Ni-Xides (B, S, and P) for Alkaline OER: Shedding Light on Reconstruction Processes and Interplay with Incidental Fe Impurities as Synergistic Activity Drivers.

Ni-Xides (X = B, P, or S) exhibit intriguing properties that have endeared them for electrocatalytic water splitting. However, the role of B, P, and S, among others, in tailoring the catalytic performance of the Ni-Xides remains vaguely understood, especially if they are studied in unpurified KOH (Un-KOH) because of the renowned impact of incidental Fe impurities. Therefore, decoupling the effect induced by Fe impurities from inherent material reconstruction processes necessitates investigation of the materials in purified KOH solutions (P-KOH). Herein, studies of the OER on Ni2B, Ni2P, and Ni3S2 in P-KOH and Un-KOH coupled with in situ Raman spectroscopy, ex situ post-electrocatalysis, and online dissolution studies by ICP-OES are used to unveil the distinctive role of Ni-Xide reconstruction and the role of Fe impurities and their interplay on the electrocatalytic behavior of the three Ni-Xide precatalysts during the OER. There was essentially no difference in the OER activity and the electrochemical Ni2+/Ni3+ redox activation fingerprints of the three precatalysts via cyclic voltammetry in P-KOH, whereas their OER activity was considerably higher in Un-KOH with marked differences in the intrinsic activity and evolution of the Ni2+/Ni3+ fingerprint redox peaks. Thus, in the absence of Fe in the electrolyte (P-KOH), neither the nature of the guest element (B, P, and S) nor the underlying reconstruction processes are decisive activity drivers. This underscores the crucial role played by incidental Fe impurities on the OER activity of Ni-Xide precatalysts, which until now has been overlooked. In situ Raman spectroscopy revealed that the nickel hydroxide derived from Ni2B exhibits higher disorder than in the case of Ni2P and Ni3S2, both exhibiting a similar degree of disorder. The guest elements thus influence the degree of disorder of the formed nickel oxyhydroxides, which through their synergistic interaction with incidental Fe impurities concertedly realize high OER performance.

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.

The redox mediated - scanning droplet cell system for evaluation of the solid electrolyte interphase in Li-ion batteries.

The so-called solid electrolyte interphase (SEI), a nanolayer formed on the negative electrode of lithium-ion batteries during the first cycles, largely influences some key performance indicators such as cycle life and specific power. The reason is due to the fact that the SEI prevents continuous electrolyte decomposition, making this protecting character extremely important. Herein, a specifically designed scanning droplet cell system (SDCS) is developed to study the protecting character of the SEI on lithium-ion battery (LIB) electrode materials. SDCS allows for automatized electrochemical measurements with improved reproducibility and time-saving experimentation. Besides the necessary adaptations for its implementation for non-aqueous batteries, a new operating mode, the so-called redox mediated-scanning droplet cell system (RM-SDCS), is established to investigate the SEI properties. By adding a redox mediator (e.g. a viologen derivative) to the electrolyte, evaluation of the protecting character of the SEI becomes accessible. Validation of the proposed methodology was performed using a model sample (Cu surface). Afterwards, RM-SDCS was employed on Si-graphite electrodes as a case study. On the one hand, the RM-SDCS shed light on the degradation mechanisms providing direct electrochemical evidence of the rupture of the SEI upon lithiation. On the other hand, the RM-SDCS was presented as an accelerated method capable of searching for electrolyte additives. The results indicate an enhancement in the protecting character of the SEI when 4 wt% of both vinyl carbonate and fluoroethylene carbonate were used simultaneously.

Enhanced Nitrate-to-Ammonia Efficiency over Linear Assemblies of Copper-Cobalt Nanophases Stabilized by Redox Polymers.

Renewable electricity-powered nitrate (NO3 - ) reduction reaction (NO3 RR) offers a net-zero carbon route to the realization of high ammonia (NH3 ) 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 NO3 - binding affinity and sluggish NO3 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 NO3 - adsorption and rapid tandem catalysis of NO3 - to NH3 , 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 (NO2 ). As a result, a stable NO3 RR with a current density of ≈450 mA cm-2 is achieved, a Faradaic efficiency of >97% for the formation of NH3 , and an unprecedented half-cell EE of ≈42%.

Simultaneous Anodic and Cathodic Formate Production in a Paired Electrolyzer by CO2 Reduction and Glycerol Oxidation.

Electrochemical CO2 conversion is a key technology to promote the production of carbon-containing molecules, alongside reducing CO2 emissions leading to a closed carbon cycle economy. Over the past decade, the interest to develop selective and active electrochemical devices for electrochemical CO2 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, CO2 reduction was coupled with glycerol oxidation: a BiOBr-modified gas-diffusion cathode and a Nix 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 .

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 .

Ag-induced Phase Transition of Bi2 O3 Nanofibers for Enhanced Energy Conversion Efficiency towards Formate in CO2 Electroreduction.

