In situ growth of M-MO (M = Ni, Co) in 3D graphene as a competent bifunctional electrocatalyst for OER and HER

Enhanced Hydrogen Evolution Reaction Using Biomass-Activated Carbon Nanosheets Derived from Eucalyptus Leaves

Carbonaceous-based metal-free catalysts are promising aspirants for effective electrocatalytic hydrogen generation. Herein, we synthesized mesoporous-activated carbon nanosheets (ELC) from biomass eucalyptus leaves through KOH activation. The microstructure, structural, and textural characteristics of the prepared materials were characterized by FE-SEM, Raman, XRD, and BET measurements. The high temperature (700 °C) KOH-activated ELC nanosheets exhibited an interconnected nanosheet morphology with a large specific surface area (1436 m2/g) and high mesoporosity. The ELC-700 catalyst exhibited an excellent electrocatalytic HER performance with a low overpotential (39 mV at 10 mA/cm2), excellent durability, and a Trivial Tafel slope (36 mV/dec) in 0.5 M H2SO4 electrolyte. These findings indicate a new approach for developing excellent biomass-derived electrocatalysts for substantially efficient green hydrogen production.

Improving the Oxygen Evolution Reaction Performance of Ternary Layered Double Hydroxides by Tuning All Three Cations' Electronic Structures

The pursuit of efficient and sustainable hydrogen production is essential in the fight against climate change. One important method for achieving this is the electrolysis of water, particularly through the oxygen evolution reaction (OER). Recent studies indicate that trimetallic layered double hydroxides (LDHs) can enhance OER performance compared to bimetallic LDHs. This improvement occurs because the third cation alters the electronic structures of the other two cations, thereby increasing the intermediates' binding energies and enhancing electrical conductivity. This study proposes an approach enabling the modulation of the electronic structures of all three cations involved in the synthesis of the trimetallic LDHs. It suggested intercalating sodium dodecyl sulfate (SDS) into the interlayer of the trimetallic NiFe-La-LDH. A successful intercalation of SDS has been confirmed through the XRD, FT-IR, EDS, and XPS. This has expanded the interlayer distance which was beneficial for the electrical conductivity. Furthermore, SDS generated sulphur, which modulated the electronic structures of all three cations enriching the active sites and improving electrical conductivity and OER performance compared to its counterparts. This approach is beneficial: 1. The interlayer can be further enlarged by using different doping ratios of SDS. 2. Sulphur can enrich the active sites and improve the OER performance.

Metal Doping Regulates Electrocatalysts Restructuring During Oxygen Evolution Reaction

High-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen, which is a clean fuel for sustainable energy conversion and storage. Among the emerging OER catalysts, transition metal dichalcogenides have exhibited superior activity compared to commercial standards such as RuO2, but inferior stability due to uncontrolled restructuring with OER. In this study, we create bimetallic sulfide catalysts by adapting the atomic ratio of Ni and Co in CoxNi1-xSy electrocatalysts to investigate the intricate restructuring processes. Surface-sensitive X-ray photoelectron spectroscopy and bulk-sensitive X-ray absorption spectroscopy confirmed the favorable restructuring of transition metal sulfide material following OER processes. Our results indicate that a small amount of Ni substitution can reshape the Co local electronic structure, which regulates the restructuring process to optimize the balance between OER activity and stability. This work represents a significant advancement in the development of efficient and noble metal-free OER electrocatalysts through a doping-regulated restructuring approach.

Ni3+-Rich Ni/NiOx@C Nanocapsules Below 4 nm Constructed by Low-Temperature Graphitization of Self-Assembled Few-Layer Coordination Polymers toward Efficient Alkaline Hydrogen Evolution Electrocatalysis

Low-cost and eco-friendly Ni/NiO heterojunctions have been theoretically proven to be the ideal candidate for stepwise electrocatalysis of alkaline hydrogen evolution reaction, attributed to the preferred OHad adsorption by incompletely filled d orbitals of NiO phase and favorable Had adsorption energy of Ni phase. Nevertheless, most Ni/NiO compounds reported so far fail to exhibit excellent catalytic activity, possibly due to the lack of efficient electron transport, limited interfacial active sites, and unregulated Nin+ ratios. To address the above bottlenecks, herein, the ultrasmall Ni/NiOx@C nanocapsules (<5 nm) are directly constructed by graphitization of four-layer Ni-based coordination polymers at record low temperatures of 400 °C. Ascribed to the accelerated electron and mass transfer by the carbon nano-onions coated around Ni/NiOx heterojunctions, the extreme rise in interfaces and Ni3+ defects with t6 2ge1 g electronic configuration owed to the ultrasmall size, the Ni/NiOx@C nanocapsules exhibit the highest catalytic activity and the lowest overpotential of η10 = 80 mV among various Ni/NiO materials (measured on the glassy carbon electrode). This work not only constructs an industrialized high-efficiency electrocatalyst toward alkaline HER, but also provides a novel strategy for the constant-scale preparation of multicomponent transition metals-based nanocrystals below 4 nm.

