Active RuO2 Research Articles

Engineering a High Stable and Active RuO2 on Ti Sheet for Kolbe Electrolysis of Acetic Acid: An Investigation to Valorize the Pyroligneous Acid

The efficient utilization of renewable sources based on the simultaneous production of energy fuels and value-added chemicals is an essential to enable a biobased circular economy that benefits carbon neutrality. As an abundant by-product during the pyrolysis of biomass, pyroligneous acid is rich in acetic acid (CH3COOH) that is difficult to handle due to its highly acidic content. However, it is essential made up of renewable carbon that can be converted into valuable chemicals and fuels (e.g. methane, ethane, propane). Herein, we fabricate a highly stable and effective ruthenium oxide electrocatalyst (RuO2/TiOx) via using available titanium sheet as the substrate, to highly selectively transform AA to ethane by electrochemical conversion with ~82 (±5)% selectivity. The obtained results demonstrate that RuO2/TiOx exhibits an efficient electrochemical performance at low temperatures (5℃), and can convert AA at various pH, current density, and different concentrations. Moreover, we mimicked the complex chemical environment of the pyroligneous acid by adding diverse hydrocarbons (e.g., phenols, aldehydes) in acid solution to evaluate the stability of RuO2/TiOx. The electrolysis was also conducted in original pyroligneous acid solution, in which the RuO2/TiOx exhibits a considerable performance to convert AA to ethane. This work, based on the selective production of ethane from AA by a RuO2/TiOx electrode as an alternative to bulk metals as anodes for oxidative upgrading of carboxylic acids, making it promise to directly turn biomass residual into fuel thus improve the biomass utilization.

Tailoring the electron redistribution of RuO2 by constructing a Ru-O-La asymmetric configuration for efficient acidic oxygen evolution

Stabilizing the highly active RuO2 electrocatalyst for the oxygen evolution reaction (OER) is critical for the application of proton exchange membrane water electrolysis, but this remains challenging due to the inevitable over-oxidation of Ru in harsh oxidative environments. Herein, we describe constructing Ru-O-La asymmetric configurations into RuO2 via a facile sol-gel method to tailor electron redistribution and thereby eliminate the over-oxidation of Ru centers. Specifically, the as-prepared optimal La0.1Ru0.9O2 shows a low overpotential of 188 mV at 10 mA cm–2, a high mass activity of 251 A at 1.6 V vs. reversible hydrogen electrode (RHE), and a long-lasting durability of 63 hours, far superior to the 8 hours achieved by standard RuO2. Experiments and density functional theory calculations jointly reveal that the Ru-O-La asymmetric configuration could trigger electron redistribution in RuO2. More importantly, electron transfer from La to Ru via the Ru-O-La configuration could lead to increased electron density around Ru, thus preventing the over-oxidation of Ru. In addition, electron redistribution tunes the Ru 4d band center’s energy level, which optimizes the adsorption and desorption of oxygen intermediates. This work offers an effective strategy for regulating electronic structure to synergistically boost the activity and stability of RuO2-based acidic OER electrocatalysts.

Ce-doped TiO2 supported RuO2 as efficient catalysts for the oxidation of HCl to Cl2

Reducing the cost of RuO2/TiO2 catalysts is still one of the urgent challenges in catalytic HCl oxidation. In the present work, a Ce-doped TiO2 supported RuO2 catalyst with a low Ru loading was developed, showing a high activity in the catalytic oxidation of HCl to Cl2. The results on some extensive characterizations of both Ce-doped TiO2 carriers and their supported RuO2 catalysts show that the doping of Ce into TiO2 can effectively change the lattice parameters of TiO2 to improve the dispersion of the active RuO2 species on the carrier, which facilitates the production of surface Ru species to expose more active sites for boosting the catalytic performance even under some harsh reaction conditions. This work provides some scientific basis and technical support for chlorine recycling.

