Elucidating the Correlation between ORR Polarization Curves and Kinetics at Metal-Electrolyte Interfaces

Gaining insight into the impact of electronic property and interface electrostatic field on ORR kinetics in alloy engineering via theoretical prognostication and experimental validation

Alloy engineering has been utilized as a potent strategy to modulate the oxygen reduction reaction （ORR） activity. However, the regulatory mechanism underpinning the ORR kinetics by means of alloy engineering is still shrouded in ambiguity. This work places emphasis on the kinetics of the ORR concerning Pt3M (M = Cr, Co, Cu, Pd, Sn, and Ir) catalysts, and integrates theoretical prognostication and experimental validation to illuminate the fundamental principles of alloy engineering. The ORR kinetic activity, as prognosticated by theory, shows significant agreement with experimental results, provided that the rate-determining step (RDS) accounts for a dominant role in the potential-independent kinetic mechanism. In essence, alloy engineering manipulates electronic properties through electron transfer to modulate intermediate adsorption and adjusts the interface electric field (Efield) to regulate hydrogen atom transport, ultimately influencing kinetics. The Efield holds greater significance in ORR kinetics compared to the intermediate adsorption (EadsO), the corresponding degrees of correlation with free energy barriers (Ea) of RDS are -0.89, and 0.75, respectively. This work highlights the nature of alloy engineering for ORR kinetics modulation and assists in the design of efficient catalysts.

Immobilization of iron phthalocyanine on MOF-derived N-doped carbon for promoting oxygen reduction in zinc-air battery

Functional carbon nanomaterials play a crucial role in the cathodic oxygen reduction reaction (ORR) for sustainable fuel cells and metal-air batteries. In this study, we propose an effective approach to immobilize iron phthalocyanines (FePc) by employing a porous N-doped carbon material, denoted as NC-1000, derived from a sheet-shaped coordination polymer. The resulting NC-1000 possesses substantial porosity and abundant pore defects. The nitrogen sites within NC-1000 not only facilitate FePc adsorption but also optimize the electron distribution at the Fe-N site. The FePc@NC-1000 composite material exhibits a significant number of active centers in the form of Fe-N4 moieties, showcasing satisfactory ORR activity. Specifically, it demonstrates an onset potential of 0.99 V, a positive half-wave potential of 0.86 V, a large limiting current of 5.96 mA cm-2, and a small Tafel slope of 44.41 mV dec-1. Additionally, theoretical calculations and experimental results confirm the favorable performance and durability of zinc-air batteries assembled using FePc@NC-1000, thereby highlighting their considerable potential for practical applications. Overall, this study provides a comprehensive exploration of the enhanced catalytic performance and increased stability of metal-organic framework-derived functional carbon nanomaterials as cost-effective, efficient, and stable catalysts for the ORR.

Photocatalytic Synthesis of Ultrafine Pt Electrocatalysts with High Stability Using TiO2 -Decorated N-Doped Carbon as Composite Support

Commercial Pt/C (Com. Pt/C) electrocatalysts are considered optimal for oxygen reduction and hydrogen evolution reactions (ORR and HER). However, their high Pt content and poor stability restrict their large-scale application. In this study, photocatalytic synthesis was used to reduce ultrafine Pt nanoparticles in-situ on a composite support of TiO2 -decorated nitrogen-doped carbon (TiO2 -NC). The nitrogen-doped carbon had a large surface area and electronic effects that ensured the uniform dispersion of TiO2 nanoparticles to form a highly photoactive and stable support. TiO2 -NC served as a composite support that enhanced the dispersibility and stability of ultrafine Pt electrocatalyst, owing to the presence of N sites and the strong metal-support interaction. Relative to Com. Pt/C, the as-obtained Pt/TiO2 -NC had positive shifts of 44 and 10 mV in the ORR half-wave potential and HER overpotential at -10 mA cm-2 , respectively. After an accelerated durability test, Pt/TiO2 -NC had lower losses in electrochemical specific area (0.7 %) and electrocatalytic activity (0 mV shift) than Com. Pt/C (25.6 %, 22 mV shift). These results indicate that the developed strategy enabled the facile synthesis and stabilization of ultrafine Pt nanoparticles, which improved the utilization efficiency and long-term stability of Pt-based electrocatalysts.

A Combination of "Push Effect" Strategy with "Triple-Phase-Boundary Engineering" on Iron Porphyrin-Based MOFs: Enhanced Selectivity and Activity for Oxygen Reduction

Herein, the "push effect" strategy combined with "triple-phase-boundary" (TPB) engineering was innovatively employed to target the single Fe-N₄ sites in an iron porphyrin-based metal-organic framework, with axially coordinated 4-octylpyridine groups on Fe-N₄ (named as PCN-224 (Fe)-1). The amphiphilic 4-octylpyridine groups donate sufficient electrons toward Fe-N₄ by the Fe-N(pyridine) coordination bond and simultaneously provide effective TBP reactive sites by the hydrophobic octyl terminals, resulting in enhanced ORR activity of the PCN-224 (Fe)-1 in hydrophobic octyl terminals, with an E_1/2 of 0.81 V and complete 4-electron selectivity. Furthermore, TPB engineering is utilized to construct the PCN-224 (Fe)-1-based Zn-air battery with a maximum power density of 98 mW cm^-2, demonstrating great practical application potential for molecule-based ORR catalysts. Meanwhile, the "push effect" mechanism on ORR is revealed by electron paramagnetic resonance, in situ UV-vis spectroelectrochemical analysis, and density functional theory.

Engineering 3d-2p-4f Gradient Orbital Coupling to Enhance Electrocatalytic Oxygen Reduction

The development of highly efficient and economical materials for the oxygen reduction reaction (ORR) plays a key role in practical energy conversion technologies. However, the intrinsic scaling relations exert thermodynamic inhibition on realizing highly active ORR electrocatalysts. Herein, a novel and feasible gradient orbital coupling strategy for tuning the ORR performance through the construction of Co 3d-O 2p-Eu 4f unit sites on the Eu₂ O₃ -Co model is proposed. Through the gradient orbital coupling, the pristine ionic property between Eu and O atoms is assigned with increased covalency, which optimizes the e_g occupancy of Co sites, and weakens the OO bond, thus ultimately breaking the scaling relation between *OOH and *OH at Co-O-Eu unit sites. The optimized model catalyst displays onset and half-wave potential of 1.007 and 0.887 V versus reversible hydrogen electrode, respectively, which are higher than those of commercial Pt/C and most Co-based catalysts ever reported. In addition, the catalyst is found to possess superior selectivity and durability. It also reveals better cell performance than commercial noble-metal catalysts in Zn-air batteries in terms of high power/energy densities and long cycle life. This study provides a new perspective for electronic modulation strategy by the construction of gradient 3d-2p-4f orbital coupling.