Ultrathin, large area β-Ni(OH)2 crystalline nanosheet as bifunctional electrode material for charge storage and oxygen evolution reaction

Bifunctional electrode materials are highly desirable for meeting increasing global energy demands and mitigating environmental impact. However, improving the atom-efficiency, scalability, and cost-effectiveness of storage systems, as well as optimizing conversion processes to enhance overall energy utilization and sustainability, remains a significant challenge for their application. Herein, we devised an optimized, facile, economic, and scalable synthesis of large area (cm2), ultrathin (∼2.9 ± 0.3 nm) electroactive nanosheet of β-Ni(OH)2, which acted as bifunctional electrode material for charge storage and oxygen evolution reaction (OER). The β-Ni(OH)2 nanosheet electrode shows the volumetric capacity of 2.82 Ah.cm-3(0.82 µAh.cm-2) at the current density of 0.2 mA.cm-2. The device shows a high capacity of 820 mAh.cm-3 with an ultrahigh volumetric energy density of 0.33 Wh.cm-3 at 275.86 W.cm-3 along with promising stability (30,000 cycles). Furthermore, the OER activity of ultrathin β-Ni(OH)2 exhibits an overpotential (η10) of 308 mV and a Tafel value of 42 mV dec-1 suggesting fast reaction kinetics. The mechanistic studies are enlightened through density functional theory (DFT), which reveals that additional electronic states near the Fermi level enhance activity for both capacitance and OER.

Iron-Cobalt Nanoparticles Embedded in B,N-Doped Chitosan-Derived Porous Carbon Aerogel for Overall Water Splitting

Given their intriguing properties, porous carbons have surfaced as promising electrocatalysts for various energy conversion reactions. This study presents a unique approach where iron-cobalt (FeCo) is confined in a boron, nitrogen-doped chitosan-derived porous carbon aerogel (BNPC-FeCo) to serve as an electrocatalyst for the hydrogen evolution and oxygen evolution reactions (HER and OER). The BNPC-FeCo-900 electrocatalyst demonstrates excellent catalyst activity, with very low overpotentials of 186 and 320 mV at 10 mA cm-2, low Tafel slopes of 82 and 55 mV dec-1, and low charge transfer resistance of 2.68 and 9.25 Ω for HER and OER, respectively. Density functional theory (DFT) calculations further reveal that the cooperation between the boron, nitrogen codoped porous carbon, and the FeCo nanoparticles reduces intermediates' energy barriers, significantly enhancing the HER and OER performance. In conclusion, this work offers significant and informative perspectives into the potential of porous carbon materials as dual-purpose electrocatalysts for water splitting.

The mechanism of enhanced electrocatalytic water splitting on S-doped NiFe2O4/Ni-Fe alloy@copper foams

The development of bifunctional catalysts with subtle structures, high efficiencies, and good durabilities for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial for overall water splitting. In this work, a multicomponent S-doped NiFe2O4/Ni-Fe micro nano flower electrocatalyst was synthesized rapidly on foam copper using a simple one-step constant current electrodeposition method. The introduction of S leads to the transformation of the microsphere structure of the Ni-Fe alloy into a cauliflower-like morphology and induces changes in the surface electronic structure, significantly enhancing the catalytic performance for the HER and OER. The S-NiFe2O4/Ni-Fe alloy/CF showed low overpotentials of 220 and 66 mV at 10 mA cm-2in 1.0 M KOH for the OER and HER, respectively. High durability OER and HER performances were demonstrated through 60 h of chronopotentiometry and 6000 CV cycles test. Excellent overall water splitting electrocatalytic activity was observed in the S-NiFe2O4/Ni-Fe alloy/CF‖S-NiFe2O4/Ni-Fe alloy/CF two-electrode system. In particular, active-phase NiOOH, a highly active substance for OER, can be controllably formed in the reaction process owing to the nanoflower structure of multi-layer sulfur which slows down the dissolution of NiFe2O4/Ni-Fe alloy. These results suggest that this composite structure is a promising bifunctional electrocatalyst.

Facile synthesis of nanostructured Ni/NiO/N-doped graphene electrocatalysts for enhanced oxygen evolution reaction

Electrocatalysts containing a Ni/NiO/N-doped graphene interface have been synthesised using the ligand-assisted chemical vapor deposition technique. NiO nanoparticles were used as the substrate to grow N-doped graphene by decomposing vapours of benzene and N-containing ligands. The method was demonstrated with two nitrogen-containing ligands, namely dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (L) and melamine (M). The structure and composition of the as-synthesized composites were characterized by XRD, Raman spectroscopy, SEM, TEM and XPS. The composite prepared using the ligand L had NiO sandwiched between Ni and N-doped graphene and showed an overpotential of 292 mV at 10 mA cm-2 and a Tafel slope of 45.41 mV dec-1 for the OER, which is comparable to the existing noble metal catalysts. The composite prepared using the ligand M had Ni encapsulated by N-doped graphene without NiO. It showed an overpotential of 390 mV at 10 mA cm-2 and a Tafel slope of 78.9 mV dec-1. The ligand-assisted CVD route demonstrates a facile route to control the microstructure of the electrocatalysts.

