Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis.

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O2 and H2 and electrochemical conversion of CO2. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Integration of Scanning Electrochemical Microscopy and Scanning Electrochemical Cell Microscopy in a Bifunctional Nanopipette toward Simultaneous Mapping of Activity and Selectivity in Electrocatalysis.

Scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) were integrated in a single bifunctional probe for simultaneous mapping of the oxygen reduction current and the oxidation current of the produced H2O2. The dual probe is fabricated from a double-barrel θ capillary, comprising one open barrel filled with the electrolyte and another filled with pyrolytic carbon. Pt is deposited with a gas injection system (GIS) at the end of the carbon barrel. The probe integrates the advantages of both SECM and SECCM by forming an electrochemical droplet cell that embeds the Pt working electrode of the carbon barrel directly into the electrolyte meniscus formed upon sample contact from the electrolyte barrel. The versatility of the dual probe is demonstrated by mapping the oxygen reduction reaction (ORR) current and the H2O2 oxidation current of a Pt microstrip on a gold substrate. This allows simultaneous localized electrochemical measurements, highlighting the potential of the dual probe for broader applications in characterizing the electrocatalytic properties of materials.

Tunable Syngas Formation at Industrially Relevant Current Densities via CO2 Electroreduction and Hydrogen Evolution over Ni and Fe-derived Catalysts obtained via One-Step Pyrolysis of Polybenzoxazine Based Composites.

Simultaneous electroreduction of CO2 and H2O to syngas can provide a sustainable feed for established processes used to synthesize carbon-based chemicals. The synthesis of MOx/M-N-Cs (M = Ni, Fe) electrocatalysts reported via one-step pyrolysis that shows increased performance during syngas electrosynthesis at high current densities with adaptable H2/CO ratios, e.g., for the Fischer-Tropsch process. When embedded in gas diffusion electrodes (GDEs) with optimized hydrophobicity, the NiOx/Ni-N-C catalyst produces syngas (H2/CO = 0.67) at -200 mA cm-2 while for the FeOx/Fe-N-C syngas production occurs at ≈-150 mA cm-2. By tuning the electrocatalyst's microenvironment, stable operation for >3 h at -200 mA cm-2 is achieved with the NiOx/Ni-N-C GDE. Post-electrolysis characterization revealed that the restructuring of the catalyst via reduction of NiOx to metallic Ni NPs still enables stable operation of the electrode at -200 mA cm-2, when embedded in an optimized microenvironment. The ionomer and additives used in the catalyst layer are important for the observed stable operation. Operando Raman measurements confirm the presence of NiOx during CO formation and indicate weak adsorption of CO on the catalyst surface.

Electrocatalytic Epoxidation of Cyclooctene on Surface Modified Ni Foam Using Water as Oxygen Source.

Electrochemical epoxidation of olefins using water as an oxygen atom source is emerging as an alternative approach for an atom economic and sustainable method towards a highly selective synthesis of epoxides. We report an electrochemical procedure for epoxidation of cyclooctene using water as the sole oxygen atom source over a sodium dodecyl sulfonate (SDS) modified nickel hydroxide Ni(OH)2 catalyst directly grown on Ni foam. The SDS modification facilitates the mass transfer of cyclooctene towards the anode, thus achieving a 2.5-fold higher conversion with more than 90 % selectivity towards the corresponding epoxide compared with pure Ni(OH)2 catalyst.

Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies.

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy.

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Gas Diffusion Electrodes for Electrocatalytic Oxidation of Gaseous Ammonia: Stepping Over the Nitrogen Energy Canyon.

