Growth of One-Dimensional RuO2 Nanowires on g-Carbon Nitride: An Active and Stable Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution Reactions at All pH Values

Exceptional Suppression of the Self-Discharge Behavior of Supercapacitors by Precisely Tuning the Surface Assets of MXene by a Spontaneous Single-Atom Doping Strategy

MXene-based supercapacitors (SCs) are widely regarded as promising energy storage devices. However, the inevitable and ignored self-discharge behavior of MXene-based SCs causes an unavoidable voltage decay and energy loss. Herein, the Ru single-atom doping strategy is used to fabricate a Ru-MXene film to modulate the surface properties of MXene, and the assembled Ru-MXene film-based SC showed a suppressed self-discharge behavior due to the simultaneously reduced activation-controlled and diffusion-controlled reactions. Regarding the self-discharge mechanism, three positive synergistic effects including increased adsorption energy, increased work function, and oxidation of the Ti atom of Ru-MXene simultaneously led to suppressing the self-discharge behavior. Benefiting from abundant electroactive sites and higher adsorption energy, the assembled Ru-MXene film-based SC exhibited an excellent electrochemical performance. This work provides a glimpse into the single-atom doping strategy to suppress the self-discharge behavior of MXene-based SCs and a prospective guide to promote their practical applications.

Metal oxide plating for maximizing the performance of ruthenium(IV) oxide-catalyzed electrochemical oxygen evolution reaction

Hydrogen production by proton exchange membrane water electrolysis requires an anode with low overpotential for oxygen evolution reaction (OER) and robustness in acidic solution. While exploring new electrode materials to improve the performance and durability, optimizing the morphology of typical materials using new methods is a big challenge in materials science. RuO2 is one of the most active and stable electrocatalysts, but further improvement in its performance and cost reduction must be achieved for practical use. Herein, we present a novel technology, named "metal oxide plating", which can provide maximum performances with minimum amount. A uniform single-crystal RuO2 film with thickness of ∼2.5 nm was synthesized by a solvothermal-post heating method at an amount (x) of only 18 μg cm-2 (ST-RuO2(18)//TiO2 NWA). OER stably proceeds on ST-RuO2(18)//TiO2 NWA with ∼100% efficiency to provide a mass-specific activity (MSA) of 341 A gcat-1 at 1.50 V (vs. RHE), exceeding the values for most of the state-of-the-art RuO2 electrodes.

Carbon Nanotube Support, Carbon Loricae and Oxygen Defect Co-Promoted Superior Activities and Excellent Durability of RuO2 Nanoparticles Towards the pH-Universal H2 Evolution

This work reports a strategy that integrates the carbon nanotube (CNT) supporting, ultrathin carbon coating and oxygen defect generation to fabricate the RuO2 based catalysts toward the pH-universal hydrogen evolution reaction (HER) with high efficiencies. Specifically, the CNT supported RuO2 nanoparticles with ultrathin carbon loricae and rich oxygen vacancies at the surface (C@OV-RuO2/CNTs-325) have been synthesized. The C@OV-RuO2/CNTs-325 shows superior activities and excellent durability for the HER. It only requires overpotentials of 36.1, 18.0, and 19.3 mV to deliver -10 mA cm-2 in the acidic, neutral, and alkaline media, respectively. Its HER activities are comparable to that of the Pt/C in the acidic media but higher than those of the Pt/C in the neutral and alkaline media. The C@OV-RuO2/CNTs-325 shows excellent HER durability with no activity losses for > 500 h in the acidic, neutral or alkaline media at -250 mA cm-2. The density-functional-theory calculations indicate that the CNT supporting, the carbon coating, and the OVs can modulate the d-band centers of Ru, increasing the HER activities of C@OV-RuO2/CNTs-325, and stabilize the Ru atoms in the catalyst, increasing the durability of the C@OV-RuO2/CNTs-325. More interestingly, the C@OV-RuO2/CNTs-325 shows great potential for practical applications toward overall seawater splitting.

Modulating *OOH Adsorption on RuO2 for Efficient and Durable Acidic Water Oxidation Electrocatalysis

Acidic water electrolysis is of considerable interest due to its higher current density operation and energy conversion efficiency, but its real industrial application is highly limited by the shortage of efficient, stable, and cost-effective acidic oxygen evolution reaction (OER) electrocatalysts. Here, an electrocatalyst consisting of Ni-implanted RuO2 supported is reported on α-MnO2 (MnO2/RuO2-Ni) that shows high activity and remarkable durability in acidic OER. Precisely, the MnO2/RuO2-Ni catalyst shows an overpotential of 198 mV at a current density of 10 mA cm-2 and can operate continuously and stably for 400 h (j = 10 mA cm-2) without any obvious attenuation of activity, making it one of the best-performing acid-stable OER catalysts. Experimental results, in conjunction with density functional theory calculations, demonstrate that the interface electron transfer effect from RuO2 to MnO2, further enhanced by Ni incorporation, effectively modulates the adsorption of OOH* and significantly reduces the overpotential, thereby enhancing catalytic activity and durability.

