Superior Oxygen Evolution Research Articles

Three-dimensional carbon network-supported black phosphorus-cobalt heterojunctions: An efficient electrocatalyst for high-rate oxygen evolution

Black phosphorus (BP), as a burgeoning two-dimensional material, has shown good electrocatalytic activity due to its unique electronic structure and abundant active sites.However, the presence of lone pair electrons in black phosphorus leads to its poor stability and rapid degradation in an oxygen/water environment, which greatly limits its practical application. Herein, BP-Co heterojunctions were synthesized on carbon nanotube@nitrogen-doped carbon (BP-Co/CNT@NC) by the pyrolysis of ZnCo-zeolitic imidazolate frameworks and subsequent solvothermal treatment. The BP-Co Schottky junction improved the electrocatalytic stability of BP, modulated its electronic structure, improved its conductivity and electron transfer during the electrocatalytic reaction. Density functional theory calculation was used to confirm the electron transfer and redistribution at the interface between BP and Co, which constructed an oppositely charged region and formed a strong built-in field. Energy band configuration analysis revealed a narrowed band gap because of the formation of BP-Co Schottky junction. Consequently, the optimized BP-Co/CNT@NC exhibited a superior oxygen evolution reaction (OER) performance, a low overpotential of 370 mV@100 mA/cm2, with a small Tafel slope of 40 mV/dec and good long-term stability. Particularly, the catalyst has an excellent OER performance at the high current density of 100–400 mA/cm2. This strategy improves the stability of BP electrocatalysts and strengthens their utilization in electrocatalytic applications.

Trace tiny NiCo alloy nanoparticles encapsulated on hierarchical porous peanut-like carbon walls for robust oxygen evolution reaction

The fine regulation of the porous structure and morphology of biomass-like carbon is a promising strategy to realize rapid catalytic reaction kinetics. Herein, we report a universal hydrothermal method to manipulate metal ions (Ni2+, Co2+, and Fe3+) into peanut-like carbon (pC) derived from sucrose. Furthermore, a simple pyrolysis strategy is developed to finely manipulate the porous structure of pC to realize the hierarchical porous structure, large specific surface area, and abundant defects of NiCo@mpNC catalyst, in which highly dispersed trace NiCo alloy nanoparticles are encapsulated with N-doped pC. Particularly, N-doped porous pC consists of massive ultrathin carbon walls, which restrict the growth of tiny NiCo alloy nanoparticles. Therefore, the fabricated NiCo@mpNC inherits superior oxygen evolution reaction (OER) activity with a low overpotential of 278 mV (vs. RHE) at a current density of 10 mA cm−2. Moreover, NiCo@mpNC enables robust electrocatalytic stability (≥ 20 h) at a multi-scale current density of 10, 20, and 50 mA cm−2. The potential decays by only 8 mV after 3000 cyclic voltammetry (CV) cycles. This work opens up a feasible way for the fine design of biomimetic carbon matrix with hierarchical porous structure as typical carriers for high-efficient OER catalysts.

Strong High Entropy Alloy-Support Interaction Enables Efficient Electrocatalytic Water Splitting at High Current Density

In electrocatalysis, the stability issue between catalyst and support still needs great attention. Here, a series of high-entropy alloy nanoparticles (HEA-NPs) embedded in carbon cloth (CC) were synthesized by using the scalable strategy-microwave heating. Among them, PtRhCoNiCu/CC exhibits outstanding hydrogen evolution reaction (HER) activity (19 and 170 mV overpotential at 10 and 1000 mA cm−2) and stability (150 h), outperforming other recently reported HEAs catalysts. IrRuCoNiCu/CC displays superior oxygen evolution reaction (OER) activity (166 and 354 mV overpotential at 10 and 1000 mA cm−2) and stability (150 h), and shows a lower overpotential than recently reported HEA catalysts. In water splitting, IrRuCoNiCu/CC(+)//PtRhCoNiCu/CC(-) electrolyzer achieves 500 mA cm−2 (1000 mA cm−2) high current density at 1.76 V (1.88 V) and exhibits excellent stability, which is one of the best catalysts currently. Therefore, the novel supported HEA catalyst with high stability is expected to be a promising candidate material for industrialized water splitting.