As ammonia continues to gain more and more interest as a promising hydrogen carrier compound, so does the electrochemical ammonia oxidation reaction (AmOR). To avoid the liberation of H2 in a reverse Haber-Bosch reaction under release of the energetically more favorable N2, we propose the oxidation of ammonia to value-added nitrite (NO2 -), which is usually obtained during the Ostwald process. We investigated the anodic oxidation of gaseous ammonia directly supplied to a gas diffusion electrode (GDE) using a variety of compositionally different multi-metal catalysts coated on Ni foam under the simultaneous formation of H2 at the cathode. This will double the amount of H2 per ammonia molecule while applying a lower overpotential than that required for water electrolysis (1.4-1.8 V vs. RHE at 50 mA ⋅ cm-2). A selectivity study demonstrated that some of the catalyst compositions were able to produce significant amounts of NO2 -, and further investigations using the most promising catalyst composition Nif_AlCoCrCuFe integrated within a GDE demonstrated up to 88 % Faradaic efficiency for NO2 - at the anode coupled to close to 100 % Faradaic efficiency for the cathodic H2 production.

Crystal Plane-Related Oxygen-Evolution Activity of Single Hexagonal Co3 O4 Spinel Particles.

The electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte of hexagonal spinel Co3 O4 nanoparticles derived using scanning electrochemical cell microscopy (SECCM) is correlated with scanning electron microscopy and atomic force microscopy images of the droplet landing sites. A unique way to deconvolute the intrinsic catalytic activity of individual crystal facets of the hexagonal Co3 O4 spinel particle is demonstrated in terms of the turnover frequency (TOF) of surface Co atoms. The top surface exposing 111 crystal planes displayed a thickness-dependent TOF with a TOF of about 100 s-1 at a potential of 1.8 V vs. RHE and a particle thickness of 100 nm. The edge of the particle exposing (110) planes, however, showed an average TOF of 270±68 s-1 at 1.8 V vs. RHE and no correlation with particle thickness. The higher atomic density of Co atoms on the edge surface (2.5 times of the top) renders the overall catalytic activity of the edge planes significantly higher than that of the top planes. The use of a free-diffusing Os complex in the alkaline electrolyte revealed the low electrical conductivity through individual particles, which explains the thickness-dependent TOF of the top planes and could be a reason for the low activity of the top (111) planes.

Reorganization energy in a polybromide ionic liquid measured by scanning electrochemical cell microscopy.

Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Understanding the effects of the electrode-electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl-N-ethyl-pyrrolidinium polybromide (MEPBr2n+1) enabled evaluation of the reorganization energy (λ), which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at a Pt ultramicroelectrode (UME) to the Marcus-Hush-Chidsey model for heterogeneous electron transfer kinetics. However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions in the form of UME, for each experiment. Herein, we report a method to evaluate the λ of MEPBr2n+1 by scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements using a single electrode with high spatial resolution. Fast mass transport in the nanosized SECCM tip is critical for achieving heterogeneous electron transfer-limited voltammograms. Furthermore, investigating λ on a high-entropy alloy materials library composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr2n+1, modulating the solvation energy of the redox-active species in the IHP.

Stability Investigations on a Pt@HGS Catalyst as a Model Material for Fuel Cell Applications: The Role of the Local pH.

Increasing the resistance of catalysts against electrochemical degradation is one of the key requirements for the wider use of Proton Exchange Membrane Fuel Cells (PEMFCs). Here, we study the degradation of one entity of a highly stable catalyst, Pt@HGS, on a nanoelectrode under accelerated mass transport conditions. We find that the catalyst degrades more rapidly than expected based on previous ensemble measurements. Corroborated by identical location transmission electron microscopy and catalyst layer experiments, we deduce that locally different pH values are likely the reason for this difference in stability. Ultimately, this work provides insights into the actual conditions present in a PEMFC and raises questions about the applicability of accelerated stress tests usually performed to evaluate catalyst stability, particularly when they are performed in half-cell setups under inert gas.

A Novel Electrode for Value-Generating Anode Reactions in Water Electrolyzers at Industrial Current Densities.

