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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 H 2 in a reverse Haber-Bosch reaction under release of the energetically more favorable N 2 , we propose the oxidation of ammonia to value-added nitrite (NO 2 - ), 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 H 2 at the cathode. This will double the amount of H 2 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 NO 2 - , and further investigations using the most promising catalyst composition Ni f _AlCoCrCuFe integrated within a GDE demonstrated up to 88 % Faradaic efficiency for NO 2 - at the anode coupled to close to 100 % Faradaic efficiency for the cathodic H 2 production., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

For the anodic H 2 O 2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H 2 O 2 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 H 2 O 2 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 HCO 3 - , at potentials where H 2 O 2 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 HCO 3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Simultaneous electroreduction of CO 2 and H 2 O to syngas can provide a sustainable feed for established processes used to synthesize carbon-based chemicals. The synthesis of MO x /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 H 2 /CO ratios, e.g., for the Fischer-Tropsch process. When embedded in gas diffusion electrodes (GDEs) with optimized hydrophobicity, the NiO x /Ni-N-C catalyst produces syngas (H 2 /CO = 0.67) at -200 mA cm -2 while for the FeO x /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 NiO x /Ni-N-C GDE. Post-electrolysis characterization revealed that the restructuring of the catalyst via reduction of NiO x 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 NiO x during CO formation and indicate weak adsorption of CO on the catalyst surface., (© 2024 The Authors. Small published by Wiley‐VCH GmbH.)

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., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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/cm 2 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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Renewable electricity-powered nitrate (NO 3 - ) reduction reaction (NO 3 RR) offers a net-zero carbon route to the realization of high ammonia (NH 3 ) productivity. However, this route suffers from low energy efficiency (EE, with a half-cell EE commonly <36%), since high overpotentials are required to overcome the weak NO 3 - binding affinity and sluggish NO 3 RR kinetics. To alleviate this, a rational catalyst design strategy that involves the linear assembly of sub-5 nm Cu/Co nanophases into sub-20 nm thick nanoribbons is suggested. The theoretical and experimental studies show that the Cu-Co nanoribbons, similar to enzymes, enable strong NO 3 - adsorption and rapid tandem catalysis of NO 3 - to NH 3 , owing to their richly exposed binary phase boundaries and adjacent Cu-Co sites at sub-5 nm distance. In situ Raman spectroscopy further reveals that at low applied overpotentials, the Cu/Co nanophases are rapidly activated and subsequently stabilized by a specifically designed redox polymer that in situ scavenges intermediately formed highly oxidative nitrogen dioxide (NO 2 ). As a result, a stable NO 3 RR with a current density of ≈450 mA cm -2 is achieved, a Faradaic efficiency of >97% for the formation of NH 3 , and an unprecedented half-cell EE of ≈42%., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The role of β-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 β-CoOOH formed on a Co microelectrode under OER conditions. We reveal that ≈6 nm β-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 β-CoOOH(10 1 ‾ ${\bar{1}}$ 3) or ≈6 nm β-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 Co III -O sites present in β-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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Electrochemical CO 2 conversion is a key technology to promote the production of carbon-containing molecules, alongside reducing CO 2 emissions leading to a closed carbon cycle economy. Over the past decade, the interest to develop selective and active electrochemical devices for electrochemical CO 2 reduction emerged. However, most reports employ oxygen evolution reaction as an anodic half-cell reaction causing the system to suffer from sluggish kinetics with no production of value-added chemicals. Therefore, this study reports a conceptualized paired electrolyzer for simultaneous anodic and cathodic formate production at high currents. To achieve this, CO 2 reduction was coupled with glycerol oxidation: a BiOBr-modified gas-diffusion cathode and a Ni x B on Ni foam anode keep their selectivity for formate in the paired electrolyzer compared to the half-cell measurements. The paired reactor here reaches a combined Faradaic efficiency for formate of 141 % (45 % anode and 96 % cathode) at a current density of 200 mA cm -2 ., (© 2023 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

