Yongfang Chen

Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst

The replacement of platinum with non-precious-metal electrocatalysts with high efficiency and superior stability for the hydrogen-evolution reaction (HER) remains a great challenge. Herein, we report the one-step synthesis of uniform, ultrafine molybdenum carbide (Mo2C) nanoparticles (NPs) within a carbon matrix from inexpensive starting materials (dicyanamide and ammonium molybdate). The optimized catalyst consisting of Mo2C NPs with sizes lower than 3 nm encapsulated by ultrathin graphene shells (ca. 1-3 layers) showed superior HER activity in acidic media, with a very low onset potential of -6 mV, a small Tafel slope of 41 mV dec(-1), and a large exchange current density of 0.179 mA cm(-2), as well as good stability during operation for 12 h. These excellent properties are similar to those of state-of-the-art 20% Pt/C and make the catalyst one of the most active acid-stable electrocatalysts ever reported for HER.

In situ growth of spinel CoFe2O4 nanoparticles on rod-like ordered mesoporous carbon for bifunctional electrocatalysis of both oxygen reduction and oxygen evolution

The lack of efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a fatal issue for the development of metal–air batteries in large-scale commercialization. In this paper, spinel CoFe2O4 (CFO) nanoparticles were successfully in situ grown onto rod-like ordered mesoporous carbon (RC) by a facile, scalable hydrothermal method, followed by annealing at different temperatures. The as-acquired CFO/RC nanohybrid pyrolyzed at 400 °C (CFO/RC-400) has a high specific surface area (150.3 m2 g−1) and two sets of uniform mesopore systems (3.38 and 19.1 nm), all of which are favorable for the improvement of the electrocatalytic activity. The hybridization of CFO nanoparticles and the RC matrix results in increased ORR and OER electrocatalytic activity of the CFO/RC nanohybrids, which is significantly superior to that of unsupported CFO nanoparticles and pure RC. CFO/RC-400 shows better catalytic activity for the ORR with a direct four-electron reaction pathway than those prepared at other temperatures in terms of the onset potential and limiting current density. Furthermore, the CFO/RC-400 nanohybrid exhibits outstanding durability for both the ORR and OER, and can outperform commercial Pt/C. The excellent bifunctional electrocatalytic activities of the CFO/RC nanohybrids are mainly owing to the hierarchical mesoporous structures of the nanohybrids and strong coupling between the CFO nanoparticles and the RC matrix.

Ditungsten carbide nanoparticles encapsulated by ultrathin graphitic layers with excellent hydrogen-evolution electrocatalytic properties

The development of efficient non-precious-metal electrocatalysts towards the hydrogen evolution reaction (HER), with superior activity and stability, remains a great challenge in the area of renewable energy. In this work, we demonstrated a facile, one-step protocol to synthesize ultrathin graphitic layer (GL)-encapsulated ultrafine ditungsten carbide (W2C) nanoparticles (W2C@GL) with sizes smaller than 10 nm, exhibiting a superior HER activity in acidic solution. An efficient W2C phase, along with an improved electron transfer process by GL wrapping, cooperatively leads to a small Tafel slope of 68 mV dec−1 and a large exchange current density of 0.24 mA cm−2 for W2C@GL, which exceeds the previous W2C materials by far. Over 91% of the current density is maintained after over 8 h of operation, which indicates a good stability of this hybrid catalyst. Thus, W2C@GL with these excellent properties has been among the best non-noble metal HER electrocatalyst reported to date.

Solvothermally synthesized graphene nanosheets supporting spinel NiFe2O4 nanoparticles as an efficient electrocatalyst for the oxygen reduction reaction

The production of efficient and low-cost electrocatalysts for the oxygen reduction reaction (ORR) is one of the key issues for the extensive commercialization of fuel cells. In this paper, we describe a facile one-pot hydrothermal synthesis route to in situ grow spinel NiFe2O4 nanoparticles onto the graphene nanosheets which were produced in advance by a scalable solvothermal reduction of chloromethane and metallic potassium. The resultant NiFe2O4/graphene nanohybrid exhibits superior electrocatalytic activity for the ORR to pure graphene nanosheets and unsupported NiFe2O4 nanoparticles, which mainly favours a desirable direct 4e− reaction pathway during the ORR process. Meanwhile, the NiFe2O4/graphene nanohybrid exhibits the outstanding long-term stability for the ORR, outperforming the commercial 20 wt% Pt/C based on the current–time chronoamperometric test. The excellent catalytic activity and stability of NiFe2O4/graphene nanohybrid are ascribed to the strong coupling and synergistic effect between NiFe2O4 nanoparticles and graphene nanosheets.

