Characterisation of non-uniform functional surfaces: towards linking basic surface properties with electrocatalytic activity

Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP–SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP–SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP–SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates.Financial support by the EU and the state NRW in the framework of the HighTech. NRW program is gratefully acknowledged. A.S.B. and W.S. additionally acknowledge financial support in the framework of Helmholtz-Energie-Allianz “Stationäre elektrochemische Speicher und Wandler” (HA-E-0002) and the Cluster of Excellence RESOLV (EXC 1069) funded by the DFG (Deutsche Forschungsgemeinschaft).Published versio

Monitoring Active Sites for Hydrogen Evolution Reaction at Model Carbon Surfaces

Carbon is ubiquitous as an electrode material in electrochemical energy conversion devices. If used as support material, the evolution of H2 is undesired on carbon. However, recently carbon-based materials are of high interest as economic and eco-conscious alternative to noble metal catalysts. The targeted design of improved carbon electrode materials requires atomic scale insight into the structure of the sites that catalyse H2 evolution. This work demonstrates that electrochemical scanning tunnelling microscopy under reaction conditions (n-EC-STM) can monitor active sites of highly oriented pyrolytic graphite for the hydrogen evolution reaction. With down to atomic resolution, the most active sites in acidic medium are pinpointed near edge sites and defects, whereas the basal planes remain inactive. Density functional theory calculations support these findings and reveal that only specific defects on graphite are active. Motivated by these results, the extensive usage of n-EC-STM on doped carbon-based materials is encouraged to locate their active sites and guide the synthesis of enhanced electrocatalysts.The authors thank Prof. Plamen Atanassov (University of California, Irvine, USA) and Dr. Jun Maruyama (Osaka Research Institute of Industrial Science and Technology, Japan) for fruitful discussion regarding some experimental results. RMK, RWH and ASB acknowledge the financial support from the German Research Foundation (DFG), in the framework of the projects BA 5795/4-1 and BA 5795/3-1, and under Germany's Excellence Strategy–EXC 2089/1–390776260, cluster of excellence ‘e-conversion’. ASB acknowledges the funding from the European Union's Horizon 2020 research and innovation programme under grant agreement HERMES No. 952184. FCV acknowledges financial support from Spanish MICIUN through RTI2018-095460-B-I00 and María de Maeztu (MDM-2017-0767) grants and a Ramón y Cajal research contract (RYC-2015-18996), and also from Generalitat de Catalunya (grants 2017SGR13 and XRQTC). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences, with financial support from NWO

On the pH Dependence of the Potential of Maximum Entropy of Ir(111) Electrodes

Studies over the entropy of components forming the electrode/electrolyte interface can give fundamental insights into the properties of electrified interphases. In particular, the potential where the entropy of formation of the double layer is maximal (potential of maximum entropy, PME) is an important parameter for the characterization of electrochemical systems. Indeed, this parameter determines the majority of electrode processes. In this work, we determine PMEs for Ir(111) electrodes. The latter currently play an important role to understand electrocatalysis for energy provision; and at the same time, iridium is one of the most stable metals against corrosion. For the experiments, we used a combination of the laser induced potential transient to determine the PME, and CO charge-displacement to determine the potentials of zero total charge, (EPZTC). Both PME and EPZTC were assessed for perchlorate solutions in the pH range from 1 to 4. Surprisingly, we found that those are located in the potential region where the adsorption of hydrogen and hydroxyl species takes place, respectively. The PMEs demonstrated a shift by ~30 mV per a pH unit (in the RHE scale). Connections between the PME and electrocatalytic properties of the electrode surface are discussed.We acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) in the framework of the cluster of excellence “Resolv” (EXC1069). This work was supported by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program

A Physical Impedance Model of Lithium Intercalation into Graphite Electrodes for a Coin‐Cell Assembly

Abstract Graphite electrodes are widely used in commercial metal‐ion batteries as anodes. Electrochemical impedance spectroscopy serves as one of the primary non‐destructive techniques to obtain key information about various batteries during their operation. However, interpretation of the impedance response of graphite electrodes in contact with common organic electrolytes can be complicated. It is especially challenging, particularly when utilizing the 2‐electrode configuration that is common in battery research. In this work, we elaborate on a physical impedance model capable of accurately describing the impedance spectra of a graphite|electrolyte|metallic Li system in a coin‐cell assembly during two initial charge/discharge cycles. We analyze the dependencies of the model parameters for graphite and metallic lithium as a function of the state of charge to verify the model. Additionally, we suggest that the double layer capacitance values obtained during specific intercalation stages could help to determine if the area‐normalized values align with the expected range. The data and the procedure necessary for calibration are provided

Correlative Electrochemical Microscopy for the Elucidation of the Local Ionic and Electronic Properties of the Solid Electrolyte Interphase in Li-Ion Batteries

The solid-electrolyte interphase (SEI) plays a key role in the stability of lithium-ion batteries as the SEI prevents the continuous degradation of the electrolyte at the anode. The SEI acts as an insulating layer for electron transfer, still allowing the ionic flux through the layer. We combine the feedback and multi-frequency alternating-current modes of scanning electrochemical microscopy (SECM) for the first time to assess quantitatively the local electronic and ionic properties of the SEI varying the SEI formation conditions and the used electrolytes in the field of Li-ion batteries (LIB). Correlations between the electronic and ionic properties of the resulting SEI on a model Cu electrode demonstrates the unique feasibility of the proposed strategy to provide the two essential properties of an SEI: ionic and electronic conductivity in dependence on the formation conditions, which is anticipated to exhibit a significant impact on the field of LIBs.The authors are grateful for financial support by the European Union's Horizon 2020 research and innovation programme under grant agreement NanoBat No 861962 as well as to the Deutsche Forschungsgemeinschaft ((DFG, German Research Foundation) under Germany's Excellence Strategy—EXC 2033-390677874—RESOLV. The authors thank Martin Trautmann for the AFM measurements. Open Access funding enabled and organized by Projekt DEAL

Electrolyte Effects on the Stabilization of Prussian Blue Analogue Electrodes in Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries based on PrussianBlue Analogues (PBA) are considered as promising and scalablecandidates for stationary energy storage systems, where longevityand cycling stability are assigned utmost importance to maintaineconomic viability. Although degradation due to active materialdissolution is a common issue of battery electrodes, it is hardlyobservable directly due to a lack of in operando techniques, makingit challenging to optimize the performance of electrodes. Byoperating Na2Ni[Fe(CN)6] and Na2Co[Fe(CN)6] model electrodesin a flow-cell setup connected to an inductively coupled plasmamass spectrometer, in this work, the dynamics of constituenttransition-metal dissolution during the charge−discharge cycleswas monitored in real time. At neutral pHs, the extraction of nickeland cobalt was found to drive the degradation process during charge−discharge cycles. It was also found that the nature of anionspresent in the electrolytes has a significant impact on the degradation rate, determining the order ClO4− > NO3− > Cl− > SO42− withdecreasing stability from the perchlorate to sulfate electrolytes. It is proposed that the dissolution process is initiated by detrimentalspecific adsorption of anions during the electrode oxidation, therefore scaling with their respective chemisorption affinity. This studyinvolves an entire comparison of the effectiveness of common stabilization strategies for PBAs under very fast (dis)chargingconditions at 300C, emphasizing the superiority of highly concentrated NaClO4 with almost no capacity loss after 10 000 cycles forNa2Ni[Fe(CN)6].KEYWORDS: Na-ion aqueous batteries, Prussian Blue Analogues, sodium nickel hexacyanoferrate, ICP-MS, active material dissolution,degradatio