Transition metal-based electrocatalysts for alkaline overall water splitting: advancements, challenges, and perspectives

Water electrolysis is a promising method for efficiently producing hydrogen and oxygen, crucial for renewable energy conversion and fuel cell technologies. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two key electrocatalytic reactions occurring during water splitting, necessitating the development of active, stable, and low-cost electrocatalysts. Transition metal (TM)-based electrocatalysts, spanning noble metals and TM oxides, phosphides, nitrides, carbides, borides, chalcogenides, and dichalcogenides, have garnered significant attention due to their outstanding characteristics, including high electronic conductivity, tunable valence electron configuration, high stability, and cost-effectiveness. This timely review discusses developments in TM-based electrocatalysts for the HER and OER in alkaline media in the last 10 years, revealing that the exposure of more accessible surface-active sites, specific electronic effects, and string effects are essential for the development of efficient electrocatalysts towards electrochemical water splitting application. This comprehensive review serves as a guide for designing and constructing state-of-the-art, high-performance bifunctional electrocatalysts based on TMs, particularly for applications in water splitting.

Elucidating the performance of hexamethylene tetra-amine interlinked bimetallic NiCo-MOF for efficient electrochemical hydrogen and oxygen evolution

Bimetallic metal-organic frameworks (MOFs) play a significant role in the electrocatalysis of water due to their large surface area and availability of increased numbers of pores. For the inaugural time, we examine the effectiveness of a hexamethylene tetra-amine (HMT)-induced 3D NiCo-MOF-based nanostructure as a potent bifunctional electrocatalyst with superior performance for overall water splitting in alkaline environments. The structural, morphological, and electrochemical properties of the as-synthesized bifunctional catalyst were examined thoroughly before analyzing its behavior towards electrochemical water splitting. The HMT-based NiCo-MOF demonstrated small overpotential values of 274 mV and 330 mV in reaching a maximum current density of 30 mA cm-2 for hydrogen and oxygen evolution mechanisms, respectively. The Tafel parameter also showed favorable HER/OER reaction kinetics, with slopes of 78 mV dec-1 and 86 mV dec-1 determined during the electrochemical evaluation. Remarkably, the NiCo-HMT electrode exhibited a double-layer capacitance of 4 mF cm-2 for hydrogen evolution and 23 mF cm-2 for oxygen evolution, while maintaining remarkable stability even after continuous operation for 20 hours. This research offers a valuable blueprint for implementing a cost-effective and durable MOF-based bifunctional catalytic system that has proven to be effective for complete water splitting. Decomposition of water under higher current densities is crucial for effective long-term generation and commercial consumption of hydrogen.

Metal-organic framework derived transition metal sulfides grown on carbon nanofibers as self-supported catalysts for hydrogen evolution reaction

Metal-organic framework (MOF) derived transition metal-based electrocatalysts have received great attention as substitutes for noble metal-based hydrogen evolution catalysts. However, the low conductivity and easy detachments from electrodes of raw MOF have seriously hindered their applications in hydrogen evolution reaction. Herein, we report the facile preparation of Co-NSC@CBC84, a porous carbon-based and self-supported catalyst containing Co9S8 active species, by pyrolysis and sulfidation of in-situ grown ZIF-67 on polydopamine-modified biomass bacterial cellulose (PDA/BC). As a binder-free and self-supported electrocatalyst, Co-NSC@CBC84 exhibits superior electrocatalytic properties to other reported cobalt-based sulfide catalytic materials and has good stability in 0.5 M H2SO4 electrolyte. At the current density of 10 mA cm-2, only an overpotential of 138 mV was required, corresponding to a Tafel slope of 123 mV dec-1, owing to the strong synergy effect between Co-NSC nanoparticles and CBC substrate. This work therefore provides a feasible approach to prepare self-supported transition metal sulfides as HER catalysts, which is helpful for the development of noble metal-free catalysts and biomass carbon materials.

Balance between Activity and Stability of Single Metal and Intermetallic Compounds for Electrocatalytic Hydrogen Evolution Reaction

The higher population of the antibonding state around the Fermi level will result in better activity yet lower stability of HER (Re vs Ru metal). There seems to be a limitation or balance for using a single metal since the bonding scheme of a single metal is relatively simple. Combining Re (strong bonding), Ru (HER active), and Zr metal (corrosion-resistant) grants ternary intermetallic compound ZrRe_1.75Ru₀₂₅, exhibiting excellent HER activity and stability in acidic and alkaline electrolytes. The overpotential at a current density of 10 mA/cm² (η₁₀) for ZrRe_1.75Ru₀₂₅ is much lower compared to that of ZrRe₂. Although the HER activity of ZrRe_1.75Ru₀₂₅ is not comparable to that of ZrRu₂, it demonstrates outstanding HER stability, while the current density of ZrRu₂ is over ca. 16% after 6 h. This suggests that intermetallic compounds can break the constraint between activity and stability in a single metal for HER, which may be applied in other fields as well.