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 ., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Polyelemental material systems, specifically high-entropy alloys, promise unprecedented properties. Due to almost unlimited combinatorial possibilities, their exploration and exploitation is hard. This challenge is addressed by co-sputtering combined with shadow masking to produce a multitude of microscale combinatorial libraries in one deposition process. These thin-film composition spreads on the microscale cover unprecedented compositional ranges of high-entropy alloy systems and enable high-throughput characterization of thousands of compositions for electrocatalytic energy conversion reactions using nanoscale scanning electrochemical cell microscopy. The exemplary exploration of the composition space of two high-entropy alloy systems provides electrocatalytic activity maps for hydrogen evolution and oxygen evolution as well as oxygen reduction reactions. Activity optima in the system Ru-Rh-Pd-Ir-Pt are identified, and active noble-metal lean compositions in the system Co-Ni-Mo-Pd-Pt are discovered. This illustrates that the proposed microlibraries are a holistic discovery platform to master the multidimensionality challenge of polyelemental systems., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte of hexagonal spinel Co 3 O 4 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 Co 3 O 4 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., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Anatase TiO 2 is a promising material for Li-ion (Li + ) batteries with fast charging capability. However, Li + (de)intercalation dynamics in TiO 2 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 TiO 2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO 2 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 TiO 2 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., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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 (NO 3 RR) based on a detailed understanding of the reaction mechanism is of high significance. We report a Cu 2 O+Co 3 O 4 tandem catalyst which enhances the NH 3 production rate by ≈2.7-fold compared to Co 3 O 4 and ≈7.5-fold compared with Cu 2 O, respectively, however, most importantly, we precisely place single Cu 2 O and Co 3 O 4 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 Cu 2 O+Co 3 O 4 nanocubes during NO 3 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., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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 H 2 O 2 , 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., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Bi-based electrocatalysts have been widely investigated in the CO 2 reduction reaction (CO 2 RR) for the formation of formate. However, it remains a challenge to achieve high Faradaic efficiency (FE) and industrial current densities at low overpotentials for obtaining both high formate productivity and energy efficiency (EE). Herein, we report an Ag-Bi 2 O 3 hybrid nanofiber (Ag-Bi 2 O 3 ) for highly efficient electrochemical reduction of CO 2 to formate. Ag-Bi 2 O 3 exhibits a formate FE of >90% for current densities from -10 to -250 mA ⋅ cm -2 and attains a yield rate of 11.7 mmol ⋅ s -1  ⋅ m -2 at -250 mA ⋅ cm -2 . Moreover, Ag-Bi 2 O 3 increased the EE (52.7%) by nearly 10% compared to a Bi 2 O 3 only counterpart. Structural characterization and in-situ Raman results suggest that the presence of Ag induced the conversion of Bi 2 O 3 from a monoclinic phase (α-Bi 2 O 3 ) to a metastable tetragonal phase (β-Bi 2 O 3 ) and accelerated the formation of active metallic Bi at low overpotentials (at > -0.3 V), which together contributes to the highly efficient formate formation., (© 2022 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH.)

Despite the success of continuous glucose measuring systems operating through the skin for about 14 days, long-term implantable biosensors are facing challenges caused by the foreign-body reaction. We present a conceptually new strategy using catalytic enzyme-based biosensors based on a measuring sequence leading to minimum disturbance of the substrate equilibrium concentration by controlling the sensor between "on" and "off" state combined with short potentiometric data acquisition. It is required that the enzyme activity can be completely switched off and no parasitic side reactions allow substrate turnover. This is achieved by using an O 2 -independent FAD-dependent glucose dehydrogenase embedded within a crosslinked redox polymer. A short measuring interval allows the glucose concentration equilibrium to be restored quickly which enables the biosensor to operate under quasi-equilibrium conditions., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The use of nanoparticles and nanostructured electrodes are abundant in electrocatalysis. These nanometric systems contain elements of nanoconfinement in different degrees, depending on the geometry, which can have a much greater effect on the activity and selectivity than often considered. In this Review, we firstly identify the systems containing different degrees of nanoconfinement and how they can affect the activity and selectivity of electrocatalytic reactions. Then we follow with a fundamental understanding of how electrochemistry and electrocatalysis are affected by nanoconfinement, which is beginning to be uncovered, thanks to the development of new, atomically precise manufacturing and fabrication techniques as well as advances in theoretical modeling. The aim of this Review is to help us look beyond using nanostructuring as just a way to increase surface area, but also as a way to break the scaling relations imposed on electrocatalysis by thermodynamics., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2022 Wiley-VCH GmbH.)

