Geometric and Electronic Engineering of Mn-Doped Cu(OH)2 Hexagonal Nanorings for Superior Oxygen Evolution Reaction Electrocatalysis

In Situ Fabrication of Mn-Doped NiMoO4 Rod-like Arrays as High Performance OER Electrocatalyst

The slow kinetics of the oxygen evolution reaction (OER) is one of the significant reasons limiting the development of electrochemical hydrolysis. Doping metallic elements and building layered structures have been considered effective strategies for improving the electrocatalytic performance of the materials. Herein, we report flower-like nanosheet arrays of Mn-doped-NiMoO₄/NF (where NF is nickel foam) on nickel foam by a two-step hydrothermal method and a one-step calcination method. The doping manganese metal ion not only modulated the morphologies of the nickel nanosheet but also altered the electronic structure of the nickel center, which could be the result of superior electrocatalytic performance. The Mn-doped-NiMoO₄/NF electrocatalysts obtained at the optimum reaction time and the optimum Mn doping showed excellent OER activity, requiring overpotentials of 236 mV and 309 mV to drive 10 mA cm^-2 (62 mV lower than the pure NiMoO₄/NF) and 50 mA cm^-2 current densities, respectively. Furthermore, the high catalytic activity was maintained after continuous operation at a current density of 10 mA cm^-2 of 76 h in 1 M KOH. This work provides a new method to construct a high-efficiency, low-cost, stable transition metal electrocatalyst for OER electrocatalysts by using a heteroatom doping strategy.

Ce-Modified Flowerlike NiFe-MOF Nanostructure Based on Ion Competitive Coordination for Enhancing the Oxygen Evolution Reaction

Metal-organic framework (MOF) has become a popular electrocatalyst for the oxygen evolution reaction (OER) because of its large specific surface area and adjustable porosity. Nevertheless, the electrochemical performance of MOFs has been greatly limited by poor intrinsic conductivity and catalytic activity. Herein, we report a Ce-doped nanoflower-like MOF material Ce@NiFe-MOF-5 via a facile ion competitive coordination effect and doping method. Benefiting from the nanoflower structure formed by the stacking of nanosheets, a large number of active sites can be exposed, which favors electron/mass transfer during water oxidation. The coordination substitution of Ce ions not only promoted an increase in the number of active sites on the surface of the nanosheets but also optimized the electronic structure of pristine NiFe-MOF. The well-designed Ce@NiFe-MOF-5 catalysts exhibited superior OER performance under basic conditions, which only required an overpotential of 258 mV at a current density of 10 mA cm^-2 and a Tafel slope of 54.44 mV dec^-1. Moreover, when Ce@NiFe-MOF-5 served as an anode and Pt/C as a cathode, the two-electrode system only needed 1.56 V to drive overall water splitting at 10 mA cm^-2.

Cobalt phosphide nanoparticles encapsulated in manganese, nitrogen co-doped porous carbon nanosheets with rich nanoholes for high-efficiency oxygen reduction reaction

It is a challenging task to research oxygen reduction electrocatalysts with cost-effectiveness, high-performance and ultra-stability to replace traditional noble metal catalysts in renewable energy conversion/storage devices. Herein, cobalt phosphide (Co₂P) nanoparticles encapsulated in Mn, N co-doped porous carbon nanosheets with abundant nanoholes (Co₂P/Mn,N-PCNS) were prepared by a alizarin complexone coordination regulated pyrolysis at 800 °C. In the controlled experiments, the pyrolysis temperature and metal types were investigated in details. The resultant catalyst exhibited rapid mass/charge transfer and superior oxygen reduction reaction (ORR) performance (E_onset = 0.96 V; E_1/2 = 0.86 V vs RHE), surpassing commercial Pt/C. This work presents some constructive guidelines for synthesis of appealing ORR electrocatalysts in renewable energy technology.

