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A critical review of classical and improved electrodes, electrocatalysts and reactors is provided. The principles governing the selection of electrochemical flow reactor or progression of a particular design for laboratory or pilot scale are reviewed integrating the principles of electrochemistry and electrochemical engineering with practical aspects. The required performance, ease of assembly, maintenance schedule and scale-up plans must be incorporated. Reactor designs can be enhanced by decorating their surfaces with nanostructured electrocatalysts. The simple parallel plate geometry design, often in modular, filter-press format, occupies a prominent position, both in the laboratory and in industry and may incorporates porous, 3D or structured electrode surfaces and bipolar electrical connections considering the reaction environment, especially potential- and current-distributions, uniformity of flow, mass transport rates, electrode activity, side reactions and current leakage. Specialised electrode geometries include capillary gap and thin film cells, rotating cylinder electrodes, 3-D porous electrodes, fluidised bed electrodes and bipolar trickle tower reactors. Applications span inorganic, organic electrosynthesis and environmental remediation. Recent developments in cell design: 3D printing, nanostructured, templating 3D porous electrodes, microchannel flow, combinatorial electrocatalyst studies, bioelectrodes and computational modelling. Figures of merit describing electrochemical reactor performance and their use are illustrated. Future research and development needs are suggested.

The two-electron water oxidation reaction (2e – WOR) is progressively gaining traction as a sustainable approach for in situ electrosynthesis of hydrogen peroxide (H2O2). State-of-the-art 2e – WOR electrocatalysts have shown great promise at low electrical currents yet exhibit diminished electrocatalytic capabilities at larger current densities. Herein, by tailoring the boron doping level of boron-doped diamond (BDD) microfilms, we have fabricated an active, selective, and stable electrocatalyst for the 2e – WOR. Experimentally, we find that our modulated BDD films achieve a peak faradaic efficiency of 87%, as well as a record H2O2 production rate of 76.4 μmol cm–2 min–1, while maintaining a stable electrochemical performance for 10 h at 200 mA cm–2 in carbonate-based solutions. The results reported in this work firmly establish BDD as a primary catalyst candidate for large-scale implementation of the 2e – WOR and synchronously unlock new research avenues for the next-generation design of sp3-structured carbonaceous materials for anodic H2O2 electrosynthesis from water.

Electrochemical production of hydrogen peroxide (H2O2) has recently gained traction as a green alternative to the unsustainable anthraquinone auto-oxidation process and the high-risk direct synthesis route. While the two-electron oxygen reduction reaction (2e – ORR) toward H2O2 has been covered extensively in the literature, the unorthodox two-electron water oxidation reaction (2e – WOR) remains far less popular, due to the thermodynamic unfavorability of the pathway. Nonetheless, the 2e – WOR constitutes a coveted procedure as it enables the electro-generation of H2O2 solely from water. A thorough understanding of the reaction mechanism, including all intermediates and competing reaction routes, is essential for the fabrication of electrocatalysts, and assembly of electrochemical reactors, capable of greater H2O2 production rates with an optimal efficiency. This review focuses exclusively on the 2e – WOR to electrochemically produce H2O2. A summary of all prevailing water oxidation mechanisms is presented, supported with computational and experimental data, and key challenges and limitations that require attention are addressed.

Using chronoamperometry at preconditioned oxide-free Pt microdisc electrodes in aqueous media, we investigated the oxygen reduction reaction (ORR) on the millisecond timescale and obtained results consistent with the reduction of oxygen species which adsorb on the electrode before the ORR is electrochemically driven. Furthermore these adsorbed species are clearly linked to oxygen in solution. At long times, the amperometric response is solely controlled by the diffusion of dissolved oxygen towards the microelectrode. However, at short times, typically below 50 ms, the reduction of pre-adsorbed oxygen produces a large extra current whose magnitude depends on the oxygen concentration in solution, deliberate electrode poisoning and the rest time before the potential step. Using sampled current voltammetry we show that this extra current affects the entire potential range of the ORR. Using microdisc electrodes made with Pt alloys we find that the amperometric response is sufficiently sensitive to distinguish oxygen coverage differences between Pt, Pt0.9Rh0.1 and Pt0.9Ir0.1 microdiscs. These unexpected and, to our knowledge, never previously reported results provide new insight into the oxygen reduction reaction on Pt. The existence over a wide potential range of irreversibly adsorbed oxygen species arising from dissolved oxygen and different from Pt oxide is particularly relevant to the development of oxygen reduction catalysts for low temperature fuel cells. A. IntroductionThe electroreduction of oxygen in aqueous media is a complex reaction which involves the cleavage of the oxygen bond, adsorption and desorption of intermediates and the transfer of four electrons and four protons. While the overall mechanism is commonly discussed in terms of a series two 2-electron peroxide pathway running in parallel with a direct 4-electron process, 1, 2 the elementary steps and, crucially, the nature of the rate determining step remain uncertain. 3,4 In mechanistic studies the two pathways are discussed in terms of the formation of OOH on the surface (associative mechanism) or of the adsorption of atomic oxygen (dissociative mechanism) 5,6 but the nature of the intermediates is still unclear; while most invoke adsorbed OH species, 7 recent studies support the existence of a soluble intermediate. 3,8,9 The complexity of the reaction is reflected by the dependence of the apparent number of electrons, n app , on experimental conditions including electrode material, 10 surface crystal planes, 11 pH, 12 and even mass transport conditions. [13][14][15] Except for a few studies, 11, 16-21 the reaction has been mostly investigated under steady state mass transport conditions with rotating disc, rotating ring-disc, hanging meniscus rotating disc and microelectrodes.Here we report the unexpected, and to our knowledge never previously reported, results obtained when studying the ORR under transient conditions on the millisecond timescale. To acquire data undistorted by the charging and discharging of the double layer we employ micro...