High Overpotentials Research Articles

Li2MoO4 Tailored Anion‐enhanced Solvation Sheath Layer Promotes Solution‐phase Mediated Li‐O2 Batteries

AbstractThe high overpotential of Li‐O2 batteries (LOBs) is primarily triggered by sluggish charge transfer kinetics at the reaction interfaces. A typical LiBr redox mediator (RM) catalyst can effectively reduce the battery's overpotential. However, it is prone to shuttling and corroding the Li anode, leading to RM loss and reduced energy efficiency. To address these challenges, we introduced Li2MoO4 into the LiBr‐containing electrolyte to promote the solution‐phase mediated LOBs. This addition tailors the anion‐enhanced Li+ solvation sheath layer and forms a robust anion‐derived solid electrolyte interphases (SEI) on the Li anode. The robust SEI effectively mitigates the corrosion of soluble Br3−/Br2 and attacks by highly reactive oxygen species. Additionally, the dispersed and high‐density Li2MoO4 exhibits strong adsorption capabilities for O2/LiO2 and Br‐related species during the discharge/charge process, thereby promoting the growth and decomposition of Li2O2 in the solution phase and inhibiting the shuttle effect of Br‐related species in LOBs. Consequently, the LOBs demonstrate exceptional cycling stability (415 cycles) and high energy efficiency (86.2 %), paving the way for the sustainable development and practical application of these battery systems.

Li2MoO4 Tailored Anion-enhanced Solvation Sheath Layer Promotes Solution-phase Mediated Li-O2 Batteries.

The high overpotential of Li-O2 batteries (LOBs) is primarily triggered by sluggish charge transfer kinetics at the reaction interfaces. A typical LiBr redox mediator (RM) catalyst can effectively reduce the battery's overpotential. However, it is prone to shuttling and corroding the Li anode, leading to RM loss and reduced energy efficiency. To address these challenges, we introduced Li2MoO4 into the LiBr-containing electrolyte to promote the solution-phase mediated LOBs. This addition tailors the anion-enhanced Li+ solvation sheath layer and forms a robust anion-derived solid electrolyte interphases (SEI) on the Li anode. The robust SEI effectively mitigates the corrosion of soluble Br3 -/Br2 and attacks by highly reactive oxygen species. Additionally, the dispersed and high-density Li2MoO4 exhibits strong adsorption capabilities for O2/LiO2 and Br-related species during the discharge/charge process, thereby promoting the growth and decomposition of Li2O2 in the solution phase and inhibiting the shuttle effect of Br-related species in LOBs. Consequently, the LOBs demonstrate exceptional cycling stability (415 cycles) and high energy efficiency (86.2 %), paving the way for the sustainable development and practical application of these battery systems.

A Segmented Along the Channel Test Cell for Locally Resolved Analysis at High Current Densities in PEM Water Electrolysis

Abstract For the scale-up of proton exchange membrane (PEM) water electrolysis, understanding the cell behavior on industrial scale is a prerequisite. A proper distribution of current and temperature in the cell can improve performance and decrease overall degradation effects. Due to water consumption as well as the concomitant gas evolution and accumulation, gradients and inhomogeneities along the reaction coordinate are expected. These effects increase along the water supply channels of a flow field and are expected to lead to spatial gradients in cell performance and temperature. At high current densities (> 5 A∙cm-2) these effects are anticipated to be critical due to the high amount of gas produced and the increased degradation rate resulting from the high overpotentials. 
In this study we present a new test cell that is segmented along the flow field channels and is designed for the operation at high current densities of up to 10 A∙cm-2. With this approach it is possible to analyze performance gradients and the spatial distribution of loss processes under industry-relevant conditions by measuring the current density and temperature distribution and by performing locally resolved electrochemical impedance spectroscopy (EIS).

