One of the key challenges for the large-scale application of electrochemical energy conversion, employing reactors for water electrolysis and CO2 reduction, is still relatively low energy-efficiency of the anodic oxygen evolution reaction (OER). Although, oxides are perceived to be representative class of compounds used as catalytic materials, after more than 40 years of intensive research on OER, it is still not clear what really determines activity and stability trends. At technically relevant current densities, the critical point for obtaining a conclusive picture about the electrode performance is to understand the dynamic behavior of the triple-phase boundary. Fluctuations in the magnitude of the active surface area, due to blockage of a fraction of the overall number of active sites by the gas, induces great uncertainty in kinetic analysis which is usually based on stationary electrochemical methods. Consequently, the task of the highest merit is to employ advanced analytical tools and create methodologies which allow realistic insight into dynamic behavior of the polarized electrode/electrolyte interface during the gas release. In this work, recently developed approaches for comprehensive activity/stability studies will be discussed. Namely, activity is estimated using scanning electrochemical microscopy (SECM) in a mode adjusted for local electrochemical noise measurements. A positioned microelectrode was used as a sensor in order to estimate/monitor the frequency of gas-bubble detachment from the electrode surface. Recorded potential-dependent frequency spectra are the fingerprints of temporal fluctuations at the electrode/electrolyte interface. At the same time stability analysis was conducted using a miniaturized electrochemical scanning flow cell coupled to the inductively coupled mass spectrometer (SFC-ICPMS). Obtained time-resolved potential dependent dissolution profiles were used as indication of the character of the dissolution process. In the final section of the work, based on the information gathered from SECM and SFC-ICPMS, was proposes a design of the electrode surface which could help to achieve the elusive goal of having simultaneously highly active and highly stable catalysts for the oxygen evolution reaction. Acknowledgment . BMBF for the financial support in the framework of the project ECCO2 (Kz: 033RC1101A). References: 1. A.R. Zeradjanin et al. ChemSusChem 5 (2012) 1905. 2. A.A. Topalov et al. Angew. Chem. Int. Ed. 51 (2012) 12613 3. A.R. Zeradjanin et al. RSC Adv. 4 (2014) 9579 4. K.J.J. Mayrhofer et al. Angew. Chem. Int. Ed. 53 (2014) 102 5. A.R. Zeradjanin et al. Int. J. Hydrogen Energ. 39 (2014) 16275
Carbon monoxide is widely known to poison Pt during heterogeneous catalysis owing to its strong donor–acceptor binding ability. Herein, we report a counterintuitive phenomenon of this general paradigm when the size of Pt decreases to an atomic level, namely, the CO-promoting Pt electrocatalysis toward hydrogen evolution reactions (HER). Compared to pristine atomic Pt catalyst, reduction current on a CO-modified catalyst increases significantly. Operando mass spectroscopy and electrochemical analyses demonstrate that the increased current arises due to enhanced H2 evolution, not additional CO reduction. Through structural identification of catalytic sites and computational analysis, we conclude that CO-ligation on the atomic Pt facilitates Hads formation via water dissociation. This counterintuitive effect exemplifies the fully distinct characteristics of atomic Pt catalysts from those of bulk Pt, and offers new insights for tuning the activity of similar classes of catalysts.
Abstract The reaction path of the Cl 2 evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. The state of the RuO 2 surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO 2 and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen‐containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl 2 formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived “volcano” curve suggests that electrocatalysis is governed by a resonance phenomenon.
The electrochemical stability of thermally prepared Ir oxide films is investigated using a scanning flow cell (SFC)–inductively coupled plasma mass-spectrometer (ICP-MS) setup under transient and stationary potential and/or current conditions. Time-resolved dissolution rates provide important insights into critical conditions for material breakdown and a fully quantitative in-situ assessment of the electrochemical stability during oxygen evolution reaction (OER) conditions. In particular, the results demonstrate that stability and OER activity of the IrOx catalysts strongly depend on the chemical and structural nature of Ir oxide species and their synthesis conditions.
Electrocatalytic water splitting is a topic of investigation of numerous research groups for decades. Oxygen evolution reaction (OER), as a more complex and more demanding reaction, is more frequently studied, despite of the fact that the simpler hydrogen evolution reaction (HER), is generally not well understood. Nowadays, recurrent approach to electrocatalysis is conquest for more active and more stable electrode materials, very often lacking significant input to better understanding of reaction mechanisms or understanding of what are the drivers of electrocatalytic activity [1]. Importantly, if one asks the key question from the conceptual point of view, and that is: what are the origins of electrocatalytic activity? - the answer will be, in the predominant majority of cases, like 70 years ago. Namely, paradigm of electrocatalysis is Sabatier principle, that suggests optimal (“not too strong, not too weak”) binding of intermediates as main precondition of high reaction rate [2]. Conventional wisdom suggests that confirmation of this should be relatively simple. Namely, Brönsted-Evans-Polanyi (BEP) relations suggest linear relationship between adsorption energy of key intermediates and activation energy. In other words, if we have reliable values of adsorption energies relevant for the electrochemical environment [3], they should form linear dependence with experimentally obtained activation energies. However, high-temperature hydrodynamic experiments on HER indicate that reducing of activation energy by tuning of adsorption energy of intermediates is not necessarily beneficial for enhancement of HER rate. Namely, HER rate is strongly influenced by preexponential frequency factor. Therefore, we propose discussion that will analyse several key aspects of HER electrocatalysis: 1) interplay of activation energy and preexponential factor as a driver of electrocatalytic activity 2) are BEP relations relevant for electrocatalysis 3) can we assess adsorption energies experimentally 4) what are relevant descriptors that shed light on HER activation process, 5) what else is important beyond Sabatier principle for rate of HER [1-7]. Figure 1. Graphical illustration of four interfacial descriptors relevant for HER. Adapted from Ref. [4] with permission from the PCCP Owner Societies, under the terms and conditions of the Creative Commons Attribution 3.0 Unported License. References [1] A.R. Zeradjanin et al, ChemElectroChem 2022, 9, e202101278 [2] A.R. Zeradjanin et al, Electrochimica Acta 2021, 388 , 138583 [3] A.R. Zeradjanin et al, Journal of Solid State Electrochemistry 2021, 25 , 33 [4] A.R. Zeradjanin et al, PCCP 2020, 22 , 8768 [5] A.R. Zeradjanin et al, PCCP 2017, 19 , 17019 [6] A.R. Zeradjanin et al, Electroanalysis 2016, 28 , 2256 [7] A.R. Zeradjanin et al, ACS Catalysis 2022 , 12, 11597 Figure 1
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