IrO2 deposited on RuO2 as core-shell structured RuO2@IrO2 for oxygen evolution reaction in electrochemical water electrolyzer

The ever-rising energy demand and environmental concerns require the application of electrochemical water splitting to produce hydrogen as a substitute energy to fossil fuel. Proton exchange membrane water electrolyzers split water into oxygen and hydrogen at the anode and cathode, respectively. Anode catalysts necessitate extensive study, because significant overpotential is required to accelerate the 4-e sluggish oxygen evolution reaction in acidic media. RuO2@IrO2 was synthesized in this work, which aims to achieve high activity and stability. In doing so, RuO2·xH2O was first prepared using the Adams method, followed by the subsequent precipitation of Ir(OH)3 on its surface via the hydrolysis of iridium precursor in a mix of NH3·H2O in ethanol. Upon heat treatment, RuO2@IrO2 with an appropriate Ru/Ir molar ratio was obtained and applied for oxygen evolution reaction. Electrochemical evaluation results show that the optimized RuO2@IrO2-20 performed the best in terms of enhanced mass activity (1.62 A mg−1oxide @1.6 V) and lower overpotential (275 mV@10 mA cm−2) compared with single RuO2 and IrO2 catalysts. In addition, both chronopotentiometry and chronoamperometry test results show that the stability of RuO2@IrO2-20 is highly competitive. The intimate contact between RuO2 and IrO2 combines both the most active RuO2 and stable IrO2 as an integrated catalyst.

Solar-driven thermo-photocatalytic CO2 methanation over a structured RuO2:TiO2/SBA-15 nanocomposite at low temperature

A new hybrid catalyst composed of mesostructured silica SBA-15 functionalized with TiO2 and further loaded with RuO2 was developed to efficiently promote thermo-photocatalytic CO2 hydrogenation into methane at low temperatures. The catalytic activity was assessed with respect to TiO2:RuO2 loading, catalyst dosage, illumination source (polychromatic sunlight and monochromatic LEDs) and power, [H2]:[CO2] molar ratio, temperature, and catalyst reusability. The best methanation yields were attained for the RuO2(6.4%):TiO2(16.9%)/SBA-15 nanocomposite at 150 ºC, under simulated sunlight (0.21 W) and stoichiometric [H2]:[CO2] molar ratio, reaching: a specific CH4 production rate of 13.6 mmol gcat-1 h-1; 99.8 % selectivity; 96.8 % CO2 conversion (110-min; 40 mL); and apparent photonic efficiency/quantum yield of 39.5 %/42.1 %. Considering only the active RuO2:TiO2 photocatalyst mass (23.3 %), the CH4 production rate increased to 58.6 mmol gactive_cat-1 h-1. Besides, this highly-active photocatalyst featured excellent UV-Vis-IR light absorbance, high surface area, and stability for reuse when moist gas was removed between cycles.

A comparative study of RuO2 and Ru reveals the role of oxygen vacancies in electrocatalytic nitrogen reduction to ammonia under ambient conditions

Nitrogen molecule reduction to ammonia requires adsorption and hydrogenation sites. In this study, the nitrogen reduction reaction (NRR) activity of RuO2 and metallic Ru catalysts was studied to explore the role of oxides and metallic dual active sites. Ruthenium oxide nanoparticles displayed an ammonia production rate of 16.5 µg h−1 cm−2 with a Faradaic efficiency (FE) of 0.26% at − 0.15 V vs. RHE in N2-saturated 0.1 M KOH which is 58% higher compared to Ru black (6.8 µg h−1 cm−2 with 0.19% FE at −0.15 Vvs.RHE). This is attributed to the formation of oxygen vacancies (Vo) on the RuO2 surface during cathodic potential polarization, which provides a facile adsorption site for N2 in addition to the Ru4+ active site while the proton supplied via hydrogen spillover from the metal hydride site to the adsorbed Vo-N2 site. This assumption was validated by detailed XPS, XRD, and N2 TPD analysis.