Hydrogen Oxidation and Evolution Reaction on Platinum in Alkaline Electrolyte: Fixed Reference vs. Overpotential and the Effect of H2 Concentration

Abstract Understanding the kinetics of the hydrogen oxidation and evolution reaction (HOR/HER) in alkaline media is of great interest, but the underlying mechanism and the dependency on the hydrogen pressure are still debated. Herein, we investigated the HOR/HER on polycrystalline platinum in alkaline electrolyte using a rotating disk electrode (RDE). Unlike typically done, we performed the data analysis on a fixed potential scale, which has implications for the assessment of RDE mass-transport corrections and the definition of reaction orders. Analyzing the kinetic currents on a fixed potential, we find hydrogen reaction orders of essentially one and zero for the HOR and HER, respectively, at larger overpotentials. In contrast, fitting the kinetic currents in a potential around equilibrium with a Butler-Volmer model yields differing kinetic parameters and fractional reaction orders. Our findings can be explained with a potential-dependent transition in the HOR mechanism from Tafel–Volmer at small overpotentials to Heyrovsky–Volmer at higher overpotentials, with a concomitant change in the H2 reaction order from fractional to unity, thus reconciling previous propositions on the alkaline HOR/HER mechanism. Our results demonstrate the strength of using a fixed potential scale, and we expect this approach to be beneficial for studies of other electrocatalytic reactions.

Comparison of oxygen evolution reaction performance for Ni and Co using isostructural trans‐cinnamate complexes

AbstractEfforts are underway to develop highly active catalysts to reduce the high overpotential of the oxygen evolution reaction (OER). Metal–organic frameworks or coordination polymers are promising candidates because of their tunable structures and high surface areas. In this study, Nickel and Cobalt trans‐cinnamate (t‐ca) were synthesized via a hydrothermal method. Their structures were analyzed and found to be isostructural. Both complexes exhibited superior electrocatalytic properties in the OER compared to those of IrO2, with overpotentials of 373 and 390 mV and Tafel slopes of 58 and 66 mV/dec. These excellent characteristics were attributed to the electron delocalization of the metal centers via interactions with π‐π delocalized organic ligands. Ni t‐ca, with stronger ligand interactions, displayed an enhanced OER catalytic performance, emphasizing the importance of metal–ligand interactions and suggesting that further exploration of diverse π–π delocalized organic ligands and metal centers may lead to further advancements in electrocatalytic activity.

Enhanced formate production from sulfur modified copper for electrocatalytic CO2 reduction

Producing liquid fuels with high selectivity and low overpotential has long been a goal of electrocatalytic CO2 reduction reaction (CO2RR). Here we present a detailed study on the structural and catalytic properties of sulfur on copper for CO2RR. Cu2S was formed by enabling sulfur to deposit on the copper surface with a simple surface impregnation method. The sulfur modified samples, especially the sulfur modified oxide-derived copper (SM-OD-Cu), have significantly higher selectivity to formate than the pure and oxide-derived copper. At a potential range of -0.51∼-0.75 V vs. RHE, the selectivity of formate for SM-OD-Cu in the liquid phase was nearly 100%. Also, the partial current density of formate reached -4.26 mA/cm2 at -0.75 V vs. RHE. By using density functional theory calculation, we found that Cu2S significantly inhibits the CO pathway and hydrogen evolution reaction, and further improves the selectivity of formate.

Cobalt nanoclusters Deposit on Nitrogen-Doped graphene Sheets as bifunctional electrocatalysts for high performance lithium – Oxygen batteries

Rechargeable lithium-oxygen (Li-O2) batteries are being considered as the next-generation energy storage systems due to their higher theoretical energy density. However, the practical application of Li-O2 batteries is hindered by slow kinetics and the formation of side products during the oxygen reduction and evolution reactions on the cathode. These reactions lead to high overpotentials during charging and discharging. To address these challenges, we propose a simple ultrasonic method for synthesizing cobalt nanoclusters embedded in nitrogen-doped graphene nanosheets (GrZnCo) derived from metal-organic frameworks (MOFs). The resulting material, due to the retention of metallic cobalt structure, exhibits better electronic conductivity. Additionally, the GrZnCo catalyst shows vigorous catalytic activity, which can improve reaction kinetics and suppress side reactions, thus lowering the charging overpotential. We have investigated the impact of different catalyst compositions (GrZnCox; x = 1, 3, 5) by varying the amounts of cobalt and zinc. The optimum catalyst, GrZnCo3, contains high cobalt-N active components, graphitic-N, pyridinic-N, pyrrolic-N, and abundant defect structures, which enhance the electrochemical performance. The defect-rich GrZnCo3 catalyst enables Li-O2 batteries to achieve a high discharge capacity of 13500 mAh·g−1 at 50 mA·g−1 and a remarkable long-term cycling performance of over 400 cycles at 100 mA·g−1 with a limited capacity of 500 mAh·g−1. This work demonstrates an effective approach to fabricate cost-effective electrocatalysts for various energy storage systems.

