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La/Fe co-doped Co3O4 nanoparticles supported on aminated CNTs have shown high catalytic activities for the ORR and OER. The specific La/Fe co-doped structure of Co3O4 is shown to be the main origin of their high catalytic activities.

Understanding of the structure changes of the catalysts during hydrogen (HER) and oxygen (OER) evolution reactions is of paramount importance for design of catalysts with enhanced performance. This work reports the structure change investigations of phosphide-based catalysts in both HER and OER. Specifically, nickel foam supported Co2P-Ni2P (Co2P-Ni2P/NF) is synthesized by in-situ growth of Co2P-Ni2P on the current collector, i.e. nickel foam, and then used for the investigation of the structure changes during HER and OER. This method avoids the use of polymer binders, allowing for the systematic analysis of the structure changes of the catalysts after HER and OER. The obtained Co2P-Ni2P/NF shows an excellent catalytic bifunctionality for HER and OER in alkaline solutions. It only needs an overpotential of 90 mV vs RHE to offer a current density of 10 mA cm−2 for HER and needs an overpotential of 230 mV vs RHE to offer a current density of 50 mA cm−2 for OER. Both HER and OER activities of Co2P-Ni2P/NF are higher than those of most bifunctional catalysts reported. Post-analysis shows the occurrence of the surface roughening and the formation of M P O (M Co and Ni) at the surface in its initial period of the HER, and the surface roughening and the formation of a thin layer of amorphous MOOH (M Co and Ni) at the surface in its initial period of OER. The surface roughening and formation of M P O and amorphous hydroxide layer can be attributed to the main reason responsible for its high performance for HER and OER. Additionally, the Co2P-Ni2P/NF is also usable as both the cathode and anode for an electrolyzer for overall water splitting and shows high performance.

Nanosized octahedral LiNi0.5Mn1.5O4 with predominant (111) facet has been successfully fabricated using Mn3O4 nanoparticles precursors via a two-step synthesis, which involves a hydrothermal treatment and the subsequent calcination. The physical properties of the Mn3O4 precursor and the resultant LiNi0.5Mn1.5O4 were characterized by XRD (X-ray diffraction), TEM (transmission electron microscopy) and SEM (scanning electron microscopy). The charge-discharge tests show that the resultant LiNi0.5Mn1.5O4 exhibits excellent cyclability and rate capability, which delivers a discharge capacity of about 117 mAh g− 1 after 300 cycles, and maintains 94% of its initial discharge capacity (124.7 mAh g− 1) at 1C, even at a rate of 40C, a specific capacity of 99.2 mAh g− 1 could be still obtained for the O-LNMO. The superior electrochemical performance of the LNMO is mainly attributed to the synergistic effect of the nanosized octahedral structure and exposed (111) facets of the prepared LiNi0.5Mn1.5O4. We found that the nanosized octahedral structure can not only accommodate the lattice stress caused by John-Teller distortion but also provide short paths for Li+ ion transportation in the material. Additionally, the obtained predominant (111) facet is helpful to the formation of protective SEI film on the spinel LiNi0.5Mn1.5O4 electrode.

Core-shell Fe3O4@CoO NCs have been demonstrated to be efficient bifunctional catalysts for the oxygen reduction (ORR) and evolution (OER) reactions. Their activities are strongly shell thickness dependent. Specifically, nanocrystals with ∼2 monolayers of CoO can exhibit a potential difference of 0.794 V at OER and ORR current densities of 10 and -3 mA cm-2, respectively. This value is competitive to those of most active bifunctional catalysts reported. In addition, they are also used as the oxygen cathode for Zn-air batteries and can deliver a peak power density of 109 mW cm-2, much higher than that of the Pt-RuO2/C (88.1 mW cm-2).

A metal-free highly torn amine functionalized nitrogen doped graphene (HT-AFNG) used as hydrogen evolution reaction (HER) catalyst is prepared by using a simple synthesis method involving the hydrothermal reaction of graphene oxide in the presence of ammonia and the subsequent ball milling. The metal-free HT-AFNG is efficient for the HER in the acid solution and can deliver an onset potential of 100 mV and an overpotential of 350 mV at the current density of 10 mA cm−2, which is much lower than those of the singly and dually doped graphene reported previously. The amine functionalized and nitrogen doped structure plays an important role in the high catalytic activity of the HT-AFNG, where the amine group can greatly reduce the |ΔGH*| value at both defect and edge sites as well as increases the electron transfer capability of nitrogen dope graphene (NG). Significantly, the highly torn structure also makes a big contribution on the high catalytic activity of the HT-AFNG, since it allows for the good accessibility of the active sites for the HER. The strategy involves the introduction of electron donating groups open a new research pathway towards the improvement of HER catalytic activity of graphene-based materials.