Suppression of Oxygen Vacancies in Rutile Ruo2 via In Situ Exsolution for Enhanced Water Electrocatalysis

AbstractElemental vacancies are proposed as an effective approach to tuning the electronic structure of catalysts that are critical for energy conversion. However, for reactions such as the sluggish oxygen evolution reaction, the excess of oxygen vacancies (VO) is inevitable and detrimental to catalysts’ electrochemical stability and activities, e.g., in the most active RuO2. While significant work is carried out to hinder the formation of VO, the development of a fast and efficient strategy is limited. Herein, a protection SrO layer produced successfully at the surface of RuO2 with the in situ exsolution method with perovskite SrRuO3 as the precatalyst, which could significantly hinder the generation of VO. Benefited from the suppression of VO, the surface‐modified RuO2 requires a low overpotential of 290 mV at 100 mA cm−2, accompanied by remarkably high electrochemical stability (100 h) and Faraday efficiency (≈100%). Theoretical investigation reveals that the formation energy of VO in RuO2 is almost doubled in the exsolved RuO2 phase as a result of the weakened RuO bond covalency. This work not only provides insight into the structural evolution of perovskite oxide catalysts but also demonstrates the feasibility of controlling vacancy formation via in situ exsolution.

Bi-Porphyrins MOF with confinement and ion-attracting effects in concert with RuO2-doped CNT as efficient electrocatalysts for HER in acidic and alkaline media

It is highly desirable and challenging to design efficient and stable hydrogen evolution reaction (HER) electrocatalysts in a variety of acidic and alkaline media. Here, a novel ternary catalyst has been proposed consisting of ruthenium dioxide doped carbon nanotubes (denoted as RuO2@CNT) wrapped in a multi-step strategy with a first synthesis of a bismuth metal porphyrin-based organic framework as co-catalyst (denoted as RuO2@CNT@MOF). The addition of CNT improves not only the conductivity of the electrocatalyst but also the distribution of the active RuO2 particles. Furthermore, the highly symmetrical conjugated structure of Bi-TCPP MOF facilitates both the adsorption of hydrogen ions and the generation of hydroxyl radicals (•OH). The Bi-TCPP MOF encapsulated in the outer layer of RuO2@CNT enhances the mechanical strength and creates the confinement effect, leading to the synthesis of RuO2@CNT@MOF as a HER electrocatalyst that exhibits excellent catalytic performance and acid and base resistance. In particular, low overpotentials of 13 mV and 50 mV can be achieved to provide current densities of 10 mA cm−2 in alkaline and acidic media, respectively. Significantly, this work provides new ideas for the careful design and development of MOFs as co-catalysts to improve the overall performance of materials in energy-related fields.

Interface engineering breaks both stability and activity limits of RuO2 for sustainable water oxidation

Designing catalytic materials with enhanced stability and activity is crucial for sustainable electrochemical energy technologies. RuO2 is the most active material for oxygen evolution reaction (OER) in electrolysers aiming at producing ‘green’ hydrogen, however it encounters critical electrochemical oxidation and dissolution issues during reaction. It remains a grand challenge to achieve stable and active RuO2 electrocatalyst as the current strategies usually enhance one of the two properties at the expense of the other. Here, we report breaking the stability and activity limits of RuO2 in neutral and alkaline environments by constructing a RuO2/CoOx interface. We demonstrate that RuO2 can be greatly stabilized on the CoOx substrate to exceed the Pourbaix stability limit of bulk RuO2. This is realized by the preferential oxidation of CoOx during OER and the electron gain of RuO2 through the interface. Besides, a highly active Ru/Co dual-atom site can be generated around the RuO2/CoOx interface to synergistically adsorb the oxygen intermediates, leading to a favourable reaction path. The as-designed RuO2/CoOx catalyst provides an avenue to achieve stable and active materials for sustainable electrochemical energy technologies.