Metal and Coordinating Atoms Synergistically Achieve High Activity and Stability in Single-Atom Catalysts within the Framework of TM-N3X for the Oxygen Evolution Reaction of Lithium Peroxide.

A critical challenge in the advancement of lithium-oxygen batteries (LOBs) is the difficulty in decomposing lithium peroxide, leading to high charge overpotentials and poor cycling stability. Single-atom catalysts (SACs), known for their ultrahigh catalytic activity in various electrochemical reactions, are expected to enhance the kinetics of the oxygen evolution reaction (OER) for LOBs. Herein, 24 SACs within the framework of TM-N3X have been designed and optimized for the OER of lithium peroxide. First-principles calculations reveal that the doped non-metal atom (X = B, C, O, or P) significantly contributes to the structural stability of the SACs while the metal atom (TM = Ru, Os, Rh, Ir, Pd, or Pt) significantly influences the catalytic activity of the SACs. Upon evaluation of their stability and catalytic activity, the Pt-N3B and Pd-N3B catalysts have been identified as promising candidates for the OER of lithium peroxide, with theoretical charge overpotentials of 0.19 and 0.18 V, respectively. This work provides new guidance for the design of efficient SACs for LOBs and inspires a fundamental understanding of the underlying structure-activity relationship.

Facile Synthesis of CuxS Electrocatalysts for CO2 Conversion into Formate and Study of Relations Between Cu and S with the Selectivity

AbstractThe conversion of CO2 into formate (HCOO−), a techno‐economically feasible product, can be achieved using earth‐abundant CuxS electrocatalysts, but questions remain regarding how catalyst structure, composition, and reaction environment influence product selectivity. A novel synthesis method based on electrodeposition of Cu foam and its subsequent sulfidation via immersion in sulfur saturated toluene solution resulted in CuxS foams. Catalytic activity studies found that HCOO− selectivity is dependent on electrochemical activation at higher overpotentials. To understand the effects of activation, determine the active forms of the catalysts, and identify the role of sulfur, the electrodes are carefully characterized as well as gaseous and sulfur dissolved in electrolyte. This included study of the effects of intentional addition of solution sulfur species, identification of the sulfur loss, determination of the electrode composition and relating sulfur speciation to observed product selectivity. It is found that residual sulfur stabilizes Cu+ during electrolysis at potentials favoring HCOO− production, in contrast to pristine Cu that undergoes complete reduction and shows poor HCOO− selectivity. Sulfur in both the catalyst and dissolved in electrolyte are of dynamic nature, and surface residues of SO42− species are identified in all activated catalysts which correspond with enhanced HCOO− production.

Bi‐Doped In2O3 Nanofiber for Efficient Electrocatalytic CO2 Reduction

AbstractElectrocatalytic carbon dioxide reduction reaction (CO2RR) to formic acid (HCOOH) is attracted for superfluous CO2 removal and HCOOH production under ambient conditions. Indium‐based catalysts has considered as a good candidate material for CO2RR to HCOOH due to their environmentally friendly features. However, the catalytic efficiency is limited by the poor HCOOH Faradaic efficiency (FE) and high reaction overpotential of electrocatalyst, and the activity and stability of indium‐based catalysts are unsatisfactory, especially in industrial current density that is critical for commercialization. Herein, a fiber Bi‐doped In2O3 was synthesized through electrospinning method, and it demonstrate a FEHCOOH of 88.2% at −1.5 V versus RHE (reversible hydrogen electrode) with partial current density of −21.8 mA cm−2 in H type cell. Specially, the Bi‐In electrocatalyst also reach the industrial current density standard, which can work at −400 mA cm−2 current density with FEHCOOH of 92.7% (yield of HCOOH is 6.9 mmol h−1) in home‐made Flow cell. Importantly, Bi‐In shows 24 h long‐term stability test in −300 mA cm−2. The improvement catalytic activity of Bi‐In catalyst is ascribed to the optimized electronic structure of In site, and the reduced work function value of Bi‐In is beneficial for reducing the formation energy of the key *OCHO intermediates.

Mechanistic insights into multimetal synergistic and electronic effects in a hexanuclear iron catalyst with a [Fe3(μ3-O)(μ2-OH)]2 core for enhanced water oxidation.

