Searcharxiv⌕ Search

arXiv subjects

Marcel Risch

Publications and source records attributed to Marcel Risch.

15 recordsLinked to original sources
arXiv2023

Measurement of Enthalpy and Entropy of the Rate-Determining Step of a Model Electrocatalyst for the Oxygen Evolution Reaction

Experimentally determined thermodynamic parameters are rarely reported for electrocatalytic reactions including the oxygen evolution reaction (OER). Yet, they contain unique and valuable mechanistic insight and present a missing link to theoretical investigations. Herein, a protocol for determining thermodynamic properties of the rate determining steps (RDS) of the OER is presented. Cobalt oxide is investigated at pH 7 as a case study. Two different approaches are employed: steady state polarization (SSP) that uses chronopotentiometry at different temperatures and current values, and potentiostatic electrochemical impedance spectroscopy (PEIS) at different DC voltages and temperatures. The data is used to fit a 3D plane from which entropy and enthalpy of the RDS are obtained. The data analysis requires an appropriate filtering of the data. Hence, we discuss suitable figures of merit for establishing appropriate filtering criteria. The values obtained are 0.72 and -0.39 eV (at 298 K) for enthalpic and entropic contributions, respectively. The obtained values are reproducible for both approaches and consistent with literature. We further highlight that the RDS entropy gives clue of the number of water molecules adsorbed or protons released prior to RDS and thereby helps identifying plausible OER mechanisms.

physics.chem-ph↗
arXiv2023

Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn$_2$O$_4$

Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LiMn$_x$O$_4$ system in aqueous alkaline electrolyte. The investigated LiMn$_2$O$_4$ particles exhibit two distinct Mn dissolution processes; one independent of OER and the other associated to OER. Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive X-ray photoelectron spectroscopy and electrochemical detection of Mn dissolution using rotating ring-disk electrode, we explore the less understood Mn dissolution mechanism during OER. We correlate near-surface oxidation with the charge attributed to dissolved Mn, which demonstrates increasing Mn dissolution with the formation of surface Mn4+ species under anodic potential. The observed stronger dissolution during the OER is attributed to the formation of additional Mn$^{4+}$ from Mn$^{3+}$ during OER. We show that control over the amount of Mn4+ in Li$_x$Mn2O$_4$ before the onset of the OER can partially mitigate the OER-triggered dissolution. Overall, our atomistic insights into the Mn dissolution processes are crucial for knowledge-guided mitigation of electrocatalyst degradation, which can be broadly extended to manganese-based oxide systems.

cond-mat.mtrl-sci↗
arXiv2022

Upgrading the Detection of Electrocatalyst Degradation During the Oxygen Evolution Reaction

Electrocatalysts for the oxygen evolution reaction (OER) are an important component for the transition from fossil to sustainable energy. Commercialization of cost-effective earth-abundant electrocatalysts is in large parts hindered by their degradation. In this short review, I identify common processes leading to a decrease in electrocatalyst activity, followed by an introduction of staple methods to determine degradation electrochemically and by additional physical characterization, which has the potential to remove ambiguities of purely electrochemical studies. I conclude by a summary of the key challenges for an accurate determination of degradation processes and highlight interesting directions to advance the understanding of degradation processes on electrocatalysts.

physics.chem-ph↗
arXiv2022

What X-ray absorption spectroscopy can tell us about the active state of earth-abundant electrocatalysts for the oxygen evolution reaction

Chemical energy storage is an attractive solution to secure a sustainable energy supply. It requires an electrocatalyst to be implemented efficiently. In order to rationally improve the electrocatalyst materials and thereby the reaction efficiency, one must reveal the nature of the electrocatalyst under reaction conditions, i.e., its active state. For a better understanding of earth-abundant metal oxides as electrocatalysts for the oxygen evolution reaction (OER), the combination of electrochemical (EC) methods and X-ray absorption spectroscopy (XAS) has been very insightful and still holds untapped potential. Herein, we concisely introduce the basics of EC and XAS and provide the necessary framework to discuss changes that electrocatalytic materials undergo, presenting manganese oxides as examples. Such changes may occur during preparation and storage, during immersion in an electrolyte, as well as during application of potentials without or with catalytic reactions. We conclude with a concise summary of how EC and XAS are currently combined to elucidate the active state as well as an outlook on future opportunities to understand the mechanisms of electrocatalysis using combined operando EC-XAS experiments.

cond-mat.mtrl-sci↗
arXiv2022

Stabilization of Co oxide during oxygen evolution in alkaline media by the introduction of Mn oxide

Improving the stability of electrocatalysts for the oxygen evolution reaction (OER) through materials design has received less attention than improving their catalytic activity. We explored the effect of Mn addition to a cobalt oxide for stabilizing the catalyst by comparing Na-containing CoOx and (Co0.7Mn0.3)Ox films electrodeposited in alkaline solution. The obtained disordered films were classified as layered oxides using X-ray absorption spectroscopy (XAS). The CoOx films showed a constant decrease in the catalytic activity during cycling, confirmed by oxygen detection, while that of (Co0.7Mn0.3)Ox slightly increased as measured by electrochemical metrics. These trends were rationalized based on XAS analysis of the metal oxidation states, which were Co2.8+ and Mn3.7+ near the surface after cycling. Thus, adding Mn to CoOx successfully stabilized the catalyst material and its activity during OER cycling. The development of stabilization approaches is essential to extend the durability of OER catalysts.

