Ultrafine TaOx/CB Oxygen Reduction Electrocatalyst Operating in Both Acidic and Alkaline Media

The high activity of non-platinum electrocatalysts for oxygen reduction reaction (ORR) in alkaline media is necessary for applications in energy conversion devices such as fuel cells and metal-air batteries. Herein, we present the electrocatalytic activity of TaOx/carbon black (CB) nanoparticles for the ORR in an alkaline atmosphere as well as in an acidic electrolyte. Ultrafine TaOx nanoparticles 1–2 nm in size and uniformly dispersed on CB supports were prepared by potentiostatic electrodeposition in a nonaqueous electrolyte and subsequent annealing treatment in an H2 flow. The TaOx/CB nanoparticles largely catalyzed the ORR with an onset potential of 1.03 VRHE in an O2-saturated 0.1 M KOH solution comparable to that of a commercial Pt/CB catalyst. ORR activity was also observed in 0.1 M H2SO4 solution. According to the rotating ring disk electrode measurement results, the oxide nanoparticles partly produced H2O2 during the ORR in 0.1 M KOH, and the ORR process was dominated by both the two- and four-electron reductions of oxygen in a diffusion-limited potential region. The Tafel slope of −120 mV dec−1 in low and high current densities revealed the surface stability of the oxide nanoparticles during the ORR. Therefore, these results demonstrated that the TaOx/CB nanoparticles were electroactive for the ORR in both acidic and alkaline electrolytes.

Effects of Metallic Impurities in Alkaline Electrolytes on Electro-Oxidation of Water and Alcohol Molecules

The presence of metallic impurities in the electrolyte greatly affects electrocatalytic performance. A systematic study on this topic can not only provide guidance for rigorous practices on electrochemical measurements, but also in-depth fundamental understanding on the mechanisms of the electrochemical reactions. Herein, nine types of metallic ions including Cu2+, Ni2+, Fe3+, Fe2+, Co2+, Mn2+, Zn2+, Ce3+ and Al3+ are intentionally introduced into the electrolytes with a controlled manner and their effects on electro-oxidation of water, 5-hydroxymethylfurfural (HMF) and glycerol are investigated in details. Among these metal ions, Co2+ has the most pronounced effects on H2O electro-oxidation while Cu2+ species displays superior activity toward HMF and glycerol electro-oxidation, but negligible effects on H2O electro-oxidation. Such a unique feature of Cu2+ can also be noted from electro-oxidation of other small molecules, such as ethylene glycol, ethanol and furfural. More importantly, the effects of metallic impurities are independent of the composition of the electrodes, only rely on the pH of the electrolytes. In-situ electrochemical Raman spectroscopy, control electrochemical experiments and X-ray photoelectron spectroscopy analyses reveal that the origin of impurity effects is attributed to the formation of hydroxides during the electrochemical measurements.

A durable and pH-universal self-standing MoC–Mo2C heterojunction electrode for efficient hydrogen evolution reaction

Efficient water electrolyzers are constrained by the lack of low-cost and earth-abundant hydrogen evolution reaction (HER) catalysts that can operate at industry-level conditions and be prepared with a facile process. Here we report a self-standing MoC–Mo2C catalytic electrode prepared via a one-step electro-carbiding approach using CO2 as the feedstock. The outstanding HER performances of the MoC–Mo2C electrode with low overpotentials at 500 mA cm−2 in both acidic (256 mV) and alkaline electrolytes (292 mV), long-lasting lifetime of over 2400 h (100 d), and high-temperature performance (70 oC) are due to the self-standing hydrophilic porous surface, intrinsic mechanical strength and self-grown MoC (001)–Mo2C (101) heterojunctions that have a ΔGH* value of −0.13 eV in acidic condition, and the energy barrier of 1.15 eV for water dissociation in alkaline solution. The preparation of a large electrode (3 cm × 11.5 cm) demonstrates the possibility of scaling up this process to prepare various carbide electrodes with rationally designed structures, tunable compositions, and favorable properties.

Bifunctional MnOx electrocatalysts for zinc–air batteries in alkaline electrolytes

The growing need for rechargeable zinc–air batteries has led researchers to look for low-cost and robust catalytic candidates. Manganese oxides are promising for their low toxicity, natural abundance and low cost. In this review, we summarize the versatility of manganese oxides which is achievable by adjusting synthetic parameters. In addition, we highlight how manganese oxides impact zinc–air electrochemistry.

Zinc–air batteries in neutral/near-neutral electrolytes

To fully utilize intermittent renewable energy and have energy security, large-scale batteries are necessary. The aqueous zinc–air battery (ZAB) is a promising potential candidate for its safety, low-cost, and theoretical capacity. This research on ZAB mainly focused on alkaline electrolytes. These are favored for their conductivity, but greatly reduce the stability of the battery by zinc corrosion from the hydrogen evolution reaction (HER), dendrite formation, and carbonate formation. To address these issues, recently neutral and near-neutral electrolytes have been applied to aqueous ZAB to suppress HER, dendrite formation, and minimize carbonate formation. These include the use of chloride-based, potassium nitrate, aqueous organic, solid-state electrolytes supplemented with additives to improve the performance and stability. This field is still young with significant opportunities available for research.