Multinuclear molecular catalysts mimicking natural photosynthesis have been shown to facilitate water oxidation; however, such catalysts typically operate in organic solutions, require high overpotentials and have unclear catalytic mechanisms. Herein, a bio-inspired hexanuclear iron(III) complex I, Fe6(μ3-O)2(μ2-OH)2(bipyalk)2(OAc)8 (H2bipyalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol); OAc = acetate) with desirable water solubility and stability was designed and used for water oxidation. Our results showed that I has high efficiency for water oxidation via the water nucleophilic attack (WNA) pathway with an overpotential of only ca. 290 mV in a phosphate buffer of pH 2. Importantly, key high-oxidation-state metal-oxo intermediates formed during water oxidation were identified by in situ spectroelectrochemistry and oxygen atom transfer reactions. Theoretical calculations further supported the above identification. Reversible proton transfer and charge redistribution during water oxidation enhanced the electron and proton transfer ability and improved the reactivity of I. Here, we have shown the multimetal synergistic and electronic effects of catalysts in water oxidation reactions, which may contribute to the understanding and design of more advanced molecular catalysts.

DFT modeling of the oxygen electroreduction reaction on SiN3-doped carbon nanotubes

The thermodynamic features and mechanism of the electrocatalytic oxygen reduction reaction were studied using the revPBE0-D3(BJ)/Def2-TZVP method on the example of (6,6)-armchair carbon nanotube doped with a tricoordinated silicon atom and nitrogen atoms of pyridinic and graphitic nature. Irreversible oxidation of the silicon center as a result of the formation of stable oxygen-containing adsorbates was shown. It was found that Si-poisoned structures are capable of participating in the catalysis of the target reaction along two- and four-electron routes at high overpotentials. For a nanotube doped simultaneously with pyridinic and graphitic nitrogens the potential possibility of eliminating the silicon atom from the catalyst composition in the form of orthosilicic acid and the participation of a silicon-free nitrogen-doped framework in the oxygen electroreduction reaction, for which the stage of tautomerization of pyridin-2(1H)-one to pyridin-2-ol is the limiting step was shown.

Coupled Lattice‐Expanded Ni and MoO2 Array for Efficient Alkaline Hydrogen Evolution

AbstractTransition metal oxides have attracted much attention due to their good electrical conductivity, high chemical stability, and easy electronic structure modulation. However, it is still a great challenge to solve the problems of sluggish reaction kinetics and high overpotential in the alkaline hydrogen evolution reaction (HER) due to their poor intrinsic activity. Herein, a lattice‐expanded Ni‐decorated MoO2 array (Ni‐MoO2‐700) catalyst is prepared for the alkaline HER. Unlike most of the reported literature, the decoration of metal heteroatoms may lead to lattice expansion of the carrier catalyst. In contrast, lattice expansion of Ni is also achieved by adjusting the annealing temperature in this work. The Ni‐induced lattice‐expanding effect can not only modulate the electronic structure of the catalyst but also accelerate electron transfer during the reaction, thus increasing its intrinsic activity for the HER. As a result, Ni‐MoO2‐700 exhibits significantly enhanced catalytic activity with an overpotential of 74.9 mV at 10 mA cm−2 in 1.0 m KOH, which is far superior to that of most of the reported advanced Ni‐ and Mo‐based materials. This work provides deep intrinsic insight and a great opportunity to design efficient transition metal oxide catalysts for the alkaline HER.

Probing the effect of Se doping in S cathode for high performance Mg-S batteries

Magnesium-sulfur (Mg-S) batteries present an attractive option as a new type of energy storage device given their high energy density, safety and the use of low-cost original materials. However, magnesium polysulfide (MgPS) is susceptible to dissolution and shuttling, and the insulating nature of sulfur and MgS leads to high overpotentials and low columbic efficiencies. The selenium-sulfur system is noted to offer enhanced electronic conductivity and improved sulfur utilization. In this study, S K-edge X-ray absorption near-edge structure (XANES) spectra demonstrate that Se doping mitigates irreversibility in Mg-S batteries. Mg-K spectra demonstrate that Se dopants induce local negative charges on S, strengthening the Mg-S bond compared to that of pure sulfur, thereby promoting Mg/Mg2+ redox reactions. Operando Raman spectroscopy was also used to reveal Mg-S0.9Se0.1/C battery redox mechanisms. Hence, kinetically favored S0.9Se0.1/C cathodes deliver an initial capacity of ∼1350 mAh·g−1 at 0.2C. Furthermore, separators modified with layered MoS2 entrap polysulfides, thereby postponing cell death. Thus, Mg-S battery with a S0.9Se0.1/C cathode and a MoS2@polypropylene (PP) separator exhibit a reversible capacity of ∼200 mAh·g−1 at 0.2C after 250 cycles, with a low fade rate of 4 mAh·g−1/cycle. This study delineates the overall impact of Se doped cathodes on Mg-S batteries. Moreover, the MoS2 modified separator provides a route to chemically immobilize MgPS and accelerate sulfur redox reactions, consequently enabling high capacities and sustained cycling.

