Utilization of the catalyst layer of dimensionally stable anodes—Interplay of morphology and active surface area

Review on Bubble Dynamics in Proton Exchange Membrane Water Electrolysis: Towards Optimal Green Hydrogen Yield

Water electrolysis using a proton exchange membrane (PEM) holds substantial promise to produce green hydrogen with zero carbon discharge. Although various techniques are available to produce hydrogen gas, the water electrolysis process tends to be more cost-effective with greater advantages for energy storage devices. However, one of the challenges associated with PEM water electrolysis is the accumulation of gas bubbles, which can impair cell performance and result in lower hydrogen output. Achieving an in-depth knowledge of bubble dynamics during electrolysis is essential for optimal cell performance. This review paper discusses bubble behaviors, measuring techniques, and other aspects of bubble dynamics in PEM water electrolysis. It also examines bubble behavior under different operating conditions, as well as the system geometry. The current review paper will further improve the understanding of bubble dynamics in PEM water electrolysis, facilitating more competent, inexpensive, and feasible green hydrogen production.

Efficient Alkaline Hydrogen Evolution Reaction Using Superaerophobic Ni Nanoarrays with Accelerated H2 Bubble Release

Despite the adverse effects of H2 bubbles adhering to catalyst's surface on the performance of water electrolysis, the mechanisms by which H2 bubbles are effectively released during the alkaline hydrogen evolution reaction (HER) remain elusive. In this study, a systematic investigation on the effect of nanoscale surface morphologies on H2 bubble release behaviors and HER performance by employing earth-abundant Ni catalysts consisting of an array of Ni nanorods (NRs) with controlled surface porosities is performed. Both aerophobicity and hydrophilicity of the catalyst's surface vary according to the surface porosity of catalysts. The Ni catalyst with the highest porosity of ≈52% exhibits superaerophobic nature as well as the best HER performance among the Ni catalysts. It is found that the Ni catalyst's superaerophobicity combined with the effective open pore channels enables the accelerated release of H2 bubbles from the surface, leading to a significant improvement in geometric activities, particularly at high current densities, as well as intrinsic activities including both specific and mass activities. It is also demonstrated that the superaerophobicity enabled by highly porous Ni NRs can be combined with Pt and Cr having optimal binding abilities to further optimize electrocatalytic performance.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

The electro-oxidation of tetracycline hydrochloride in commercial DSA® modified by electrodeposited platinum

Tetracycline hydrochloride (TCH) electro-oxidation by commercial DSA® and commercial DSA® modified by platinum electrodeposition was evaluated. The electrodeposition was carried out at constant potential (E = - 0.73 V vs RHE) in different times (1200, 2400, and 4800 s). Scanning electron microscopy (SEM) images show that Pt electrodeposits have elongated shape particle forming a uniform surface, and energy dispersive spectroscopy (EDS) data confirms the presence of Pt on the surface. The electrochemical characterization by cyclic voltammetry showed an increase of the electrochemically active area (E_AA) in function of the Pt electrodeposition time. The electro-oxidation of the TCH 0.45 mmol L^-1 in H₂SO₄ 0.1 mol L^-1 solution was evaluated according to the applied current densities (j = 25, 50, 100 mA cm^-2). Both the amount of platinum deposited and j showed a slight improvement in the efficiency of TCH removal, reaching 97.2% of TCH removal to DSA®/Pt₄₈₀₀ and 100 mA cm^-2. The TCH mineralization (TOC removal), the percentage of mineralization current efficiency (MCE%), and energy consumption were 15.8%, 0.2649%, and 7.4138 kWh (g TOC)^-1, respectively. The DSA®/Pt electrodes showed higher stability to TCH electro-oxidation, indicating to be a promising material for the electro-oxidation of organic pollutants.

