Trace Level Co–N Doped Graphite Foams as High-Performance Self-Standing Electrocatalytic Electrodes for Hydrogen and Oxygen Evolution

Electron-deficient Co7Fe3 induced by interfacial effect of molybdenum carbide boosting oxygen evolution reaction

Developing a high-activity and low-cost catalyst to reduce the anodic overpotential is essential for hydrogen production from water splitting. In this work, a hetero-structured Co7Fe3/Mo2C@C catalyst has been developed to efficiently catalyze oxygen evolution reaction (OER), the overpotential (ƞ10) of Co7Fe3/Mo2C@C-catalyzed OER with current density of 10 mA/cm2 is about 254 mV, substantially lower than the counterparts of Co7Fe3@C-catalyzed OER (ƞ10, 308 mV) and Mo2C@C-catalyzed OER (ƞ10, 439 mV), close to that of OER catalyzed by commercial RuO2. The mechanistic studies reveal that the distinct electron transfer across the Co7Fe3/Mo2C interface results in electron-deficient Co7Fe3, which has been identified as the highly active catalytic sites. Density functional theory (DFT) calculations manifest that Mo2C induces a distinct decrease in electron density on Co7Fe3 and upgrades the d-band centers of Co and Fe in Co7Fe3 towards Fermi energy level, thus substantially lowering the energy barrier of the rate-determining reaction step and conferring significantly improved OER activity on the Co7Fe3/Mo2C@C catalyst.

One-Step Synthesis of a Binder-Free, Stable, and High-Performance Electrode; Cu-O|Cu3P Heterostructure for the Electrocatalytic Methanol Oxidation Reaction (MOR)

Although direct methanol fuel cells (DMFCs) have been spotlighted in the past decade, their commercialization has been hampered by the poor efficiency of the methanol oxidation reaction (MOR) due to the unsatisfactory performance of currently available electrocatalysts. Herein, we developed a binder-free, copper-based, self-supported electrode consisting of a heterostructure of Cu₃P and mixed copper oxides, i.e., cuprous-cupric oxide (Cu-O), as a high-performance catalyst for the electro-oxidation of methanol. We synthesized a self-supported electrode composed of Cu-O|Cu₃P using a two-furnace atmospheric pressure-chemical vapor deposition (AP-CVD) process. High-resolution transmission electron microscopy analysis revealed the formation of 3D nanocrystals with defects and pores. Cu-O|Cu₃P outperformed the MOR activity of individual Cu₃P and Cu-O owing to the synergistic interaction between them. Cu₃P|Cu-O exhibited a highest anodic current density of 232.5 mAcm^-2 at the low potential of 0.65 V vs. Hg/HgO, which is impressive and superior to the electrocatalytic activity of its individual counterparts. The formation of defects, 3D morphology, and the synergistic effect between Cu₃P and Cu-O play a crucial role in facilitating the electron transport between electrode and electrolyte to obtain the optimal MOR activity. Cu-O|Cu₃P shows outstanding MOR stability for about 3600 s with 100% retention of the current density, which proves its robustness alongside CO intermediate.

Solvent-free synthesis of NiCo2S4 having the metallic nature

Nickel-cobalt sulfide (NiCo2S4) is a prominent member of bimetallic transition metal sulfides. It is being widely used for a variety of applications such as electrode material, photocatalysis, and energy storage devices (like pseudo capacitors, supercapacitors, solar cells, and fuel cells) due to its better electronic conductivity, manageable morphology, and high capacitance. This work presents the one-step solventless synthesis of NiCo2S4 sheet-like nanostructures and then explores their metallic nature. Scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis show the sheet-like grown morphology. Few nanorods are also seen. Except for a recent study (Xia et al. 2015) that shows metallic behavior, most of the reports show that NiCo2S4 is a semiconductor with claimed bandgap between 1.21 and 2.4 eV. In this study, we observe from UV-Vis and diffuse reflectance spectroscopy (DRS) that NiCo2S4 has a specific band gap value between 2.02 and 2.17 eV. However, IV characteristics in the temperature range of 300–400 K show that NiCo2S4 is a metal with a positive temperature coefficient of resistance consistent with a recent report. Furthermore, we see the ohmic conduction mechanism. The Arrhenius plot is drawn, and the activation energy is calculated to be 3.45 meV. The metallic nature is attributed to the coupling of two metal species (nickel and cobalt), which accounts for its superior conductivity and performance in a variety of essential applications.