Porous nanorods by stacked NiO nanoparticulate exhibiting corn-like structure for sustainable environmental and energy applications

A porous 1D nanostructure provides much shorter electron transport pathways, thereby helping to improve the life cycle of the device and overcome poor ionic and electronic conductivity, interfacial impedance between electrode-electrolyte interface, and low volumetric energy density. In view of this, we report on the feasibility of 1D porous NiO nanorods comprising interlocked NiO nanoparticles as an active electrode for capturing greenhouse CO₂, effective supercapacitors, and efficient electrocatalytic water-splitting applications. The nanorods with a size less than 100 nm were formed by stacking cubic crystalline NiO nanoparticles with dimensions less than 10 nm, providing the necessary porosity. The existence of Ni²⁺ and its octahedral coordination with O^2- is corroborated by XPS and EXAFS. The SAXS profile and BET analysis showed 84.731 m² g^-1 surface area for the porous NiO nanorods. The NiO nanorods provided significant surface-area and the active-surface-sites thus yielded a CO₂ uptake of 63 mmol g^-1 at 273 K via physisorption, a specific-capacitance (C_S) of 368 F g^-1, along with a retention of 76.84% after 2500 cycles, and worthy electrocatalytic water splitting with an overpotential of 345 and 441 mV for HER and OER activities, respectively. Therefore, the porous 1D NiO as an active electrode shows multifunctionality toward sustainable environmental and energy applications.

Iron, Cobalt, and Nickel Phthalocyanine Tri-Doped Electrospun Carbon Nanofibre-Based Catalyst for Rechargeable Zinc-Air Battery Air Electrode

The goal of achieving the large-scale production of zero-emission vehicles by 2035 will create high expectations for electric vehicle (EV) development and availability. Currently, a major problem is the lack of suitable batteries and battery materials in large quantities. The rechargeable zinc-air battery (RZAB) is a promising energy-storage technology for EVs due to the environmental friendliness and low production cost. Herein, iron, cobalt, and nickel phthalocyanine tri-doped electrospun carbon nanofibre-based (FeCoNi-CNF) catalyst material is presented as an affordable and promising alternative to Pt-group metal (PGM)-based catalyst. The FeCoNi-CNF-coated glassy carbon electrode showed an oxygen reduction reaction/oxygen evolution reaction reversibility of 0.89 V in 0.1 M KOH solution. In RZAB, the maximum discharge power density (P_max) of 120 mW cm^-2 was obtained with FeCoNi-CNF, which is 86% of the P_max measured with the PGM-based catalyst. Furthermore, during the RZAB charge-discharge cycling, the FeCoNi-CNF air electrode was found to be superior to the commercial PGM electrocatalyst in terms of operational durability and at least two times higher total life-time.

Evaluation of electrosynthesized reduced graphene oxide-Ni/Fe/Co-based (oxy)hydroxide catalysts towards the oxygen evolution reaction

In this work, the specific role of the addition of graphene oxide (GO) to state-of-the-art nickel-iron (NiFe) and cobalt-nickel-iron (CoNiFe) mixed oxides/hydroxides towards the oxygen evolution reaction (OER) is investigated. Morphology, structure, and OER catalytic activity of the catalysts with and without GO were studied. The catalysts were fabricated via a two-step electrodeposition. The first step included the deposition of GO flakes, which, in the second step, were reduced during the simultaneous deposition of NiFe or CoNiFe. As a result, NiFe-GO and CoNiFe-GO were fabricated without any additives directly on the nickel foam substrate. A significant improvement of the OER activity was observed after combining NiFe with GO (OER overpotential η(10 mA·cm-2): 210 mV) compared to NiFe (η: 235 mV) and GO (η: 320 mV) alone. A different OER activity was observed for CoNiFe-GO. Here, the overall catalytic activity (η: 230 mV) increased compared to GO alone. However, it was reduced in comparison to CoNiFe (η: 224 mV). The latter was associated with the change in the morphology and structure of the catalysts. Further OER studies showed that each of the catalysts specifically influenced the process. The improvement in the OER by NiFe-GO results mainly from the structure of NiFe and the electroactive surface area of GO.