Spinel nickel ferrite nanoparticles strongly cross-linked with multiwalled carbon nanotubes as a bi-efficient electrocatalyst for oxygen reduction and oxygen evolution

It is of great concern to explore new electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, spinel NiFe2O4 nanoparticles cross-linked with the outer walls of multiwalled carbon nanotubes (MWCNTs) were successfully prepared by a simple, scalable hydrothermal method. The as-synthesized NiFe2O4/MWCNT nanohybrid shows not only a better ORR catalytic activity than pure NiFe2O4 and MWCNTs, but also a close four-electron reaction pathway. Meanwhile, the NiFe2O4/MWCNT nanohybrid exhibits a much higher OER catalytic activity when compared to NiFe2O4, MWCNTs and commercial Pt/C in terms of the onset potential and current density. Moreover, the NiFe2O4/MWCNT nanohybrid demonstrates the preeminent long-term durability measured by the current–time chronoamperometric test for both the ORR and OER, which evidently outperforms commercial Pt/C. The excellent bi-functional electrocatalytic activities of the NiFe2O4/MWCNT nanohybrid are attributed to the strong coupling between the NiFe2O4 nanoparticles and the MWCNTs as well as the network structure.

Carbon dioxide activated carbon nanofibers with hierarchical micro-/mesoporosity towards electrocatalytic oxygen reduction

Polyacrylonitrile (PAN)-based carbon nanofibers prepared by electrospinning were physically activated using carbon dioxide as the oxidizing agent. The activation procedure was performed at 800 °C for different periods of time ranging from 15 to 60 min. The activated materials have a hierarchical structure with two sets of pore systems in the micropore range centered at ∼0.8 nm and small mesopore range centered at ∼2.8 nm. The activation not only increased the specific surface area and pore volume to 1123 m2 g−1 and 0.64 cm3 g−1, respectively, but also resulted in the evident loss of doped N atoms. The pyridinic and graphitic nitrogen groups are dominant among various N functional groups in the activated samples. CACNF-60, prepared by activating the carbon nanofibers (CNFs) for 60 min, showed excellent electrocatalytic activity for the oxygen reduction reaction (ORR) as well as superior long-term stability and methanol tolerance compared to commercial Pt/C in alkaline media. The excellent electrocatalytic activity of the activated sample is mainly due to its high N content (6.9 at%), unique hierarchical micro-/mesoporosity, and large specific surface area.

The direct growth of highly dispersed CoO nanoparticles on mesoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction

Cobalt monoxide (CoO) nanoparticles (NPs) and mesoporous carbon (MC) with large specific surface area were combined as a novel nanocomposite (CoO/MC) using a hydrothermal method to reveal outstanding electrocatalytic activity in the oxygen reduction reaction (ORR). The addition of polyvinylpyrrolidone (PVP) as the surfactant during the hydrothermal process is beneficial for the high dispersion of CoO NPs on the surface of MC. Among the as-acquired products, the CoO/MC nanocomposite prepared with 1.5 g PVP as the surfactant (CoO/MC-1.5) exhibits much better catalytic activity for the ORR with a more positive onset potential, a highly efficient four-electron transfer pathway and a larger current density than the others. Furthermore, the CoO/MC-1.5 nanocomposite demonstrates outstanding durability based on current–time chronoamperometric tests, which significantly prevails over a commercial Pt/C catalyst. The eminent catalytic activities of the CoO/MC-1.5 nanocomposite should be a result of the synergistic effect of the highly dispersed CoO nanoparticles and the ordered mesostructures with large specific surface area, which are advantageous for increasing the exposure of the active sites and promoting fast transfer of the reactants and products.