Effect of the Morphology of the High-Surface-Area Support on the Performance of the Oxygen-Evolution Reaction for Iridium Nanoparticles

The development of affordable, low-iridium-loading, scalable, active, and stable catalysts for the oxygen-evolution reaction (OER) is a requirement for the commercialization of proton-exchange membrane water electrolyzers (PEMWEs). However, the synthesis of high-performance OER catalysts with minimal use of the rare and expensive element Ir is very challenging and requires the identification of electrically conductive and stable high-surface-area support materials. We developed a synthesis procedure for the production of large quantities of a nanocomposite powder containing titanium oxynitride (TiON x ) and Ir. The catalysts were synthesized with an anodic oxidation process followed by detachment, milling, thermal treatment, and the deposition of Ir nanoparticles. The anodization time was varied to grow three different types of nanotubular structures exhibiting different lengths and wall thicknesses and thus a variety of properties. A comparison of milled samples with different degrees of nanotubular clustering and morphology retention, but with identical chemical compositions and Ir nanoparticle size distributions and dispersions, revealed that the nanotubular support morphology is the determining factor governing the catalyst's OER activity and stability. Our study is supported by various state-of-the-art materials' characterization techniques, like X-ray photoelectron spectroscopy, scanning and transmission electron microscopies, X-ray powder diffraction and absorption spectroscopy, and electrochemical cyclic voltammetry. Anodic oxidation proved to be a very suitable way to produce high-surface-area powder-type catalysts as the produced material greatly outperformed the IrO2 benchmarks as well as the Ir-supported samples on morphologically different TiON x from previous studies. The highest activity was achieved for the sample prepared with 3 h of anodization, which had the most appropriate morphology for the effective removal of oxygen bubbles.

Straight-Parallel Electrodes and Variable Gap for Hydrogen and Oxygen Evolution Reactions

The challenges to be overtaken with alkaline water electrolysis are the reduction of energy consumption, the maintenance, and the cost as well as the increase of durability, reliability, and safety. Having these challenges in mind, this work focused on the reduction of the electrical resistance of the electrolyte which directly affects energy consumption. According to the definition of electrical resistance of an object, the reduction of the space between electrodes could lower the electrical resistance but, in this process, the formation of bubbles could modify this affirmation. In this work, the performance analyses of nine different spaces between stainless steel 316L electrodes were carried out, although the spaces proposed are not the same as those from the positive electrode (anode) to the separator and from the separator to the negative electrode (cathode). The reason why this is studied is that stoichiometry of the reaction states that two moles of hydrogen and one mole of oxygen can be obtained per every two moles of water. The proposed spaces were 10.65, 9.20, 8.25, 7.25, 6.30, 6.05, 4.35, 4.15, and 3.40 millimetres. From the nine different analysed distances between electrodes, it can be said that the best performance was reached by one of the smallest distances proposed, 4.15 mm. When the same distance between electrodes was compared (the same and different distance between electrodes and separator), the one that had almost twice the distance (negative compartment) presented an increase in current density of approximately 33% with respect to that where both distances (from electrodes to separator) are the same. That indicates that the stichometry of the electrolysis reaction influenced the performance.

Bi2WO6 and FeWO4 Nanocatalysts for the Electrochemical Water Oxidation Process

Polyvinylpyrrolidone (PVP)-assisted nanocatalyst preparation was succeeded by employing a controlled solvothermal route to produce efficient electrodes for electrochemical water-splitting applications. Bi₂WO₆ and FeWO₄ nanocatalysts have been confirmed through the strong signature of (113) and (111) crystal planes, respectively. The binding natures of Bi-W-O and Fe-W-O have been thoroughly discussed by employing X-ray photoelectron spectroscopy which confirmed the formation of Bi₂WO₆ and FeWO₄. The freestanding nanoplate array morphology of Bi₂WO₆ and the fine nanosphere particle morphology of FeWO₄ nanocatalysts were revealed by scanning electron microscopy images. With these confirmations, the fabrication of durable, long-term electrodes for electrochemical water splitting has been subjected to efficient oxidation of water, confirmed by obtaining 2.79 and 1.96 mA/g for 0.5 g PVP-assisted Bi₂WO₆ and FeWO₄ nanocatalysts, respectively. The water oxidation mechanism of both nanocatalysts has been revealed with the support of 24 h stability test over continuous water oxidation and faster charge transfer achieved by the smaller Tafel slope values of 75 and 78 mV/dec, respectively. Generally, these nanocatalysts are utilized for photocatalytic applications. The present study revealed the PVP-assisted synthesis to produce electrocatalytically active nanocatalysts and their electrochemical water-splitting mechanism which will offer a pathway for research interests with regard to the production of multifunctional nanocatalysts for both electro- and photocatalytic applications in the near future.

