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The universal linear scan voltammogram measurement on the rotating disk electrode (RDE) has been identified as a simple method to investigate the oxygen reduction activity of electrocatalysts. The steady‐state limiting current density Ilim indicates the maximum diffusion current density in the oxygen reduction reaction (ORR) during RDE measurement, which should be a fixed value in theory for a 4e ORR in a particular concentration solution and at a certain rotate speed. However, in experiments, Ilim is always variable and smaller than theoretical value even though with the same the catalyst, electrode, and rotator. So the impact of various experimental operating parameters on Ilim is highly necessary to be investigated. In this paper, factors, such as catalyst loading, O2 inlet condition, O2 flow rate, gas tightness, solution concentration, and purity, have been investigated for their effects on the Ilim of ORR on three typical catalysts (20 % commercial Pt/C, Iron/Nitrogen/Carbon‐catalyst and N‐doped carbon nanotubes). The results indicate that the catalyst loading and O2 inlet condition are the key factors influencing the Ilim of ORR. While, the O2 flow rate, gas tightness, solution concentration, and purity have little influence on the Ilim of ORR. The correct Ilim could be obtained under the optimized catalyst loading and the O2 inlet with an extended sand core tube.

Developing low cost, high-performance, and durable bifunctional catalysts for oxygen reduction and oxygen evolution reactions is critical for a commercial application of fuel cells and metal−air batteries. Nitrogen-doped carbon nanotubes encapsulated nickel nanoparticles are prepared through a simple pyrolysis procedure with melamine and nickel chloride hexahydrate as precursors. The catalyst is featured by nickel nanoparticles encapsulated inside nitrogen-doped carbon nanotubes, with abundant surface nitrogen doping. The optimized catalyst exhibits proximate oxygen reduction activity to platinum/carbon catalyst, comparable oxygen evolution activity to ruthenium dioxide catalyst, and better stability to noble metal catalysts in alkaline medium. The oxygen electrode activity parameter (the gap between the potential of oxygen evolution at 10 mA cm −2 and the half-wave potential of oxygen reduction) of the as-prepared catalyst is 0.754 V, which is among the state-ofthe-art bifunctional electrocatalysts reported to date. To explore the active sites, a series of catalysts with different bulk nickel and surface nitrogen contents are synthesized and served as the oxygen reduction and oxygen evolution reactions catalysts. The results reveal that the oxygen reduction activity of this catalyst arises from the doped nitrogen, while the oxygen evolution activity originates from the encapsulated nickel nanoparticles.

Humins, waste from biomass hydrolysis, are the main factor limiting the utilization efficiency of biomass carbon. In the present study, waste humins were employed for activated carbon production though KOH activation in a temperature range of 500-900 • C. The structure and properties of the activated carbons were studied, and a honeycomb-like macropore structure was observed. High activation temperature was demonstrated to be capable of promoting the formation of activated carbon with high surface area, high pore volume and high adsorption capacity. The activated carbon obtained by carbonization at 800 • C (KOH800) was selected as sorbent to adsorb methylene blue (MB) and phenol in aqueous solution, and the adsorption process can be explained by pseudo-second-order kinetic model. The adsorption behavior complies with Langmuir isotherm model and exhibits superior adsorption capacity of 1195 and 218 mg/g for MB and phenol, respectively. The impacts of surface area, acidic active sites and pore structures were also investigated, and it was found that the adsorption of approximately 44.0% MB and 39.7% phenol were contributed by the pores with apertures from 1.7 nm to 300 nm.