Electronic modulation and reaction-pathway optimization on three-dimensional seaweed-like NiSe@NiMn LDH heterostructure to trigger effective oxygen evolution reaction

The development of low-cost and high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is essential to produce high-purity hydrogen in large scale. Herein, a three-dimensional (3D) seaweed-like hierarchical structure was fabricated using two-dimensional (2D) NiMn LDH nanosheets wrapped on one-dimensional (1D) NiSe nanowires with nickel foam (NF) as a substrate (NiSe@NiMn LDH/NF) via hydrothermal and electrodeposition processes. Owing to the strong interfacial synergy, 3D seaweed-like hierarchical structure, higher conductivity, and strong structural stability, the NiSe@NiMn LDH/NF exhibited superior OER performance with an overpotential of 287 mV at 100 mA cm-2, and stably operated for 160 h at large current. Moreover, the overall water splitting system with NiSe@NiMn LDH/NF as the anode and Pt/C/NF as the cathode exhibited a low cell voltage of 1.59/1.64 V to reach 50/100 mA cm-2, and excellent stability for 110 h at 300 mA cm-2. The density function theory (DFT) calculations unveiled that NiSe@NiMn LDH enabled the interfacial synergy, reallocating the electron density at the interface, and further weakening the energy barrier of OH* by strengthening chemical bonds with OH* intermediates to improve the intrinsic OER activity.

Immobilizing Bimetallic RuCo Nanoalloys on Few-Layered MXene as a Robust Bifunctional Electrocatalyst for Overall Water Splitting

Doping Co atoms into Ru lattices can tune the electronic structure of active sites, and the conductive MXene can adjust the electrical conductivity of catalysts, which are both favorable for improving the electrocatalytic activity of the catalyst for water splitting. Here, ruthenium-cobalt bimetallic nanoalloys coupled with exfoliated Ti3 C2 Tx MXene (RuCo-Ti3 C2 Tx ) have been constructed by ice-templated and thermal activation. Due to the strong interaction between the RuCo nanoalloys and conductive MXene, RuCo-Ti3 C2 Tx not only exhibits an excellent hydrogen evolution reaction (HER) performance with a low overpotential and Tafel slope (60 mV, 34.8 mV dec-1 in 0.5 M H2 SO4 and 52 mV, 38.7 mV dec-1 in 1 M KOH), but also good oxygen evolution reaction (OER) performance in an alkaline electrolyte (266 mV, 111.1 mV dec-1 in 1 M KOH). The assembled RuCo-Ti3 C2 Tx ||RuCo-Ti3 C2 Tx electrolyzer requires a lower potential (1.56 V) than does the Pt/C||RuO2 electrolyzer at 10 mA cm-2 . A boosted catalytic HER activity from immobilizing the RuCo nanoalloys on MXene was unveiled by density functional theory calculations. This study provides a feasible and efficient strategy for developing MXene-based catalysts for overall water splitting.

Correlating Single-Atomic Ruthenium Interdistance with Long-Range Interaction Boosts Hydrogen Evolution Reaction Kinetics

Correlated single-atom catalysts (c-SACs) with tailored intersite metal-metal interactions are superior to conventional catalysts with isolated metal sites. However, precise quantification of the single-atomic interdistance (SAD) in c-SACs is not yet achieved, which is essential for a crucial understanding and remarkable improvement of the correlated metal-site-governed catalytic reaction kinetics. Here, three Ru c-SACs are fabricated with precise SAD using a planar organometallic molecular design and π-π molecule-carbon nanotube confinement. This strategy results in graded SAD from 2.4 to 9.3 Å in the Ru c-SACs, wherein tailoring the Ru SAD into 7.0 Å generates an exceptionally high turnover frequency of 17.92 H2 s-1 and a remarkable mass activity of 100.4 A mg-1 under 50 and 100 mV overpotentials, respectively, which is superior to all the Ru-based catalysts reported previously. Furthermore, density functional theory calculations confirm that Ru SAD has a negative correlation with its d-band center owing to the long-range interactions induced by distinct local atomic geometries, resulting in an appropriate electrostatic potential and the highest catalytic activity on c-SACs with 7.0 Å Ru SAD. The present study promises an attractive methodology for experimentally quantifying the metal SAD to provide valuable insights into the catalytic mechanism of c-SACs.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Low Ruthenium Content Confined on Boron Carbon Nitride as an Efficient and Stable Electrocatalyst for Acidic Oxygen Evolution Reaction