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 H 2 O 2 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., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The number of active sites and their intrinsic activity are key factors in designing high-performance catalysts for the oxygen evolution reaction (OER). The synthesis, properties, and in-depth characterization of a homogeneous CoNiFeCu catalyst are reported, demonstrating that multimetal synergistic effects improve the OER kinetics and the intrinsic activity. In situ carbon corrosion and Cu leaching during the OER lead to an enhanced electrochemically active surface area, providing favorable conditions for improved electronic interaction between the constituent metals. After activation, the catalyst exhibits excellent activity with a low overpotential of 291.5 ± 0.5 mV at 10 mA cm -2 and a Tafel slope of 43.9 mV dec -1 . It shows superior stability compared to RuO 2 in 1 m KOH, which is even preserved for 120 h at 500 mA cm -2 in 7 m KOH at 50 °C. Single particles of this CoNiFeCu after their placement on nanoelectrodes combined with identical location transmission electron microscopy before and after applying cyclic voltammetry are investigated. The improved catalytic performance is due to surface carbon corrosion and Cu leaching. The proposed catalyst design strategy combined with the unique single-nanoparticle technique contributes to the development and characterization of high-performance catalysts for electrochemical energy conversion., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO 2 reduction, so as to maximize the rate of conversion to C-C-coupled products. Considering this, a novel Cu/C-Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest CO 2 conversion to C 2+ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO 2 gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC-H bands. These findings suggest that two mechanisms operate simultaneously towards the production of HCO 2 H and C-C-coupled products on the Cu/Ag bimetallic surface., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The formation of a vast number of different multielement active sites in compositionally complex solid solution materials, often more generally termed high-entropy alloys, offers new and unique concepts in catalyst design, which mitigate existing limitations and change the view on structure-activity relations. We discuss these concepts by summarising the currently existing fundamental knowledge and critically assess the chances and limitations of this material class, also highlighting design strategies. A roadmap is proposed, illustrating which of the characteristic concepts could be exploited using which strategy, and which breakthroughs might be possible to guide future research in this highly promising material class for (electro)catalysis., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co 3 O 4 , MgCo 2 O 4 , Co 2 FeO 4 , Co 2 AlO 4 and CoFe 2 O 4 . The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H 2 O 2 decomposition. Studying the effect of dominant surface termination, isotropic Co 3 O 4 and CoFe 2 O 4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co 3+ plays the major role for high activity. In H 2 O 2 decomposition, Co 2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCo x Fe 1-x O 3 . Phase pure perovskites were synthesized up to x = 0 . 300 in 0.025/0.050 steps. HAADF STEM and EDX analysis points towards FeO 2 -terminated (001)-facets in LaFeO 3 , in accordance with the stability diagram obtained from density functional theory calculations with a Hubbard U term (DFT+U). Linear sweep voltammetry conducted in a rotating disk electrode setup shows a reduction of the OER overpotential and a nonmonotonic trend with x, with double layer capacitance measurements indicating an intrinsic nature of activity. This is supported by DFT+U results that show reduced overpotentials for both Fe and Co reaction sites with the latter reaching values of 0.32-0.40 V, ∼0.3 V lower than for Fe. This correlates with a stronger reduction of the binding energy difference of the *O and *OH intermediates towards an optimum value of 1.6 eV for x = 0 . 