Cu@S-Co(OH)2 hybrid electrocatalysts with high performance for alkaline hydrogen evolution reaction

Transition metal hydroxides have superior oxygen evolution reaction (OER) activity but poor hydrogen evolution reaction (HER) performance, which greatly limits the water splitting for hydrogen production. Herein, copper modification of sulfur doped Co(OH)2 hybrid catalyst (Cu@S-Co(OH)2) was prepared by one-step of electrodeposition method which can serve as efficient hydrogen evolution reaction catalyst. In the presence of sulfur and copper, Cu@S-Co(OH)2 hybrid catalyst exhibits the best catalytic activity, showing ultralow overpotential of 42 mV at a current density of 10 mA cm−2. Compared with Co(OH)2, the overpotential is improved by nearly 100 mV as well as enhanced stability. In addition, the active site poison experiments proved that the active center is cobalt metal. Combined with the adsorption strength of the catalyst for OH and H species, it is considered that both S and Cu can enhance the adsorption of OH for Co(OH)2, while S is contributed to the adsorption of H, and Cu can promote the desorption of H. This work provides a facile method and new insight to enhance the HER activity for transitional metal hydroxides via metal and non-metallic elements co-modulation.

Fe doped NiS2 derived from metal-organic framework embedded into g-C3N4 for efficiently oxygen evolution reaction

The exploitation of the superior oxygen evolution reaction (OER) electrocatalysts is of great paramount importance to the investigation of developing hydrogen production from water splitting to alleviate the energy crisis. Herein, Fe3+ ion doped NiS2 for OER electrocatalysts obtained from a metal-organic framework (MOF) as a self-template, with the graphitic phase carbon nitride (g-C3N4) as a chain armor, was prepared by the two-step solvothermal-pyrolysis method. The MOF-derived Fe–NiS2@g-C3N4 composite achieves a current density of 10 mA cm−2 at a tiny OER overpotential (280 mV) and shows good stability in duration tests over 60 h. Compared with the undoped sample, the overpotential is significantly lower by 60 mV and the stability is substantially improved. In this study, two strategies, iron ion doping and g-C3N4 coating, were implemented to target the defects of NiS2, promoting its catalytic activity and achieving a significant improvement in stability at the cost of a bit of activity. Balancing activity and stability maximized has been achieved by our work. This study presents a potential approach to improve the catalyst activity and stability in an integrated strategy.

Electrochemical Reconstruction Engineering: Metal–Organic Gels as Pre‐Catalysts for NiOOH/FeOOH Heterostructure to Boost Oxygen Evolution Reaction

Metal–organic gels (MOG) as new types of soft materials have shown promising applications in various fields such as chemosensors, environmental remediation, and gas adsorption/separation, owing to their high porosity, low density, and high surface area. However, the application of MOG materials in energy electrocatalysis and the active components made from them are rarely perceived. Herein, a new electrochemistry‐driven reconstruction strategy to synthesize the NiOOH/FeOOH heterostructure from MOG materials is reported. The reconstructed NiOOH/FeOOH exhibits superior oxygen evolution reaction activity and excellent stability, owing to the synergistic effect of bimetallic centers, the abundant interface between NiOOH and FeOOH, and the plentiful defects. Impressively, the activated Re–FeNi–MOG‐4 electrocatalyst displays remarkable catalytic activity with a low overpotential of 220 mV at a current density of 10 mA cm−2 and a small Tafel slope of 48 mV dec−1 in alkaline electrolyte, outperforming most recently reported electrocatalysts. Herein, a facile and effective electrochemical reconstruction engineering of pre‐catalysts is provided and the evolution of self‐reconstruction of MOG materials for accelerating the kinetics of the electrocatalytic process is highlighted.

Synthesis of Urchin-like Ni@NP@NCP Composites with Three Solvothermal Systems for Highly Efficient Overall Seawater Splitting.

