Search

Developing new forms of flexible zinc–air (Zn–air) batteries is crucial to economic growth. Nevertheless, the absence of an oxygen evolution reaction catalyst with high catalytic efficiency and low-cost causes instability of Zn–air battery cycling and economic unviability, respectively. In this paper, a leaf-like N-doped carbon structure with CoS2 nanoparticles self-supported on carbon fiber paper (CoS2-CZ@CFP) was developed as an air electrode for Zn–air batteries, which not only uses nonprecious metal elements, but also has a self-supporting configuration, enhancing the contact between the catalyst and electrode while facilitating the transfer of electrons. Sulfur as the source gas was introduced into Co-doped metal–organic skeletons (Co-ZIFs) anchored on carbon fiber paper (CFP) for in situ vulcanizing transition metal Co, which successfully ensures the uniform distribution of sulfide particles in the carbon matrix, hence enriching active catalyst sites. The CoS2-CZ@CFP demonstrated an overpotential of 1.611 V at J = 50 mA cm−2, superior to RuO2@CFP (1.814 V). A Zn–air battery fabricated using the synthesized CoS2-CZ@CFP exhibited an extended life cycle of 117 h. This research paves the way for a novel method of designing self-supporting configurations of sulfides and carbon matrix composites as promising air electrodes applicable to Zn–air batteries. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks (MOF) were used as templates to prepare transition metal sulfides and phosphides in order to expose the active site of the material to a greater extent. Utilizing the synergistic effect of bimetals, Ni2+ and Mo6+ were selected as metal ion centers and trimesic acid was selected as the ligand to prepare NiMo-MOFs by the hydrothermal method. Then, NiMo-P, NiMo-S and NiMo-P/S were prepared using NiMo-MOFs as the precursor, and the preparation conditions of the abovementioned hybrids were optimized through linear sweep voltammetry. The results show that when the molar ratio of NiMo-MOF to NaH2PO2 is 1: 5, and the as-prepared NiMo-P is further doped with S in a proportion of 1: 1, the overpotential of the final NiMo-P/S hybrid is 313 mV at a current density of 10 mA cm-2, and its electrochemical impedance (Rct), Tafel slope and electrochemical active surface areas are 22.38 Ω, 63.8 mV dec-1, and 707.5 cm-2, respectively. The optimized NiMo-P/S hybrid provides more active sites and reaction areas along with good stability, which is conducive to promoting the kinetics of the oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

Cellulose nanofiber (CNF) is an environmentally friendly material with a high specific surface area, which is an ideal candidate for cathode catalysts in zinc–air batteries (ZABs). However, untreated CNF-based carbon materials suffer from low intrinsic activity and fewer micro-mesopores. Herein, we developed an N-doped porous carbon aerogel from CNF and graphite phase carbon nitride (g-C3N4) via hydrothermal pretreatment and the carbonization process. The CNF can serve as the skeleton of the aerogel that leads to an interconnected porous structure, while g-C3N4 guarantees the in situ N doping and the formation of more nanopores. Therefore, the as-prepared carbon aerogel exhibits excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances owing to the successful N doping with sufficient graphite-N and pyridine-N active sites (69.1%) and high specific surface areas (785.4 m2 g−1). The assembled ZABs with a carbon aerogel deliver a specific capacity of 790 mA h g−1 and an energy density of 945.6 W h kg−1 with a long-term stability over 100 h. This work opens a new avenue for the effective conversion of abundant biomass into bifunctional air cathodes for energy-related applications. [ABSTRACT FROM AUTHOR]

Medium entropy alloys, consisting of four elements in close to isotropic proportions, have attracted a great deal of attention because of their unique properties (excellent strength, corrosion resistance, etc.). Here, we present a series of quaternary medium entropy alloy FeCoNiW electrocatalysts loaded onto nitrogen-doped carbon (NC) skeletons. The prepared medium entropy alloy catalysts have excellent OER catalytic activity and stability. In alkaline media, the FeCoNiW-NC electrocatalysts annealed at 350 °C were able to achieve an overpotential as low as 225 mV at a current density of 10 mA cm−2, which is 55 mV lower than that of the commercial RuO2, while it has a smaller Tafel slope of 54 mV dec−1. The introduction of W leads to a change in the electronic structure of the FeCoNi alloy and increases the specific surface area of the catalyst, exhibiting excellent performance. This work contributes to the exploration of OER catalysts by opening new avenues for the design of efficient and economical OER electrocatalysts. [ABSTRACT FROM AUTHOR]