Enhanced electrocatalytic performance for oxygen evolution reaction via active interfaces of Co3O4arrays@FeOx/Carbon cloth heterostructure by plasma-enhanced atomic layer deposition

Oxygen evolution reaction (OER) is a necessary procedure in various devices including water splitting and rechargeable metal-air batteries but required a higher potential to improve oxygen evolution efficiency due to its slow reaction kinetics. In order to solve this problem, a heterostructured electrocatalyst (Co₃O₄@FeO_x/CC) is synthesized by deposition of iron oxides (FeO_x) on carbon cloth (CC) via plasma-enhanced atomic layer deposition, then growth of the cobalt oxide (Co₃O₄) nanosheet arrays. The deposition cycle of FeO_xon the CC strongly influences thein situgrowth and distribution of Co₃O₄nanosheets and electronic conductivity of the electrocatalyst. Owing to the high accessible and electroactive areas and improved electrical conductivity, the free-standing electrode of Co₃O₄@FeO_x/CC with 100 deposition cycles of FeO_xexhibits excellent electrocatalytic performance for OER with a low overpotential of 314.0 mV at 10 mA cm^-2and a small Tafel slope of 29.2 mV dec^-1in alkaline solution, which is much better than that of Co₃O₄/CC (448 mV), and even commercial RuO₂(380 mV). This design and optimization strategy shows a promising way to synthesize ideally designed catalytic architectures for application in energy storage and conversion.

Surface Modulation of 3D Porous CoNiP Nanoarrays In Situ Grown on Nickel Foams for Robust Overall Water Splitting

The careful design of nanostructures and multi-compositions of non-noble metal-based electrocatalysts for highly efficient electrocatalytic hydrogen and oxygen evolution reaction (HER and OER) is of great significance to realize sustainable hydrogen release. Herein, bifunctional electrocatalysts of the three-dimensional (3D) cobalt-nickel phosphide nanoarray in situ grown on nickel foams (CoNiP NA/NF) were synthesized through a facile hydrothermal method followed by phosphorization. Due to the unique self-template nanoarray structure and tunable multicomponent system, the CoNiP NA/NF samples present exceptional activity and durability for HER and OER. The optimized sample of CoNiP NA/NF-2 afforded a current density of 10 mA cm⁻² at a low overpotential of 162 mV for HER and 499 mV for OER, corresponding with low Tafel slopes of 114.3 and 79.5 mV dec⁻¹, respectively. Density functional theory (DFT) calculations demonstrate that modulation active sites with appropriate electronic properties facilitate the interaction between the catalyst surface and intermediates, especially for the adsorption of absorbed H* and *OOH intermediates, resulting in an optimized energy barrier for HER and OER. The 3D nanoarray structure, with a large specific surface area and abundant ion channels, can enrich the electroactive sites and enhance mass transmission. This work provides novel strategies and insights for the design of robust non-precious metal catalysts.

Optimized nano-metal particles filled into carbon nanohorns to achieve high N-doping amount and high porosity for enhanced oxygen evolution reaction

Nano-metal-filled N-doped carbon materials have been actively verified as promising alternatives for precious-metal catalysts in the oxygen evolution reaction (OER). Herein, Ni/Fe/Cu-filled N-doped carbon nanohorns (CNHs) are synthesized via a positive pressure assisted arc discharge method using a Ni/Fe/Cu rod charged in an anode hole in a N2 and Ar mixture. We first found that the amount of N atom doping can be controlled by the types of nano-metal particles encapsulated by CNHs. The content of N atoms on CNHs uniquely depended on the initial Ni wires inserted into the anode graphite; increasing the number of Ni wires induced the enrichment of N atoms until 3.56 at%, whereas the content of N atoms for Cu- and Fe-filled CNHs is against the results; loading Cu and Fe nanoparticles decreases the N-doping amount. And the morphologies and N-configurations can be changed by the types of metal nanoparticles. Furthermore, the OER performance of Ni-filled CNHs is much superior to that of Cu- and Fe-filled CNHs, which can be significantly enhanced by the tip opening structure, and the increase in Ni loading amount and the N atom content.

Excellent Electrocatalytic Hydrogen Evolution Reaction Performances of Partially Graphitized Activated-Carbon Nanobundles Derived from Biomass Human Hair Wastes

Carbonaceous materials play a vital role as an appropriate catalyst for electrocatalytic hydrogen production. Aiming at realizing the highly efficient hydrogen evolution reaction (HER), the partially graphitized activated-carbon nanobundles were synthesized as a high-performance HER electrocatalyst by using biomass human hair ashes through the high-temperature KOH activation at two different temperatures of 600 and 700 °C. Due to the partial graphitization, the 700 °C KOH-activated partially graphitized activated-carbon nanobundles exhibited higher electrical conductivity as well as higher textural porosity than those of the amorphous activated-carbon nanobundles that had been prepared by the KOH activation at 600 °C. As a consequence, the 700 °C-activated partially graphitized activated-carbon nanobundles showed the extraordinarily high HER activity with the very low overpotential (≈16 mV at 10 mA/cm² in 0.5 M H₂SO₄) and the small Tafel slope (≈51 mV/dec). These results suggest that the human hair-derived partially graphitized activated-carbon nanobundles can be effectively utilized as a high-performance HER electrocatalyst in future hydrogen-energy technology.