In this work, an urchin-like Ni@Ni2P@NiCoP (Ni@NP@NCP) composite was prepared on nickel foam by a simple hydrothermal treatment process. Using the prepared NiO nanosheets as templates, the NiCo precursor was prepared in the presence of three solvothermal systems of water/dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) by the hydrothermal process. After mixing and calcining with sodium hypophosphite under a nitrogen atmosphere at a high temperature for phosphating, an urchin-like Ni@NP@NCP(F/SO/H) nanostructured catalyst was obtained with superior hydrogen evolution and oxygen evolution performance. To further explore their efficiency in seawater splitting. Ni@NP@NCP(F/SO/H) composites were used as the cathode and anode of an electrolytic cell, which delivered 1.822 V potential at 300 mA cm-2 in simulated seawater (1 M KOH and 0.5 M NaCl). This may provide an effective way of developing clean energy.

Electrocatalytic Overall Water Splitting Induced by Surface Reconstruction of an Iron-Modified Ni2P/Ni5P4 Heterojunction Array Encapsulated into a N-Doped Carbon Layer

Reasonable development of high-efficiency and robust electrocatalysts for efficient electrocatalytic water splitting at high current density is hopeful for renewable energy, but the real challenge is substituting the precious metal catalysts. Herein, ultrathin Fe-modified Ni2P/Ni5P4 nanosheet arrays hybridized with N-doped carbon grown on Ni foam (Fe-Ni2P/Ni5P4@N-C) were synthesized via a solvothermal-pyrolysis strategy. Theoretical calculations and in situ Raman characterizations confirm that the Fe sites can facilitate the surface reconstruction of highly active NiOOH species and significantly lower the energy barrier for the formation of the *OOH intermediate owing to the electron coupling effect between Fe and the Ni2P/Ni5P4 heterostructure. On account of the structural advantages and compositional synergy, the optimized Fe-Ni2P/Ni5P4@N-C exhibits superior hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities with an overpotential of 105 and 280 mV to reach 10 and 50 mA cm-2, respectively, and can work stably for 60 h at 100 mA cm-2. Impressively, the electrolyzer with Fe-Ni2P/Ni5P4@N-C only needs 1.56 V to achieve 10 mA cm-2 current density for water splitting. This protocol not only provides inspiration for designing transitional metal electrocatalysts for water splitting but also puts forward a pathway for practical application.

Ultrathin fan-like multimetallic oxide as a superior oxygen evolution electrocatalyst in alkaline water and seawater

Multimetallic systems exhibit high catalytic activity because of the flexibility to change the electronic and crystal structures of the materials. On the other hand, the preparation of homogeneous catalysts and the revelation of synergistic interactions in the catalytic process limit the further development of multimetallic catalysts due to the increase in chemical complexity. Here, atomically thin quinary CNWFVOx (CoNiWFeVOx) fan-like nanobelts are successfully synthesized as an OER electrocatalyst by a simple colloidal chemistry strategy. Compared with the ternary and quaternary systems, the quinary CNWFVOx shows more excellent catalytic performance, requiring only low overpotentials of 245 mV and 272 mV to achieve a current density of 10 mA cm−2 under alkaline conditions and simulated seawater, respectively, and a small Tafel slope of 47.17 mV dec−1 under alkaline conditions. Moreover, the electrode activity remained almost constant after more than 83 h of stability testing. The OER activity of the catalyst was significantly enhanced and the kinetics were accelerated thanks to the synergistic effect of the multi-metals and the amorphous ultra-thin structure. This work may provide opportunities for the design and study of atomically thin multinary transition metal oxide nanomaterials.

Modulation in the d Band of Ir by Core-Shell Construction for Robust Water Splitting Electrocatalysts in Acid.

