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Energy conversion technologies like fuel cells and metal-air batteries require oxygen reduction reaction (ORR) electrocatalysts with low cost and high catalytic activity. Herein, N-doped carbon spheres (N-CS) with rich micropore structure have been synthesized by a facile two-step method, which includes the polymerization of pyrrole and formaldehyde and followed by a facile pyrolysis process. During the preparation, zinc chloride (ZnCl2) was utilized as a catalyst to promote polymerization and provide a hypersaline environment. In addition, the morphology, defect content and activity area of the resultant N-CS catalysts could be regulated by controlling the content of ZnCl2. The optimum N-CS-1 catalyst demonstrated much better catalytic activity and durability towards ORR in alkaline conditions than commercial 20 wt% Pt/C catalysts, of which the half-wave potential reached 0.844 V vs. RHE. When applied in the Zn-air batteries as cathode catalysts, N-CS-1 showed a maximum power density of 175 mW cm-2 and long-term discharging stability of over 150 h at 10 mA cm-2, which outperformed 20 wt% Pt/C. The excellent performance could be due to its ultrahigh specific surface area of 1757 m2 g-1 and rich micropore channels structure. Meanwhile, this work provides an efficient method to synthesize an ultrahigh surface porous carbon material, especially for catalyst application.

Although direct glucose fuel cell (DGFC) is widely regarded as one of the most promising energy systems, the low catalytic activity and inferior instability of most anode catalysts during electro-oxidation of glucose has greatly hampered its potential applications. In this work, an efficient and durable anode catalyst of nanoporous bismuth (Bi) for the alkaline electro-oxidation of glucose was proposed just by a simple de-alloying method. The microstructure and catalytic performance of nanoporous bismuth could be finely tuning through actively controlling the composition of precursor Mg–Bi alloy. A three-dimension structure was formed after de-alloying Mg–Bi precursor, giving rise to an increased specific surface area and correspondingly resulting in an enhanced electro-catalytic performance. It has intimated that the optimal nanoporous Bi catalyst with an open, bi-continuous interpenetrating pore-to-ligament structure was constructed based on Mg65Bi35 alloy etching and exhibited an enhanced current density (as high as 8.04 mA/cm2) during alkaline electro-oxidation of glucose, together with the lowest poisoning rate of 5.6 × 10−3%. The remarkable electrochemical performance of the nanoporous Bi catalyst, coupling with facile dealloying strategy may facilitate design and development of renewable energy device.

The MnFe2O4/C catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were prepared by a facile microwave sintering method, and the MnFe2O4/C electrode showed good performance for both ORR and OER. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the phase and morphology of the product. The current density of the MnFe2O4/C electrode was 141.8 mA cm−2 at −0.2 V (vs. Hg/HgO) and 92.5 mA cm−2 at 0.6 V (vs. Hg/HgO) in 6 mol L−1 KOH electrolyte at room temperature. A primary zinc-air battery was assembled using MnFe2O4/C electrode, the discharge voltage could reach up to 816 mV at a high current density of 80 mA cm−2.

Manganese(II)/Manganese(III) with a standard electrode potential of 1.51 V has been identified as a potential positive electrode for redox flow cells. The current work is to evaluate the use of methane sulfonic acid (MSA) as a solvent for Manganese(II)/Manganese(III) electric pairs as cathode materials applied to redox flow batteries (RFB). The electrochemical behavior of the MSA medium in Manganese(II)/Manganese(III) was studied by linear scanning voltammetry and cyclic voltammetry. The results show that the reaction of Manganese(II)/Manganese(III) on the graphite electrode is a quasi reversible single electron transfer step. The diffusion coefficient of Mn2+ was identified as 0.982 × 10−6 cm2 s−1 in 1 M MSA. As the concentration of MSA increases from 2 to 5 M, Mn3+ would be more stable, and the electrochemical kinetics of Manganese(II)/Manganese(III) couple would be better. When the MSA concentration reached 5 M, reduction kinetics of oxidized species Mn3+ decreased obviously. The results support the selecting of 4 M MSA. When the current density was 20 mA cm−2, the average energy efficiency and coulomb efficiency of the battery were 74.0% and 92.2% respectively. The average discharge voltage was 1.91 V and the average energy efficiency was 59%, which was higher than the total V cell. At the same time, the battery got a good charging and discharging cycle performance, which indicates that the zinc manganese oxide redox battery has a wide application prospect.

