•α-Fe2O3@PPy and α-Fe2O3@NC pseudo-nanocubes composites have been synthesized.•The composite catalysts own lower intrinsic and charge transfer resistances.•The composites exhibit more pyridinic-N/FeN, graphitic-N and oxygen vacancy.•The composite catalysts display enhanced ORR catalytic performances.•α-Fe2O3@NC shows promising applications on metal-air batteries and fuel cells.Exploring of high-efficient, low-cost and eco-friendly catalysts for oxygen reduction reaction (ORR) is one of the vital issues for fuel cells and metal-air batteries. Herein, α-Fe2O3 pseudo-nanocubes have been synthesized with a facile solvothermal method, and then α-Fe2O3@NC composite catalysts were prepared through chemical polymerization of pyrrole on α-Fe2O3, following with pyrolyzed in nitrogen atmosphere. The synthesized composite catalysts display enhanced ORR catalytic performances, including the positive shifting of the onset potential, improving of the limited current density with long-term stability compared to those of the pristine α-Fe2O3. The enhanced electrocatalytic performances could be ascribed to the low intrinsic and charge transfer resistances, the high content of pyridinic-N and/or FeN, the emergence of graphitic-N and abundant oxygen vacancy on the composite surface. This study here implies that the catalytic activity and stability of metal oxides with poor conductivity could be controlled and improved by simply coating with a nanoscale conductive layer, which shows promising potential applications as precious-metal free catalysts for various metal-air batteries and fuel cells.Download high-res image (218KB)Download full-size image

•Hierarchical, freestanding bowl-like NiCo2O4 have been designed and prepared on the carbon fiber papers directly.•The synthesis approach is electrochemical deposition combined with hard template.•The NiCo2O4@CFPs could be applied as air electrode directly without binder and conductive agents.•This novel air electrode exhibits excellent cell performances for Lithium-O2 batteries.In this paper, hierarchical, freestanding bowl-like NiCo2O4 growing directly on carbon fiber paper (NiCo2O4@CFPs) are designed and synthesized through a controllable and facile electro-deposition combined with hard template method, and then applied as air electrode for long-life Lithium-O2 batteries. This novel air electrode could deliver a high discharge capacity of 9624.2 mAh g−1, low discharge/charge over-potentials (over-potential gap < 1.0 V), excellent cycleability (∼100 cycles). Moreover, it exhibits extraordinarily rate capability and reversibility. Specifically, amorphous nanosheet-like Li2O2 have been observed as the main discharge products, which might be tailored by the special bowl-like catalyst. The excellent cell performances could be ascribed to the special open structure of the synthesized bowl-like catalyst, which could provide large specific surface area, enough space for accommodating the discharge products and facilitate the transport of oxygen and electrolyte in the Lithium-O2 batteries. This facile, low-cost synthesis approach further sheds light on fabrication of various hierarchical bowl-like air electrodes including metal oxides, metals or their hybrids, and possesses promising applications on metal-air batteries and fuel cells.Hierarchical, freestanding bowl-like NiCo2O4 has been designed and synthesized on the carbon fiber papers directly and applied as air electrodes for Lithium-O2 batteries, which exhibits excellent cell performances with a high discharge capacity of 9624.2 mAh g−1, low discharge/charge over-potentials (over-potential gap < 1.0 V, charge potential < 4.0 V), excellent cycleability (100 cycles), superior rate capability and reversibility.Download high-res image (321KB)Download full-size image

Ni-doped spinel oxides NixCo1-xFe2O4 (x = 0, 0.25, 0.5, 0.75) hollow nanospheres electrocatalysts are synthesized with a simple hydrothermal approach. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the morphology, hollow and spinel structures of the cobalt ferrites remain unchanged with doping. The electrocatalytic activity of the Ni-doped CoFe2O4 with different doping contents has been studied and compared with the pure CoFe2O4 hollow nanospheres in alkaline solution by using rotating ring-disk electrode (RRDE) technique. For ORR, the Ni0.5Co0.5Fe2O4 (x = 0.5) exhibits as the most active catalyst with the highest diffusion limited current density and more positive onset potential. Whereas, the Ni0.75Co0.25Fe2O4 (x = 0.75) shows the best catalytic activity for OER with more negative onset potential (0.27 V vs. Ag/AgCl) and maximum current density (36.0 mA/cm2 at 1.0 V). X-ray photoelectron spectra (XPS) measurements reveal that the oxygen vacancy on the oxide surfaces increases, while the cations occupied ratio on octahedral/tetrahedral sites in spinel structures decreases along with the increasing of the Ni doping content. Combining with the charge transfer resistance measured by electrochemical impedance spectroscopy (EIS), these three factors work synergistically on the catalytic activities of the Ni-doped CoFe2O4 hollow nanospheres.

