Single-Entity Electrocatalysis of Individual "Picked-and-Dropped" Co3 O4 Nanoparticles on the Tip of a Carbon Nanoelectrode.

Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co3 O4 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 Co3 O4 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.

Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag-Core-Porous-Cu-Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules.

Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as -0.6 V vs RHE.

Oxygen Evolution Electrocatalysis of a Single MOF-Derived Composite Nanoparticle on the Tip of a Nanoelectrode.

Determination of the intrinsic electrocatalytic activity of nanomaterials by means of macroelectrode techniques is compromised by ensemble and film effects. Here, a unique "particle on a stick" approach is used to grow a single metal-organic framework (MOF; ZIF-67) nanoparticle on a nanoelectrode surface which is pyrolyzed to generate a cobalt/nitrogen-doped carbon (CoN/C) composite nanoparticle that exhibits very high catalytic activity towards the oxygen evolution reaction (OER) with a current density of up to 230 mA cm-2 at 1.77 V (vs. RHE), and a high turnover frequency (TOF) of 29.7 s-1 at 540 mV overpotential. Identical location transmission electron microscopy (IL-TEM) analysis substantiates the "self-sacrificial" template nature of the MOF, while post-electrocatalysis studies reveal agglomeration of Co centers within the CoN/C composite during the OER. "Single-entity" electrochemical analysis allows for deriving the intrinsic electrocatalytic activity and furnishes insight into the transient behavior of the electrocatalyst under reaction conditions.

Electrocatalytic Nanoparticles That Mimic the Three-Dimensional Geometric Architecture of Enzymes: Nanozymes.

Enzymes are characterized by an active site that is typically embedded deeply within the protein shell thus creating a nanoconfined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer that blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nanoconfined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores.

Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel-Iron Layered Double Hydroxide.

NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (>80 °C) and high current densities (>500 mA cm-2 ) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline ß-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions.

Evaluation of the intrinsic catalytic activity of nanoparticles without prior knowledge of the mass loading.

The quantitative characterisation of electrocatalytic properties of nanoparticle catalyst materials is so far only performed for layers typically comprising additionally conducting additives and binders. We propose a method enabling the evaluation of intrinsic catalytic activity of nanoparticles based on the diffusion-limited steady-state current. In a step-after-step process, the influence of coverage on kinetic and diffusion limited current is evaluated to highlight the challenges of sub-monolayer electroanalysis. Conclusions are used to point out strategies and their limitations for qualitative and quantitative comparison of intrinsic catalytic properties. Particularly, the impact of coverage, electrode geometry, altered diffusion profile for nanoparticles and the catalyst activity and selectivity are discussed. Fundamental information about electrochemical sub-monolayer nanoparticle analysis is provided.

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.

Gaining the Freedom of Scalable Gas Diffusion Electrodes for the CO2 Reduction Reaction.

Gas diffusion electrodes (GDEs) in CO2 reduction reaction (CO2RR) alleviate the mass transfer limitation of gaseous reagents, which is beneficial for reducing CO2 into valuable chemicals. GDEs offer higher current densities compared to electrodes immersed in the electrolyte. Disclosing the roles of different structural parameters in tuning the performance of the GDEs is essential to exert the potential of catalysts and to meet potential large-scale industrial applications of the CO2RR. A novel layer structure for the airbrush-type spray fabrication of GDEs was designed and optimised, comprising a carbon-based gas-diffusion layer, a PEEK fabric, a Ni mesh, a carbon-integrated catalyst layer, and a PTFE top layer. It was shown that adjusting the carbon material in the gas diffusion and the catalyst layer impacts the selectivity of the CO2RR due to the modulation of the pore network. This work disclosed a practical and scalable but also an easily transferable pathway for preparing GDEs and offered an idea of how to tune the significant parameters of GDEs for optimising their CO2RR performance.

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.

Is Cu instability during the CO2 reduction reaction governed by the applied potential or the local CO concentration?

Cu-based catalysts have shown structural instability during the electrochemical CO2 reduction reaction (CO2RR). However, studies on monometallic Cu catalysts do not allow a nuanced differentiation between the contribution of the applied potential and the local concentration of CO as the reaction intermediate since both are inevitably linked. We first use bimetallic Ag-core/porous Cu-shell nanoparticles, which utilise nanoconfinement to generate high local CO concentrations at the Ag core at potentials at which the Cu shell is still inactive for the CO2RR. Using operando liquid cell TEM in combination with ex situ TEM, we can unequivocally confirm that the local CO concentration is the main source for the Cu instability. The local CO concentration is then modulated by replacing the Ag-core with a Pd-core which further confirms the role of high local CO concentrations. Product quantification during CO2RR reveals an inherent trade-off between stability, selectivity and activity in both systems.

Tuning Light-Driven Water Oxidation Efficiency of Molybdenum-Doped BiVO4 by Means of Multicomposite Catalysts Containing Nickel, Iron, and Chromium Oxides.

Mo-doped BiVO4 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:BiVO4 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:BiVO4 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.

Ultrathin 2D Cobalt Zeolite-Imidazole Framework Nanosheets for Electrocatalytic Oxygen Evolution.

2D layered materials, including metal-di-chalcogenides and transition metal layered double hydroxides, among others, are intensively studied because of new properties that emerge from their 2D confinement, which are attractive for advanced applications. Herein, 2D cobalt ion (Co2+) and benzimidazole (bIm) based zeolite-imidazole framework nanosheets, ZIF-9(III), are reported as exceptionally efficient electrocatalysts for the oxygen evolution reaction (OER). Specifically, liquid-phase ultrasonication is applied to exfoliate a [Co4(bIm)16] zeolite-imidazole framework (ZIF), named as ZIF-9(III) phase, into nanoscale sheets. ZIF-9(III) is selectively prepared through simple mechanical grinding of cobalt nitrate and benzimidazole in the presence of a small amount of ethanol. The resultant exfoliated nanosheets exhibit significantly higher OER activity in alkaline conditions than the corresponding bulk phases ZIF-9 and ZIF-9(III). The electrochemical and physicochemical characterization data support the assignment of the OER activity of the exfoliated nanosheet derived material to nitrogen coordinated cobalt oxyhydroxide N4CoOOH sites, following a mechanism known for Co-porphyrin and related systems. Thus, exfoliated 2D nanosheets hold promise as potential alternatives to commercial noble metal electrocatalysts for the OER.