The Mn/C catalysts for oxygen reduction reaction (ORR) were prepared by a facile method, the as-prepared material synthesized at 300 °C for 2 h exhibits splendid catalytic activity for ORR along with ascendant long-term durability, the current density of the Mn-300-2/C electrode is 119 mA cm−2 at −0.2 V (vs. Hg/HgO) in 6 mol L−1 KOH electrolyte. A primary Zn-air battery was assembled using Mn-300-2/C electrode, the discharge voltage could reach up to 1.17 V at 50 mA cm−2 and 1.07 V at a high current density of 75 mA cm−2.

The Pd ZnO/C catalytic material for ethanol oxidation reaction is prepared by microwave heating-glycol reduction method. Pd ZnO is well polymerized and dispersed on XC72. The results demonstrate that Pd ZnO/C has better electro catalytic activity and stability for ethanol oxidation reaction than Pd/C at room temperature. ZnO/C shows no catalysis for ethanol oxidation. The oxidation peak potential of Pd ZnO/C electrode is shifted negatively to 0.21 V. The current density of Pd ZnO/C electrode is 145 mA cm−2, while that of the Pd/C electrode is 60 mA cm−2. Moreover, single cell discharge test shows that discharge voltage of the Pd ZnO/C electrode reaches to 0.41 V at 30 mA cm−2. In summary, ZnO as a co-catalyst significantly improves the activity of Pd ZnO/C catalyst for ethanol oxidation reaction.

The perovskite-type oxide La0.2Sr0.8CoO3 was prepared by sol-gel process, which particle was irregular cube with particle size between 50 and 60 nm. A novel air electrode for oxygen evolution reaction prepared by using La0.2Sr0.8CoO3 as a catalyst has a current density of 152 mA cm−2 when the potential is 0.7 V (vs. Hg/HgO) in an alkaline medium (6 mol L−1 KOH) under the air atmosphere. The research results show that n (C6H8O7): n (M) = 1:1, pH = 9, calcination temperature 800 °C, 5 h calcination time is the best preparation process conditions.

The hematite type Fe2O3/C catalyst for oxygen evolution reaction (OER) was prepared using a facile urea combustion method. The Fe2O3/C electrode showed excellent electrocatalytic ability towards OER in alkaline medium. The phase and morphology of the product were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The sintering temperature is 600 °C, and the calcination time is 3 h, which is the best preparation process condition. The particles were irregular spherical with the size of 10–30 nm and dispersed uniformly. The current density of the Fe2O3/C electrode was 147 mA cm−2 at 0.6 V (vs. HgO/Hg) in 6 mol L−1 KOH electrolyte at room temperature.

The thermal decomposition behavior of NiFe layered double hydroxide (LDH) was investigated by thermogravimetric analysis-differential scanning calorimetry (TG-DSC). The calcined product at 500 °C was mainly NiO/FeOx composite oxide, of which FeOx was amorphous oxide; the calcined product at 650 °C was mainly NiO/NiFe2O4 composite oxide. The polarization curves and chronopotentiometry stated that the NiO/FeOx and NiO/NiFe2O4 showed good electrocatalytic OER and ORR activity; the OER activity of NiO/FeOx was better than that of NiO/NiFe2O4; the ORR activity of NiO/NiFe2O4 was better than that of NiO/FeOx.

Beta-cyclodextrin (β-CD) was first ground among the single stranded linear polyethylene glycol (PEG), and then the ZnO/PVA/β-CD/PEG composite electrode was prepared by chemical crosslinking method. From the SEM diagram of the composite electrode after 80 cycles, it can be seen that the internal and surface of the electrode was distributed a large number of holes within the average diameter of 150 μm. These holes have formed the interpenetrated three-dimensional framework with the active ZnO embedded in and no dendrite formation. The results show that the discharge capacity of the composite electrode is reduced to 80% of the maximum capacity after 130 cycles, which indicates that the cycle life of the composite electrode is longer than that of the blank electrode. The results of polarization curve test shows that the maximum dissolution current of the composite electrode was increased from 60 to 280 mA·cm−2 after 80 cycles, indicating that the activity of the composite electrode was still good after 80 cycles. Compared with it, the blank electrode had few activities after 80 cycles.

Exploring cost-effective and high-efficiency noble-metal free electrocatalysts for oxygen evolution reaction (OER) are of critical importance to the area of renewable energy storage and conversion systems. Herein, Ni–Fe–Mo films were fabricated via electrodeposition method with a very high current density. The film exhibits excellent activity towards OER with a overpotentials of 306 mV at current density of 10 mA·cm−2 and a lower Tafel slope of 77.1 mV·dec−1 under alkaline conditions (30 wt.% KOH). The synergistic effect of the Mo, Ni, and Fe contribute the improved catalytic activity for OER.