250 , the OH deprotonation being the potential limiting step in most cases. Significant variations of the magnetic moments of both surface and subsurface Co and Fe during OER demonstrate that the beneficial effect is a result of a concerted action involving many surrounding ions, which extends the concept of the active site., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Active, selective and stable catalysts are imperative for sustainable energy conversion, and engineering materials with such properties are highly desired. High-entropy alloys (HEAs) offer a vast compositional space for tuning such properties. Too vast, however, to traverse without the proper tools. Here, we report the use of Bayesian optimization on a model based on density functional theory (DFT) to predict the most active compositions for the electrochemical oxygen reduction reaction (ORR) with the least possible number of sampled compositions for the two HEAs Ag-Ir-Pd-Pt-Ru and Ir-Pd-Pt-Rh-Ru. The discovered optima are then scrutinized with DFT and subjected to experimental validation where optimal catalytic activities are verified for Ag-Pd, Ir-Pt, and Pd-Ru binary alloys. This study offers insight into the number of experiments needed for optimizing the vast compositional space of multimetallic alloys which has been determined to be on the order of 50 for ORR on these HEAs., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Co 3 O 4 nanocubes are evaluated concerning their intrinsic electrocatalytic activity towards the oxygen evolution reaction (OER) by means of single-entity electrochemistry. Scanning electrochemical cell microscopy (SECCM) provides data on the electrocatalytic OER activity from several individual measurement areas covering one Co 3 O 4 nanocube of a comparatively high number of individual particles with sufficient statistical reproducibility. Single-particle-on-nanoelectrode measurements of Co 3 O 4 nanocubes provide an accelerated stress test at highly alkaline conditions with current densities of up to 5.5 A cm -2 , and allows to derive TOF values of up to 2.8×10 4  s -1 at 1.92 V vs. RHE for surface Co atoms of a single cubic nanoparticle. Obtaining such high current densities combined with identical-location transmission electron microscopy allows monitoring the formation of an oxy(hydroxide) surface layer during electrocatalysis. Combining two independent single-entity electrochemistry techniques provides the basis for elucidating structure-activity relations of single electrocatalyst nanoparticles with well-defined surface structure., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Developing highly efficient and selective electrocatalysts for the CO 2 reduction reaction to produce value-added chemicals has been intensively pursued. We report a series of Cu x O y C z nanostructured electrocatalysts derived from a Cu-based MOF as porous self-sacrificial template. Blending catalysts with polytetrafluoroethylene (PTFE) on gas diffusion electrodes (GDEs) suppressed the competitive hydrogen evolution reaction. 25 to 50 wt % teflonized GDEs exhibited a Faradaic efficiency of ≈54 % for C 2+ products at -80 mA cm -2 . The local OH - ions activity of PTFE-modified GDEs was assessed by means of closely positioning a Pt-nanoelectrode. A substantial increase in the OH - /H 2 O activity ratio due to the locally generated OH - ions at increasing current densities was determined irrespective of the PTFE amount., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Electroreduction of CO 2 to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO 2 conversion to C 2+  products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C 2+  products formation at a current density of -200 mA cm -2 and a potential of -0.45 V vs. RHE. The long-term stability for C 2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu + species under CO 2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO 2 to C 2+ products on B-Cu-Zn GDEs., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH - and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO 2 RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH - and H 2 O activity that in turn can possibly affect activity, stability, and selectivity of the CO 2 RR. We determine the local OH - and H 2 O activity in close proximity to a CO 2 -converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO 2 RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity., (© 2021 The Authors. Published by Wiley-VCH GmbH.)