The effect of ruthenium content on the stability and activity of Ti/RuO2-Sb2O5-SnO2 for oxygen evolution

The effects of ruthenium content on Ti/RuO2-Sb2O5-SnO2 anodes are investigated in terms of catalytic activity and stability for oxygen evolution. The best electrochemical stability appears at nominal Ru content around 30%, giving average accelerated lives of 419 and 165 h at 25 and 70 °C, respectively, under the current density of 500 mA cm−2 in 3 M H2SO4 solution. These values are respectively ~8 and >22 times longer than any types of RuO2-based electrodes reported previously. The electrocatalytic activity of Ti/RuO2-Sb2O5-SnO2 anodes is found to increase with nominal Ru content approaching 75%. The physicochemical and electrochemical analyses reveal that the homogeneous intermixture of RuO2, SnO2 and Sb2O5 with a smooth and compact surface decreases the dissolution rate of RuO2, leading to a significant improvement on the electrode service life. Despite the numerous pores and cracks found in the coating, Ti/RuO2-Sb2O5-SnO2 anode with a nominal Ru content of 75% exhibits better electrocatalytic performance for O2 evolution, which is attributed to more active RuO2 sites exposed to electrolyte. Therefore, the existence of the optimal Ru content for Ti/RuO2-Sb2O5-SnO2 is the balanced effect between electrochemical stability and activity for O2 evolution, as well as cost for specific applications.

Double Approach Towards 3D Electrodeposited RuOx Porous Structure for High Energy/High Power Micro-Supercapacitors

Hydrous RuOx has been one of the well-studied materials for application in supercapacitors owing to its excellent properties (high conductivity similar to metals, redox activity capable of fast faradaic reactions and structural water enabling swift proton transfer and decreased diffusion distance). The main issue regarding supercapacitors is that, they suffer from low energy density compared to batteries. One way to overcome this problem is to increase surface area of the active material and hence the energy stored through 3D structuration of current collector. Multiple techniques have been used in this regard like 3D printing, pholitography methods...etc.However, these techniques are pretty complex, time consuming and have constraining conditions. Hence, it’s necessary to develop economic and simple routes for 3D structuration of electrodes for micro-supercapacitors. One simple, time efficient and easy to set up method that can be used under ambient conditions is the electrochemical structuration using dynamic hydrogen bubble template (DHBT). With this strategy, 3D scaffolds which can hold small quantities of intrinsic pseudocapacitve materials like RuOx can be fabricated. The challenge in this latter case is magnified, needing controlled decoration of active materials for efficient utilization and full benefit of the 3D framework. Hence, the pursuit of superior micro-supercapacitors not only needs controlled deposition, but also requires stable 3D scaffolds to maximize capacitance per footprint area1. We have previously shown electrodeposition of active RuO2 on porous gold electrodes for micro-supercapacitors achieving a capacitance of 3 F/cm².One other approach would be to deposit the active material directly by the DHBT method. As RuOx is a good conductor this would not hinder its efficiency (this would not be the case for MnO2 for example). The difficulty here is that, not all metals can be deposited by the DHBT technique as other factors play a crucial role such as the exchanged current density towards H2 evolution and mechanical stability3 In the current work, we report two different strategies for the 3D deposition of RuOx. The first is the successful fabrication of highly porous 3D platinum current collector using DHBT followed by a conformal coating of hydrous RuOx to achieve a specific capacitance as high as 10 F/cm². The second is the direct electrodeposition of the active material, RuOx, in a 3D porous structure. These two approaches are characterized and discussed within the framework of fabricating superior micro-supercapacitors with excellent capacitance and low internal resistance. References N. A. Kyeremateng, T. Brousse, and D. Pech, Nat. Nanotechnol., 12, 7–15 (2017)A. Ferris, S. Garbarino, D. Guay, and D. Pech, Adv. Mater., 27, 6625–6629 (2015)Plowman, B. J., Jones, L. A., & Bhargava, S. K. Chem. Commun., 51(21), 4331–4346. Figure 1

