Controllable synthesis of ultrathin Co9S8 nanosheets as a highly efficient electrocatalyst for overall water splitting

Boosting Efficient and Sustainable Alkaline Water Oxidation on a W-CoOOH-TT Pair-Sites Catalyst Synthesized via Topochemical Transformation

The development of facile methods for constructing highly active, cost-effective catalysts that meet ampere-level current density and durability requirements for an oxygen evolution reaction is crucial. Herein, a general topochemical transformation strategy is posited: M-Co9S8 single-atom catalysts (SACs) are directly converted into M-CoOOH-TT (M = W, Mo, Mn, V) pair-sites catalysts under the role of incorporating of atomically dispersed high-valence metals modulators through potential cycling. Furthermore, in situ X-ray absorption fine structure spectroscopy is used to track the dynamic topochemical transformation process at the atomic level. The W-Co9S8 breaks through the low overpotential of 160 mV at 10 mA cm-2. A series of pair-site catalysts exhibit a large current density of approaching 1760 mA cm-2 at 1.68 V vs reversible hydrogen electrode (RHE) in alkaline water oxidation and achieve a ≈240-fold enhancement in the normalized intrinsic activity compare to that reported CoOOH, and sustainable stability of 1000 h. Moreover, the O─O bond formation is confirmed via a two-site mechanism, supported by in situ synchrotron radiation infrared and density functional theory (DFT) simulations, which breaks the limit of adsorption-energy scaling relationship on conventional single-site.

Co-Co Dinuclear Active Sites Dispersed on Zirconium-doped Heterostructured Co9 S8 /Co3 O4 for High-current-density and Durable Acidic Oxygen Evolution

Developing cost-effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in-depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co-Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9 S8 /Co3 O4 heterostructure. The X-ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co-Co DASs layer with stretching of cobalt oxygen bond and S-Co-O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm-2 , a breakthrough of 500 mA cm-2 high current density, and water splitting stability of 500 hours in acid, making it one of the best-performing acid-stable OER non-noble metal materials. The optimized catalyst with interatomic Co-Co distance (ca. 2.80 Å) followed oxo-oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm-1 ), confirmed by in situ SR-FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g-1 high mass current density using the first reported noble metal-free cobalt anode catalyst of Co-Co DASs/ZCC in PEM-WE at 2.14 V was recorded.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Controlled synthesis of M doped NiVS (M = Co, Ce and Cr) as a robust electrocatalyst for urea electrolysis

Urea electrolysis can be used to treat wastewater containing urea and alleviate the energy crisis, so it is one of the best ways to solve environmental and energy problems. This paper reports the synthesis of M doped NiVS (M = Co, Ce and Cr) composites by a simple hydrothermal process for the first time. What is noteworthy is that the Ce-NiVS material as a catalytic electrode requires only 141 mV overpotential for the hydrogen evolution reaction (HER) and 1.291 V potential for the urea oxidation reaction (UOR) at a current density of 10 mA cm-2 in 1.0 M KOH and 0.5 M urea mixed alkaline solution. Using Ce-NiVS/NF as both the anode and cathode for urea electrolysis, a current density of 10 mA cm-2 is driven by a voltage of only 1.55 V, which is better than most previous catalysts. Experimental results demonstrate that the excellent catalytic activity of Ce-NiVS materials is due to the formation of a large number of active sites and the improvement of conductivity due to doping with Ce. Density functional theory calculation shows that the VS4 material has a small Gibbs free energy of hydrogen adsorption, which plays a major role in the hydrogen production process, and Ce-NiS has a higher density of states (DOS) near the Fermi level, indicating that Ce-NiS has better electronic conductivity. The synergistic catalysis of VS4 and Ce-NiS promoted the hydrogen production performance of the Ce-NiVS material. This work provides guidance for the optimization and design of low-cost electrocatalysts to replace expensive precious metal-based electrocatalysts for overall urea electrolysis.