The realization of commercialization of proton electrolyte membrane water splitting technology significantly depends on the anodic electrocatalyst working at a high potential and strong acidic conditions requiring superior oxygen evolution reaction activity and stability. In this work, we devise the construction of ultrasmall Pd@Ir core-shell nanoparticles (5 nm) with atomic layer Ir (3 atomic layers) on carbon nanotubes (Pd@Ir/CNT) as an exceptional bifunctional electrocatalyst in acidic water splitting. Due to the core-shell structure, strain generated at heterointerfaces leads to an upshifted d band center of Ir atoms contributing to a 62-fold better mass activity at 1.63 V vs RHE than commercial IrO2; besides, the electronic hybridization suppresses the electrochemical dissolution of Ir; as a result, robust stability is also achieved. In hydrogen evolution reaction catalysis, Pd@Ir/CNT exhibits a 3.7 times higher mass activity than Pt/C. Furthermore, only 1.7 V is required to reach a water splitting current density of 100 mA cm-2, 251 mV lower than that of Pt/C-IrO2, indicating its superiority in acidic water splitting.

In situ facile fabrication of ultrathin Co(OH)2-CoO/graphene oxide nanosheet hybrids with superior oxygen evolution reaction performance

As representative electrocatalysts for the oxygen evolution reaction (OER), Co-based hydroxides and oxides generally suffer from poor electron conductivity and severe self-agglomeration, substantially hindering their wide application in catalysis. To overcome such issues, we herein report a convenient electrostatic adsorption-assisted solvothermal method for the in situ fabrication of ultrathin Co(OH)2-CoO/graphene oxide nanosheet hybrids (denoted as Co-O-H NSs/GO-x) as efficient electrocatalysts for the OER in alkaline media. Notably, the optimal Co-O-H NSs/GO-1.4 mg sample with a thickness of 3.5–3.8 nm demonstrated outstanding electrocatalytic OER performance, with a low overpotential of only 283 mV at 10 mA cm−2 and a small Tafel slope of 57 mV dec−1, making it comparable to state-of-the-art Co-based non-self-supporting electrocatalysts. Comprehensive analysis revealed that the superior OER electrocatalytic performance of Co-O-H NSs/GO-1.4 mg could be ascribed to the synergistic contribution from the enlarged electroactive surface area (approximately 146 mF.cm−2 for its double-layer capacitance), higher electron concentration (approximately 8.1 × 1017 cm−3), enriched oxygen vacancies (accounting for approximately 74.3 % of the oxygen content) and the upshift of the 3d-band centre caused by the high-quality coupling between Co-O-H NSs and GO NSs. This work provides a universal synthetic strategy for the facile construction of high-performance hybrid nanosheet-based catalysts, showing particular importance for establishing efficient energy conversion reactions.

Modification of Ti/Sb-SnO2/PbO2 Electrode by Active Granules and Its Application in Wastewater Containing Copper Ions

Active granule (WC/Co3O4) doping Ti/Sb-SnO2/PbO2 electrodes were successfully synthesized by composite electrodeposition. The as-prepared electrodes were systematically characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical performance, zeta potential, and accelerated lifetime. It was found that the doping of active granules (WC/Co3O4) can reduce the average grain size and increase the number of active sites on the electrode surface. Moreover, it can improve the proportion of surface oxygen vacancies and non-stoichiometric PbO2, resulting in an outstanding conductivity, which can improve the electron transfer and catalytic activity of the electrode. Electrochemical measurements imply that Ti/Sb-SnO2/Co3O4-PbO2 and Ti/Sb-SnO2/WC-Co3O4-PbO2 electrodes have superior oxygen evolution reactions (OERs) relative to those of Ti/Sb-SnO2/PbO2 and Ti/Sb-SnO2/WC-PbO2 electrodes. A Ti/Sb-SnO2/Co3O4-PbO2 electrode is considered as the optimal modified electrode due to its long lifetime (684 h) and the remarkable stability of plating solutions. The treatment of copper wastewater suggests that composite electrodes exhibit low cell voltage and excellent extraction efficiency. Furthermore, pilot simulation tests verified that a composite electrode consumes less energy than other electrodes. Therefore, it is inferred that composite electrodes may be promising for the treatment of wastewater containing high concentrations of copper ions.