Preparation, Characterization and CO Oxidation Performance of Ag2O/γ-Al2O3 and (Ag2O+RuO2)/γ-Al2O3 Catalysts

This research dealt with the preparation and characterization of silver oxide (SLO) nanomaterials (NMs) and their composite catalysts (i.e., silver and ruthenium oxide (SLORUO)). The prepared materials were tested for their catalytic performance in carbon monoxide (CO) oxidation. Generally, silver in its pure state is not widely used for CO oxidation due to stability and structural issues. However, the usage of subsurface oxygen and oxygen-induced reconstruction could be effective as an oxidation catalyst at a slightly high temperature. The low-temperature reaction of highly active RuO2 (RUO) is a well-known phenomenon. Thus, the possibility of using it with SLO to observe the combined catalytic behavior was investigated. The wet chemically prepared SLO and SLORUO NMs exhibited spherical and rods in spherical aggregate-type surface morphology belonging to cubic and rutile crystalline structures, respectively. The NMs and catalysts (i.e., the NMs on γ-Al2O3 catalyst support at 0.5 and 1.0 wt.% ranges) showed good thermal stability. The dry and wet CO oxidation using RUO and SLO showed concentration-dependent catalytic activity. The RUO, SLO, and SLORUO composites using 0.5 wt.% showed full CO oxidation at 200, 300, and 225 °C, respectively. The reasons for the observed activity of the catalysts are explained based on the pore characteristics, chemical composition, and dispersion using H2 temperature-programmed reduction (TPR) behaviors.

A comparative study on enhancer and inhibitor of glycine–nitrate combustion ZnO screen-printed sensor: detection of low concentration ammonia at room temperature

We report a comparative study on enhancing and inhibiting the sensing performance of Sr-doped ZnO (Sr0.01 Zn0.99O) and RuO2-activated Sr-doped ZnO heterostructured sensors towards the low concentration (≤ 50 ppm) of ammonia gas at ambient. Sub-microns sized with high specific surface area, high reactive, oxygen-deficient Sr-doped ZnO particles were synthesized at low temperature (196 °C) through facile glycine–nitrate solution combustion synthesis (SCS) method. Porous, adhered screen-printed film of Sr-doped ZnO with optical bandgap (3.22 eV) was dip-coated using 0.02 M RuCl3 aqueous solution to obtain RuO2 activation. Smaller crystallite size and lesser lattice distortion obtained with Sr-doping in ZnO enhance the gas response (S = 71) towards the 50 ppm of ammonia gas at room temperature. RuO2-activated Sr-doped ZnO sensor associated with lesser oxygen vacancies and a lower concentration of chemisorbed oxygen species due to passivation layer and no-spill-over activity of RuO2, which inhibits the gas response from 71 to 3. Sr-doped ZnO-based sensor shows high selectivity towards ammonia against 50 ppm of volatile organic compound (VOCs) vapor. Expeditious sensor kinetics (response time and recovery time) in the Sr-doped ZnO sensor was observed, in which smaller crystallite size offers a shorter distance for the diffusion of oxygen vacancies (Vo). Ultra-high-sensitive and selective sensors with ease and economical fabrication offer feasibility in industries and domestic applications where detection of the less concentration ammonia vapor is crucial.

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS2 nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co-Ru-MoS2 hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T' phase MoS2 nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm-2 and Tafel slopes of 55 and 50 mV decade-1 in 1.0 m KOH, respectively. Analysis of X-ray photoelectron and absorption spectra of the catalysts show that the MoS2 well retained its metallic 1T' phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co-doped MoS2 greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO2 nanoparticles that are produced via oxidation of Ru nanoparticles.

