalytic process. Theoretical studies demonstrate that the enhanced Ni(OH)O formation kinetics originates from the reduced dehydrogenation energy of Ni(OH)2 . The product analysis and transition state simulation prove that the Pt also reduces adsorption energy of HMF with optimized adsorption behavior as Pt can act as the adsorption site of HMF. Overall, this work here provides a strategy to design an efficient and universal nickel-based catalyst for HMF electro-oxidation, which can also be extended to other Ni-based catalysts such as Ni(HCO3 )2 and NiO.

Electrochemical hydrogenation (ECH) of biomass-derived platform molecules is a burgeoning route for the sustainable utilization of hydrogen. However, the noble-metal-catalyzed ECH of phenolic compounds suffers from intense competition with hydrogen evolution reaction. We prepared PtRh bimetallic nanoparticles dispersed on highly ordered mesoporous carbon nanospheres, which improves the utilization efficiency of adsorbed hydrogen (Had) to ECH in H–UPD region (>0 V vs. RHE). Further analysis reveals (i) the strong overlapping between the d-orbitals of Pt and Rh enhances specific adsorption of phenol; (ii) incorporation of Rh devotes an electronic effect on weakening the alloy–Had interaction to increase the FE of ECH. DFT calculations confirm the selectivity difference and the ECH parallel pathways: cyclohexanol and cyclohexanone are formed via hydrogenation/dehydrogenation of the intermediate *C6H10OH. These findings deepen our fundamental understanding of the ECH process, and cast new light on exploration of highly efficient electrocatalysts for biomass upgrading.

High-entropy oxides (HEOs), a new concept of entropy stabilization, exhibit unique structures and fascinating properties, and are thus important class of materials with significant technological potential. However, the conventional high-temperature synthesis techniques tend to afford micron-scale HEOs with low surface area, and the catalytic activity of available HEOs is still far from satisfactory because of their limited exposed active sites and poor intrinsic activity. Here we report a low-temperature plasma strategy for preparing defect-rich HEOs nanosheets with high surface area, and for the first time employ them for 5-hydroxymethylfurfural (HMF) electrooxidation. Owing to the nanosheets structure, abundant oxygen vacancies, and high surface area, the quinary (FeCrCoNiCu)3 O4 nanosheets deliver improved activity for HMF oxidation with lower onset potential and faster kinetics, outperforming that of HEOs prepared by high-temperature method. Our method opens new opportunities for synthesizing nanostructured HEOs with great potential applications.

Electronic regulation of carbon is essential for developing non-platinum electrocatalysts for oxygen reduction reactions (ORRs). In this work, we used Cs to further regulate the electronic structure of nitrogen-doped (N-doped) carbon. The Cs atoms coordinated with the nitrogen atom in the N-doped carbon for injecting electrons into the carbon conjugate structure and reducing the work function of the carbon network. The low-work-function surface improved electron donation, facilitated O2 dissociation, and enhanced the adsorption of an OOH* intermediate. Thus, electrocatalytic performance for the ORR was improved. The material shows potential as an ORR electrocatalyst comparable with Pt/C.

Single-atom catalysts (SACs) have enormous significance in heterogeneous catalysis. However, understanding how SACs function at the molecular level remains a huge challenge. Here, we report a general approach to anchor Pt single-atom intercalated in layered double hydroxide (LDH) and decipher the alternating synergy between Pt single-atom and Ni3Fe LDH support for overall water splitting. Aided with Tafel slope, interface species evolution and control experiments, operando electrochemical impedance spectroscopy (EIS) can distinguish interface charge transport and elementary reactions during hydrogen and oxygen evolution reactions (HER and OER). For HER, interlayer Pt single-atom vastly enhances electron transferability of LDH support, and Ni3Fe LDH support accelerates water dissociation, thus resulting in a mixture of mechanisms (Heyrovsky–Volmer and Tafel–Volmer) in 1 M KOH. For OER, interlayer Pt single-atom not only prompts active phase transition from NiFe LDH to Ni2+δFe3+ζOxHy, but also optimizes OER intrinsic activity of Ni2+δ–O–Fe3+ζ in Ni2+δFe3+ζOxHy. Overall, we provide a referential paradigm for SACs synthesis strategy and unscrambling its alternating synergy.