cond-mat.mtrl-sci↗
arXiv2021

Calculation of the Tafel slope and reaction order of the oxygen evolution reaction between pH 12 and pH 14 for the adsorbate mechanism

Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction, the mechanism of the OER on transition metal oxides remains controversial. This is in part owed to the ambiguity of electrochemical parameters of the mechanism such as the Tafel slope and reaction orders. We took the most commonly assumed adsorbate mechanism and calculated the Tafel slopes and reaction orders with respect to pH based on microkinetic analysis. We demonstrate that number of possible Tafel slopes strongly depends on a number of preceding steps and surface coverage. Furthermore, the Tafel slope becomes pH dependent when the coverage of intermediates changes with pH. These insights complicate the identification of a rate-limiting step by a single Tafel slope at a single pH. Yet, simulations of reaction orders complementary to Tafel slopes can solve some ambiguities to distinguish between possible rate-limiting steps. The most insightful information can be obtained from the low overpotential region of the Tafel plot. The simulations in this work provide clear guidelines to experimentalists for the identification of the limiting steps in the adsorbate mechanism using the observed values of the Tafel slope and reaction order in pH-dependent studies.

physics.chem-ph↗
arXiv2021

Requirements for beneficial electrochemical restructuring: A model study on a cobalt oxide in selected electrolytes

The requirements for beneficial materials restructuring into a higher performance OER electrocatalyst are still a largely open question. Here we use Erythrite (Co$_3$(AsO$_4$)$_2\cdot$8H$_2$O) as a Co-based OER electrocatalyst to evaluate its catalytic properties during in-situ restructuring into an amorphous Co-based catalyst in four different electrolytes at pH 7. Using diffraction, microscopy and spectroscopy, we observed a strong effect in the restructuring kinetics depending of the anions in the electrolyte. Only carbonate electrolyte could activate the catalyst electrode, which we relate to its slow restructuring kinetics. While its turnover frequency (TOF) reduced from 2.84 O$_2$ Co$^{-1}$ s$^{-1}$ to a constant value of 0.10 O$_2$ Co$^{-1}$ s$^{-1}$ after 300 cycles, the number of redox active sites continuously increased, which explained the current increase of around 100%. The final activated material owns an adequate local order, a high Co oxidation state and a high number of redox-active Co ions, which we identify as the trinity for enhancing the OER activity. Thus, this work provides new insights into for the rational design of high-performance OER catalysts by electrochemical restructuring.

physics.chem-ph↗
arXiv2021

Nafion-induced reduction of manganese and its impact on the electrocatalytic properties of a highly active MnFeNi oxide for bifunctional oxygen conversion

Electrocatalysts for bifunctional oxygen reduction (ORR) and oxygen evolution reaction (OER) are commonly studied under hydrodynamic conditions, rendering the use of binders necessary to ensure the mechanical stability of the electrode films. The presence of a binder, however, may influence the properties of the materials under examination to an unknown extent. Herein, we investigate the impact of Nafion on a highly active ORR/OER catalyst consisting of MnFeNi oxide nanoparticles supported on multi-walled carbon nanotubes. Electrochemical studies revealed that, in addition to enhancing the mechanical stability and particle connectivity, Nafion poses a major impact on the ORR selectivity, which correlates with a decrease in the valence state of Mn according to X-ray absorption spectroscopy. These findings call for awareness regarding the use of electrode additives, since in some cases the extent of their impact on the properties of electrode films cannot be regarded as negligible.

physics.chem-ph↗
arXiv2020

Seven steps to reliable cyclic voltammetry measurements for the determination of double layer capacitance

Discovery of electrocatalytic materials for high-performance energy conversion and storage applications relies on the adequate characterization of their intrinsic activity, which is currently hindered by the dearth of a protocol for consistent and precise determination of double layer capacitance (CDL). Herein, we propose a seven-step method that aims to determine CDL reliably by scan rate-dependent cyclic voltammetry. The method considers three aspects that strongly influence the outcome of the analysis: measurement settings, data collection, and data processing. To illustrate the proposed method, two systems were studied: a resistor-capacitor electric circuit and a glassy carbon disk in an electrochemical cell. With these studies it is demonstrated that when any of the mentioned aspects of the procedure are neglected, substantial deviations of the results are observed with misestimations as large as 61% in the case of the investigated electrochemical system. Moreover, we propose allometric regression as a more suitable model than linear regression for the determination of CDL for both the ideal and the non-ideal systems investigated. We stress the importance of assessing the accuracy of not only highly specialized electrochemical methods, but also of those that are well-known and commonly used as it is the case of the voltammetric methods. The methodology proposed herein is not limited to the determination of CDL, but can be effectively applied to any other voltammetry-based analysis that aims to deliver quantitative results.

physics.ed-ph↗
arXiv2020

Characterization of a modular flow cell system for electrocatalytic experiments and comparison to a commercial RRDE system