Hydrogen generated in electrolyzers is discussed as a key element in future energy scenarios, but oxygen evolution as the standard anode reaction is a complex multi-step reaction requiring a high overpotential. At the same time,it does not add value-the oxygen is typically released into the atmosphere. Alternative anode reactions which can proceed at similar current densities as the hydrogen evolution are, therefore, of highest interest. We have discovered a high-performance electrode based on earth-abundant elements synthesized in the presence of H2 O2 , which is able to sustain current densities of close to 1 A cm-2 for the oxidation of many organic molecules, which are partly needed at high production volumes. Such anode reactions could generate additional revenue streams, which help to solve one of the most important problems in the transition to renewable energy systems, i.e. the cost of hydrogen electrolysis.

A Flexible Theory for Catalysis: Learning Alkaline Oxygen Reduction on Complex Solid Solutions within the Ag-Pd-Pt-Ru Composition Space.

Compositionally complex materials such as high-entropy alloys and oxides have the potential to be efficient platforms for catalyst discovery because of the vast chemical space spanned by these novel materials. Identifying the composition of the most active catalyst materials, however, requires unraveling the descriptor-activity relationship, as experimentally screening the multitude of possible element ratios quickly becomes a daunting task. In this work, we show that inferred adsorption energy distributions of *OH and *O on complex solid solution surfaces within the space spanned by the system Ag-Pd-Pt-Ru are coupled to the experimentally observed electrocatalytic performance for the oxygen reduction reaction. In total, the catalytic activity of 1582 alloy compositions is predicted with a cross-validated mean absolute error of 0.042 mA/cm2 by applying a theory-derived model with only two adjustable parameters. Trends in the discrepancies between predicted electrochemical performance values of the model and the measured values on thin film surfaces subsequently provide insight into the alloys' surface compositions during reaction conditions. Bridging this gap between computationally modeled and experimentally observed catalytic activities, not only reveals insight into the underlying theory of catalysis but also takes a step closer to realizing exploration and exploitation of high-entropy materials.

Single-entity Electrochemistry Unveils Dynamic Transformation during Tandem Catalysis of Cu2 O and Co3 O4 for Converting NO3 - to NH3.

Electrochemically converting nitrate to ammonia is an essential and sustainable approach to restoring the globally perturbed nitrogen cycle. The rational design of catalysts for the nitrate reduction reaction (NO3 RR) based on a detailed understanding of the reaction mechanism is of high significance. We report a Cu2 O+Co3 O4 tandem catalyst which enhances the NH3 production rate by ≈2.7-fold compared to Co3 O4 and ≈7.5-fold compared with Cu2 O, respectively, however, most importantly, we precisely place single Cu2 O and Co3 O4 cube-shaped nanoparticles individually and together on carbon nanoelectrodes provide insight into the mechanism of the tandem catalysis. The structural and phase evolution of the individual Cu2 O+Co3 O4 nanocubes during NO3 RR is unveiled using identical location transmission electron microscopy. Combining single-entity electrochemistry with precise nano-placement sheds light on the dynamic transformation of single catalyst particles during tandem catalysis in a direct way.

Scalable Synthesis of Multi-Metal Electrocatalyst Powders and Electrodes and their Application for Oxygen Evolution and Water Splitting.

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm-2 , and 24 h measurements demonstrated no loss of current density of 1 A cm-2 .

Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy.

Anatase TiO2 is a promising material for Li-ion (Li+ ) batteries with fast charging capability. However, Li+ (de)intercalation dynamics in TiO2 remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO2 with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li+ storage in TiO2 particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

Acidic Hydrogen Evolution Electrocatalysis at High-Entropy Alloys Correlates with its Composition-Dependent Potential of Zero Charge.

The vast possibilities in the elemental combinations of high-entropy alloys (HEAs) make it essential to discover activity descriptors for establishing rational electrocatalyst design principles. Despite the increasing attention on the potential of zero charge (PZC) of hydrogen evolution reaction (HER) electrocatalyst, neither the PZC of HEAs nor the impact of the PZC on the HER activity at HEAs has been described. Here, we use scanning electrochemical cell microscopy (SECCM) to determine the PZC and the HER activities of various elemental compositions of a Pt-Pd-Ru-Ir-Ag thin-film HEA materials library (HEA-ML) with high statistical reliability. Interestingly, the PZC of Pt-Pd-Ru-Ir-Ag is linearly correlated with its composition-weighted average work function. The HER current density in acidic media positively correlates with the PZC, which can be explained by the preconcentration of H+ in the electrical double layer at potentials negative of the PZC.