The nucleophile oxidation reaction (NOR) is of enormous significance for organic electrosynthesis and coupling for hydrogen generation. However, the nonuniform NOR mechanism limits its development. For the NOR, involving electrocatalysis and organic chemistry, both the electrochemical step and non-electrochemical process should be taken into account. The NOR of nickel-based hydroxides includes the electrogenerated dehydrogenation of the Ni

Perovskite solar cells (PSCs) have been developed as a promising photovoltaic technology because of their excellent photovoltaic performance. However, interfacial recombination and charge carrier transport losses at the surface greatly limit the performance and stability of PSCs. In this work, the fabrication of high-quality PSCs based on methylammonium lead iodide with excellent ambient stability is reported. An anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), is introduced to simultaneously passivate the defect states and stabilize the cubic phase of the perovskite film. The SDBS located at grain boundaries and the surface of the active layer can effectively passivate under-coordinated lead ions and protect the perovskite components from water-induced degradation. As a result, a champion power conversion efficiency (PCE) of 19.42% is achieved with an open-circuit voltage (VOC) of 1.12 V, a short-circuit current (JSC) of 23.23 mA cm-2, and a fill factor (FF) of 74% in combination with superior moisture stability. The SDBS-passivated devices retain 80% of their initial average PCE after 2112 h of storage under ambient conditions.

Oxygen evolution reaction (OER) is a bottleneck process for many electrochemical devices due to the sluggish kinetics, for which advanced electrocatalysts should be carefully designed. Nickle-based materials have been extensively studied to catalyze OER. However, their performances are still below the expectation and the active sites are often controversial. Herein, we have successfully modulated the electronic and surface properties of layered •-Ni(OH)2 by the interlayer ligand engineering, aiming to design novel efficient electrocatalysts and unveil the catalysis mechanism. By one-step solvothermal reaction, alkoxyl substituted •-Ni(OH)2 with variable interlayer distances is obtained, and the ethoxyl substituted one (NiEt) shows great potential for efficient OER. With the assistance of powder X-ray diffraction and crystalline structure computational simulation, the formula of alkoxyl substituted •-Ni(OH)2 are determined. Operando X-ray absorption spectroscopy studies combined with ex-situ analyses revealed that the critical active species of NiEt is formed via hydroxylation and subsequent de-protonation, with high valent Niδ+ (3

Heterogeneous catalysis plays an important role in modern industries. Exploring catalysts with high efficiency, low-cost and high stability is an important issue for the research of heterogeneous c...

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure-activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO-Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co-OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

Ni-based electrocatalysts with strong redox abilities are active for the electrochemical oxidation of 5-hydroxymethylfurfural (HMF). Interface engineering is an efficient way to modulate the electronic structure, tune the intermediate adsorption, and expose more active sites. Herein, we increased the concentration of interfacial sites with rich defects in a 3D hierarchical nanostructured NiO-Co3O4 electrocatalyst and investigated its catalytic performance for HMF electro-oxidation. The interface effect created abundant cation vacancies, modulated the electronic properties of Co and Ni atoms, and raised the oxidation state of Ni species. The NiO-Co3O4 catalysts show superb HMF oxidation activities with a low onset potential of 1.28 VRHE. Meanwhile, in-situ surface-selective vibrational spectroscopy of sum-frequency generation was performed to study the reaction pathway during the oxidation process on the electrocatalysts. The current study offers an efficient way to create cation vacancies and proves the decisive role of cation vacancies in catalyzing the HMF electro-oxidation.