Bi-based electrocatalysts have been widely investigated in the CO2 reduction reaction (CO2 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-Bi2 O3 hybrid nanofiber (Ag-Bi2 O3 ) for highly efficient electrochemical reduction of CO2 to formate. Ag-Bi2 O3 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-Bi2 O3 increased the EE (52.7%) by nearly 10% compared to a Bi2 O3 only counterpart. Structural characterization and in-situ Raman results suggest that the presence of Ag induced the conversion of Bi2 O3 from a monoclinic phase (α-Bi2 O3 ) to a metastable tetragonal phase (ß-Bi2 O3 ) and accelerated the formation of active metallic Bi at low overpotentials (at > -0.3 V), which together contributes to the highly efficient formate formation.

Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia.

Electrocatalytic recycling of waste nitrate (NO3-) to valuable ammonia (NH3) at ambient conditions is a green and appealing alternative to the Haber-Bosch process. However, the reaction requires multi-step electron and proton transfer, making it a grand challenge to drive high-rate NH3 synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO3--to-NH3 conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu-Co binary sulfides into potential-dependent core-shell Cu/CuOx and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in-situ Raman spectra reveal that the inner Cu/CuOx phases preferentially catalyze NO3- reduction to NO2-, which is rapidly reduced to NH3 at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO3--to-NH3 Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO3- concentrations at pH 13, a high NH3 yield rate of 1.17 mmol cm-2 h-1 in 0.1 M NO3- at -0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.

In Situ Carbon Corrosion and Cu Leaching as a Strategy for Boosting Oxygen Evolution Reaction in Multimetal Electrocatalysts.

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 RuO2 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.

Redox Replacement of Silver on MOF-Derived Cu/C Nanoparticles on Gas Diffusion Electrodes for Electrocatalytic CO2 Reduction.

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO2 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 CO2 conversion to C2+ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO2 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 HCO2 H and C-C-coupled products on the Cu/Ag bimetallic surface.

A Metal-Organic Framework derived Cux Oy Cz Catalyst for Electrochemical CO2 Reduction and Impact of Local pH Change.

Developing highly efficient and selective electrocatalysts for the CO2 reduction reaction to produce value-added chemicals has been intensively pursued. We report a series of Cux Oy Cz 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 C2+ 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- /H2 O activity ratio due to the locally generated OH- ions at increasing current densities was determined irrespective of the PTFE amount.

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes.

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 (CO2 RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH- and H2 O activity that in turn can possibly affect activity, stability, and selectivity of the CO2 RR. We determine the local OH- and H2 O activity in close proximity to a CO2 -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/CO2 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.

B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ †Products at High Current Densities.

Electroreduction of CO2 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 CO2 conversion to C2+  products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C2+  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 C2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu+ species under CO2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO2 to C2+ products on B-Cu-Zn GDEs.

Combining Nanoconfinement in Ag Core/Porous Cu Shell Nanoparticles with Gas Diffusion Electrodes for Improved Electrocatalytic Carbon Dioxide Reduction.

Bimetallic silver-copper electrocatalysts are promising materials for electrochemical CO2 reduction reaction (CO2RR) to fuels and multi-carbon molecules. Here, we combine Ag core/porous Cu shell particles, which entrap reaction intermediates and thus facilitate the formation of C2+ products at low overpotentials, with gas diffusion electrodes (GDE). Mass transport plays a crucial role in the product selectivity in CO2RR. Conventional H-cell configurations suffer from limited CO2 diffusion to the reaction zone, thus decreasing the rate of the CO2RR. In contrast, in the case of GDE-based cells, the CO2RR takes place under enhanced mass transport conditions. Hence, investigation of the Ag core/porous Cu shell particles at the same potentials under different mass transport regimes reveals: (i) a variation of product distribution including C3 products, and (ii) a significant change in the local OH- activity under operation.

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure.

Large scale CO2 electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, e.g. for CO2 reduction to CO. We have used shear-force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO2 reduction as a function of catalyst loading and CO2 back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under operando conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO2 back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO2 back pressure on their performance.

Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in†situ Generation of Atomic Hydrogen.

Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of "nascent" hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity.

Electrocatalytic Oxidation of 5-(Hydroxymethyl)furfural Using High-Surface-Area Nickel Boride.

The electrochemical oxidation of the biorefinery product 5-(hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), an important platform chemical for the polymer industry, is receiving increasing interest. FDCA-based polymers such as polyethylene 2,5-furandicarboxylate (PEF) are sustainable candidates for replacing polyethylene terephthalate (PET). Herein, we report the highly efficient electrocatalytic oxidation of HMF to FDCA, using Ni foam modified with high-surface-area nickel boride (Nix B) as the electrode. Constant potential electrolysis in combination with HPLC revealed a high faradaic efficiency of close to 100 % towards the production of FDCA with a yield of 98.5 %. Operando electrochemistry coupled to ATR-IR spectroscopy indicated that HMF is oxidized preferentially via 5-hydroxymethyl-2-furancarboxylic acid rather than via 2,5-diformylfuran, which is in agreement with HPLC results. This study not only reports a low-cost active electrocatalyst material for the electrochemical oxidation of HMF to FDCA, but additionally provides insight into the reaction pathway.