N, S-doped hollow carbon nanosheet encapsulated Co9S8 nanoparticles as high-efficient bifunctional electrocatalyst for rechargeable zinc-air battery

The development of non-noble metal-based oxygen reduction/evolution reaction (ORR/OER) bifunctional electrocatalyst with reasonably designed structure and inexpensive component is of practical significance for commercialization of rechargeable zinc-air batteries. Here, we...

Promoting electrocatalytic overall water splitting by sulfur incorporation into CoFe-(oxy)hydroxide

The design and fabrication of highly cost-effective electrocatalysts with high activity, and stability to enhance the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered to be one of the most promising approaches toward overall water splitting. In this study, sulfur-incorporated cobalt-iron (oxy)hydroxide (S-(Co,Fe)OOH) nanosheets were directly grown on commercial iron foam via galvanic corrosion and hydrothermal methods. The incorporation of sulfur into (Co,Fe)OOH results in superior catalytic performance and high stability in both the HER and OER conducted in 1 M KOH. The incorporation of sulfur enhanced the electrocatalytic activity by modifying the electronic structure and chemical states of (Co,Fe)OOH. An alkaline water electrolyzer for overall water splitting was fabricated using a two-electrode configuration utilizing the S-(Co,Fe)OOH bifunctional electrocatalyst in both the HER and OER. The fabricated electrolyzer outperformed a precious metal-based electrolyzer using Pt/C as the HER electrocatalyst and IrO2 as the OER electrocatalyst, which are the benchmark catalysts. This electrolyzer provides a lower potential of 1.641 V at 10 mA cm-2 and maintains 98.4% of its performance after 50 h of durability testing. In addition, the S-(Co,Fe)OOH-based electrolyzer successfully generated hydrogen under natural illumination upon its combination with a commercial silicon solar cell and exhibited a solar to hydrogen (STH) efficiency of up to 13.0%. This study shows that S-(Co,Fe)OOH is a promising candidate for application in the future renewable energy industry due to its high cost-effectiveness, activity, and stability during overall water splitting. In addition, the combination of a commercial silicon solar cell with an alkaline water electrolyzer has great potential for the production of hydrogen.

A Janus cobalt nanoparticles and molybdenum carbide decorated N-doped carbon for high-performance overall water splitting

The fabrication of high-performance and stable electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of importance for sustainable water-splitting technologies. Herein, the cobalt (Co) nanoparticles and molybdenum carbide (Mo₂C) heterostructures anchored N-doped carbon (Co/Mo₂C@NC-800) was designed as bifunctional electrocatalyst for overall water splitting via a simple pyrolysis approach for metal organic frameworks (MOFs) precursor. This composite shows a remarkable performance for HER and OER with a small overpotential of 121 mV and 311 mV at 10 mA cm^-2, respectively. When the optimized electrocatalyst was employed as both anode and cathode for overall water splitting in a two-electrode system, the electrolyzer achieves a low cell voltage of 1.67 V at 10 mA cm^-2 in 1 M KOH, as well as a superior and stable long-time operation of 30 h. The promising hybrid material demonstrates excellent electrocatalysis performance due to effective combination of the best of both worlds: Mo₂C with remarkable HER performance and Co nanoparticles with excellent OER activity. The Mo₂C possesses strong hydrogen binding energy and Co exhibits prominent electrical conductivity, thus the construction of heterostructures achieves more active sites with different functions and significantly boosts HER and OER process. The novel and effective synthesis strategy provides new insights into the design of outstanding non-noble metal bifunctional electrocatalysts for overall water splitting.