To date, only a few noble metal oxides exhibit the required efficiency and stability as oxygen evolution reaction (OER) catalysts under the acidic, high-voltage conditions that exist during proton exchange membrane water electrolysis (PEMWE). The high cost and scarcity of these catalysts hinder the large-scale application of PEMWE. Here, we report a novel OER electrocatalyst for OER comprised of uniformly dispersed Ru clusters confined on boron carbon nitride (BCN) support. Compared to RuO2 , our BCN-supported catalyst shows enhanced charge transfer. It displays a low overpotential of 164 mV at a current density of 10 mA cm-2 , suggesting its excellent OER catalytic activity. This catalyst was able to operate continuously for over 12 h under acidic conditions, whereas RuO2 without any support fails in 1 h. Density functional theory (DFT) calculations confirm that the interaction between the N on BCN support and Ru clusters changes the adsorption capacity and reduces the OER energy barrier, which increases the electrocatalytic activity of Ru.

One-dimensional carbon based nanoreactor fabrication by electrospinning for sustainable catalysis

An efficient and economical electrocatalyst as kinetic support is key to electrochemical reactions. For this reason, chemists have been working to investigate the basic changing of chemical principles when the system is confined in limited space with nanometer-scale dimensions or sub-microliter volumes. Inspired by biological research, the design and construction of a closed reaction environment, namely the reactor, has attracted more and more interest in chemistry, biology, and materials science. In particular, nanoreactors became a high-profile rising star and different types of nanoreactors have been fabricated. Compared with the traditional particle nanoreactor, the one-dimensional (1D) carbon-based nanoreactor prepared by the electrospinning process has better electrolyte diffusion, charge transfer capabilities, and outstanding catalytic activity and selectivity than the traditional particle catalyst which has great application potential in various electrochemical catalytic reactions.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Tuning Mass Transport in Electrocatalysis Down to Sub-5 nm through Nanoscale Grade Separation

Nano and single-atom catalysis open new possibilities of producing green hydrogen (H₂ ) by water electrolysis. However, for the hydrogen evolution reaction (HER) which occurs at a characteristic reaction rate proportional to the potential, the fast generation of H₂ nanobubbles at atomic-scale interfaces often leads to the blockage of active sites. Herein, a nanoscale grade-separation strategy is proposed to tackle mass-transport problem by utilizing ordered three-dimensional (3d) interconnected sub-5 nm pores. The results reveal that 3d criss-crossing mesopores with grade separation allow efficient diffusion of H₂ bubbles along the interconnected channels. After the support of ultrafine ruthenium (Ru), the 3d mesopores are on a superior level to two-dimensional system at maximizing the catalyst performance and the obtained Ru catalyst outperforms most of the other HER catalysts. This work provides a potential route to fine-tuning few-nanometer mass transport during water electrolysis.

Phase Transformation from Amorphous RuSx to Ru-RuS2 Hybrid Nanostructure for Efficient Water Splitting in Alkaline Media

Ruthenium (Ru)-based materials, as a class of efficient hydrogen evolution reaction (HER) catalysts, play an important role in hydrogen generation by electrolysis of water in an alkaline solution for clean hydrogen energy. Hybrid nanostructure (HN) materials, which include two or more components with distinct functionality, show better performance than their individual materials, since HN materials can potentially integrate their advantages and overcome the weaknesses. However, it remains a challenge to construct Ru-based HN materials with desired crystal phases for enhanced HER performances. Herein, a series of new Ru-based HN materials (t-Ru-RuS₂, S-Ru-RuS₂, and T-Ru-RuS₂) through phase engineering of nanomaterials (PEN) and chemical transformation are designed to achieve highly efficient HER properties. Owing to the plentiful formation of heterojunctions and amorphous/crystalline interfaces, the t-Ru-RuS₂ HN delivers the most outstanding overpotential of 16 mV and owns a small Tafel slope of 29 mV dec^-1 at a current density of 10 mA cm^-2, which exceeds commercial Pt/C catalysts (34 mV, 38 mV dec^-1). This work shows a new insight for HN and provides alternative opportunities in designing advanced electrocatalysts with low cost for HER in the hydrogen economy.

Interfacial Carbon Makes Nano-Particulate RuO2 an Efficient, Stable, pH-Universal Catalyst for Splitting of Seawater

An electrocatalyst composed of RuO₂ surrounded by interfacial carbon, is synthesized through controllable oxidization-calcination. This electrocatalyst provides efficient charge transfer, numerous active sites, and promising activity for pH-universal electrocatalytic overall seawater splitting. An electrolyzer with this catalyst gives current densities of 10 mA cm^-2 at a record low cell voltage of 1.52 V, and shows excellent durability at current densities of 10 mA cm^-2 for up to 100 h. Based on the results, a mechanism for the catalytic activity of the composite is proposed. Finally, a solar-driven system is assembled and used for overall seawater splitting, showing 95% Faraday efficiency.