Increasing the number of active sites is critical for developing N-doped carbon electrocatalysts towards oxygen reduction reaction (ORR) in fuel cells and metal-air batteries applications. Herein, we prepared N-doped carbon nanotubes (N-CNT) with enriched pyridinic N and abundant defects resulted from the etching of KMnO4 of the precursor (polypyrrole). It was observed that the content of pyridinic N could be well controlled by regulating the etching time. The resultant catalyst displayed a superior ORR activity compared commercial Pt/C in an alkaline solution, which was further confirmed by home-made Zn-air batteries. Density functional theory (DFT) computations showed that the superior catalytic activity originated from the second nearest carbon atom to the pyridinic-N at the edge. This work provides a simple etching approach to alter the N configuration and the amount of defects in N-doped CNT, which can be extended to many other energy conversion materials.

A core/shell polyaniline (PANI)/MnO2 composite (denoted as “PANI@MnO2”) was prepared and evaluated as electrode material for supercapacitor applications. In the composite, α-MnO2 showed chrysanthemum-like and spherical structure, which were coated in situ with PANI. In this process, α-MnO2 serves as a scaffold and also acts as an oxidant to induce aniline polymerization. Scanning electron microscopy and transmission electron microscopy images clearly showed the core/shell structure of the PANI@MnO2 composite, which was further characterized by X-ray powder diffraction, Fourier transformation infrared spectroscopy and Raman spectroscopy. In addition, the thermo-gravimetric analysis illustrates that the composite has a good thermal stability and the mass ratio of PANI and MnO2 is 0.11. Electrochemical properties of the composite and its components were evaluated using cyclic voltammetry, electrochemical impedance spectra and chronopotentiometry. The specific capacitance of the high-mass loading PANI@MnO2 electrode is 75 Fg−1, 1.1 times higher than that of MnO2 electrode (67 Fg−1), at a current density of 0.1Ag−1. Moreover, the composite electrode retains 90% of the initial capacitance after 3000 cycles, while this is only 75% for α-MnO2. The improved electrochemical performance may be attributed to the synergistic effect between PANI and α-MnO2.

The main barrier preventing the commercial application of alkaline aluminum-air battery is the severe self-corrosion of Al anode. Herein, three kinds of novel inorganic-organic hybrid corrosion inhibitors are constructed, which containing ZnO and organic acids (acetic acid, citric acid and EDTA). Electrochemical tests demonstrate that the combination of ZnO and organic acid effectively suppresses the self-corrosion of Al foam anode in alkaline solution. Among them, the Al-air battery exhibits a high voltage platform (1.13 V), a high specific capacity (1902 mA h g−1) and a high anode utilization (58.23%) within 0.3 M ZnO+0.03 M citric acid hybrid inhibitors system. Surface analyses confirm that the organic acid greatly boosts the formation of uniform and dense Zn film on Al surface. A combined experimental research and theory computation reveal that the corrosion inhibition mechanism of ZnO + organic acid hybrid system is: the phase-forming corrosion inhibitor (ZnO) deposit on the Al surface to form Zn protective film and cover the active sites of hydrogen evolution; the active sites of organic acids are the O atoms of carbonyl groups, which adsorbing on the surface of Zn protective film and Al foam, thereby forming the composite protective films of RCOO-Al and RCOO-Zn.

Zinc electrode with polyvinyl alcohol(PVA)/poly(diallyldimethylammonium chloride) (PDDA) film coated ZnO was prepared by a chemical cross-linking method. The PVA/PDDA polymer film has an interpenetrating network structure where PVA works as polymer matrix and PDDA as anion charge carriers. Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal that a polymer film with well-connected three dimensional network structure is formed around ZnO nanoparticles. Linear sweep voltammetry, cyclic voltammetry (CV) and charge/discharge cycling tests were carried out on the ZnO/PVA/PDDA composite electrode, which exhibits higher electrochemical activity and dramatically superior cycle stability than the bare ZnO electrode without polymer films. The ZnO/PVA/PDDA electrode showed remarkably lower solubility during cycling.

It goes without saying that the oxygen reduction reaction (ORR) on Pt-based catalyst is one of the most significant cathode reactions in fuel cells. Although high price and low stability, Pt-based catalyst still enjoys its widespread researches for its relatively high activity. Recently, there has been some impressive progress in strategies like alloying, single atoms, morphology and component regulation. This paper aims to introduce the preparation strategies of Pt-based catalyst and further expound its inactivation mechanism and the essence of active sites. It also shows some challenges as well as methods applying in fuel cells, which provides some reference for the practical application in the future.