Selective Electrochemical Reduction of CO2 to CO on Zn-Based Foams Produced by Cu2+ and Template-Assisted Electrodeposition

In this work, we aim to develop a Zn-based metal foam catalyst with very large specific area suitable for efficient CO production. Its manufacture is based on the dynamic hydrogen bubble template method that consists of the superposition of metal deposition and hydrogen evolution at the solid-liquid interface. We employed Cu ions in the Zn²⁺-rich electroplating bath as foaming agent. The concentration of Cu as foaming agent was systematically studied and an optimized Zn₉₄Cu₆ foam alloy was developed, which, to the best of our knowledge, is the most selective Zn-based CO₂ electrocatalyst toward CO in aqueous bicarbonate solution (FE_CO = 90% at -0.95 V vs reversible hydrogen electrode). This high efficiency is ascribed to the combination of high density of low-coordinated active sites and preferential Zn(101) over Zn(002) texturing. X-ray photoelectron spectroscopy investigations demonstrate that the actual catalyst material is shaped upon reduction of an oxide/hydroxide-terminating surface under CO₂ electrolysis conditions. Moreover, intentional stressing by oxidation at room conditions proved to be beneficial for further activation of the catalyst. Identical location scanning electron microscopy imaging before and after CO₂ electrolysis and long-term electrolysis experiments also showed that the developed Zn₉₄Cu₆ foam catalyst is both structurally and chemically stable at reductive conditions.

Toward CO2 Electroreduction under Controlled Mass Flow Conditions: A Combined Inverted RDE and Gas Chromatography Approach

The use of rotating disk electrodes (RDEs) is probably the most convenient way of studying simple electrode reactions under well-defined transport conditions. Standard RDEs become, however, less expedient when the studied electrode process is a complex one, leading to the formation of various reaction products. In these cases, the accurate detection and quantification of the formed products are desirable. If the formed products are gaseous, then the usual way of quantifying them is the use of online gas chromatography (GC), a method that is not compatible with open RDE cells. In order to overcome these difficulties, we present here a sophisticated inverted RDE (iRDE) cell design. The design combines various advantages: it is amenable to the same mathematical treatment as standard (downward-facing) RDEs; it can be operated airtight and coupled to online GC; and due to its upward-facing design, the electrode surface is less prone to blockage by any formed gas bubbles. The iRDE&GC design is tested using simple model reactions and is demonstratively used for studying the electrochemical reduction of CO₂, accompanied by parasitic hydrogen evolution, on a silver electrode.

Testing a Silver Nanowire Catalyst for the Selective CO₂ Reduction in a Gas Diffusion Electrode Half-cell Setup Enabling High Mass Transport Conditions

In this work, we discuss the application of a gas diffusion electrode (GDE) setup for benchmarking electrocatalysts for the reductive conversion of CO₂ (CO₂ RR: CO₂ reduction reaction). Applying a silver nanowire (Ag-NW) based catalyst, it is demonstrated that in the GDE setup conditions can be reached, which are relevant for the industrial conversion of CO₂ to CO. This reaction is part of the so-called 'Rheticus' process that uses the CO for the subsequent production of butanol and hexanol based on a fermentation approach. In contrast to conventional half-cell measurements using a liquid electrolyte, in the GDE setup CO₂ RR current densities comparable to technical cells (>100 mA cm^-2) are reached without suffering from mass transport limitations of the CO₂ reactant gas. The results are of particular importance for designing CO₂ RR catalysts exhibiting high faradaic efficiencies towards CO at technological reaction rates.

Unwrap Them First: Operando Potential- induced Activation Is Required when Using PVP-Capped Ag Nanocubes as Catalysts of CO₂ Electroreduction