Development of advanced electrocatalysts to drive efficient hydrogen evolution reaction through water splitting is vital in many energy related technologies. Herein the development of nitrogen-doped carbon supported fewer-layer MoSe2 (FL-MoSe2/NC) through calcining the hydrothermal product of the Se and Mo precursors in the presence of ammonium citrate is reported. This FL-MoSe2/NC is highly efficient for the hydrogen evolution reaction (HER) and exhibits a remarkably low onset potential of 15.2 mV, an extremely small overpotential of 74.6 mV (vs. RHE) at 10 mA cm−2, a greatly reduced Tafel slope of 45.2 mV dec−1, and a higher exchange current density of 0.266 mA cm−2. The performance of the FL-MoSe2/NC is much higher than that of most Mo-based HER electrocatalysts reported to date and even comparable to that of other most active non-precious metal based catalysts, suggesting the great potential of using the FL-MoSe2/NC as an efficient catalyst for the HER. The NC is believed to play an important role in the high performance of the FL-MoSe2/NC. It does not only promote a strong interaction between MoSe2 and the NC, but facilitates the formation of MoSe2 with few layers and shorter lattice fringe lengths, which expose more active sites accessible to the HER.

A simple co-polymerization method has been employed to prepare polysilicic-zirconium (PSZr), consisting of chemical species of ZrSi24O50, ZrSiO4, and Zr5Si3. The obtained PSZr is found to be highly active as the coagulant for the purification of the raw water and humic acid wastewater. The performance of PSZr on the wastewater purification is greatly dependent upon the synthetic temperature of PSZr, and the mole ratio of Si:Zr, and the basicity of the PSZr. It shows that PSZr synthesized at the temperature of 20–25 °C with the mass fraction of polysilicic acid of 1.5%, the mole ratio of Si:Zr of 1:1, and a basicity of 0.25, is relatively more efficient for water purification and could achieve the UV254 (Ultraviolet wavelength less than 254 nm) and total organic carbon removals of 81.23% and 66.85% and residual turbidity of 0.86 NUT in the raw water, and the UV254 and TOC removals of 99.20% and 74.21% in simulated humic acid wastewater, respectively. These results clearly demonstrate that PSZr could be used as the coagulant for the wastewater treatments.

A simple synthetic method is developed for the synthesis of CoS2/N, S-codoped graphene. The result shows the existence of a strong electronic coupling between CoS2 and N, S-codoped graphene. The pyrrolic and pyridinic type nitrogen and S in the form of C-S-C in N, S-codoped graphene are found to be the anchoring sites of the CoS2 nanoparticles. As a bifunctional catalyst, the CoS2/N, S-codoped graphene exhibits an oxygen reduction onset potential of 0.963 V vs. RHE and delivers an oxygen evolution overpotential of 393 mV at the current density of 10 mA cm−2. Its oxygen reduction and evolution catalytic activities are comparable to those of the Pt/C and the state-of-art RuO2/C, respectively. Most impressively, the CoS2/N, S-codoped graphene exhibits a potential gap of 771 mV. This value is lower than those of most bifuntional catalysts reported, clearly indicating its potential use as the bifunctional catalyst to replace the noble-metal based catalysts for practical applications. Additionally, our results also suggest a great importance to prepare a single pure phase CoS2 in improving the catalytic bifunctionality of the CoS2/N, S-codoped graphene. The primary Zn-air battery with CoS2/N, S-codoped graphene shows a higher discharge peak power density than that with Pt/C.

A composite with a hierarchical structure consisting of nitrogen doped carbon nanosheets with the deposition of nitrogen doped carbon coated Co–CoO nanoparticles (Co–CoO@NC/NC) has been synthesized by a simple procedure involving the drying of the reaction mixture containing Co(NO3)2, glucose, and urea and its subsequent calcination. The drying step is found to be necessary to obtain a sample with small and uniformly sized Co–CoO nanoparticles. The calcination temperature has a great effect on the catalytic activity of the final product. Specifically, the sample prepared at the calcination temperature of 800 °C shows better catalytic activity of the oxygen reduction reaction (ORR). Urea in the reaction mixture is crucial to obtain the sample with the uniformly sized Co–CoO nanoparticles and also plays an important role in improving the catalytic activity of the Co–CoO@NC/NC. Additionally, there exists a strong electronic interaction between the Co–CoO nanoparticles and the NC. Most interestingly, the Co–CoO@NC/NC is highly efficient for the ORR and can deliver an ORR onset potential of 0.961 V vs. RHE and a half-wave potential of 0.868 V vs. RHE. Both the onset and half-wave potentials are higher than those of most catalysts reported previously and even close to those of the commercial Pt/C (the ORR onset and half-wave potential of the Pt/C are 0.962 and 0.861 V vs. RHE, respectively). This, together with its high stability, strongly suggests that the Co–CoO@NC/NC could be used as an efficient catalyst for the ORR.