MgCo2 O4 , CoCr2 O4 , and Co2 TiO4 were selected, where only Co3+ in the center of octahedron (Oh), Co2+ in the center of tetrahedron (Td), and Co2+ in the center of Oh, can be active sites for the oxygen evolution reaction (OER). Co3+ (Oh) sites are the best geometrical configuration for OER. Co2+ (Oh) sites exhibit better activity than Co2+ (Td). Calculations demonstrate the conversion of O* into OOH* is the rate-determining step for Co3+ (Oh) and Co2+ (Td). For Co2+ (Oh), it is thermodynamically favorable for the formation of OOH* but difficult for the desorption of O2 . Co3+ (Oh) needs to increase the lowest Gibbs free energy over Co2+ (Oh) and Co2+ (Td), which contributes to the best activity. The coexistence of Co3+ (Oh) and Co2+ (Td) in Co3 O4 can promote the formation of OOH* and decrease the free-energy barrier. This work screens out the optimal geometrical configuration of cobalt cations for OER and gives a valuable principle to design efficient electrocatalysts.

Integrating biomass upgrading and hydrogen production in an electrocatalytic system is attractive both environmentally and in terms of sustainability. Conventional electrolyser systems coupling anodic biosubstrate electrooxidation with hydrogen evolution reaction usually require electricity input. Herein, we describe the development of an electrocatalytic system for simultaneous biomass upgrading, hydrogen production, and electricity generation. In contrast to conventional furfural electrooxidation, the employed low-potential furfural oxidation enabled the hydrogen atom of the aldehyde group to be released as gaseous hydrogen at the anode at a low potential of approximately 0 V

The exploitation of highly efficient, low-cost, and stable electrocatalysts is a key issue of the broad application of green electrocatalytic reactions and efficient energy devices. Recently, modulating the surface structure of electrocatalysts to improve the catalytic activity has attracted a lot of attention. In particular, defect engineering is an important strategy to modulate the surface electronic structure of electrocatalysts. In this review, an overview of defects in metal, carbon materials, transition metal compounds, and defect-decorated catalysts is presented. The defect species, synthesis methods, characterization, and essential defect catalytic mechanism are introduced. Notably, tuning electronic structure to modulate the intermediates’ adsorption energy is highlighted throughout the review. Finally, the design principles for defect electrocatalysts are proposed. The in-depth understanding of the structure–reactivity relationship will provide more profound guidance for the design of defect electrocatalysts and potential application in energy conversion and green synthesis.

Oxygen evolution reactions (OERs) as core components of energy conversion and storage technology systems, such as water splitting and rechargeable metal–air batteries, have attracted considerable attention in recent years. Transition metal compounds, particularly layered double hydroxides (LDHs), are considered as the most promising electrocatalysts owing to their unique two-dimensional layer structures and tunable components. However, heir poor intrinsic electrical conductivities and the limited number of active sites hinder their performances. The regulation of the electronic structure is an effective approach to improve the OER activity of LDHs, including cationic and anionic regulation, defect engineering, regulation of intercalated anions, and surface modifications. In this review, we summarize recent advances in the regulation of the electronic structures of LDHs used as electrocatalysts in OERs. In addition, we discuss the effects of each regulation type on OER activities. This review is expected to shed light on the development and design of effective OER electrocatalysts by summarizing various electronic structure regulation pathways and the effects on their catalytic performances.

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) provides an efficient way to obtain high-value-added biomass-derived chemicals. Compared with other transition metal oxides, CuO exhibits poor oxygen evolution reaction performance, leading to high Faraday efficiency for HMF oxidation. However, the weak adsorption and activation ability of CuO to OH

Nickel hydroxide (Ni(OH)

The electrooxidation of 5-hydroxymethylfurfural (HMF) offers a promising green route to attain high-value chemicals from biomass. The HMF electrooxidation reaction (HMFOR) is a complicated process involving the combined adsorption and coupling of organic molecules and OH- on the electrode surface. An in-depth understanding of these cooperative adsorption behaviors and reaction processes is fundamentally essential. Herein, the adsorption behavior of HMF and OH-, and the role of oxygen vacancy on Co3O4 are initially unraveled. Correspondingly, instead of the competitive adsorption of OH- and HMF on the metal sites, it is observed that the OH- could fill into oxygen vacancy (Vo) before couple with organic molecules through the lattice oxygen oxidation reaction process, which could accelerate the rate-determining step of the dehydrogenation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and enhance the overall conversion of HMF on Vo-Co3O4. This work sheds a depth insight on the catalytic mechanism of oxygen vacancy, which benefits designing a novel strategy to modulate the multi-molecules combined adsorption behaviors.