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

Modulating the Cavity Size of Carbon Nanobelts for Enhanced Oxygen Reduction Reaction

The preactivation of reactants within the cavities of carbon nanotubular materials has remained largely unexplored due to the scarcity of materials with well-defined sizes and precisely engineered doping sites. Herein, we demonstrate that the catalytic activity toward the oxygen reduction reaction (ORR) is primarily governed by the cavity sizes of well-defined nanobelt materials with precisely doped sp2-nitrogen atoms. Our results show that the confinement effect induced by cavity size and the electron-rich chemical environment within the cavity are crucial for O2 adsorption and preactivation, leading to enhanced catalytic activity. Belt2, with its medium-sized cavity (6.3 Å), exhibits superior ORR catalytic performance compared to Belt1 with its narrower cavity and Belt3/Belt4 with its larger cavities. Notably, Belt2 achieves high half-wave and onset potentials of 0.84 and 0.97 V, respectively, along with an open-circuit voltage of 1.32 V and a peak power density of 181 mW cm-2 in a zinc-air battery. This work not only provides a deeper understanding of the geometric factors influencing the ORR electrocatalysis of nanocarbon materials but also offers insights into the future design of nanocarbon electrocatalysts for enhancing catalytic efficiency. These findings may also be beneficial for other energy conversion and catalytic materials.

Effects of porous structure and oxygen functionalities on electrochemical synthesis of hydrogen peroxide on ordered mesoporous carbon

Two-electron oxygen reduction reaction (2e- ORR) is a promising alternative to energy-intensive anthraquinone process for hydrogen peroxide (H2O2) production. Metal-free nanocarbon materials have garnered intensive attention as highly prospective electrocatalysts for H2O2 production, and an in-depth understanding of their porous structure and active sites have become a critical scientific challenge. The present research investigates a range of porous carbon catalysts, including non-porous, microporous, and mesoporous structures, to elucidate the impacts of porous structures on 2e- ORR activity. The results highlighted the superiority of mesoporous carbon over other porous materials, demonstrating remarkable H2O2 selectivity. Furthermore, integration of X-ray photoelectron spectroscopy (XPS) data analysis with electrochemical assessment results unravels the moderate surface oxygen content is the key to increase 2e- ORR activity. These results not only highlight the intricate interplay between pore structure and oxygen content in determining catalytic selectivity, but also enable the design of carbon catalysts for specific electrochemical reactions.

Carbon Surface-Influenced Heterogeneity of Ni and Co Catalytic Sites as a Factor Affecting the Efficiency of Oxygen Reduction Reaction

Highly porous carbon black and micro/mesoporous activated carbon were impregnated with cobalt and nickel nitrates, followed by heat treatment at 850 °C in nitrogen. Detailed information about chemistry and porosity was obtained using XPS, XRD, TEM/EDX, and nitrogen adsorption. The samples were used as ORR catalysts. Marked differences in the performance were found depending on the type of carbon. Differences in surface chemistry and porosity affected the chemistry of the deposited metal species that governed the O₂ reduction efficiency along with other features of the carbon supports, including electrical conductivity and porosity. While dissociating surface acidic groups promoted the high dispersion of small metal species, carbon reactivity with oxygen and acidity limited the formation of the most catalytically active Co₃O₄. Formation of Co₃O₄ on the highly conductive carbon black resulted in an excellent performance with four electrons transferred and a current density higher than that on Pt/C. When Co₃O₄ was not formed in a sufficient quantity, nickel metal nanoparticles promoted ORR on the Ni/Co-containing samples. The activity was also significantly enhanced by small pores that increased the ORR efficiency by strongly adsorbing oxygen, which led to its bond splitting, followed by the acceptance of four electrons.