Bioinspired inhibition of aggregation in metal-organic frameworks (MOFs)

Different from traditional procedures of using solid stabilizers like polymers and surfactants, here we demonstrate that water, as a very "soft" matter, could function as a "spacer" to prevent the aggregation of metal-organic frameworks (MOFs) in aqueous dispersions. Our theoretical calculations reveal in case of an excess of positively charged metal nodes of MOFs, where water molecules are ligated to metal nodes that greatly enhance MOFs' solution dispersibility through electrostatic stabilization. This discovery has motivated us to develop a facile experimental approach for producing a category of "clean" MOF dispersions without foreign additives. Potential application has been demonstrated for the size fractionation of MOFs, which results in small-size MOFs (50-80nm) characteristic of superior electrocatalytic oxygen evolution activities (256mV at 10mA cm-2, Tafel slope of 49mV dec-1 and durability >30h). This work would provide new clues for aqueous processing of MOFs for many emerging applications.

Density‐Controlled Metal Nanocluster with Modulated Surface for pH‐Universal and Robust Water Splitting

AbstractReducing the particle sizes of transition metals (TMs) and avoiding their aggregation are crucial for increasing the TMs atom utilization and enhancing their industrial potential. However, it is still challenging to achieve uniform distributed and density‐controlled TMs nanoclusters (NCs) under high temperatures due to the strong interatomic metallic bonds and high surface energy of NCs. Herein, a series of TMs NCs with controllable density and nitrogen‐modulated surface are prepared with the assistance of a selected covalent organic polymer (COP), which can provide continuous anchoring sites and size‐limited skeletons. The prepared Ir NCs show superior hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities than commercial Pt/C and Ir/C in both acid and alkaline media. In particular, the as‐prepared Ir NCs exhibit remarkable full water splitting performance, reaching a current density of 10 mA cm−2 at ultralow overpotentials of 1.42 and 1.43 V in alkaline and acidic electrolyte, respectively. The excellent electrocatalytic activities are attributed to the increased surface atom utilization and the improved intrinsic activity of Ir NCs. More importantly, the Ir NCs catalyst shows superior long‐term stability due to the strong interaction between Ir NCs and the N‐doped carbon layer.

Carbon-Encapsulated Multimetallic Hybrid Electrocatalyst for Overall Water Splitting and Urea Oxidation

Overall water splitting is a hot topic in the development of green energy, but it cannot be generalized on a large scale due to the lack of low-cost and highly efficient overall water-splitting catalysts. As a result, currently, the exploitation of stable and nonprecious metal catalysts with elevated performance has become a research focus in overall water splitting. Here, we introduce a multicomponent material (Ni/MoC/Ti3C2Tx@C) by a strategy of the combination of electrostatic assembly and elevated-temperature calcination to employ it as an electrocatalyst for water splitting and urea oxidation. The Ni/MoC/Ti3C2Tx@C electrocatalyst presents a superior hydrogen evolution reaction and oxygen evolution reaction ability with an overpotential of 128 and 338 mV at 10 mA cm–2 in 1.0 M KOH, respectively, along with long-term stability over 24 h. The Ni/MoC/Ti3C2Tx@C catalyst has a turnover frequency of 1.64 s–1 at 100 mV. Improved catalytic behavior of Ni/MoC/Ti3C2Tx@C is principally due to the synergistic effect of Ni and MoC nanoparticles, the introduction of the conductive Ti3C2Tx MXene matrix with a layered porous structure, and the protection of the carbon layer. In addition, when Ni/MoC/Ti3C2Tx@C electrodes are used to electrolyze water with urea assistance, a voltage of 1.56 V is required for both electrodes to reach 10 mA cm–2, which is 80 mV lower than that required for conventional water electrolysis. Our work provides a strategy for the exploitation of elevated-performance MXene-based electrocatalysts for overall water splitting as well as future advancement of green energy.

Nitrogen doped and carbon coated CoP hollow nanospheres with enhanced electrocatalytic activity towards the oxygen evolution reaction

CoP@NC HNS manifest superior electrocatalytic oxygen evolution performance with low overpotential (320 mV, 10 mA cm−2), small Tafel slope (68 mV dec−1), and remarkable stability in alkaline electrolyte.

