Theoretical Studies of Potential-Dependent and Competing Mechanisms of the Electrocatalytic Oxygen Reduction Reaction on Pt(111)
Theoretical Studies of Potential-Dependent and Competing Mechanisms of the Electrocatalytic Oxygen Reduction Reaction on Pt(111)
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DOI：
10.1002/anie.201004794
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Jacob, Timo
中科院分区：
文献类型：
--
作者：
Keith, John A.;Jacob, Timo
氧还原反应（ORR）是燃烧、腐蚀、细胞呼吸和能源技术中的关键过程。在具有氢气供应的电化学条件下，电催化ORR [1]也是允许聚合物电解质（或质子交换）膜燃料电池（PEMFC）运行的反应。经济和环境因素正在推动研究开发实用和环境可持续的能源以及长寿命的非均相催化剂。预计ORR将在这些技术中发挥核心作用，但需要对ORR有更基本的了解。由于ORR动力学的高度复杂性，对ORR机制的原子级理解仍处于早期阶段。然而，已知的是，完整的电化学ORR涉及在阴极处到分子氧的四个净耦合质子和电子转移（CPH）。尽管理想化的电化学反应每个电子产生1.23 V（方案1），在Pt（111）上的电催化ORR的标准操作电位低于0.9V。确定约0.3V的该过电位的原因并改进总体活性（增加电流密度）是改进ORR催化剂设计和更好地利用ORR作为能量转化的实用手段的关键。一些ORR过程被认为会导致表面氧化物和/或强结合中间体，这反过来又会成为ORR的障碍。[1]虽然可能的ORR中间体仅由H和O原子组成，但ORR机制是难以捉摸的，即使在高度研究的Pt（111）电极上也是如此。在过去的十年中，人们已经做出了大量的努力，使用第一性原理量子力学（QM）计算来确定铂和其他过渡金属表面上氧的结合能（BE），并将这些数据扩展到研究ORR机制的各个方面，[2]甚至研究施加的电极电位对反应机制的作用。[3]最相关的是，QM计算可以提供化学键合能的准确描述，其可以用于预测ORR速率常数。计算这些值是从第一原理理解完整电催化ORR的第一步，这可能需要多尺度分析，明确解决电化学双层，电子动力学，表面覆盖效应和运输问题。虽然这样一个完整的模拟是不可行的，能量和障碍的基础上QM计算可以用于动力学模型，然后比较实验观测。如果从第一性原理计算获得的速率常数使ORR的特征合理化，那么这将是从根本上理解这种反应和其他高度复杂反应的一大步。构建物理上合理的多相ORR机制需要明确确定一系列可能的中间体的BE：O*，H*，O2*，OH*，OOH*，H2 O2 * 和H2O* 以及将中间体相互连接的过渡态。一般来说，我们认为电化学反应可以通过Langmuir-Hinshelwood（LH）或Eley-Rideal（ER）机制进行。LH机制涉及表面上的所有反应中间体，而ER机制涉及来自电解质的物质与表面中间体（例如H3 O+）反应。在理想化的Pt（111）表面上的关键步骤的表征允许在不完美或修饰的表面上进行类似的步骤。
The oxygen reduction reaction (ORR) is a key process in combustion, corrosion, cellular respiration, and energy technology. Under electrochemical conditions with a supply of hydrogen, the electrocatalytic ORR [1] is also the reaction that allows polymer-electrolyte (or proton-exchange) membrane fuel cells (PEMFCs) to operate. Economic and environmental factors are driving research to develop practical and environmentally sustainable energy sources as well as long-living heterogeneous catalysts. The ORR is expected to play a central role in these technologies, but a more fundamental understanding of the ORR is needed. An atomic-level understanding of the ORR mechanism is still in its early stages because of the high complexity of ORR kinetics. What is known, however, is that the complete electrochemical ORR involves four net coupled proton and electron transfers (CPETs) to molecular oxygen at the cathode. Although the idealized electrochemical reaction generates 1.23 V per electron (Scheme1), the standard operating potential for the electrocatalytic ORR on Pt (111) is below 0.9 V. Determining the cause of this overpotential of about 0.3 V and improving the overall activity (increasing the current density) are the keys to improving ORR catalyst design and to better harness the ORR as a practical means for energy conversion. Some ORR processes are believed to lead to surface oxides and/or strongly binding intermediates, which in turn are expected as hindrances of the ORR.[1] Thus, designing new heterogeneous catalysts that destabilize these intermediates without changing the overall mechanism is desired.Whilst possible ORR intermediates only consist of H and O atoms, the ORR mechanism is elusive, even on the highly studied Pt (111) electrodes. There have been substantial efforts over the past decade to use first-principles quantum mechanics (QM) calculations to determine binding energies (BEs) of oxygen on platinum and other transition metal surfaces and to extend those data to investigate aspects of the ORR mechanism,[2] and even investigate the role of an applied electrode potential on a reaction mechanism.[3] Most relevantly, QM calculations can provide accurate descriptions of chemical bonding energies, which can be used to predict ORR rate constants. Calculating these values is a first step towards understanding the complete electrocatalytic ORR from first principles, something that will likely require a multiscale analysis explicitly addressing the electrochemical double layer, electron dynamics, surface-coverage effects, and transport issues. Although such a complete simulation is not yet feasible, energies and barriers based on QM calculations can be used in a kinetic model and then compared to experimental observables. If the rate constants obtained from firstprinciples calculations rationalized features of the ORR, this would be a large step towards fundamentally understanding this and other highly complex reactions. Constructing a physically reasonable heterogeneous ORR mechanism requires explicitly determining the BEs of a list of possible intermediates: O*, H*, O2*, OH*, OOH*, H2O2*, and H2O* as well as the transition states that link the intermediates to one another. For generality, we consider that electrochemical reactions may operate by Langmuir–Hinshelwood (LH) or Eley–Rideal (ER) mechanisms. LH mechanisms involve all the reacting intermediates on the surface, whereas ER mechanisms involve species from the electrolyte reacting with a surface intermediate (for example H3O+). Characterization of key steps on the idealized Pt (111) surface allows analogous steps on imperfect or modified …