Complex solid solutions ("high entropy alloys"), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data-driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition-spread thin-film libraries are synthesised. High-throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru. The method can identify optimal complex-solid-solution materials for electrocatalytic reactions in an unprecedented manner., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co 3 O 4 nanoparticle supported on a carbon-based nanoelectrode is determined by employing an efficient SEM-controlled robotic technique for picking and placing a single catalyst particle onto a modified carbon nanoelectrode surface. The stable nanoassembly is microscopically investigated and subsequently electrochemically characterized. The hexagonal-shaped Co 3 O 4 nanoparticles demonstrate size-dependent electrochemical activity and exhibit very high catalytic activity with a current density of up to 11.5 A cm -2 at 1.92 V (vs. RHE), and a turnover frequency of 532±100 s -1 at 1.92 V (vs. RHE) towards catalyzing the oxygen evolution reaction., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

We report on a photobioelectrochemical fuel cell consisting of a glucose-oxidase-modified BiFeO 3 photobiocathode and a quantum-dot-sensitized inverse opal TiO 2 photobioanode linked to FAD glucose dehydrogenase via a redox polymer. Both photobioelectrodes are driven by enzymatic glucose conversion. Whereas the photobioanode can collect electrons from sugar oxidation at rather low potential, the photobiocathode shows reduction currents at rather high potential. The electrodes can be arranged in a sandwich-like manner due to the semi-transparent nature of BiFeO 3 , which also guarantees a simultaneous excitation of the photobioanode when illuminated via the cathode side. This tandem cell can generate electricity under illumination and in the presence of glucose and provides an exceptionally high OCV of about 1 V. The developed semi-artificial system has significant implications for the integration of biocatalysts in photoactive entities for bioenergetic purposes, and it opens up a new path toward generation of electricity from sunlight and (bio)fuels., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H 2 evolution., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Producing stable but active materials is one of the enduring challenges in electrocatalysis and other types of catalysis. Producing branched nanoparticles is one potential solution. Controlling the number of branches and branch size of faceted branched nanoparticles is one of the major synthetic challenges to achieve highly active and stable nanocatalysts. Herein, we use a cubic-core hexagonal-branch mechanism to synthesize branched Ru nanoparticles with control over the size and number of branches. This structural control is the key to achieving high exposure of active {10-11} facets and optimum number of Ru branches that enables improved catalytic activity for oxygen evolution reaction while maintaining high stability., (© 2020 Wiley-VCH GmbH.)

Batteries have become an integral part of everyday life-from small coin cells to batteries for mobile phones, as well as batteries for electric vehicles and an increasing number of stationary energy storage applications. There is a large variety of standardized battery sizes (e.g., the familiar AA-battery or AAA-battery). Interestingly, all these battery systems are based on a huge number of different cell chemistries depending on the application and the corresponding requirements. There is not one single battery type fulfilling all demands for all imaginable applications. One battery class that has been gaining significant interest in recent years is polymer-based batteries. These batteries utilize organic materials as the active parts within the electrodes without utilizing metals (and their compounds) as the redox-active materials. Such polymer-based batteries feature a number of interesting properties, like high power densities and flexible batteries fabrication, among many more., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

Cryptomelane (α-(K)MnO 2 ) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce 4+ were compared. The decisive factors that explain this difference were identified in the catalysts' microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H 2 O 2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm -2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

The incorporation of highly active but also highly sensitive catalysts (e.g. the [FeFe] hydrogenase from Desulfovibrio desulfuricans) in biofuel cells is still one of the major challenges in sustainable energy conversion. We report the fabrication of a dual-gas diffusion electrode H 2 /O 2 biofuel cell equipped with a [FeFe] hydrogenase/redox polymer-based high-current-density H 2 -oxidation bioanode. The bioanodes show benchmark current densities of around 14 mA cm -2 and the corresponding fuel cell tests exhibit a benchmark for a hydrogenase/redox polymer-based biofuel cell with outstanding power densities of 5.4 mW cm -2 at 0.7 V cell voltage. Furthermore, the highly sensitive [FeFe] hydrogenase is protected against oxygen damage by the redox polymer and can function under 5 % O 2 ., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

We report an organic redox-polymer-based electroenzymatic nitrogen fixation system using a metal-free redox polymer, namely neutral-red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate) with a low redox potential of -0.58 V vs. SCE. The stable and efficient electric wiring of nitrogenase within the redox polymer matrix enables mediated bioelectrocatalysis of N 3 - , NO 2 - and N 2 to NH 3 catalyzed by the MoFe protein via the polymer-bound redox moieties distributed in the polymer matrix in the absence of the Fe protein. Bulk bioelectrosynthetic experiments produced 209±30 nmol NH 3  nmol MoFe -1  h -1 from N 2 reduction. 15 N 2 labeling experiments and NMR analysis were performed to confirm biosynthetic N 2 reduction to NH 3 ., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Controlling the formation of nanosized branched nanoparticles with high uniformity is one of the major challenges in synthesizing nanocatalysts with improved activity and stability. Using a cubic-core hexagonal-branch mechanism to form highly monodisperse branched nanoparticles, we vary the length of the nickel branches. Lengthening the nickel branches, with their high coverage of active facets, is shown to improve activity for electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF), as an example for biomass conversion., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The use of renewable energy by means of electrochemical techniques by converting H 2 O, CO 2 and N 2 into chemical energy sources and raw materials, is the basis for securing a future sustainable "green" energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