Bioactive NAD+ Regeneration Promoted by Multimetallic Nanoparticles Based on Graphene-Polymer Nanolayers

The concentration of nicotinamide adenine dinucleotide oxidized form (NAD⁺) changes during aging, and the production of NAD⁺ can significantly affect both health span and life span. However, it is still of great challenge to regenerate NAD⁺ from its precursors. Herein, we introduce a method to prepare multimetallic nanoparticles (including Au, Pt, Cu, and MgO) that can efficiently promote the conversion of NADH to NAD⁺. The nanoparticles are made by mixing reduced graphene oxide-polyethyleneimine-polyacrylic acid nano-films with metallic salts, where four different metal ions are reduced and grow at the surface of the nanolayers. The morphology, size, and growth rate of nanoparticles can be controlled by adding surfactants, applying an electric field, and so forth. Our multimetallic nanoparticles exhibit excellent catalytic performance that a complete conversion of NADH to NAD⁺ can be finished in 3 min without introducing additional oxygen. This work presents a way for the preparation of multimetallic nanoparticles to promote NAD⁺ regeneration, which shows great promise for the future design of high-performance materials for antiaging.

Enhanced Electrocatalytic Activity of Nickel Cobalt Phosphide Nanoparticles Anchored on Porous N-Doped Fullerene Nanorod for Efficient Overall Water Splitting

Design and fabrication of bifunctional efficient and durable noble-metal-free electrocatalyst for hydrogen and oxygen evolution is highly desirable and challenging for overall water splitting. Herein, a novel hybrid nanostructure with Ni₂P/CoP nanoparticles decorated on a porous N-doped fullerene nanorod (p-NFNR@Ni-Co-P) was developed as a bifunctional electrocatalyst. Benefiting from the electric current collector (ECC) effect of FNR for the active Ni₂P/CoP nanoparticles, the p-NFNR@Ni-Co-P exhibited outstanding electrocatalytic performance for overall water splitting in alkaline medium. To deliver a current density of 10 mA cm^-2, the electrolytic cell assembled by p-NFNR@Ni-Co-P merely required a potential as low as 1.62 V, superior to the benchmark noble-metal-based electrocatalyst. Experimental and theoretical results demonstrated that the surface engineered FNR serving as an ECC played a critical role in accelerating the charge transfer during the electrocatalytic reaction. The present work paves the way for fullerene nanostructures in the realm of energy conversion and storage.

Surface and Interface Engineering: Molybdenum Carbide-Based Nanomaterials for Electrochemical Energy Conversion

Molybdenum carbide (Mo_x C)-based nanomaterials have shown competitive performances for energy conversion applications based on their unique physicochemical properties. A large surface area and proper surface atomic configuration are essential to explore potentiality of Mo_x C in electrochemical applications. Although considerable efforts are made on the development of advanced Mo_x C-based catalysts for energy conversion with high efficiency and stability, some urgent issues, such as low electronic conductivity, low catalytic efficiency, and structural instability, have to be resolved in accordance with their application environments. Surface and interface engineering have shown bright prospects to construct highly efficient Mo_x C-based electrocatalysts for energy conversion including the hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. In this Review, the recent progresses in terms of surface and interface engineering of Mo_x C-based electrocatalytic materials are summarized, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and others electronic conductive species. Finally, the challenges and prospects for energy conversion on Mo_x C-based nanomaterials are discussed in terms of key performance parameters for the catalytic performance.