N,S co-doped porous carbon with Co9S8 prepared with a Co-FF-derived Co3O4 template: a bi-functional electrocatalyst for rechargeable zinc-air batteries.

To achieve broad commercialization of rechargeable metal-air batteries, the development of non-precious metal-based bi-functional oxygen electrocatalysts is critical. In this study, we prepared N,S co-doped porous carbon materials containing Co9S8 nanoparticles (Co9S8/NSC) through a one-step pyrolysis process. The process involved the pyrolysis of a polydopamine (PDA) coated Co-formic acid framework (Co-FF) derived Co3O4 and thiourea. The improved catalyst Co9S8/NSC-1 exhibited satisfactory long-term durability and superior oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity, the half-wave potential (E1/2) of the ORR reached 0.83 V, and the OER overpotential at 10 mA cm-2 (η10) was 300 mV. The zinc-air battery (ZAB) based on Co9S8/NSC-1 assembly had a maximum power density of 102.0 mW cm-2 and the cycle life reached 500 cycles. The material preparation method was simple, environmentally friendly and inexpensive, providing a feasible strategy for the development of non-precious metal-based electrocatalysts.

Laves phase Ir2Sm intermetallic nanoparticles as a highly active electrocatalyst for acidic oxygen evolution reaction.

Rare earth (RE) intermetallic nanoparticles (NPs) are significant for fundamental explorations and promising for practical applications in electrocatalysis. However, they are difficult to synthesize because of the unusually low reduction potential and extremely high oxygen affinity of RE metal-oxygen bonds. Herein, intermetallic Ir2Sm NPs were firstly synthesized on graphene as a superior acidic oxygen evolution reaction (OER) catalyst. It was verified that intermetallic Ir2Sm is a new phase belonging to the C15 cubic MgCu2 type in the Laves phase family. Meanwhile, intermetallic Ir2Sm NPs achieved a mass activity of 1.24 A mgIr-1 at 1.53 V and stability of 120 h at 10 mA cm-2 in 0.5 M H2SO4 electrolyte, which corresponds to a 5.6-fold and 12-fold enhancement relative to Ir NPs. Experimental results together with density functional theory (DFT) calculations show that in the structurally ordered intermetallic Ir2Sm NPs, the alloying of Sm with Ir atoms modulates the electronic nature of Ir, thereby reducing the binding energy of the oxygen-based intermediate, resulting in faster kinetics and enhanced OER activity. This study provides a new perspective for the rational design and practical application of high-performance RE alloy catalysts.

Nickel-doped iridium echinus-like nanosheets for stable acidic water splitting.

Nickel-doped iridium echinus-like nanosheets (NiIr-ENS) have a superior acidic oxygen evolution reaction (OER) activity with a TOF of 1.72 s-1 at an overpotential of 300 mV, 8.6-fold higher than that of IrO2.

Ligand leaching enabling improved electrocatalytic oxygen evolution performance.

Design and fabrication of cost-effective (pre-)catalysts are important for water splitting and metal-air batteries. In this direction, various metal-organic frameworks (MOFs) have been investigated as pre-catalysts for oxygen evolution. However, the activation process and the complex reconstruction behaviour of these MOFs are not well understood. Herein, square-like MOF nanosheets in which carbon nanotubes were embedded were prepared by introducing an amine ligand to coordinate with Ni ions and then reacting with [Fe(CN)6]3-. The formed MOF nanosheets containing nickel and iron species were then activated by NaBH4, inducing the leaching of ligands and the formation of tiny active species in situ loaded on carbon nanotubes. The prepared catalyst shows superior oxygen evolution performance with an ultralow overpotential of 231 mV for 10 mA cm-2, a fast reaction kinetics with a small Tafel slope of 52.3 mV dec-1, and outstanding catalysis stability. The excellent electrocatalytic performance for oxygen evolution can be attributed to the structural advantage of in situ derived small sized active species and one-dimensional conductive networks. This work provides a new thought for the enhancement of the electrocatalytic performance of MOF materials.