Direct urea fuel cells (DUFCs) offer an environmentally friendly, and cost-effective way to turn wastewater into energy. However, the paucity of efficient, cost-effective, and electrochemically stable urea electro-oxidation reaction (UOR) specific electrocatalysts continues to impede the design and development of practically useful DUFCs. The present work explores the design and development of N-doped graphene (NGr)-supported platinum (Pt) plus silver (Ag) alloy nanohybrids (PtxAg100−x-NGr) as potential UOR electrocatalysts. Our results suggest that the electrocatalytic performance of PtxAg100−x-NGr is very sensitive toward the composition of this nanohybrid. The nanohybrid with a Pt : Ag ratio of 1 : 1, referred to as Pt50Ag50-NGr in the MS, exhibits the best UOR electrocatalytic performance. The Pt50Ag50-NGr composite exhibits a Tafel slope of just ∼12.92 mV dec−1, and a UOR-specific activity of nearly 4028 mA cm−2 mg cat−1 at 1.673 V (vs. RHE) and requires an overpotential of just 1.617 V (vs. RHE) to maintain a UOR specific current density of 10 mA cm−2. These parameters qualify the Pt50Ag50-NGr as a promising anode material for DUFCs. This we demonstrate through the design of a Pt50Ag50-NGr anode-based prototype urea-H2O2 fuel cell that delivers an open circuit voltage (OCV) of 750 mV and a power density of ∼5.75 mW cm−2. [ABSTRACT FROM AUTHOR]

Single-atom catalysts (SACs) are widely used as catalysts for overall water splitting due to their maximum utilization of surface active sites. However, until now, efficient SACs for water splitting are still lacking. Herein, a systematic screening of the overall water splitting performance of Fe, Co, and Ni atoms anchored on M2C, nitrogen modified M2C, and Fe and N co-modification M2C MXenes is performed by using first-principles calculations. The results show that Fe/Mo2C exhibits better hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances among the studied TM/Mo2C structures, with the corresponding value of ΔGH* and ηOER being 0.03 eV and 0.66 V, respectively. In addition, there is an obvious volcanic curve relationship between the d-band center (εd) of TM/M2C and the overpotential of the OER (ηOER). Low N doping concentrations (25%) can also adjust the HER and OER activity of M2C. Interestingly, N doping and Fe loading co-modification Mo2C (Fe/N-Mo2C) shows the best HER and OER properties among the studied TM/N-Mo2C structures, with the corresponding ΔGH* and ηOER being 0.01 eV and 0.39 V, respectively. The thermodynamics binding energy and dynamics simulation results reveal that TM/M2C and TM/N-M2C are dynamically stable at the reaction temperature. This work shows that Fe/N-Mo2C can be a potential catalyst for overall water splitting. [ABSTRACT FROM AUTHOR]

