image: A Csp–π–d Conjugated System Enables a Breakthrough in Electrocatalytic Oxygen Reduction Performance
Credit: ©Science Bulletin
The oxygen reduction reaction (ORR) is a key electrochemical process in energy conversion devices such as fuel cells and meta-air batteries. However, its inherently sluggish kinetics generally require the assistance of catalysts to accelerate the reaction rate. At present, the most efficient ORR catalysts are primarily based on precious metals such as platinum, yet their high cost and limited natural abundance have severely hindered large-scale practical applications. Therefore, the development of efficient and stable non-precious-metal catalysts has remained a major research focus in the field of energy catalysis.
Among the various candidates, transition metal-nitrogen-carbon (M-N-C) catalysts have attracted extensive attention because of their low cost and structural tunability. Nevertheless, their catalytic performance is often limited by the adsorption strength of the active centers toward the key intermediate *OH. Excessively strong *OH adsorption impedes intermediate desorption and slows down the reaction kinetics, whereas overly weak adsorption is unfavorable for oxygen activation. Accordingly, precise control over *OH adsorption at active sites represents a central scientific challenge for improving ORR performance.
To address this issue, the research team proposed constructing a Csp–π–d conjugated structure using carbon–carbon triple bonds (C≡C) as electronically functional linkers. Unlike conventional sp2-carbon-based conjugated systems, the sp-hybridized carbon atoms in C≡C bonds not only participate in forming an extended conjugated skeleton, but also further regulate the electronic environment of the metal active centers. Based on this concept, the team synthesized a series of p-M-TEPP/rGO catalysts using alkynyl-functionalized metalloporphyrin monomers through deprotection and Hay coupling reactions, followed by hybridization with reduced graphene oxide (rGO).
The results showed that the introduction of C≡C bonds not only enabled intermolecular coupling, but also established the targeted Csp–π–d conjugated structure. In this structure, the 2pz orbitals of the C≡C units interact with the 3dz2 orbitals of the metal centers, inducing charge redistribution around the active sites. This electronic reconfiguration optimizes the adsorption strength of *OH at the catalytic centers, thereby promoting the ORR process.
Among the synthesized catalysts, p-Co-TEPP/rGO exhibited the best catalytic performance. In 0.1 M KOH electrolyte, it achieved an onset potential of 0.96 V and a half-wave potential of 0.87 V, demonstrating excellent ORR activity. In addition, the catalyst showed a low Tafel slope, good long-term stability, and excellent resistance to methanol poisoning. To further elucidate the origin of enhanced catalytic activity, the team carried out a systematic analysis combining experimental investigation with density functional theory (DFT) calculations. The results indicated that the rate-determining step of the ORR is closely associated with *OH desorption. The Csp–π–d conjugated structure facilitates electron transfer toward the Co active center and shifts the d-band center away from the Fermi level, thereby moderately weakening *OH adsorption. This effect promotes intermediate desorption and accelerates ORR kinetics.
The critical role of the C≡C units was further verified by control experiments. After selective reduction was used to disrupt the C≡C structure in the catalyst, the catalytic performance decreased markedly. This result confirms that the C≡C moiety serves not merely as a structural linkage unit, but also as the key factor responsible for electronic regulation and performance enhancement.
In device-level applications, the team employed p-Co-TEPP/rGO as the cathode catalyst for rechargeable zinc–air batteries. The assembled battery delivered an open-circuit voltage of 1.52 V, a peak power density of 138.24 mW cm−2, and a specific capacity of 799 mA h g Zn−1. Its overall performance surpassed that of the benchmark Pt/C–RuO2 system and was accompanied by excellent cycling stability.
This study demonstrates that constructing a Csp–π–d conjugated structure through the incorporation of C≡C units is an effective strategy for regulating the electronic structure of non-precious-metal active centers, thereby enhancing both ORR catalytic performance and device-level performance. These findings open a new avenue for the development of highly active, stable, and low-cost non-precious-metal catalysts for energy-related applications.