Improving the utilization efficiency of waste biomass resources and industrial electric energy is the key research direction in the clean energy field. Here, rhododendron petals with extensive resources and low cost were used as precursors, the nitrogen-doped derived renewable carbon prepared by simple processing technology demonstrates its feasibility and application value in electrical energy storage systems. For the potassium-ion batteries anode, the renewable carbon delivers superior electrochemical properties: the initial reversible capacity has 294.7 mAh/g, and the retention rate after 150 cycles is 83.5%. Even at the high rate of 2000 mA/g, it still maintains 72.4% after a long cycle. In addition, the energy storage mechanism of the material was verified by different characterization methods: the high potential slope area corresponds to the adsorption behavior of potassium-ion on the surface active sites, and the low potential plateau area shows the characteristics of potassium-ion inserted into graphitic layers. The increased active functional groups and expanded graphitic layer spacing are important factors for the excellent cycle performance and rate capacity of the renewable carbon. These design and application methods have provided strategic guidance and new development directions for reusing biomass waste resources.

The influence of fluoroethylene carbonate (FEC) as an additive to the conventional electrolyte of ethylene carbonate and dimethyl carbonate with lithium hexafluorophosphate on the electrochemical performance of the Li/ZnCo2O4 cell has been investigated. The addition of FEC is found to promote the formation of a more uniform and stable solid electrolyte interphase (SEI) layer with the thinner thickness on the surface of ZnCo2O4, which could reduce the charge transfer resistance and SEI resistance, increase the diffusivity of the lithium ions in the ZnCo2O4 electrode, and improve the stability of ZnCo2O4 during the repeated discharge/charge process. In addition, the formation of less LiF on the ZnCo2O4 surface could also be observed when FEC is added to the electrolyte, which decreases the interfacial impedance of the electrode materials. The Li/ZnCo2O4 cell in the electrolyte with FEC therefore exhibits substantially improved coulombic efficiency, reversible capacity, and long-term stability. The results make us believe that the addition of FEC could be a promising method to fabricate LIBs with improved performance.

The micrometer-sized ZnCo2O4 parallelepipeds with a hierarchical porous structure have been fabricated by a simple two-step procedure, i.e., the synthesis of the Zn1/3Co2/3CO3 parallelepipeds and the subsequent calcination. When tested in lithium-ion batteries (LIBs), the hierarchical porous ZnCo2O4 parallelepipeds could exhibit a reversible capacity of >860 mAh g−1 at a current density of 0.1 C. This clearly demonstrates the potential use of the hierarchical porous ZnCo2O4 parallelepipeds in LIBs. The high electrochemical performance of the hierarchical porous ZnCo2O4 parallelepipeds might originate from the unique porous structure which consists of the secondary ZnCo2O4 particles. First, the porous structure allows for a better accessibility of the active material to the Li+ ion storage, favoring easier diffusion of electrolyte in and out of electrode material. Second, the presence of the secondary particles shortens a pathway of Li+ diffusion in ZnCo2O4, facilitating the better utilization of the active material.

Supercapacitors with cobalt oxide (Co3O4) electrode are a kind of strong candidates for achieving high-capacity storage energy devices. However, the reported capacitances lower than the theoretical values are undesirable. In this study, a high-performance supercapacitor by using the Co3O4 electrode with a porous nanostructure is enabled by the calcination treatment of metal organic framework ZIF-67. Of note, the resulting Co3O4 electrodes shows high specific capacitance of 1015 F/g under a current density of 1 A/g. Additionally, it exhibits energy density of 36.6 Wh/kg at a power density of 189.5 W/kg and maintains capacitance retention of 78.2% after 5000 cycles at 8 A/g. The strategy proposed here provides a good way to synthesize nanostructured Co3O4, a candidate for constructing high-capacitance energy storage devices.

The design and synthesis of hierarchical micro-nano structures of transition metal oxides have played an essential role in the supercapacitor field. In this work, in situ three-dimensional construction of nanoporous cobalt oxide (Co3O4) has been derived from the metal-organic framework (MOF) distributed evenly in electrospun polyacrylonitrile nanofibers. Due to large specific surface area and network architectures, the as-synthesized Co3O4 electrode notably presents a high specific capacitance of 970 F/g at a current density of 1 A/g. Besides, the as-obtained electrode exhibits a high energy density of 54.6 Wh/kg at a power density of 360.6 W/kg and maintains a capacitance retention of 77.5% after 5000 cycles at 6 A/g. Therefore, this method paves a way to produce the nanoporous MOF-derived Co3O4 network architecture as advanced electrodes materials, which shows an application potential for the energy storage industry.

Core-shell Fe3O4@CoO NCs have been demonstrated to be efficient bifunctional catalysts for the oxygen reduction (ORR) and evolution (OER) reactions. Their activities are strongly shell thickness dependent. Specifically, nanocrystals with ∼2 monolayers of CoO can exhibit a potential difference of 0.794 V at OER and ORR current densities of 10 and -3 mA cm-2, respectively. This value is competitive to those of most active bifunctional catalysts reported. In addition, they are also used as the oxygen cathode for Zn-air batteries and can deliver a peak power density of 109 mW cm-2, much higher than that of the Pt-RuO2/C (88.1 mW cm-2).