Metallic nanoparticles of different shape can be used as efficient electrocatalysts for many technologically and environmentally relevant processes, like the electroreduction of CO₂. Intense research is thus targeted at finding the morphology of nanosized features that best suits catalytic needs. In order to control the shape and size distribution of the designed nanoobjects, and to prevent their aggregation, synthesis routes often rely on the use of organic capping agents (surfactants). It is known, however, that these agents tend to remain adsorbed on the surface of the synthesized nanoparticles and may significantly impair their catalytic performance, both in terms of overall yield and of product selectivity. It thus became a standard procedure to apply certain methods (e.g. involving UV-ozone or plasma treatments) for the removal of capping agents from the surface of nanoparticles, before they are used as catalysts. Proper design of the operating procedure of the electrocatalysis process may, however, render such cleaning steps unnecessary. In this paper we use poly-vinylpyrrolidone (PVP) capped Ag nanocubes to demonstrate a mere electrochemical, operando activation method. The proposed method is based on an observed hysteresis of the catalytic yield of CO (the desired product of CO₂ electroreduction) as a function of the applied potential. When as-synthesized nanocubes were directly used for CO₂ electroreduction, the CO yield was rather low at moderate overpotentials. However, following a potential excursion to more negative potentials, most of the (blocking) PVP was irreversibly removed from the catalyst surface, allowing a significantly higher catalytic yield even under less harsh operating conditions. The described hysteresis of the product distribution is shown to be of transient nature, and following operando activation by a single 'break-in' cycle, a truly efficient catalyst was obtained that retained its stability during long hours of operation.

Practical electrochemical hydrogenation of nitriles at the nickel foam cathode

We report a scalable hydrogenation method for nitriles based on cost-effective materials in a very simple two-electrode setup under galvanostatic conditions. All components are commercially and readily available. The method is very easy to conduct and applicable to a variety of nitrile substrates, leading exclusively to primary amine products in yields of up to 89% using an easy work-up protocol. Importantly, this method is readily transferable from the milligram scale in batch-type screening cells to the multi-gram scale in a flow-type electrolyser. The transfer to flow electrolysis enabled us to achieve a notable 20 g day-1 productivity of phenylethylamine at a geometric current density of 50 mA cm-2 in a flow-type electrolyser with 48 cm2 electrodes. It is noteworthy that this method is sustainable in terms of process safety and reusability of components.

Simple and scalable electrosynthesis of 1H-1-hydroxy-quinazolin-4-ones

Cathodic synthesis provides sustainable access to 1-hydroxy- and 1-oxy-quinazolin-4-ones from easily accessible nitro starting materials. Mild reaction conditions, inexpensive and reusable carbon-based electrode materials, an undivided electrochemical setup, and constant current conditions characterise this method. Sulphuric acid is used as a simple supporting electrolyte as well as a catalyst for cyclisation. The broad applicability of this protocol is demonstrated in 27 differently substituted derivatives in high yields of up to 92%. Moreover, mechanistic studies based on cyclic voltammetry measurements highlight a selective reduction of the nitro substrate to hydroxylamine as a key step. The relevance for preparative applications is demonstrated by a 100-fold scale-up for gram-scale electrolysis.

Simple electrochemical synthesis of cyclic hydroxamic acids by reduction of nitroarenes

The electrochemical reduction of nitroarenes allows direct access to manifold nitrogen containing heterocycles. This work reports the simple and direct electro-organic synthesis of 18 different examples of 2H,4H-4-hydroxy-1,4-benzoxazin-3-ones in up to 81% yield. The scalability of the method was demonstrated on a gram-scale.

Highly Selective Electrosynthesis of 1H-1-Hydroxyquinol-4-ones-Synthetic Access to Versatile Natural Antibiotics

1H-1-Hydroxyquinolin-4-ones represent a broad class of biologically active heterocycles having an exocyclic N,O motif. Electrosynthesis offers direct, highly selective, and sustainable access to 1-hydroxyquinol-4-ones by nitro reduction. A versatile synthetic route starting from easily accessible 2-nitrobenzoic acids was established. The broad applicability of this protocol was demonstrated on 26 examples with up to 93% yield, highlighted by the naturally occurring antibiotics Aurachin C and HQNO. The practicability and technical relevance were underlined by multigram scale electrolysis.

Simple and versatile electrochemical synthesis of highly substituted 2,1-benzisoxazoles

A sustainable, general and scalable electrochemical protocol for direct access to 3-(acylamidoalkyl)-2,1-benzisoxazoles by cathodic reduction of widely accessible nitro arenes is established. The method is characterised by a simple undivided set-up under constant current conditions, inexpensive and reusable carbon-based electrodes, and environmentally benign reaction conditions. The versatility of the developed protocol is demonstrated on 39 highly diverse examples with up to 81% yield. A 50-fold scale-up electrolysis highlights its relevance for preparative applications.