Three-dimensional self-supporting superstructured double-sided nanoneedles arrays of iron carbide nanoclusters embedded in manganese, nitrogen co-doped carbon for highly efficient oxygen reduction reaction

Nitrogen- and transition metal-dual doped carbon materials with low cost and high catalytic performances are considered as one of promising alternatives for noble metal catalysts in acceleration of oxygen reduction reaction (ORR). In this work, three-dimensional (3D) self-supporting superstructures of iron carbide (Fe₃C) nanoclusters entrapped in manganese (Mn)- and nitrogen (N)-dual doped carbon nanosheets covered with double-sided nanoneedles carbon arrays (Fe₃C/Mn,N-NCAs) are simply synthesized by a coordination pyrolysis method, in which dicyandiamide mainly behaves as nitrogen source and 1-(2-pyridylazo)-2-naphthol (PAN) as carbon source. Integration of the unique 3D self-supporting superstructures and synergistic effects of the multi-compositions, the as-obtained catalyst displays appealing ORR performance such as the much positive onset potential (E_onset = 0.98 V vs. RHE) and half-wave potential (E_1/2 = 0.88 V vs. RHE), as well as a just 10 mV negative shift in E_1/2 after 2000 cycles, surpassing commercial Pt/C. This work provides some valuable perspectives for preparation of high-efficiency and low-cost non-noble metal ORR electrocatalysts in energy transformation and storage correlated systems.

Recent advances in fuel cell reaction electrocatalysis based on porous noble metal nanocatalysts

As the center of fuel cells, electrocatalysts play a crucial role in determining the conversion efficiency from chemical energy to electrical energy. Therefore, the development of advanced electrocatalysts with both high activity and stability is significant but challenging. Active site, mass transport, and charge transfer are three central factors influencing the catalytic performance of electrocatalysts. Endowed with rich available surface active sites, facilitated electron transfer and mass diffusion channels, and highly active components, porous noble metal nanomaterials are widely considered as promising electrocatalysts toward fuel cell-related reactions. The past decade has witnessed great achievements in the design and fabrication of advanced porous noble metal nanocatalysts in the field of electrocatalytic fuel oxidation reaction (FOR) and oxygen reduction reaction (ORR). Herein, the recent research advances regarding porous noble metal nanocatalysts for fuel cell-related reactions are reviewed. In the discussions, the inherent structural features of porous noble metal nanostructures for electrocatalytic reactions, advanced synthetic strategies for the fabrication of porous noble metal nanostructures, and the structure-performance relationships are also provided.

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 produced Co9S8 nanoclusters/Co/Mn-S, N multi-doped 3D porous carbon derived from eriochrome black T as an effective bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

Construction of high-efficiency, low cost and stable non-noble metal catalyst on air cathode is of great importance for design and assembly of rechargeable Zn-air battery. Eriochrome black T (EBT) has phenolic hydroxyl and -N=Ν- groups, which provides multiple coordination sites for metal ions. Herein, Co₉S₈ nanoclusters implanted in Co/Mn-S,N multi-doped porous carbon (Co₉S₈@Co/Mn-S,N-PC) are fabricated with the mixture (i.e. EBT, metal precursors and dicyandiamide) by a coordination regulated pyrolysis strategy. Specifically, EBT effectively chelates with the Co and Mn ions, resulting in multiple incorporation and fine modulation of the carbon electronic structures. Meanwhile, its sulfonic acid groups are reduced at such high temperature, accompanied by simultaneously embedding S element in the carbon, ultimately in situ forming Co₉S₈ nanoclusters. The Co₉S₈@Co/Mn-S,N-PC performs as an effective bifunctional oxygen catalyst, displaying a positive half-wave potential of 0.85 V and a large limiting current density of 5.89 mA cm^-2 for oxygen reduction reaction (ORR) in alkaline media, coupled with a small overpotential of 320 mV at 10 mA cm^-2 towards oxygen evolution reaction (OER), outperforming commercial Pt/C and RuO₂ catalysts, respectively. Furthermore, the assembled rechargeable Zn-air battery with Co₉S₈@Co/Mn-S,N-PC exhibits the much better charge/discharge performance and long-term durability (210 h, 630 cycles). This research opens an instructive avenue to develop high-efficient and stable bifunctional oxygen electrocatalysts in energy transformation and storage devices.

Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co₉S₈@CoS₂ heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹.