Highly selective photocatalytic oxidation of CH4 to CH3OH: A theoretical study

Photooxidation of methane (CH4) in aqueous solution to generate high-value organic compounds is an effective way to replace traditional industrial methods. It is crucial to select catalysts that can avoid competitive reactions and achieve high selectivity. Using first principles calculations, a novel one-dimensional (1D) Ti3C2O2 nanoribbon (NR)/two-dimensional (2D) monolayer MoS2 (MS) heterojunction photocatalyst was ingenious designed in this work, which can achieve the oxidation of CH4 molecules and suppress the execution of competitive reactions. The results indicate that the constructed 1D NR exhibits semiconductor properties and its energy level position meet the requirements of photocatalytic oxidation for CH4. The heterojunction structure composed of NR and MS forms an efficient S-Scheme electron transfer mechanism under illumination, which not only provides sufficient internal driving force for CH4 oxidation, but also effectively avoids the competitive reaction between water molecules and photo-generated holes. Specifically, the photocatalysts exhibits high selectivity and low reaction overpotential for the generation of methanol (CH3OH). This study provides a new perspective for photocatalytic CH4 oxidation and demonstrates the potential application value of 1D MXene in the future field of photocatalysis.

Introducing a new model for solid-state batteries: Parameter estimation and sensitivity analysis on diffusion, concentration, and electrochemical kinetics

Despite their potential, solid-state batteries (SSBs) are challenging to optimize due to the complexity of their materials and the early stage of their modeling, especially compared to liquid-electrolyte technologies. Many existing models rely on empirical equations that cannot represent internal mass-transport and kinetic phenomena. Even studies based on mechanistic models often focus on theoretical explanations of experimental phenomena or generating hard-to-obtain experimental data. In this work, a simple yet versatile mechanistic model – able to simulate any battery composed of a metallic anode, solid electrolyte and intercalation cathode – is proposed and used in a parameter estimation routine to identify the material properties of a Li/LiPON/LiCoO2 battery. After validation, a parametric study is made to address how each material property impacts concentration profiles, discharge curves, overpotentials, and core key-performance indexes (KPIs) related to energy efficiency and battery capacity. Findings reveal that cathode diffusion is critical to enhancing Extraction Efficiency, in some cases reaching 98.9% of the theoretical capacity without enhancing any other parameter. In the case of Energy Efficiency being a priority over capacity, electrolyte diffusion and concentration showed a pivotal role, in the range of parameters studied, to increase Voltaic Efficiency up to 97.8%. Despite not directly affecting the Extraction Efficiency, it is discussed how high overpotentials caused by Electrolyte Concentration and Kinetic Constants still can harm battery capacity, for charge–discharge cycles limited by boundaries in electrical potential. Finally, studies simultaneously varying two or more parameters show that, despite extreme property enhancements (up to ca 500x higher than the standard case) indeed lead to exceptional results (up to 96.4% for Voltaic Efficiency and 98.9% for Extraction Efficiency), to use such materials at its full potential will result in other zones of the device usually neglected – such as the overpotentials from the metal anode or even the current collector – may become the new stumbling block, underscoring the significance of considering the battery as a holistic system that will inherently exhibit bottlenecks. This also implies, however, in the perpetual potential for enhancement across its components.

Citrate ions-modified NiFe layered double hydroxide for durable alkaline seawater oxidation

Seawater electrolysis taking advantage of coastal/offshore areas is acknowledged as a potential way of large-scale producing H2 to substitute traditional technology. However, anodic catalysts with high overpotentials and limited lifespans (caused by chloride-induced competitive chemical reactions) hinder the system of seawater electrolysis for H2 production. Herein, we present a citrate anion (CA) modified NiFe layered double hydroxide nanosheet array on nickel foam (NiFe LDH@NiFe-CA/NF), which serves as an efficient and stable electrocatalyst towards long-term alkaline seawater oxidation. It requires only a low overpotential of 387 mV to achieve a current density of 1000 mA cm−2, outperforming NiFe LDH/NF (414 mV). Moreover, NiFe LDH@NiFe-CA/NF exhibits continuous oxygen evolution testing for 300h at 1000 mA cm−2 due to its anti-corrosion characterization. Additionally, the fabricated cell can stably operate at 300 mA cm−2 (60 °C, 6 M KOH + seawater) and only require 1.69 V, achieving low energy consumption of seawater splitting.