Generator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using a rotating ring-disk electrode (RRDE). We developed a double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. Commonalities and differences between the RRDE and DEFC are discussed based on analytical solutions, numerical simulations and measurements of the ferri-/ferrocyanide redox couple on Pt electrodes in a potassium chloride electrolyte. The analytical solutions agree with the measurements using electrode widths of 5 and 2 mm. Yet, we find an unexpected dependence on the exponent of the width so that wider electrodes cannot be analysed using the conventional analytical solution. In contrast, all the investigated electrodes show a collection efficiency of close to 35.4% above a minimum rotation speed or flow rate, where the narrowest electrode is most accurate at the cost of precision and the widest electrode the least accurate but most precise. Our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs due to the flexibility to optimize the electrode materials and geometry for the desired reaction.

physics.chem-ph↗
arXiv2020

Trends of epitaxial perovskite oxide films catalyzing the oxygen evolution reaction in alkaline media

The oxygen evolution reaction (OER) is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in electrolyzers, metal-air batteries and other applications that rely on sustainable protons from water oxidation. In this review, the prospect of epitaxial perovskite oxides for the OER at room temperature in alkaline media is reviewed with respect to fundamental insight into systematic trends of the activity. First, we thoroughly define the perovskite structure and its parameter space. Then, the synthesis methods used to make electrocatalytic epitaxial perovskite oxide are surveyed, and we classify the different kinds of electrodes that can be assembled for electrocatalytic investigations. We discuss the semiconductor physics of epitaxial perovskite electrodes and their consequences for the interpretation of catalytic results. Prototypical mechanisms of the OER are introduced and comparatively discussed. OER investigations on epitaxial perovskite oxides are comprehensively surveyed and selected trends are graphically highlighted. The review concludes with a short perspective on opportunities for future electrocatalytic research on epitaxial perovskite oxide systems.

cond-mat.mtrl-sci↗
arXiv2020

Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction

Manganese oxides have received much attention over the years among the wide range of electrocatalysts for the oxygen evolution reaction (OER) due to their low toxicity, high abundance and rich redox chemistry. While many previous studies focused on the activity of these materials, a better understanding of the material transformations relating to activation or degradation is highly desirable, both from a scientific perspective and for applications. We electrodeposited Na-containing MnOx without long-range order from an alkaline solution to investigate these aspects by cyclic voltammetry, scanning electron microscopy and x-ray absorption spectroscopy at the Mn-K and Mn-L edges. The pristine film was assigned to a layered edge-sharing Mn3+/4+ oxide with Mn-O bond lengths of mainly 1.87 {\AA} and some at 2.30 {\AA} as well as Mn-Mn bond lengths of 2.87 {\AA} based on fits to the extended x-ray fine structure. The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186 \pm 48 to 114 \pm 18 mV dec-1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edge showed no change of the microstructure, bulk local structure or bulk Mn valence. Yet, the surface region of MnOx oxidized toward Mn4+, which explains the reduction of the currents in agreement with literature. Surprisingly, we find that MnOx reactivates after 30 min at open-circuit (OC), where the currents and also the Tafel slope increase. Reactivation processes during OC are crucial because OC is unavoidable when coupling the electrocatalysts to intermittent power sources such as solar energy for sustainable energy production.

cond-mat.mtrl-sci↗
arXiv2020

A criticial view on e$_g$ occupancy as a descriptor for oxygen evolution catalytic activity in LiMn$_2$O$_4$ nanoparticles

We investigate the effect of the surface electronic structure and composition of LiMn$_2$O$_4$ nanoparticles on the electrocatalytic oxygen evolution reaction (OER). Scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) studies combined with density functional theory (DFT) based simulations of the EEL spectra reveal in pristine nanoparticles a 4 nm thick surface layer with reduced average Mn oxidation state and increased Mn concentration. This is attributed to Mn$^{2+}$ partially replacing Li$^+$ at the tetrahedral sites of the spinel lattice accompanied by Mn 3d-state filling of octahedrally coordinated Mn. During electrocatalytic OER cycling, this near-surface tetrahedral Mn is leached out, thereby increasing the oxidation state of octahedrally coordinated Mn. Using rotating ring-disc electrode (RRDE) based detection of O and Mn during the OER, we show that the oxygen evolution remains constant while the Mn$^{2+}$ is removed, revealing that near-surface tetrahedrally coordinated Mn has no effect on the OER activity of LiMn$_2$O$_4$. This is surprising since the e$_g$ occupancy of Mn in octahedral sites, which is widely used as a descriptor of OER activity, changes significantly in the surface layer during cycling. The fact that e$_g$ emptying fails to correlate with OER activity here indicates that octahedral cation valence is not a fundamental measure of activity, either because the active surface state is not affected by tetrahedral Mn or because other details of the band structure or metal-oxygen bonding character, more strongly regulate the rate-limiting steps for OER.

cond-mat.mtrl-sci↗
arXiv2020

Closing the gap between theory and experiment for lithium manganese oxide spinels using a high-dimensional neural network potential

Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing X-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment.

cond-mat.mtrl-sci↗