Controlled introduction of defects into single-walled carbon nanotubes via a fluorination-defluorination strategy using xenon difluoride and their alkaline oxygen reduction reaction catalytic activity

The defect edges in carbon nanomaterials have attracted attention as catalytic active sites for the oxygen reduction reaction (ORR) of the cathode in electrolyte fuel cells, and the defect control in carbon nanomaterials is becoming increasingly important. This study evaluates a fluorination-defluorination strategy for the controlled introduction of defects into single-walled carbon nanotubes (SWCNTs) involving the fluorination of SWCNTs using xenon difluoride (XeF₂) and their subsequent defluorination through thermal annealing. We synthesized fluorinated SWCNTs with different fluorine contents using gaseous XeF₂ and annealed the fluorinated SWCNTs at 1000 °C for 3 h under nitrogen gas flow. Structural analyses revealed that SWCNTs derived from fluorinated SWCNTs with low fluorine contents primarily had single point defects. In contrast, SWCNTs derived from fluorinated SWCNTs with high fluorine contents had vacancy defects with edges. According to the ORR catalyst evaluation in alkaline aqueous solution, SWCNTs with edge defects, rather than point defects, can improve the efficiency of ORR catalytic activity. The proposed fluorination-defluorination strategy using gaseous XeF₂ is expected to enable the controlled introduction of defects in different types of carbon materials.

Cyanogel-Derived Synthesis of Porous PdFe Nanohydrangeas as Electrocatalysts for Oxygen Reduction Reaction

It is important to develop cost-efficient electrocatalysts used in the oxygen reduction reaction (ORR) for widespread applications in fuel cells. Palladium (Pd) is a promising catalyst, due to its more abundant reserves and lower price than platinum (Pt), and doping an earth-abundant 3d-transition metal M into Pd to form Pd-M bimetallic alloys may not only further reduce the use of expensive Pd but also promote the electrocatalytic performance of ORR, owing to the synergistic effect between Pd and M. Here we report a cyanogel-derived synthesis of PdFe alloys with porous nanostructure via a simple coinstantaneous reduction reaction by using K2PdIICl4/K4FeII(CN)6 cyanogel as precursor. The synthesized PdFe alloys possess hydrangea-like morphology and porous nanostructure, which are beneficial to the electrochemical performance in ORR. The onset potential of the porous PdFe nanohydrangeas is determined to be 0.988 V, which is much more positive than that of commercial Pt/C catalyst (0.976 V) and Pd black catalyst (0.964 V). Resulting from the unique structural advantages and synergetic effect between bimetals, the synthesized PdFe nanohydrangeas with porous structure have outstanding electrocatalytic activity and stability for ORR, compared with the commercial Pd black and Pt/C.

Biomass Related Highly Porous Metal Free Carbon for Gas Storage and Electrocatalytic Applications

In this paper we report the synthesis of a N, S co-doped metal free carbon cryogel obtained from a marine biomass derived precursor using urea as nitrogen source. Natural carrageenan intrinsically contains S and inorganic salt. The latter also serves as an activating agent during the pyrolytic step. The overall 11.6 atomic % surface heteroatom concentration comprises 5% O, 4.6% N and 1% S. The purified and annealed final carbon (CA) has a hierarchical pore structure of micro-, meso- and macropores with an apparent surface area of 1070 m2/g. No further treatment was applied. The gas adsorption potential of the samples was probed with H2, CO2 and CH4, while the electrocatalytic properties were tested in an oxygen reduction reaction. The atmospheric CO2 and CH4 storage capacity at 0 °C in the low pressure range is very similar to that of HKUST-1, with the CO2/CH4 selectivity below 20 bar, even exceeding that of the MOF, indicating the potential of CA in biogas separation. The electrocatalytic behavior was assessed in an aqueous KOH medium. The observed specific gravimetric capacitance 377 F/g was exceeded only in B, N dual doped and/or graphene doped carbons from among metal free electrode materials. The CA electrode displays almost the same performance as a commercial 20 wt% Pt/C electrode. The oxygen reduction reaction (ORR) exhibits the 4-electron mechanism. The 500-cycle preliminary stability test showed only a slight increase of the surface charge.