A Co-MOF-derived Co9S8@NS-C electrocatalyst for efficient hydrogen evolution reaction

The exploitation of efficient hydrogen evolution reaction (HER) electrocatalysts has become increasingly urgent and imperative; however, it is also challenging for high-performance sustainable clean energy applications. Herein, novel Co9S8 nanoparticles embedded in a porous N,S-dual doped carbon composite (abbr. Co9S8@NS-C-900) were fabricated by the pyrolysis of a single crystal Co-MOF assisted with thiourea. Due to the synergistic benefit of combining Co9S8 nanoparticles with N,S-dual doped carbon, the composite showed efficient HER electrocatalytic activities and long-term durability in an alkaline solution. It shows a small overpotential of -86.4 mV at a current density of 10.0 mA cm-2, a small Tafel slope of 81.1 mV dec-1, and a large exchange current density (J 0) of 0.40 mA cm-2, which are comparable to those of Pt/C. More importantly, due to the protection of Co9S8 nanoparticles by the N,S-dual doped carbon shell, the Co9S8@NS-C-900 catalyst displays excellent long-term durability. There is almost no decay in HER activities after 1000 potential cycles or it retains 99.5% of the initial current after 48 h.

N, S and Transition-Metal Co-Doped Graphene Nanocomposites as High-Performance Catalyst for Glucose Oxidation in a Direct Glucose Alkaline Fuel Cell

In this work, reduced graphene oxide (rGO) nanocomposites doped with nitrogen (N), sulfur (S) and transitional metal (Ni, Co, Fe) were synthesized by using a simple one-step in-situ hydrothermal approach. Electrochemical characterization showed that rGO-NS-Ni was the most prominent catalyst for glucose oxidation. The current density of the direct glucose alkaline fuel cell (DGAFC) with rGO-NS-Ni as the anode catalyst reached 148.0 mA/cm2, which was 40.82% higher than the blank group. The DGAFC exhibited a maximum power density of 48 W/m2, which was more than 2.08 folds than that of blank group. The catalyst was further characterized by SEM, XPS and Raman. It was speculated that the boosted performance was due to the synergistic effect of N, S-doped rGO and the metallic redox couples, (Ni2+/Ni3+, Co2+/Co3+ and Fe2+/Fe3+), which created more active sites and accelerated electron transfer. This research can provide insights for the development of environmental benign catalysts and promote the application of the DGAFCs.

Heterogeneous Co–CN nanofibers with controlled active terminal N sites for hydrogen evolution reaction

The terminal N sites derived from the thermal fusion process exhibit excellent performance for hydrogen evolution reaction.

A multi-interfacial FeOOH@NiCo2O4 heterojunction as a highly efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic water decomposition is the key to sustainable energy, and the design and synthesis of cost-effective electrocatalysts is the main objective of electrocatalytic water splitting. In this paper, multi-interfacial FeOOH@NiCo2O4 hybrid nanoflowers are prepared through a two-step hydrothermal reaction. In such heterostructures, NiCo2O4 nanoflowers are coated with a layer of FeOOH nanoparticles. In addition, the obtained electrocatalyst could provide abundant electroactive sites and the formation of FeOOH@NiCo2O4 nanointerfaces can also improve the charge transfer rate. As a result, under the HER and OER conditions, the prepared catalysts show an outstanding electrocatalytic performance. Moreover, in a two-electrode water splitting system, the FeOOH@NiCo2O4 heterostructure, as a dual-function electrocatalyst, needs a cell voltage of only 1.58 V at a current density of 10 mA cm-2. This study provides a facile and feasible method to construct different kinds of heterostructures as bifunctional electrocatalysts with multiple interfaces by a simple hydrothermal method.