Ultralight, Safe, Economical, and Portable Oxygen Generators with Low Energy Consumption Prepared by Air-Breathing Electrochemical Extraction

Pure oxygen is vital in medical treatment, first aid, and chemical synthesis. Hypoxia can cause severe damage to the organ systems such as respiratory, digestive, and nervous systems and even directly cause death. Notably, the severe Coronavirus disease 2019 (COVID-19) pandemic has exacerbated the shortage of medical oxygen in the world. Hence, a safe, economical, and portable oxygen supply device is urgently needed. Here, we have successfully prepared a device with air-breathing electrochemical extraction of pure oxygen (ABEEPO) with light weight and high energy efficiency. By renovating the structure of the electrolytic cell, the components bipolar plate and end plate are replaced with a plastic membrane, and the component current collector is replaced with a highly conductive graphene composite membrane electrode. Due to the use of the plastic membrane and graphene composite membrane electrode, the weight of the electrolytic cell is reduced from 1319.4 to 1.6 g, and the flexibility of the electrolytic cell is successfully realized. Through optimizing anode catalysts, working area, and operating voltage, a high flow rate per mass (234 mL h^-1 g^-1) was achieved at a voltage of 1.2 V. The device exhibits high stability in 2 h. The new portable oxygen production device would be effective for hypoxia treatment.

Bifunctional WC-Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media

We report the strong catalyst-support interaction in WC-supported RuO₂ nanoparticles (RuO₂ -WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η₁₀ of 347 mV and a mass activity of 1430 A g_Ru ^-1 , eight-fold higher than that of commercial RuO₂ (176 A g_Ru ^-1 ). Theoretical calculations demonstrate that the strong catalyst-support interaction between RuO₂ and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO₂ -WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm^-2 . The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

RuCo Alloy Nanoparticles Embedded into N-Doped Carbon for High Efficiency Hydrogen Evolution Electrocatalyst

For large-scale and sustainable water electrolysis, it is of great significance to develop cheap and efficient electrocatalysts that can replace platinum. Currently, it is difficult for most catalysts to combine high activity and stability. To solve this problem, we use cobalt to regulate the electronic structure of ruthenium to achieve high activity, and use carbon matrix to protect alloy nanoparticles to achieve high stability. Herein, based on the zeolitic imidazolate frameworks (ZIFs), a novel hybrid composed of RuCo alloy nano-particles and N-doped carbon was prepared via a facile pyrolysis-displacement-sintering strategy. Due to the unique porous structure and multi-component synergy, the optimal RuCo500@NC750 material in both acidic and alkaline media exhibited eminent HER catalytic activity. Notably, the 3-RuCo500@NC750 obtained a current density of 10 mA cm−2 at 22 mV and 31 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively, comparable to that of the reference Pt/C catalyst. Furthermore, the Tafel slopes of the catalyst are 52 mV Dec−1 and 47 mV Dec−1, respectively, under acid and alkali conditions, and the catalyst has good stability, indicating that it has broad application prospects in practical electrolytic systems. This work contributes to understanding the role of carbon-supported polymetallic alloy in the electrocatalytic hydrogen evolution process, and provides some inspiration for the development of a high efficiency hydrogen evolution catalyst.

Investigation of enhanced electro-catalytic HER/OER performances of copper tungsten oxide@reduced graphene oxide nanocomposites in alkaline and acidic media

In this paper, we investigate the electro-catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of synthesized copper tungsten oxide@reduced graphene oxide (CuWO4@rGO) nanocomposites.

Highly efficient and stable Ru nanoparticle electrocatalyst for the hydrogen evolution reaction in alkaline conditions

Ru nanoparticles are prepared via solvothermal synthesis with allotropism control. Both fcc and hcp samples are active catalysts for the hydrogen evolution reaction, but the hcp sample is stable during 12 hour operation.

Highly efficient catalysts of ruthenium clusters on Fe3O4/MWCNTs for the hydrogen evolution reaction

Anchoring of Ru onto the Fe3O4/MWCNTs composite, its HER mechanism and DFT calculated change in Gibbs free energy of adsorbed hydrogen are described.