•Hierarchical NiCo2O4 hollow nanospheres have been prepared via a hard template method.•The synthesized NCO hollow consists of ultrathin nanosheets.•The NCO hollow exhibits excellent catalytic activity for both ORR and OER.•The NCO hollow also owns superior long-term stability.High efficient, cost-effective catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the key issue of the rechargeable lithium-air batteries. Herein, we report the facile synthesis of hierarchical NiCo2O4 (NCO) hollow nanospheres via a hard template method and following heat treatment. The synthesized hollow NCO consists of ultrathin nanosheets, which could provide larger specific surface area, more active sites and easy access channels for oxygen and electrolyte transport. The electrochemical studies reveal that the NCO hollow owns excellent ORR and OER catalytic activities, characterized with lower onset potential, higher diffusion limiting current density. Moreover, the NCO hollow shows superior stability compared to that of NCO urchin, Pt/C and RuO2 for ORR and OER, respectively. The excellent electrocatalytic activities, long-term stability for both the ORR and OER and the facile synthesis procedure make the NCO hollow a potential promising bi-functional catalyst for the metal-air batteries and fuel cells.

Phosphorus (P) and cobalt (Co) co-doped reduced graphene oxide (P-Co-rGO) has been developed and studied through a facile electrostatic assembly followed by a pyrolysis process. The prepared P-Co-rGO catalyst shows a great enhancement in the electrocatalytic activity and stability towards the oxygen reduction reaction (ORR) in alkaline solution, characterized with a positive onset potential of 0.89 V (vs. RHE), a negative shifting of only about 12.8 mV of the half-wave potential and the closest diffusion limiting current density (−5.5 mA cm−2) as compared to those of the commercial Pt/C (20 wt%). More interestingly, the prepared P-Co-rGO also exhibits excellent catalytic activity and stability for the oxygen evolution reaction (OER), with a low potential of 1.62 V (vs. RHE) at the current density of 10 mA cm−2 and a maximum current density of almost 30 mA cm−2 at 1.66 V (vs. RHE). Specifically, the prepared P-Co-rGO shows much higher activity and stability than the mono-doped reduced graphene oxide either with P or Co, respectively. This could be ascribed to the modification of the charge and spin densities and the edge and defect effects of the rGO after the co-doping of P and Co, thus resulting in a remarkable enhancement of the electrocatalytic properties for both the ORR and OER.

Boron-doped ordered mesoporous carbons (B-OMCs) with a tunable and high level of doping content (>1 wt%) have been synthesized via a one-pot solvent evaporation induced self-assembly (EISA) process. The as-prepared B-OMCs show a highly ordered 2D hexagonal mesostructure with an average pore size of 3.4–4.0 nm, which could facilitate an efficient mass transport of O2 and electrolyte during the oxygen reduciton reaction (ORR) process. The electrochemical investigations demonstrate that B-doping could significantly enhance the electrocatalytic activity of the carbon materials for the ORR in alkaline media. Specifically, the B-OMCs with a boron doping content of 1.17 wt% show the highest electrocatalytic activity and best long-term durability for ORR as compared to the non-doped OMCs and the B-OMCs with other doping contents. Combined with various physical characetrizations including X-ray diffraction, small angle X-ray scattering, N2 physisorption, Raman spectroscopy and X-ray photoelectron spectroscopy, the enhanced catalytic performance of the B-OMCs could be ascribed to the synergistic effects of the ordered mesostructure, specific surface area and moderate boron doping. This work not only helps the fundamental understanding of the correlation between the catalytic performance and the morphology, structure of the OMCs and the doping extent of heteroatoms, but also shows the promising potential applications of the B-OMCs as efficient, low-cost catalysts in metal-air batteries and fuel cells.