TriMOF synergized on the surface of activated carbon produced from pineapple leaves for the environmental pollutant reduction and oxygen evolution process

Facile and modest synthesis of significantly effective and less-cost catalysts for environmental pollutant degradation and oxygen evolution holds substantial potential in environmental and energy fields. Hereby, Trimetallic organic frameworks (TriMOF) consisting of Fe, Co, and Zn synergized on the surface of activated carbon (AC) from pineapple leaves tend to show exponential catalytic activity due to the more excellent ionic conductivity, catalytic stability and multiple active sites provided by different metal precursors. Furthermore, the developed nanocomposite was coated on the stainless-steel electrode substrate at room temperature, delivering greater electrocatalytic surface area and numerous active sites. The oxidation reaction kinetics drives the catalytic reduction of 4-nitrophenol to 4-aminophenol with a minimal time of 12 min @ >97 % efficiency. Furthermore, on electrocatalytic oxidation of water splitting process due to the presence of multiple metallic, active sites, the overpotential is at 370 mV having Tafel slope of 40 mV/dec and electrochemically active surface area of is 9.9 mF/cm². This superior catalytic reduction of 4-nitrophenol and electrocatalytic water oxidation process is attributed to the developed composite's active centre and conductivity.

Function of Doping Ru Element in the Hydrogen Evolution Reaction in Rare-Earth Transition-Metal Intermetallics

Transition metal-based intermetallics are promising electrocatalysts for replacing the commercial Pt metal in the hydrogen evolution reaction (HER). In this work, RENi₂ and RERu_0.25Ni_1.75 (RE = Pr, Tb, and Er) were synthesized and their electrocatalytic HER activities were explored. Among undoped compounds, PrNi₂ exhibits the best performance and requires an overpotential of 55 mV, while partially replacing Ni with Ru element (PrRu_0.25Ni_1.75) can greatly reduce the overpotential to 20 mV at a current density of 10 mA/cm². Such enhancement was recognized that belongs to their extrinsic property, and their intrinsic HER activities were similar after normalizing the electrocatalytic surface area. Further investigation on ScM₂ and ScRu_0.25M_1.75 (M = Co and Ni) suggests that doping Ru element in ScCo₂ will significantly enhance antibonding character around the Fermi level (E_F) and weaken hydrogen adsorption energy. On the other hand, the antibonding population for ScNi₂ and ScRu_0.25Ni_1.75 is similar at E_F, which accounts for their close intrinsic HER activities.

In Situ Grown CoMn2 O4 3D-Tetragons on Carbon Cloth: Flexible Electrodes for Efficient Rechargeable Zinc-Air Battery Powered Water Splitting Systems

The integration of energy conversion and storage systems such as electrochemical water splitting (EWS) and rechargeable zinc-air battery (ZAB) is on the vision to provide a sustainable future with green energy resources. Herein, a unique strategy for decorating 3D tetragonal CoMn₂ O₄ on carbon cloth (CMO-U@CC) via a facile one-pot in situ hydrothermal process, is reported. The highly exposed morphology of 3D tetragons enhances the electrocatalytic activity of CMO-U@CC. This is the first demonstration of such a bifunctional activity of CMO-U@CC in an EWS system; it achieves a nominal cell voltage of 1.610 V @ 10 mA cm^-2 . Similarly, the fabricated rechargeable ZAB delivers a specific capacity of 641.6 mAh g_zn ^-1 , a power density of 135 mW cm^-2 , and excellent cyclic stability (50 h @ 10 mA cm^-2 ). Additionally, a series of flexible solid-state ZABs are fabricated and employed to power the assembled CMO-U@CC-based water electrolyzer. To the best of the authors' knowledge, this is the first demonstration of an in situ-grown binder-free CMO-U@CC as a flexible multifunctional electrocatalyst for a built-in integrated rechargeable ZAB-powered EWS system.

Recent advances in activating surface reconstruction for the high-efficiency oxygen evolution reaction

A climax in the development of cost-effective and high-efficiency transition metal-based electrocatalysts has been witnessed recently for sustainable energy and related conversion technologies. In this regard, structure-activity relationships based on several descriptors have already been proposed to rationally design electrocatalysts. However, the dynamic reconstruction of the surface structures and compositions of catalysts during electrocatalytic water oxidation, especially during the anodic oxygen evolution reaction (OER), complicate the streamlined prediction of the catalytic activity. With the achievements in operando and in situ techniques, it has been found that electrocatalysts undergo surface reconstruction to form the actual active species in situ accompanied with an increase in their oxidation state during OER in alkaline solution. Accordingly, a thorough understanding of the surface reconstruction process plays a critical role in establishing unambiguous structure-composition-property relationships in pursuit of high-efficiency electrocatalysts. However, several issues still need to be explored before high electrocatalytic activities can be realized, as follows: (1) the identification of initiators and pathways for surface reconstruction, (2) establishing the relationships between structure, composition, and electrocatalytic activity, and (3) the rational manipulation of in situ catalyst surface reconstruction. In this review, the recent progress in the surface reconstruction of transition metal-based OER catalysts including oxides, non-oxides, hydroxides and alloys is summarized, emphasizing the fundamental understanding of reconstruction behavior from the original precatalysts to the actual catalysts based on operando analysis and theoretical calculations. The state-of-the-art strategies to tailor the surface reconstruction such as substituting/doping with metals, introducing anions, incorporating oxygen vacancies, tuning morphologies and exploiting plasmonic/thermal/photothermal effects are then introduced. Notably, comprehensive operando/in situ characterization together with computational calculations are responsible for unveiling the improvement mechanism for OER. By delivering the progress, strategies, insights, techniques, and perspectives, this review will provide a comprehensive understanding of the surface reconstruction in transition metal-based OER catalysts and future guidelines for their rational development.