NiFe Layered Double Hydroxide/FeOOH Heterostructure Nanosheets as an Efficient and Durable Bifunctional Electrocatalyst for Overall Seawater Splitting

Electrolysis of seawater can not only desalinate seawater but also produce high-purity hydrogen. Nevertheless, the presence of chloride ions in seawater will cause electrode corrosion and also undergo a chlorine oxidation reaction (ClOR) that competes with the oxygen evolution reaction (OER). Therefore, highly efficient and long-term stable electrocatalysts are needed in this field. In this work, an advanced bifunctional electrocatalyst based on NiFe layered double hydroxide (LDH)/FeOOH heterostructure nanosheets (NiFe LDH/FeOOH) was synthesized on nickel-iron foam (INF) via a simple electrodeposition method. The NiFe LDH/FeOOH electrode demonstrates excellent electrocatalytic activity and stability, which results from the strong interaction between FeOOH and NiFe LDH. Furthermore, ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy revealed the catalytic process and also demonstrated that the NiFe LDH/FeOOH heterostructure could facilitate the formation of active NiOOH species in the reaction. The obtained NiFe LDH/FeOOH catalyst displays low overpotentials of 181.8 mV at 10 mA·cm^-2 for hydrogen evolution reaction (HER) and 286.2 mV at 100 mA·cm^-2 for OER in the 1.0 M KOH + 0.5 M NaCl electrolyte. Furthermore, it also exhibits a low voltage of 1.55 V to achieve the current density of 10 mA·cm^-2 and works steadily for 105 h at 100 mA·cm^-2 for overall alkaline simulated seawater splitting. This work will afford a valid strategy for designing a non-noble metal catalyst for seawater splitting.

Electrocatalytic Oxygen Evolution Reaction in Acidic Conditions: Recent Progress and Perspectives

The electrochemical oxygen evolution reaction (OER) is an important half-cell reaction in many renewable energy conversion and storage technologies, including electrolyzers, nitrogen fixation, CO₂ reduction, metal-air batteries, and regenerative fuel cells. Among them, proton exchange membrane (PEM)-based devices exhibit a series of advantages, such as excellent proton conductivity, high durability, and good mechanical strength, and have attracted global interest as a green energy device for transport and stationary sectors. Nevertheless, with a view to rapid commercialization, it is urgent to develop highly active and acid-stable OER catalysts for PEM-based devices. In this Review, based on the recent advances in theoretical calculation and in situ/operando characterization, the OER mechanism in acidic conditions is first discussed in detail. Subsequently, recent advances in the development of several types of acid-stable OER catalysts, including noble metals, non-noble metals, and even metal-free OER materials, are systematically summarized. Finally, the current key issues and future challenges for materials used as acidic OER catalysis are identified and potential future directions are proposed.

Electronic modulation and proton transfer by iron and borate co-doping for synergistically triggering the oxygen evolution reaction on a hollow NiO bipyramidal prism

Designing an Earth-abundant and inexpensive electrocatalyst to drive the oxygen evolution reaction (OER) for high-purity hydrogen production is of great importance. Herein, the cation (iron) and anion (borate) co-doping strategy was proposed to effectively trigger the OER performance on a low-cost NiO material. The optimal hollow Fe/B_i-NiO bipyramidal prism shows superior OER performance, and displays a low overpotential (261 mV) at 10 mA cm^-2, accompanied by a low Tafel slope (46 mV dec^-1), excellent intrinsic activity and robust stability. The overall alkaline water splitting using Fe/B_i-NiO/NF as an anode affords low cell voltages of 1.50 and 1.63 V at 10 and 100 mA cm^-2, and operates steadily at a high current density of 100 mA cm^-2 for 55 h without decay. The excellent electrocatalytic activity could be ascribed to the hollow structure to shorten the mass transfer pathway, the electronic modulation by Fe doping, the increased accessible electroactive sites created by oxygen vacancies through borate doping, and the formation of BO₃^3--OH^- to accelerate the deprotonation of OH_ads.

Decoration of Ru/RuO2 hybrid nanoparticles on MoO2 plane as bifunctional electrocatalyst for overall water splitting

Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the two branches of artificial overall water splitting (OWS), in which the reaction efficiency usually depends on different specific catalysts. Although effective bifunctional electrocatalyst for OWS (HER and OER) are highly desired, designing and constructing such suitable materials is full of challenges to overcome several difficulties, involving slow kinetics, differences in catalytic mechanisms, large overpotential values, and low round-trip efficiencies. In this work, we reported a new bifunctional electrocatalyst Ru/RuO₂-MoO₂ catalyst (RRMC) via a redox solid phase reaction (RSPR) strategy to achieve the high electrocatalytic activity of OWS. Briefly, due to the restricted transport behavior of atoms in solid state precursor, the designed redox reaction occurred between the adjacent part of RuO₂ and MoS₂, forming Ru/RuO₂ hybrid NPs and MoO₂ plane. Therefore, the newly formed Ru/RuO₂ hybrid NPs and MoO₂ plane were tightly combined and used as an electrocatalyst for OWS. Benefiting from the exposed active sites and optimized electronic structure, the RRMC sample annealed at 500 °C (RRMC-500) exhibited low overpotential for HER (18 mV) and for OER (260 mV) at 10 mA cm^-2 under alkaline conditions. Especially, a low cell voltage of 1.54 V was required at 10 mA cm^-2 under alkaline condition.