Recent progress in water-splitting electrocatalysis mediated by 2D noble metal materials

Two-dimensional (2D) nanostructures have enabled noble-metal-based nanomaterials to be promising electrocatalysts toward overall water splitting due to their inherent structural advantages, including a high specific surface active area, numerous low-coordinated atoms, and a high density of defects and edges. Moreover, it is also disclosed that the electronic effect and strain effect within 2D nanostructures also benefit the further promotion of the electrocatalytic performance. In this review, we have focused on the recent progress in the fabrication of advanced electrocatalysts based on 2D noble-metal-based nanomaterials toward water splitting electrocatalysis. First, fundamental descriptions about water-splitting mechanisms, some promising engineering strategies, and major challenges in electrochemical water splitting are given. Then, the structural merits of 2D nanostructures for water splitting electrocatalysis are also highlighted, including abundant surface active sites, lattice distortion, abundant surface defects, electronic effects, and strain effects. Additionally, some representative water-splitting electrocatalysts have been discussed in detail to highlight the superiorities of 2D noble-metal-based nanomaterials for electrochemical water splitting. Finally, the underlying challenges and future opportunities for the fabrication of more advanced electrocatalysts for water splitting are also highlighted. We hope that this review article provides guidance for the fabrication of more efficient electrocatalysts for boosting industrial hydrogen production via water splitting.

A general MOF-intermediated synthesis of hollow CoFe-based trimetallic phosphides composed of ultrathin nanosheets for boosting water oxidation electrocatalysis

Engineering an electrode material for boosting reaction kinetics is highly desired for the oxygen evolution reaction (OER) in the anodic half reaction, and is still a grand challenge for energy conversion technologies. By taking inspiration from the catalytic properties of transition metal phosphides (TMPs) and metal-organic frameworks (MOFs), we herein propose a general MOF-intermediated synthesis of a series of hollow CoFeM (M = Bi, Ni, Mn, Cu, Ce, and Zn) trimetallic phosphides composed of ultrathin nanosheets as advanced electrocatalysts for the OER. A dramatic improvement of electrocatalytic performance toward the OER is observed for hollow CoFeM trimetallic phosphides compared to bimetallic CoFe phosphides. Remarkably, composition-optimized CoFeBiP hollow microspheres could deliver superior electrocatalytic performance, achieving a current density of 10 mA cm^-2 with an overpotential of only 273 mV. Mechanistic investigations reveal that the Bi and P doping effectively optimizes the electronic structure of Co and Fe by charge redistribution, which significantly lowers the adsorption energy of oxygen intermediates. Moreover, the hollow microsphere structures composed of ultrathin nanosheets also enable them to provide rich surface active sites to boost the electrocatalytic OER.

One-step pyrolysis synthesis of nitrogen, manganese-codoped porous carbon encapsulated cobalt-iron nanoparticles with superior catalytic activity for oxygen reduction reaction

Replacing precious metal catalysts with low-price and abundant catalysts is one of urgent goals for green and sustainable energy development. It is imperative yet challenging to search low-cost, high-efficiency, and long-durability electrocatalysts for oxygen reduction reaction (ORR) in energy conversion devices. Herein, three-dimensional low-cost Co₃Fe₇ nanoparticles/nitrogen, manganese-codoped porous carbon (Co₃Fe₇/N, Mn-PC) was synthesized with the mixture of dicyandiamide, cobalt (II) tetramethoxyphenylporphyrin (Co(II)TMOPP), hemin, and manganese acetate by one-step pyrolysis and then acid etching. The resultant Co₃Fe₇/N, Mn-PC exhibited excellent durability and prominent ORR activity with more positive onset potential (E_onset, 0.98 V) and half-wave potential (E_1/2, 0.87 V) in 0.1 M KOH electrolyte, coupled with strong methanol resistance. The pyrolysis temperature and optimal balance of graphite with pyridine-nitrogen are of significance for the ORR performance. The prepared Co₃Fe₇/N, Mn-PC displayed excellent ORR performance over commercial Pt/C in the identical environment. It was ascribed to the uniform 3D architecture, Mn- and N-doping effects by finely adjusting the electronic structures, coupled with the synergistic catalytic effects of multi-compositions and multi-active sites. This work provides some constructive guidelines for preparation of low-cost and high-efficiency ORR electrocatalysts.