Computational Insight into Transition Metal Atoms Anchored on B2C3P as Single‐Atom Electrocatalysts for Nitrogen Reduction Reaction

AbstractNH3 is not only an important chemical raw material but also a high‐energy storage chemical with zero carbon. Electrocatalytic nitrogen reduction reaction (NRR), which can be driven by clean electric energy under ambient conditions, has become a promising technology for NH3 synthesis due to its environmentally friendly properties. Because of the limitations of low yield and high overpotential, efficient catalysts are urgently needed to solve this problem. In this study, based on density functional theory method and high throughput screening strategy, the NRR was investigated on transition metal single atom anchored to 2D B2C3P surface (TM@B2C3P) as single‐atom catalysts (SACs). The results showed that V@B2C3P and Ti@B2C3P have good catalytic properties, and the limiting potentials were −0.10 and −0.24 V, respectively. Furthermore, the charge density difference and crystal orbital Hamilton population calculations demonstrated that the high catalytic activity can be attributed to the obvious charge transfer between TM@B2C3P and the adsorption intermediates. It is hoped that this work can play a certain role in exploring the application of SACs in NRR.

Boosting synergistic effects between PtGe nanoalloys and 2D materials for PEMFC applications

In the present work we have investigated the stability, CO poisoning tendency and HOR and ORR activities of small-sized monometallic Pt and bimetallic PtGe nanoclusters deposited on a series of 2D materials. By means of global minima search and mechanistic studies at the density functional theory (DFT) level with continuum implicit solvent, a screening of size, composition and support material has been carried out to obtain the optimal catalyst compositions that may boost the catalytic performance. Alloying and support effects work in two directions. On the one hand, the support increases the stability and modifies the electronic structure of the cluster via metal-support interaction. On the other hand, the cluster can change the electronic properties and catalytic activity of the support, which can be modulated with its size and composition. In addition, the 2D support assists pure Pt clusters in reducing the propensity to undergo CO poisoning, which can be further reduced by Ge alloying. This effect can be exploited to the point that the interaction with CO is not even favorable, as in Pt5Ge5/germanene, thus completely inhibiting the CO poisoning on the catalyst. Regarding the catalytic activities, all the catalysts studied in this work yield too high overpotentials for the ORR in acidic conditions, due to the presence of too oxophilic active sites which results in the overbinding of oxygenated species. However, the combination of Ge alloying and 2D support yield remarkable results in the HOR performance. The simultaneous tailoring of catalyst composition and support is demonstrated to be an effective strategy to successfully adjust the catalytic properties.

Concurrent oxygen evolution reaction pathways revealed by high-speed compressive Raman imaging.

Transition metal oxides are state-of-the-art materials for catalysing the oxygen evolution reaction (OER), whose slow kinetics currently limit the efficiency of water electrolysis. However, microscale physicochemical heterogeneity between particles, dynamic reactions both in the bulk and at the surface, and an interplay between particle reactivity and electrolyte makes probing the OER challenging. Here, we overcome these limitations by applying state-of-the-art compressive Raman imaging to uncover concurrent bias-dependent pathways for the OER in a dense, crystalline electrocatalyst, α-Li2IrO3. By spatially and temporally tracking changes in stretching modes we follow catalytic activation and charge accumulation following ion exchange under various electrolytes and cycling conditions, comparing our observations with other crystalline catalysts (IrO2, LiCoO2). We demonstrate that at low overpotentials the reaction between water and the oxidized catalyst surface is compensated by bulk ion exchange, as usually only found for amorphous, electrolyte permeable, catalysts. At high overpotentials the charge is compensated by surface redox active sites, as in other crystalline catalysts such as IrO2. Hence, our work reveals charge compensation can extend beyond the surface in crystalline catalysts. More generally, the results highlight the power of compressive Raman imaging for chemically specific tracking of microscale reaction dynamics in catalysts, battery materials, or memristors.