Interface- and Surface-Engineered PdO-RuO2 Hetero-Nanostructures with High Activity for Hydrogen Evolution/Oxidation Reactions

Active catalysts for HER/HOR are crucial to develop hydrogen-based renewable technologies. The interface of hetero-nanostructures can integrate different components into a single synergistic hybrid with high activity. Here, the synthesis of PdO-RuO₂ -C with abundant interfaces/defects was achieved for the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR). It exhibited a current density of 10 mA cm^-2 at 44 mV with a Tafel slope of 34 mV dec^-1 in 1 m KOH. The HER mass activity was 3 times higher in base and comparable to Pt/C in acid. The stability test confirmed high HER stability. The catalyst also exhibited excellent HOR activity in both media; in alkaline HOR it outperformed Pt/C. The exchange current density i_0,m of PdO-RuO₂ /C was 522 mA mg^-1 in base, which is 58 and 3.4 times higher than those of Pd/C and Pt/C. The HOR activity of PdO-RuO₂ /C was 22 and 300 times higher than those of PdO/C in acid and base. Improvement of HER/HOR kinetics in different alkaline electrolytes was observed in the order K⁺ <Na⁺ <Li⁺ , and increase of HER as well decrease of HOR kinetics was observed with increasing Li⁺ concentration. It was proposed that OH_ad -M⁺ -(H₂ O)_x in the double-layer region could influence HER/HOR activity in base. Based on the hard and soft acid and base (HSAB) theory, the OH_ads -M⁺ -(H₂ O)_x could help to remove more OH_ads into the bulk, leading to increase in HER/HOR activity in alkaline electrolyte (K⁺ <Na⁺ <Li⁺ ) and increasing the HER with increasing Li⁺ concentration. The decrease of HOR activity of PdO-RuO₂ /C with increasing M⁺ was due to M⁺ -induced OH_ads destabilization through the bifunctional mechanism. The high HER/HOR activity of PdO-RuO₂ /C could be attributed, among other factors, to interface engineering and strong synergistic interaction. This work provides an opportunity to design oxide-based catalysts for renewable energy technologies.

Well-dispersed Pt/RuO2-decorated mesoporous N-doped carbon as a hybrid electrocatalyst for Li-O2 batteries

Despite their high energy density, the poor cycling performance of lithium-oxygen (Li-O2) batteries limits their practical application. Therefore, to improve cycling performance, considerable attention has been paid to the development of an efficient electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Catalysts that can more effectively reduce the overpotential and improve the cycling performance for the OER during charging are of particular interest. In this study, porous carbon derived from protein-based tofu was investigated as a catalyst support for the oxygen electrode (O2-electrode) of Li-O2 batteries, wherein ORR and OER occur. The porous carbon was synthesized using carbonization and KOH activation, and RuO2 and Pt electrocatalysts were introduced to improve the electrical conductivity and catalytic performance. The well-dispersed Pt/RuO2 electrocatalysts on the porous N-doped carbon support (Pt/RuO2@ACT) showed excellent ORR and OER catalytic activity. When incorporated into a Li-O2 battery, the Pt/RuO2@ACT O2-electrode exhibited a high specific discharge capacity (5724.1 mA h g-1 at 100 mA g-1), a low discharge-charge voltage gap (0.64 V at 2000 mA h g-1), and excellent cycling stability (43 cycles with a limit capacity of 1000 mA h g-1). We believe that the excellent performance of the Pt/RuO2@ACT electrocatalyst is promising for accelerating the commercialization of Li-O2 batteries.

Partial-Single-Atom, Partial-Nanoparticle Composites Enhance Water Dissociation for Hydrogen Evolution

The development of an efficient electrocatalyst toward the hydrogen evolution reaction (HER) is of significant importance in transforming renewable electricity to pure and clean hydrogen by water splitting. However, the construction of an active electrocatalyst with multiple sites that can promote the dissociation of water molecules still remains a great challenge. Herein, a partial-single-atom, partial-nanoparticle composite consisting of nanosized ruthenium (Ru) nanoparticles (NPs) and individual Ru atoms as an energy-efficient HER catalyst in alkaline medium is reported. The formation of this unique composite mainly results from the dispersion of Ru NPs to small-size NPs and single atoms (SAs) on the Fe/N codoped carbon (Fe-N-C) substrate due to the thermodynamic stability. The optimal catalyst exhibits an outstanding HER activity with an ultralow overpotential (9 mV) at 10 mA cm-2 (η 10), a high turnover frequency (8.9 H2 s-1 at 50 mV overpotential), and nearly 100% Faraday efficiency, outperforming the state-of-the-art commercial Pt/C and other reported HER electrocatalysts in alkaline condition. Both experimental and theoretical calculations reveal that the coexistence of Ru NPs and SAs can improve the hydride coupling and water dissociation kinetics, thus synergistically enhancing alkaline hydrogen evolution performance.