Developing cost-efficient, stable, and environmentally benign electrocatalysts is a strong enticement towards the realization of commercial clean energy devices. Ni–Co-based systems have been particularly interesting owing to the well-recognized catalytically active redox pairs of Co3+/Co2+ and Ni3+/Ni2+. The sulphides of the latter have been frequently investigated catalyst systems with a particular focus on the sulphide-based heterostructured unions that offer better interfacial contacts. In this study, we report an electrochemically efficient hetero-catalyst (Co9S8–NiCo2S4/N-rGO) consisting of a binary sulphide–sulphide system, Co9S8/NiCo2S4, anchored on N-doped rGO) through a simple sulphurization strategy. The hetero-catalyst with ideally integrated interfaces ensures facile interfacial electron movement within the sulphide components and the rGO backbone, rendering excellent electrocatalytic properties. The highly exposed surface and N-doping in rGO further offer to expedite the mass transfer rate, and the suitable integration with the latter reduces the electrolyte corrosion to improve the activity and stability. Co9S8–NiCo2S4/N-rGO exhibits impressive bifunctional activity with an overpotential requirement of 149 mV for a benchmark hydrogen evolution reaction (HER) current of 10 mA cm−2 and an overpotential requirement of 230 mV for a benchmark oxygen evolution reaction (OER) current of 20 mA cm−2. The enhanced HER and OER kinetics are further supported by low Tafel slopes of 89 mV dec−1 and 93 mV dec−1, respectively. The full cell constructed from Co9S8–NiCo2S4/N-rGO exhibits water-splitting to the tune of 10 mA cm−2 and 100 mA cm−2 at cell voltages of 1.59 V and 2.1 V, respectively. Moreover, excellent catalytic stability was demonstrated with negligible loss in the catalytic performance for 15 continuous hours of electrocatalysis. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) is involved in various renewable energy systems, such as water-splitting cells and metal–air batteries. Cobalt phosphide nanomaterials have been reported as promising OER electrocatalysts. Herein, the rational design of advanced nanostructures for nitrogen doped and carbon coated cobalt phosphide hollow nanospheres (CoP@NC HNS) was highly desirable to optimize their electrocatalytic performance. SiO2 nanospheres were used as a sacrificial template for the synthesis of hollow nanospheres composed of ultrathin CoP@NC nanosheet arrays. The hollow nanostructure CoP@NC was coated with a layer of carbon material and the thickness of the shell was about 40 nm. The resultant CoP@NC HNS manifested high electrocatalytic activity towards the OER with low overpotential (320 mV, 10 mA cm−2), small Tafel slope (68 mV dec−1), and remarkable stability in alkaline electrolyte. Remarkably, the CoP@NC HNS still maintained an outstanding performance for the OER at 60 °C. [ABSTRACT FROM AUTHOR]

In the current work, TiO2 nanoarchitectures with high surface area and charge density have been synthesized by a facile hydrothermal method and further modified via co-doping with a transition metal (nickel) and rare-earth metals (europium, cerium, and holmium). Different characterization tools (i.e., XRD, BET surface area, FTIR, Raman, PL, SEM, TEM, XAS, XPS, and DRS) have been used to check the effect of co-doping on the electrical, morphological, and structural properties of the TiO2 nanoarchitectures. The FTIR, Raman, and XRD results confirmed the successful co-doping of the TiO2 nanoarchitectures. The high surface area of the co-doped nanoarchitectures as compared to undoped and singly doped TiO2 was confirmed from BET measurements. The presence of dopants in various oxidation states was further elucidated via XPS and XAS studies. FE-SEM and TEM revealed the formation of interconnected, mesoporous-type nanoarchitectures, with particle sizes less than 100 nm. The supercapacitive behavior of synthesized nanoarchitectures was evaluated by performing cyclic voltammetry, linear scan voltammetry, galvanostatic charge–discharge, and electrochemical impedance measurements. The specific capacitance values (Csp) indicate that the co-doping of rare-earth metals into the TiO2 structure results in enhanced capacitance of the target materials. The highest capacitive behavior was observed in the Ni-Ho-3 sample (740 F g−1), followed by Ni-Eu-3 (630 F g−1), Ni-Ce-3 (552 F g−1), and Ni-5 (545 F g−1), as compared to that of pure TiO2 (240 F g−1). Furthermore, the Ni-Ho-3 electrode also showed better cyclic stability with 97% capacity retention after 1000 cycles at a high current density of 5 A g−1. The water oxidation studies also showed that all the doped TiO2 nano-catalysts show enhanced oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity. The OER overpotential values at 10 mA cm−2 for Ni-Eu-3, Ni-5, Ni-Ce-3, and Ni-Ho-3 are 520, 550, 600, and 620 mV, respectively. The Tafel slopes of undoped TiO2, Ni-5, Ni-Eu-3, Ni-Ce-3, and Ni-Ho-3 are measured to be 151, 143, 134, 141, and 137 mV dec−1, respectively. The smallest Tafel slope of Ni-Eu-3 indicated its highest OER efficiency among all the selected samples. As compared with the other samples, Ni-Ho-3 sample also showed a lower value for the Tafel slope. The superior performance of co-doped nanoarchitectures was credited to the development of mesoporous structures with more electroactive sites, high charge density, and better charge transfer (EIS results). [ABSTRACT FROM AUTHOR]