Self-templated formation of (NiCo)9S8 yolk-shelled spheres for high-performance hybrid supercapacitors

Rational materials design for the synthesis of desirable hollow micro- and nanostructures has recently revealed the remarkable potential for high-performance energy storage and conversion devices. Owing to their unique "core-void-shell" structural configurations, yolk-shell-structured electrode materials can achieve intimate contact with the electrolyte and alleviate the volume expansion issue during electrochemical cycling, which is therefore poised to further boost the electrochemical properties of hybrid supercapacitors. Herein, a facile self-templated strategy, consisting of a hydrothermal step and a high-temperature sulfurization process, has been developed for the construction of yolk-shell (NiCo)9S8 spheres in situ coated by graphite carbon ((NiCo)9S8/GC) due to the non-equilibrium thermal treatment of alkali metal alkoxides. The as-synthesized yolk-shelled sphere exhibits a high specific capacitance of 1434.4 F g-1 (179.3 mA h g-1) at a current density of 1 A g-1, and good rate capability and cycling stability with 83.1% capacitance retention at 8 A g-1 over 5000 cycles. To further demonstrate its practical application, a hybrid supercapacitor device was assembled using (NiCo)9S8/GC as the battery-type positive electrode and activated carbon (AC) as the capacitive-type electrode. The as-fabricated device can reach a wide voltage window of up to 1.6 V, deliver a high energy density of 55.6 W h kg-1 at a power density of 800.3 W kg-1 and maintain 90.2% of specific capacitance after 3000 cycles.

MoS2/Au0/N-CNT derived from Au(III) extraction by polypyrrole/MoS4 as an electrocatalyst for hydrogen evolution reaction

The nanotube type MoS₄^2-/polypyrrole (MoS₄-Ppy) is newly synthesized which exhibits exceptional uptake capability for Au(III) with a high removal rate of >99.99% and large K_d of >10⁷ mL/g. Through simple calcination, the Au-loaded MoS₄-Ppy is successfully converted to nitrogen-doped carbon nanotubes (N-CNT) decorated with MoS₂ nanosheets as well as Au⁰ nanoparticles, forming a novel composite of MoS₂/Au⁰/N-CNT. The conductivity of MoS₂/Au/N-CNT is substantially improved by the highly conductive N-CNT and Au⁰ metal. In addition, the N-CNT can effectively eliminate the aggregation of MoS₂ sheets and Au⁰ particles, thus endowing more exposed active sites. The optimized MoS₂/Au⁰/N-CNT displays good electrocatalytic performance towards hydrogen evolution reaction (HER). For the first time, we demonstrate the MoS₄-Ppy is a promising material for efficient extraction and recovery of trace amounts of precious metal Au from gold-bearing leachates, and the subsequent product MoS₂/Au/N-CNT can act as an effective HER electrocatalyst. The fabrication process of the composite material can not only remediate water contamination but also reuse precious metals. This dual-use approach opens a new way to explore effective sorbents for recovery of trace of gold and apply them in catalysis and sensing fields.

Graphene-Encapsulated Co9 S8 Nanoparticles on N,S-Codoped Carbon Nanotubes: An Efficient Bifunctional Oxygen Electrocatalyst

An inexpensive and efficient bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is central to the rechargeable zinc-air battery. Herein, a nanohybrid, in which N,S-codoped carbon nanotubes were decorated with Co₉ S₈ nanoparticles encapsulated in porous graphene layers, was fabricated by a one-step heat-treatment process. The N,S dopant species were the major active sites for the ORR, and Co₉ S₈ nanoparticles were mainly responsible for the OER. Compared with commercial 20 wt % Pt/C and Ir/C electrocatalysts, this nanohybrid exhibited a comparable ORR half-wave potential (0.831 V vs. reversible hydrogen electrode) and OER potential (1.591 V at 10 mA cm^-2 ), better long-term stability in an alkaline medium, and a narrower potential gap (0.76 V) between ORR and OER. Furthermore, as air electrode of the rechargeable zinc-air battery, it delivered a low charge-discharge voltage gap (0.65 V at 5 mA cm^-2 ), high open-circuit potential (1.539 V), good specific capacity (805 mA h g - 1 Zn at 5 mA cm^-2 ), and excellent cycling stability (48 h), superior to those of commercial Pt/C and Ir/C catalysts, and thus showed promise for applications in renewable energy conversion devices.