Tuning the Electronic Structures of Multimetal Oxide Nanoplates to Realize Favorable Adsorption Energies of Oxygenated Intermediates

Highly active oxygen evolution reaction (OER) electrocatalysts are important to effectively transform renewable electricity to fuel and chemicals. In this work, we construct a series of multimetal oxide nanoplate OER electrocatalysts through successive cation exchange followed by electrochemical oxidation, whose electronic structure and diversified metal active sites can be engineered via the mutual synergy among multiple metal species. Among the examined multimetal oxide nanoplates, CoCeNiFeZnCuO_x nanoplates exhibit the optimal adsorption energy of OER intermediates. Together with the high electrochemical active surface area, the CoCeNiFeZnCuO_x nanoplates manage to deliver a small overpotential of 211 mV at an OER current density of 10 mA cm^-2 (η₁₀) with a Tafel slope as low as 21 mV dec^-1 in 1 M KOH solution, superior to commercial IrO₂ (339 mV at η₁₀, Tafel slope of 55 mV dec^-1), which can be stably operated at 10 mA cm^-2 (at an overpotential of 211 mV) and 100 mA cm^-2 (at an overpotential of 307 mV) for 100 h.

Ru single atoms and nanoparticles on carbon nanotubes as multifunctional catalysts

In the last decade we have witnessed increasing interest in the production of renewable energy and value-added chemicals through sustainable and low-cost technologies where catalysts play a crucial role. Herein, we report the application of a Ru/CNT material containing a mixture of Ru single atoms and Ru nanoparticles as a multifunctional catalyst for both the catalytic reduction of nitroarenes and the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The catalytic activity of the Ru-CNT material was evaluated in the reduction of 4-nitrophenol (4-NP), 4-nitroaniline (4-NA) and 2-nitrophenol (2-NP) in the presence of sodium borohydride as a reducing agent at room temperature, showing high catalytic activity with normalized rate constants (k_nor) of 19.0 × 10³, 57.7 × 10³ and 16.6 × 10³ min^-1 mmol^-1 respectively. Furthermore, the catalyst could be reused in at least 10 cycles without catalytic activity loss, confirming the high stability and robustness of the material. The Ru/CNT material also showed good ORR electrocatalytic activity in alkaline medium with E_onset of 0.76 V vs. RHE, a diffusion-limited current density of 3.89 mA cm^-2 and ñ_O2 of 3.3. In addition, Ru/CNT was remarkably insensitive to methanol with a current retention of 93% (51% for Pt/C) and competitive electrochemical stability of 80% after 20 000 s. Moreover, Ru/CNT was active for the OER with j_max = 29.5 mA cm^-2 at E = 1.86 V vs. RHE, η₁₀ = 0.50 V and good stability (η₁₀ changed to 0.01 V and j_max only decreased by ≈12% after 500 cycles).

Efficient Multifunctional Catalytic and Sensing Properties of Synthesized Ruthenium Oxide Nanoparticles

Ruthenium oxide is one of the most active electrocatalyst for oxygen evolution (OER) and oxygen reduction reaction (ORR). Herein, we report simple wet chemical route to synthesize RuO2 nanoparticles at controlled temperature. The structural, morphological and surface area studies of the synthesized nanoparticles were conducted with X-ray diffraction, electron microscopy and BETsurface area studies. The bifunctional electrocatalytic performance of RuO2 nanoparticles was studied under different atmospheric conditions for OER and ORR, respectively, versus reversible hydrogen electrode (RHE) in alkaline medium. Low Tafel slopes of RuO2 nanoparticles were found to be ~47 and ~49 mV/dec for OER and ORR, respectively, in oxygen saturated 0.5 M KOH system. Moreover, the catalytic activity of RuO2 nanoparticles was examined against the Horseradish peroxidase enzyme (HRP) at high temperature, and the nanoparticles were applied as a sensor for the detection of H2O2 in the solution.

Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts

The oxygen evolution reaction (OER), as the anodic reaction of water electrolysis (WE), suffers greatly from low reaction kinetics and thereby hampers the large-scale application of WE. Seeking active, stable, and cost-effective OER catalysts in acidic media is therefore of great significance. In this perspective, studying the reaction mechanism and exploiting advanced anode catalysts are of equal importance, where the former provides guidance for material structural engineering towards a better catalytic activity. In this review, we first summarize the currently proposed OER catalytic mechanisms, i.e., the adsorbate evolution mechanism (AEM) and lattice oxygen evolution reaction (LOER). Subsequently, we critically review several acidic OER electrocatalysts reported recently, with focus on structure-performance correlation. Finally, a few suggestions on exploring future OER catalysts are proposed.