NiCo2O4 nanoparticles inlaid on sulphur and nitrogen doped and co-doped rGO sheets as efficient electrocatalysts for the oxygen evolution and methanol oxidation reactions

The present work depicts the fabrication of NiCo₂O₄ decorated on rGO, and doped and co-doped rGO and its electrocatalytic activity towards the oxygen evolution reaction and methanol oxidation reaction. The NiCo₂O₄ catalyst with S-doped rGO outperformed the other catalysts, indicating that the sulphur atoms attached on rGO possess low oxophilicity and optimum free energy. This results in facile adsorption of the intermediate products formed during the OER and a rapid release of O₂ molecules. The same catalyst requires an overpotential of 1.51 V vs. RHE to attain the benchmark current density value of 10 mA cm^-2 and shows a Tafel slope of 57 mV dec^-1. It also reveals outstanding stability during its operation for 10 h with a minimum loss in potential. On the other hand, NiCo₂O₄/S,N-rGO reveals superior activity with high efficiency and stability in catalyzing methanol oxidation. The catalyst delivered a low onset potential of 0.12 V vs. Hg/HgO and high current density of 203.4 mA cm^-2 after addition of 0.5 M methanol, revealing the outstanding performance of the electrocatalyst.

Double-shelled carbon nanocages grafted with carbon nanotubes embedding Co nanoparticles for enhanced hydrogen evolution electrocatalysis

Herein, small Co nanoparticles (NPs) encapsulated in N-doped double-shelled carbon nanocages grafted with thin carbon nanotubes (Co@CNTs@DSCNCs) were synthesized from yolk-shell bimetallic zeolitic imidazolate framework (BMZIF). For HER electrocatalysis, they exhibit higher activity (η₁₀ = 214 mV) and more favorable kinetics than Co@CNTs@PC (PC = porous carbon) with thick CNTs and large Co NPs derived from solid BMZIF cubes.

3D porous Ni/NiOx as a bifunctional oxygen electrocatalyst derived from freeze-dried Ni(OH)2

Bifunctional electrocatalytic properties of freeze-dried Ni/NiOx, freeze-dried NiO, and freeze-dried Ni(OH)2 are reported. Freeze-dried Ni(OH)2 was synthesized by the freeze-drying method. Freeze-dried Ni/NiOx and freeze-dried Ni were obtained from the thermal annealing of the material. Both Ni(OH)2 and Ni/NiOx could sustain with freestanding freeze-dried 3D structures without any carbon support. Freeze-dried Ni/NiOx exhibited excellent bifunctional electrocatalytic properties with the ORR performance at 0.62 V (half-wave potential) and OER at 1.47 V (η = 10 mA cm-2). Using freeze-dried metal hydroxides can be considered useful in a wide range of carbon-free applications and can improve the electrocatalytic performance. The bifunctional catalytic activities were calculated to be 0.86, 0.98 and 1.14 V for freeze-dried Ni/NiOx, freeze-dried NiO and freeze-dried Ni(OH)2, respectively. The stacking of 2D sheets into 3D mass seemed to play a vital role behind this excellent bifunctionality of freeze-dried Ni/NiOx. The material reveals possible applications in Zn-air batteries. Besides, the strategy developed herein could be justified to obtain other transition metal-oriented bifunctional electrocatalysts as alternatives to Pt- and Ir/Ru-based expensive benchmark catalysts.

Hierarchical ultrathin defect-rich CoFe2O4@BC nanoflowers synthesized via a temperature-regulated strategy with outstanding hydrogen evolution reaction activity

Graphical illustration of the synthesis and the electrocatalytic performance of CoFe₂O₄@BC 500 °C nanoflowers.

Aggregates of Ni/Ni(OH)2/NiOOH Nanoworms on Carbon Cloth for Electrocatalytic Hydrogen Evolution