Variants of the highly active [NiFeSe] hydrogenase from D. vulgaris Hildenborough that exhibit enhanced O 2 tolerance were used as H 2 -oxidation catalysts in H 2 /O 2 biofuel cells. Two [NiFeSe] variants were electrically wired by means of low-potential viologen-modified redox polymers and evaluated with respect to H 2 -oxidation and stability against O 2 in the immobilized state. The two variants showed maximum current densities of (450±84) μA cm -2 for G491A and (476±172) μA cm -2 for variant G941S on glassy carbon electrodes and a higher O 2 tolerance than the wild type. In addition, the polymer protected the enzyme from O 2 damage and high-potential inactivation, establishing a triple protection for the bioanode. The use of gas-diffusion bioanodes provided current densities for H 2 -oxidation of up to 6.3 mA cm -2 . Combination of the gas-diffusion bioanode with a bilirubin oxidase-based gas-diffusion O 2 -reducing biocathode in a membrane-free biofuel cell under anode-limiting conditions showed unprecedented benchmark power densities of 4.4 mW cm -2 at 0.7 V and an open-circuit voltage of 1.14 V even at moderate catalyst loadings, outperforming the previously reported system obtained with the [NiFeSe] wild type and the [NiFe] hydrogenase from D. vulgaris Miyazaki F., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The fabrication and electrochemical evaluation of transparent photoelectrodes consisting of Photosystem I (PSI) or Photosystem II (PSII) is described, which are embedded and electrically wired by a redox polymer. The fabrication process is performed by an automated airbrush-type spray coating system, which ensures controlled and scalable electrode preparation. As proof of concept, electrodes with a surface area of up to 25 cm 2 were prepared. The macro-porous structure of the indium tin oxide electrodes allows a high loading of the photoactive protein complexes leading to enhanced photocurrents, which are essential for potentially technologically relevant solar-powered devices. In addition, we show that unpurified crude PSII extracts, which can be provided in comparatively high yields for electrode modification, are suitable for photoelectrode fabrication with comparable photocurrent densities., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Robotic electroanalysis in 24-well microplates was used to determine Paracetamol (PCT) release from thin films of chitosan and two pH-sensitive synthetic polymers as well as blends of the polymers with each other and with agarose. Square-wave voltammograms were recorded automatically in a potential window of 0.35 V-0.85 V vs. Ag/AgCl/0.1 M KCl and their evaluation revealed time-dependent PCT release into acidic and basic media. Comparison of the release profiles showed that pure chitosan layers released PCT quickly in a single-phase process while liberation from synthetic polymer thin films was slower with a sigmoidal shape at pH 1.2 and pH 8.0 with a maximum release of PCT after approximately 150 and 140 min, respectively. The release profile from thicker agarose films was between those of the thin films. Agarose blended with chitosan or synthetic polymers formed films with biphasic release behavior. Chitosan linearized the initial section of the release profile in chitosan/polymer blends. The automated procedure for release testing offers the advantage of low-cost, labor-effective and error-free data acquisition. The procedure has been validated as a useful microplate assay option for release profile testing., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Complex solid-solution electrocatalysts (also referred to as high-entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single-phase solid solutions from five or more constituents, they offer unique capabilities of fine-tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, Ni x B nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Mo-doped BiVO 4 has emerged as a promising material for photoelectrodes for photoelectrochemical water splitting, however, still shows a limited efficiency for light-driven water oxidation. We present the influence of an oxygen-evolution catalyst composed of Ni, Fe, and Cr oxides on the activity of Mo:BiVO 4 photoanodes. The photoanodes are prepared by spray-coating, enabling compositional and thickness gradients of the incorporated catalyst. Two different configurations are evaluated, namely with the catalyst embedded into the Mo:BiVO 4 film or deposited on top of it. Both configurations provide a significantly different impact on the photoelectrocatalytic efficiency. Structural characterisation of the materials by means of SEM, TEM and XRD as well as the photoelectrocatalytic activity investigated by means of an optical scanning droplet cell and in situ detection of oxygen using scanning photoelectrochemical microscopy are presented., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long-term functionality of electrodes. In-depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO 2 into CO 3 2- . We report the adaptation of a DEMS system for online CO 2 detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO 3 2- as CO 2 in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon-supported nickel boride (Ni x B/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)