Linking Composition, Structure and Thickness of CoOOH layers to Oxygen Evolution Reaction Activity by Correlative Microscopy.

The role of ß-CoOOH crystallographic orientations in catalytic activity for the oxygen evolution reaction (OER) remains elusive. We combine correlative electron backscatter diffraction/scanning electrochemical cell microscopy with X-ray photoelectron spectroscopy, transmission electron microscopy, and atom probe tomography to establish the structure-activity relationships of various faceted ß-CoOOH formed on a Co microelectrode under OER conditions. We reveal that ≈6 nm ß-CoOOH(01 1 ‾ ${\bar{1}}$ 0), grown on [ 1 ‾ 2 1 ‾ ${\bar{1}2\bar{1}}$ 0]-oriented Co, exhibits higher OER activity than ≈3 nm ß-CoOOH(10 1 ‾ ${\bar{1}}$ 3) or ≈6 nm ß-CoOOH(0006) formed on [02 2 ‾ 1 ] ${\bar{2}1]}$ - and [0001]-oriented Co, respectively. This arises from higher amounts of incorporated hydroxyl ions and more easily reducible CoIII -O sites present in ß-CoOOH(01 1 ‾ ${\bar{1}}$ 0) than those in the latter two oxyhydroxide facets. Our correlative multimodal approach shows great promise in linking local activity with atomic-scale details of structure, thickness and composition of active species, which opens opportunities to design pre-catalysts with preferred defects that promote the formation of the most active OER species.

Introducing Stacking Faults into Three-Dimensional Branched Nickel Nanoparticles for Improved Catalytic Activity.

Creating high surface area nanocatalysts that contain stacking faults is a promising strategy to improve catalytic activity. Stacking faults can tune the reactivity of the active sites, leading to improved catalytic performance. The formation of branched metal nanoparticles with control of the stacking fault density is synthetically challenging. In this work, we demonstrate that varying the branch width by altering the size of the seed that the branch grows off is an effective method to precisely tune the stacking fault density in branched Ni nanoparticles. A high density of stacking faults across the Ni branches was found to lower the energy barrier for Ni2+/Ni3+ oxidation and result in enhanced activity for electrocatalytic oxidation of 5-hydroxylmethylfurfural. These results show the ability to synthetically control the stacking fault density in branched nanoparticles as a basis for enhanced catalytic activity.

On the Mediated Electron Transfer of Immobilized Galactose Oxidase for Biotechnological Applications.

Invited for the cover of this issue are Felipe Conzuelo, Wolfgang Schuhmann, and co-workers at the Ruhr University Bochum. The image depicts the electrochemical conversion of glycerol and 5-(hydroxymethyl)furfural with an electrode made up of galactose oxidase electrically wired with a redox polymer. Read the full text of the article at 10.1002/chem.202200868.

On the Mediated Electron Transfer of Immobilized Galactose Oxidase for Biotechnological Applications.

The use of enzymes as catalysts in chemical synthesis offers advantages in terms of clean and highly selective transformations. Galactose oxidase (GalOx) is a remarkable enzyme with several applications in industrial conversions as it catalyzes the oxidation of primary alcohols. We have investigated the wiring of GalOx with a redox polymer; this enables mediated electron transfer with the electrode surface for its potential application in biotechnological conversions. As a result of electrochemical regeneration of the catalytic center, the formation of harmful H2 O2 is minimized during enzymatic catalysis. The introduced bioelectrode was applied to the conversion of bio-renewable platform materials, with glycerol as model substrate. The biocatalytic transformations of glycerol and 5-hydroxymethylfurfural (HMF) were investigated in a circular flow-through setup to assess the possibility of substrate over-oxidation, which is observed for glycerol oxidation but not during HMF conversion.