The investigation of the electrocatalytic and corrosion behavior of a TiO2–RuO2 anode modified by graphene oxide and reduced graphene oxide nanosheets via a sol–gel method

The participation of GO in the coating structure improved the ClER activity, selectivity, and the electrochemical stability of the electrodes significantly.

Superaerophobic RuO2 -Based Nanostructured Electrode for High-Performance Chlorine Evolution Reaction

Constructing a nanostructured electrode with superaerophobic surface property (i.e., superlow adhesion to gas bubbles) has been strikingly highlighted as an advanced technology to minimize the energy loss during various electrochemical gas evolution reactions. Herein, aiming at enhancing the performance of chlorine evolution reaction (ClER), which holds the key for chlor-alkali industry as well as water treatment, a nanostructured RuO₂ @TiO₂ electrode is demonstrated to overcome the bubble shielding effect, thereby maximizing the working area and offering a robust working condition. Benefitting from the direct growing architecture and the superaerophobic surface property, this nanostructured RuO₂ @TiO₂ electrode exhibits an excellent ClER performance, reaching 50 mA cm^-2 at a low potential of 1.10 V (vs SCE) with a Faradaic efficiency over ≈90%. Moreover, a prominent stability (250 mA cm^-2 for 10 h) is observed for this nanostructured electrode, probably due to the small vibrations and scratching forces from gas product.

Iridium Oxide Coatings with Templated Porosity as Highly Active Oxygen Evolution Catalysts: Structure-Activity Relationships

Iridium oxide is the catalytic material with the highest stability in the oxygen evolution reaction (OER) performed under acidic conditions. However, its high cost and limited availability demand that IrO2 is utilized as efficiently as possible. We report the synthesis and OER performance of highly active mesoporous IrO2 catalysts with optimized surface area, intrinsic activity, and pore accessibility. Catalytic layers with controlled pore size were obtained by soft-templating with micelles formed from amphiphilic block copolymers poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide). A systematic study on the influence of the calcination temperature and film thickness on the morphology, phase composition, accessible surface area, and OER activity reveals that the catalytic performance is controlled by at least two independent factors, that is, accessible surface area and intrinsic activity per accessible site. Catalysts with lower crystallinity show higher intrinsic activity. The catalyst surface area increases linearly with film thickness. As a result of the templated mesopores, the pore surface remains fully active and accessible even for thick IrO2 films. Even the most active multilayer catalyst does not show signs of transport limitations at current densities as high as 75 mA cm(-2) .

The electrocatalytic activity of IrO2–Ta2O5 anode materials and electrolyzed oxidizing water preparation and sterilization effect

Ti/IrO₂–Ta₂O₅ anodes with different contents and preparation temperatures were prepared for electrolyzed oxidizing water's preparation and sterilization in this work.

On the faradaic selectivity and the role of surface inhomogeneity during the chlorine evolution reaction on ternary Ti–Ru–Ir mixed metal oxide electrocatalysts

Faradaic selectivity of the chlorine and oxygen evolution (left) is linked to the spatial inhomogeneity of the surface reactivity of Ti–Ru–Ir mixed metal oxide catalysts.