RuO2 nanoparticles decorate belt-like anatase TiO2 for highly efficient chlorine evolution

Chlorine gas, mainly produced by the chlor-alkali industry, plays a crucial role in chemical synthesis, water treatment and electronics industry. Herein, facile hydrothermal growth followed by electrodeposition is applied to fabricate a RuO2@TiO2 electrode with nanostructured morphology toward the chlorine evolution reaction (CER), which contributes to reducing energy consumption and environmental pollution in comparison with the traditional thermal decomposition route to form a “mud-crack” structure. The appropriate amount of RuO2 nanoparticles (NPs) decorated on TiO2 nanobelt arrays (NBs) is determined by controlling the electrodeposition time to realize aligned morphological features instead of typically calculating various molar ratios of Ti and Ru to determine the best catalytic activity. Benefitting from the nanostructure of broadly outstretched TiO2 NBs and the perfect decoration of highly catalytically active RuO2 NPs roughening the nanomaterial surface, the RuO2 NPs@TiO2 NBs electrode exhibits enhanced catalytic activity with splendid stability and high selectivity toward the CER. It requires an extremely low potential of 1.34 V (overpotential 80 mV) to achieve a catalytic current density of 50 mA/cm2 and the current efficiency of the material electrode for chlorine evolution is up to 90%, making it among the best CER anodes reported so far.

Highly catalytic flexible RuO2 on carbon fiber cloth network for boosting chlorine evolution reaction

In this study, a novel flexible electrode consisting of homogeneous RuO2 nanoparticles (NPs) wrapped around the carbon fiber cloth (CC) is fabricated by combining the cathodic electro-deposition in the aqueous acidic RuCl3·xH2O solution and annealing in air atmosphere. The as-fabricated RuO2 NPs/CC electrode with electro-deposition duration of 10 min demonstrates a superior electrocatalytic activity with a lower overpotential of ca. 1.05 V vs. SCE and its current density reaches up to 175 mA cm−2 at 1.2 V vs. SCE for chlorine evolution reaction (CER) in 5.0 M NaCl aqueous solution, outperforming many of the reported counterparts. The as-revealed outstanding electrocatalytic activity should be attributed to several beneficial factors such as its typical integrated binder-free 3D architecture, higher usage efficiency of active RuO2 NPs, greatly decreased interfacial resistance, etc. This work paves a way to utilize the carbon fiber cloth decorated with active noble metal oxides for boosting electrochemical Cl2 production.

Nanoscale Architecture of RuO2/La0.9Fe0.92Ru0.08–xO3−δ Composite via Manipulating the Exsolution of Low Ru-Substituted A-Site Deficient Perovskite

The exsolution of noble metal nanoparticles (NPs) from perovskite usually requires high doping ratio of noble metal. Herein, we constructed a RuO2/LFRO composite by the exsolution of a low Ru-substituted A-site deficient perovskite, La0.9Fe0.92Ru0.08O3 (LFRO). In this process, pure Ru NPs are in situ exsolved from LFRO via a relatively low temperature heat treatment in 5% H2/Ar. Then the exsolved Ru NPs were oxidized to RuO2 for oxygen evolution reaction (OER) applications. The RuO2/LFRO composite achieved a high OER performance compared with the pristine LFRO, which is mainly originated from the generation of electrochemically active RuO2 NPs and the improvement of conductivity. In addition, the exsolution is a reversible process that the exsolved Ru NPs can disappear into the perovskite lattice at 550 °C in air. Our work thereof demonstrates an effective strategy to minimize the dosage of precious metals for catalytic applications in different fields.

Electrocatalysts of Mn and Ru oxides loaded on MWCNTS with 3D structure and synergistic effect for rechargeable Li-O2 battery