Efficient electrocatalyst for oxygen reduction reaction (ORR) is crucial for the performance improvement of fuel cells and metal-air batteries. However, catalyst with high activity, easy fabrication process, and low cost is still a daunting challenge. In this work, low-cost BaCO3 nanorods have been demonstrated as efficient electrocatalysts toward the ORR in alkaline media for the first time. The activity of BaCO3 nanorods can be further enhanced by hybridizing with reduced graphene oxide (BaCO3/rGO). The mechanism of ORR on the surface of BaCO3 catalyst was investigated via in situ electrochemical Raman spectroscopy (in situ EC-Raman). Our findings suggest that the barium ions on the surface of catalyst play a key role in the adsorption of oxygen molecules and the formation/decomposition of intermediates. This work provides an important insight into the catalytic activity of BaCO3 for ORR, which can serve as a guide for the design of alkali-earth metal-carbonate-based catalyst.

•A non-rare-earth element based perovskite BaMnO3 nanorods as an active electrocatalyst for the ORR and OER have been prepared and investigated for the first time.•A thin carbon-coating layer with thickness of approximately 10 nm has been successfully introduced to enhance the electrical conductivity and the electrocatalytic activities of the bare perovskite for both ORR and OER.•The stabilities of bare BaMnO3 nanorods for both ORR and OER have also been improved dramatically with the help of carbon coating, especially for the OER process.Highly efficient, low-cost catalysts, especially with bifunctional electrocatalytic capabilities for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for the wide commercialization of fuel cells and metal-air batteries. In this study, BaMnO3 - a non-rare-earth element based perovskite nanorods have been prepared and investigated for the first time, and a thin carbon-coating with a thickness of approximately 10 nm has been successfully introduced to enhance the electrical conductivity of the bare perovskite. Electrochemical tests reveal that bare BaMnO3 nanorods exhibit very good catalytic activity. More interestingly, a remarkably enhanced ORR activity for the perovskite BaMnO3 nanorods was observed after coating with a thin layer of carbon, which dominated with a direct four-electron pathway. Meanwhile, the OER process has also been enhanced extraordinarily with the carbon-coating, reaching a maximum of 14.8 mA cm−2 at 1.0 V (vs. Ag/AgCl), which is far superior to both the bare BaMnO3 nanorods and commercial Pt/C (20 wt%) catalysts. Furthermore, the stabilities of bare BaMnO3 nanorods for both ORR and OER have also been improved dramatically with the help of carbon coating. These results shed light on the significant potential application of the carbon-coated perovskite BaMnO3 porous nanorods on fuel cells and metal-air batteries as an efficient bi-functional catalyst.A thin carbon layer has been introduced to coat on the perovskite BaMnO3 nanorods by a facile method, which exhibit significantly enhanced electrocatalytic activity for both the ORR and OER with excellent stability.

Lithium-air battery has attracted extensively attention and now developing catalysts with high electrocatalytic activity is one of the challenges for lithium-air battery. In this paper, 3D hierarchical porous spinel CoFe2O4 hollow nanospheres were first prepared by a facile hydrothermal method. The hollow CoFe2O4 nanospheres have unique bimodal porous structure which consists of micropores and mesopores. The catalytic activity of the CoFe2O4 hollow nanospheres for oxygen reduction reaction (ORR) has been studied and compared with the acetylene black, the solid CoFe2O4 nanospheres and the commercial Pt/C by using rotating ring-disk electrode (RRDE) technique. The spinel CoFe2O4 hollow nanospheres exhibit superior catalytic activity for the ORR compared to the acetylene black and the solid CoFe2O4 nanospheres. Besides, the spinel CoFe2O4 hollow nanospheres also afford high catalytic activity for the oxygen evolution reaction (OER). Furthermore, the hollow CoFe2O4 nanospheres show the smallest overpotential between ORR and OER. The chronoamperometric studies show that the CoFe2O4 hollow nanospheres exhibit excellent stability for both the ORR and OER. The high ORR and OER activities and stabilities of CoFe2O4 hollow nanospheres could be attributed to their special 3D hierarchical porous structure. This material shows a significant potential application on lithium-air battery.