Cobalt Colloid-derived Efficient and Durable Nanoscale Electrocatalytic Films for High-Activity Water Oxidation

Oxygen evolution reaction is of immense importance and is vitally necessary for devices such as electrolyzers, fuel cells, and other solar and chemical energy conversion devices. The major challenges that remain in this quest are due to the lack of effective catalytic assemblages operating with optimum efficiency and obtainable following much simpler setups and easily accessible methods. Here, we demonstrate that the robust electrocatalytic activity toward water oxidation can be achieved employing straightforwardly obtainable nanoscale electrocatalysts derived from easily made colloidal-cobalt nanoparticles (Co-CNPs) prepared in clean carbonate systems. Thin-film non-noble metal nanoscale electrocatalysts such as simple Co-CNPs/FTO and annealed Co-CNPs/FTO250 and Co-CNPs/FTO500 obtained by depositing Co-CNPs on the FTO substrate are shown to initiate water oxidation at much lower overpotentials such as just 240 mV for Co-CNPs/FTO250 under mildly alkaline conditions while demonstrating an impressive Tafel slope of just 40 mV dec-1. Furthermore, the robust catalyst demonstrated a high electrochemical surface area of 91 cm2 and high turnover frequency and mass activity of 0.26 s-1 and 18.84 mA mg-1, respectively, just at 0.35 V, and superior durability during long-term electrolysis. These outstanding catalytic outcomes using easily prepared Co-CNPs/FTO250-type catalytic systems are comparable and even better than other noble and non-noble metal-based nanoscale catalytic assemblages obtained by much difficult methods. Most advantageously, the colloidal route also offers the easiest approach of incorporating carbon contents in the catalytic layer, which can ultimately increase mechanical stability and mass transfer capability of the system.

A layer-by-layer strategy for the scalable preparation of uniform interfacial electrocatalysts with high structural tunability: a case study of a CoNP/N,P-graphene catalyst complex

Electrocatalysts are important for providing clean energy and have attracted intense research attention. All electrocatalysts must function on electrode surfaces; however, interfacial engineering strategies for electrocatalytic structures remain understudied, and scalable preparation methods are especially rare. In this study, we propose a strategy that employs a layer-by-layer (LbL) assembly, subsequent in-film deposition, and calcination to prepare a complex Co nanoparticle (CoNP)/N,P-graphene catalyst on various 2D and 3D electrode surfaces. We delicately adjusted a variety of parameters and demonstrated that our LbL-based method can finely tune the total amount, the doping fraction, and the electronic structure of the complex catalysts, and provide optimal catalytic performance. When prepared on a large piece of carbon cloth, the catalysts showed a highly uniform catalytic performance, demonstrating the capability of our method for scalable fabrication. Our study emphasizes the delicate nature of the interfacial engineering of electrocatalysts, and promotes functional interface design and mechanism studies.

Boosting the activity of Prussian-blue analogue as efficient electrocatalyst for water and urea oxidation

The design and fabrication of intricate hollow architectures as cost-effective and dual-function electrocatalyst for water and urea electrolysis is of vital importance to the energy and environment issues. Herein, a facile solvothermal strategy for construction of Prussian-blue analogue (PBA) hollow cages with an open framework was developed. The as-obtained CoFe and NiFe hollow cages (CFHC and NFHC) can be directly utilized as electrocatalysts towards oxygen evolution reaction (OER) and urea oxidation reaction (UOR) with superior catalytic performance (lower electrolysis potential, faster reaction kinetics and long-term durability) compared to their parent solid precursors (CFC and NFC) and even the commercial noble metal-based catalyst. Impressively, to drive a current density of 10 mA cm-2 in alkaline solution, the CFHC catalyst required an overpotential of merely 330 mV, 21.99% lower than that of the solid CFC precursor (423 mV) at the same condition. Meanwhile, the NFHC catalyst could deliver a current density as high as 100 mA cm-2 for the urea oxidation electrolysis at a potential of only 1.40 V, 24.32% lower than that of the solid NFC precursor (1.85 V). This work provides a new platform to construct intricate hollow structures as promising nano-materials for the application in energy conversion and storage.