Crucial role for oxygen functional groups in the oxygen reduction reaction electrocatalytic activity of nitrogen-doped carbons

Roles of Oxygen-Containing Functional Groups in Carbon for Electrocatalytic Two-Electron Oxygen Reduction Reaction

Recent years have witnessed great research interests in developing high-performance electrocatalysts for the two-electron (2e-) oxygen reduction reaction (ORR) that enables the sustainable and flexible synthesis of H2O2. Carbon-based electrocatalysts exhibit attractive catalytic performance for the 2e- ORR, where oxygen-containing functional groups (OFGs) play a decisive role. However, current understanding is far from adequate, and the contribution of OFGs to the catalytic performance remains controversial. Therefore, a critical overview on OFGs in carbon-based electrocatalysts toward the 2e- ORR is highly desirable. Herein, we go over the methods for constructing OFGs in carbon including chemical oxidation, electrochemical oxidation, and precursor inheritance. Then we review the roles of OFGs in activating carbon toward the 2e- ORR, focusing on the intrinsic activity of different OFGs and the interplay between OFGs and metal species or defects. At last, we discuss the reasons for inconsistencies among different studies, and personal perspectives on the future development in this field are provided. The results provide insights into the origin of high catalytic activity and selectivity of carbon-based electrocatalysts toward the 2e- ORR and would provide theoretical foundations for the future development in this field.

Oil palm leaf-derived hierarchical porous carbon for "water-in-salt" based supercapacitors: the effect of anions (Cl- and TFSI-) in superconcentrated conditions

This study investigates the use of a hierarchical porous carbon electrode derived from oil palm leaves in a "water-in-salt" supercapacitor. The impact of anion identity on the electrical performance of the carbon electrode was also explored. The results show that the prepared carbon had a hierarchical porous structure with a high surface area of up to 1840 m2 g-1. When a 20 m LiTFSI electrolyte was used, the carbon electrode had a specific capacitance of 176 F g-1 with a wider potential window of about 2.6 V, whereas the use of a cheaper 20 m LiCl electrolyte showed a higher specific capacitance of 331 F g-1 due to the smaller size of the Cl- anion, which enabled inner capacitance. Therefore, the anion identity has an effect on the electrochemical performance of porous carbon, and this research contributes to the understanding of using "water-in-salt" electrolytes in carbon-based supercapacitors. The study's findings provide insights into developing low-cost, high-performance supercapacitors that can operate in a wider voltage range.

Exploring the structural dependence of metal-free carbon electrocatalysts on zinc-based metal-organic framework types

Metal-organic frameworks (MOFs) have been widely used as precursors to derive carbon-based electrocatalysts for the oxygen reduction reaction (ORR) due to their high porosity and tunable chemical composition/structure. However, the influence of MOF type on the structure and further ORR activity of derived metal-free carbon catalysts is still elusive. In the present work, a series of different Zn-based MOFs were employed as precursors to explore this issue. Meanwhile, prepare N-doped metal-free carbon catalysts were prepared for the ORR under the activation of sacrificial urea (which is effective to enhance the ORR activity of carbon-based catalysts). By analyzing the intermediates during pyrolysis, it is found that the decisive role of MOF types on the doped N and the morphology of derived carbon catalysts was played by the Zn coordination environment of MOFs and its reactivity with the decomposition intermediate of urea. Although the structure and porosity of derived carbon catalysts from different MOFs are very different, they all showed superior ORR activity and Zn-air battery performance up to 20 wt% Pt/C benchmark catalysts. From the above analyses, the combination of urea and compounded Zn is also a promising activation method for the preparation of highly-efficient metal-free carbon electrocatalysts.

N- and O-doped hollow carbons constructed by self- and extrinsic activation for the oxygen reduction reaction and flexible zinc-air Batteries

Zinc-air batteries (ZAB), especially those assembled on flexible substrates, have attracted great research attention in electronics and wearable electronics. However, the air-cathode reaction-oxygen reduction reaction (ORR) has limited the development of ZAB technology. In this study, a hollow carbon catalyst, NOC-1000-1, was prepared by pyrolysis of a mixture of a N-enriched Zn/bispyrozolate-based metal-organic framework and urea to replace the labile Pt-based catalysts for ORR. The employment of sacrifical urea eliminated the requirement for complicated post-treatment compared to the template method. Combined with self-activation (Zn evaporation), the obtained carbon showed a micro- and mesopore-dominant hierarchical structure coexisting with some macropores. Moreover, the doped N and O species were also tailored in a preferable configuration for ORR by simply screening the pyrolysis conditions. Under the synergistic effect of the preferable N and O configurations and pore structure, the derived carbon catalyst displayed superior ORR activity of 0.977 V onset potential and 0.867 V half-wave potential; these values are slightly better than those of the 20% Pt/C benchmark catalyst (0.985 and 0.861 V, respectively). Flexible solid-state ZABs were further assembled by employing the derived carbon catalyst as an air-cathode, and they exhibited a higher peak power density of 100.92 mW cm^-2 than a 20% Pt/C-RuO₂ battery as well as previously reported similar batteries and very high stability for up to 30 h. The flexible solid-state ZABs could drive a red light-emitting diode and run a 130-type motor for hours, which indicates their promising applications in real-world technologies.

Alternative view of oxygen reduction on porous carbon electrocatalysts: the substance of complex oxygen-surface interactions

Electrochemical oxygen reduction reaction (ORR) is an important energy-related process requiring alternative catalysts to expensive platinum-based ones. Although recently some advancements in carbon catalysts have been reported, there is still a lack of understanding which surface features might enhance their efficiency for ORR. Through a detailed study of oxygen adsorption on carbon molecular sieves and using inelastic neutron scattering, we demonstrated here that the extent of oxygen adsorption/interactions with surface is an important parameter affecting ORR. It was found that both the strength of O₂ physical adsorption in small pores and its specific interactions with surface ether functionalities in the proximity of pores positively influence the ORR efficiency. We have shown that ultramicropores and hydrophobic surface rich in ether-based groups and/or electrons enhance ORR on carbon electrocatalysts and the performance parameters are similar to those measured on Pt/C with the number of electron transfer equal to 4.

The joint effect of electrical conductivity and surface oxygen functionalities of carbon supports on the oxygen reduction reaction studied over bare supports and Mn–Co spinel/carbon catalysts in alkaline media

Mn–Co spinel/carbon electrocatalyst performance exhibits a volcano-type shape which results from a trade-off between electrical conductivity and the amount of oxygen groups.