The structure and electrochemical activity of electrocatalysts play a vital role in lithium-oxygen battery. Herein, three-dimensional Mn and Ru oxides loaded on multiwall carbon nanotubes (RuO2-MnO2/MWCNTs) was synthesized via a one-pot hydrothermal approach and served as cathode of rechargeable lithium-oxygen (Li-O2) batteries. SEM and TEM images both illustrated that RuO2 nanoparticles and MnO2 nanoflakes are well dispersed on the surface of MWCNTs and formed a three-dimensional structure, the cyclic voltammetry studies revealed that the RuO2-MnO2/MWCNTs nanocomposite can effectively integrate the oxygen reduction reaction (ORR) activity of MnO2 and oxygen evolution reaction (OER) activity of RuO2. Electrochemical results demonstrated that the RuO2-MnO2/MWCNTs as oxygen cathode of Li–O2 batteries could maintain a reversible capacity of 1000 mAh g−1 within 94 cycles at a rate of 200 mA g−1 and within 120 cycles at a rate as high as 1000 mA g−1. Moreover, the cell with the catalyst RuO2-MnO2/MWCNTs presented much lower voltage polarization (about 0.68 V at a rate of 50 mA g−1) than that measured with MnO2/MWCNTs during charge/discharge processes. Ex-situ SEM were conducted to investigate the morphology and decomposition of Li2O2, which were formed after discharge and decomposed almost after charge process. The excellent electrochemical performance could be contributed to the three-dimensional structure which benefited the diffusion of oxygen and the storage of Li2O2 as well as the synergy effect of MWCNTs as an ideal conductive support for RuO2-MnO2 as co-catalyst in Li-O2 battery.

Combined effect of inherent residual chloride and bound water content and surface morphology on the intrinsic electron-transfer activity of ruthenium oxide

RuO2 is an unparalleled electrode with wider applications. Chloride, bound water, and surface stoichiometry are the inherent residues of RuO2 left upon the high-temperature pyrolysis of a RuCl3⋅xH2O precursor. Although the electrocatalytic properties of RuO2 for specific applications have been studied extensively, there is a paucity of studies linking the intrinsic electron-transfer (ET) activity of RuO2 with the oxide preparation temperature-dependent inherent residual parameters. This paper presents the intrinsic ET activity-oxide residue correlations for RuO2 electrodes. The ET kinetic parameters were estimated using a surface oxide-sensitive Fe3+/Fe2+ redox probe by rotating disc electrode voltammetry. Oxide powder-based electrodes (RuO2 powder-PVC/Pt-modified electrodes), which were fabricated conveniently at room temperature even with high-temperature oxides, were used instead of the traditional thermally prepared electrodes to circumvent the primary problems at the coating|support interface and inefficient chloride removal. RuO2 powders prepared at five temperatures (Tprep), i.e., 300, 400, 500, 600, and 700 °C, were used for electrode fabrication. The results showed that the electron exchange rate was highest for the 400 °C RuO2 electrode, and it was independent of the Tprep in the range 500 to 700 °C. The oxide powders were characterized using a range of techniques. The measured intrinsic ET activity and the associated structural correlation over the Tprep range 300–700 °C suggest that the best activity of the 400 °C electrode can be attributed to the optimal chloride and bound water contents in a completely formed rutile surface layer containing the catalyst sites of a particular nature with the highest electroactivity.

Reaction mechanism for oxygen evolution on RuO2, IrO2, and RuO2@IrO2 core-shell nanocatalysts

Iridium dioxide, IrO2, is second to the most active RuO2 catalyst for the oxygen evolution reaction (OER) in acid, and is used in proton exchange membrane water electrolyzers due to its high durability. To improve the activity of IrO2-based catalysts, we prepared RuO2@IrO2 core-shell nanocatalysts using carbon-supported Ru as the template. At 1.48V, the OER specific activity of RuO2@IrO2 is threefold that of IrO2. While the activity volcano plots over wide range of materials have been reported, zooming into the top region to clarify the rate limiting steps of most active catalysts is important for further activity enhancement. Here, we verified theory-proposed sequential water dissociation pathway in which the OO bond forms on a single metal site, not via coupling of two adsorbed intermediates, by fitting measured polarization curves using a kinetic equation with the free energies of adsorption and activation as the parameters. Consistent with theoretical calculations, we show that the OER activities of IrO2 and RuO2@IrO2 are limited by the formation of O adsorbed phase, while the OOH formation on the adsorbed O limits the reaction rate on RuO2.