Huang Qing'e, Chen Jun, Luan Peng, Ding Chunmei, Li Can
Department of Chemical Physics, University of Science and Technology of China Hefei 230026 China
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences Dalian 116023 China.
Chem Sci. 2022 Jul 8;13(30):8797-8803. doi: 10.1039/d2sc02213c. eCollection 2022 Aug 4.
The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm, much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH). However, the substrate nucleophilically attacked by OH for bi-CoPc is the phthalocyanine cation-radical species (Co-Pc-Pc˙-Co-OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc-Co-OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH on Co-Pc-Pc˙-Co-OH is 1.67 eV, lower than that of mono-CoPc with Pc-Co-OH nucleophilically attacked by OH (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.
用于电化学水氧化的高效催化剂的合理设计高度依赖于对反应途径的理解，而这仍然是一个挑战。在此，选择具有明确分子结构的单核和双核钴酞菁（单钴酞菁和双钴酞菁）作为模型电催化剂来研究水氧化机理。我们发现，碳载体上的双钴酞菁（双钴酞菁/碳）在10 mA cm时的过电位为357 mV，远低于单钴酞菁/碳（>450 mV）。动力学分析表明，两种电催化剂上析氧反应（OER）的速率决定步骤（RDS）是涉及羟基阴离子（OH）的亲核攻击过程。然而，双钴酞菁被OH亲核攻击的底物是由酞菁环氧化形成的酞菁阳离子自由基物种（Co-Pc-Pc˙-Co-OH），而通过原位紫外可见光谱电化学技术证明，单钴酞菁涉及钴氧化物种（Pc-Co-OH）。密度泛函理论（DFT）计算表明OH对Co-Pc-Pc˙-Co-OH亲核攻击的反应势垒为1.67 eV，低于OH对Pc-Co-OH亲核攻击的单钴酞菁的反应势垒（1.78 eV）。实验和理论结果之间的良好一致性表明，双钴酞菁可以有效地稳定酞菁环中积累的氧化电荷，因此具有更高的OER性能。