The development of an efficient electrocatalyst for hydrogen evolution reaction (HER) is essential to facilitate the practical application of water splitting. Here, we aim to develop an electrocatalyst, Ni/Ni(OH)₂/NiOOH, via electrodeposition technique on carbon cloth, which shows efficient activity and durability for HER in an alkaline medium. Phase purity and morphology of the electrodeposited catalyst are determined using powder X-ray diffraction and electron microscopic techniques. The compositional and thermal stability of the catalyst is checked using X-ray photoelectron spectroscopy and thermogravimetry analysis. Electrodeposited Ni/Ni(OH)₂/NiOOH material is an efficient, stable, and low-cost electrocatalyst for hydrogen evolution reaction in a 1.0 M KOH medium. The catalyst exhibits remarkable performance, achieving a current density of 10 mA/cm² at a potential of -0.045 V vs reversible hydrogen electrode (RHE), and the Tafel slope value is 99.6 mV/dec. The overall electrocatalytic water splitting mechanism using Ni/Ni(OH)₂/NiOOH catalyst is well explained, where formation and desorption of OH^- ion on the catalyst surface are significant at alkaline pH. The developed electrocatalyst shows significant durability up to 200 h in a negative potential window in a highly corrosive alkaline environment along with efficient activity. The electrocatalyst can generate 165.6 μmol of H₂ in ∼145 min of reaction time with 81.5% faradic efficiency.

Loading of individual Se-doped Fe2O3-decorated Ni/NiO particles on carbon cloth: facile synthesis and efficient electrocatalysis for the oxygen evolution reaction

The synthesis of competitive, affordable and sustainable electrocatalysts via simple and scalable methods is highly desirable for the oxygen evolution reaction (OER). Usually, expensive, complex, time-consuming methods are applied to prepared suitable electrocatalysts for the OER. In contrast, a single-step thermal method is simple and inexpensive. Nickel and iron-based composite materials are potential candidates as OER catalysts. Accordingly, herein, Se-doped Fe2O3-decorated Ni/NiO particles on carbon cloth (Se-Fe2O3@Ni/NiO/CC) were synthesized via a facile and scalable one-step thermal method. The individual Se-Fe2O3@Ni/NiO particles were accommodated in holes in the carbon fibers of CC. The optimized Se-Fe2O3@Ni/NiO/CC-2 sample exhibited an outstanding OER performance with an overpotential of 205 mV at the current density 10 mA cm-2, small Tafel slope of 36 mV dec-1, and good stability in 1.0 M KOH electrolyte. The outstanding catalytic performance was mainly attributed to the heterointerfaces between Se-Fe2O3 and Se-Ni/NiO. Moreover, the accommodation of the Se-Fe2O3@Ni/NiO particles in the holes of CC restricted the aggregation of the particles, and CC provided a conductive substrate for the OER process. Thus, this work provides a simple, scalable and effective strategy for designing and engineering of outstanding electrocatalysts for the OER.

Co/FeC core-nitrogen doped hollow carbon shell structure with tunable shell-thickness for oxygen evolution reaction

Herein, multi-core-shell structure Co/FeC@N-doped hollow carbon (Co/FeC@NHC) with tunable carbon shell thickness is well crafted by a novel and simple strategy. Novel core-shell structure consisting of polydopamine (PDA) shell with different thickness and bimetal-based metal-organic frameworks (MOFs) Co/Fe core with cubic morphology are first prepared, followed by a thermal and etching treatments to fabricate hollow composite materials composed of multiple Co/FeC cores evenly distributed in N-doped carbon shell. PDA acts as a carbon and nitrogen source simultaneously to form N-doped hollow carbon in this procedure. Creatively, the N-doped hollow carbon shell protects Co/FeC core and against the degradation, in addition to enhance the conductivity of Co/FeC@NHC. Through adjusting the PDA shell thickness simply, Co/FeC@NHC with tunable N-doped carbon shell thickness is well crafted. The Co/FeC@NHC-1 with the best electrocatalytic performance is obtained by optimizing the thickness of N-doped hollow carbon shell. The Co/FeC@NHC-1 exhibits the highest activity for the oxygen evolution reaction (OER) and outstanding stability and durability. Hence, this research may establish a promising path for the rational design of metals@carbon composite with controllable structure which can act as highly efficient electrocatalysts for (OER).

Excellent Oxygen Evolution Reaction of Activated Carbon-Anchored NiO Nanotablets Prepared by Green Routes

A sustainable and efficient electrocatalyst for the oxygen evolution reaction (OER) is vital to realize green and clean hydrogen production technology. Herein, we synthesized the nanocomposites of activated carbon-anchored nickel oxide (AC-NiO) via fully green routes, and characterized their excellent OER performances. The AC-NiO nanocomposites were prepared by the facile sonication method using sonochemically prepared NiO nanoparticles and biomass-derived AC nanosponges. The nanocomposites exhibited an aggregated structure of the AC-NiO nanotablets with an average size of 40 nm. When using the nanotablets as an OER catalyst in 1 M KOH, the sample displayed superb electrocatalytic performances, i.e., a substantially low value of overpotential (320 mV at 10 mA/cm2), a significantly small Tafel slope (49 mV/dec), and a good OER stability (4% decrease of overpotential after 10 h). These outstanding OER characteristics are considered as attributing to the synergetic effects from both the ample surface area of the electrochemically active NiO nanoparticles and the high electrical conductivity of the AC nanosponges. The results pronounce that the fully ecofriendly synthesized AC-NiO nanotablets can play a splendid role as high-performance electrocatalysts for future green energy technology.