Rhodium/graphitic-carbon-nitride composite electrocatalyst facilitates efficient hydrogen evolution in acidic and alkaline electrolytes

Exploring the highly efficient and durable electrocatalysts for hydrogen evolution reaction (HER) is vitally necessary for sustainable energy conversion and storage system. Herein, we fabricate an interfacial engineered Rh-carbon nitride as advanced electrocatalysts for HER in the acidic and alkaline electrolytes. The interface between Rh nanocrystals and carbon nitride may adjust the electronic structure of Rh, which results in high activity for HER. The optimal Rh-carbon nitride shows low overpotential at current density of -10 mA·cm^-2 and small Tafel slope (13 mV and 25.0 mV dec^-1 in 0.5 M H₂SO₄, 46 mV and 42.0 mV dec^-1 in 1.0 M KOH, respectively), which is superior to that of commercial Pt/C (21 mV and 28.5 mV dec^-1 in 0.5 M H₂SO₄, 55 mV and 44.0 mV dec^-1 in 1.0 M KOH, respectively). Importantly, this composite also exhibits long-term stability in 0.5 M H₂SO₄ and 1.0 M KOH. The excellent HER performances can be attribute to the interface between Rh and carbon nitride, which downshifts their d-band center positions, tuning the adsorption ability for hydrogen and accelerating the HER kinetics. This work may open up an efficient method to design metal/carbon hybrid for electrocatalysis.

Designing transition-metal-boride-based electrocatalysts for applications in electrochemical water splitting

Investigating renewable and clean energy materials as alternatives to fossil fuels can be foreseen as a potential solution to the global problems of energy shortages and environmental pollution. Recently, transition metal boride (TMB)-based materials have emerged as the rising star as efficient electrocatalysts for hydrogen evolution reaction (HER) and/or oxygen evolution reaction (OER). In this review, an overview of the most recent developments in the use of TMB-based materials as electrocatalysts for HER/OER or overall water splitting has been presented. Initially, we provide a comprehensive introduction of the fundamentals of electrochemical water splitting. Then, the synthesis approaches of TMB materials are summarized and compared. Emphasis is put on the various strategies for further improving the electrocatalytic performance of TMBs. Finally, challenges and future perspectives for TMBs in water-splitting applications are proposed.

Cobalt Phosphide-Embedded Reduced Graphene Oxide as a Bifunctional Catalyst for Overall Water Splitting

It is highly desirable to design high-efficiency stable and low-price catalysts in the electrocatalysis field. Herein, we reported a cobalt phosphide (Co₂P)-loaded reduced graphene oxide (rGO) composite catalyst (rGO/Co₂P) prepared via the convenient hydrothermal and H₂ reduction methods. The rGO/Co₂P catalyst reduced at 800 °C (rGO/Co₂P-800) shows superior electrocatalytic activities for hydrogen evolution reaction and oxygen evolution reaction in 1.0 M KOH solution, achieving an overpotential of 134 and 378 mV, respectively, at a current density of 10 mA cm^-2. Moreover, the catalyst can not only maintain stability for a long time in alkaline solution but also in acid media because of the protection of the rGO layers. The superior performance of this catalyst is attributed to the synergy between the carbon layer and transition-metal phosphides. The Co₂P nanoparticles have a high degree of dispersion, which prevents agglomeration, thereby exposing more active sites. Moreover, rGO protects the exposed metal particles while providing more electroconductivity to the material. This work provides an efficient route for the development of bifunctional electrocatalysts with excellent performance and stability, which provides new ideas toward overall water splitting.

Insights into Correlation among Surface-Structure-Activity of Cobalt-Derived Pre-Catalyst for Oxygen Evolution Reaction

Rational design of unique pre-catalysts for highly active catalysts toward catalyzing the oxygen evolution reaction (OER) is a great challenge. Herein, a Co-derived pre-catalyst that allows gradual exposure of CoOOH that acts as the active center for OER catalysis is obtained by both phosphate ion surface functionalization and Mo inner doping. The obtained catalyst reveals an excellent OER activity with a low overpotential of 265 mV at a current density of 10 mA cm-2 and good durability in alkaline electrolyte, which is comparable to the majority of Co-based OER catalysts. Specifically, the surface functionalization produces lots of Co-PO4 species with oxygen vacancies which can trigger the surface self-reconstruction of pre-catalyst for a favorable OER reaction. Density functional theory calculations reveal that the Mo doping optimizes adsorption-free energy of *OOH formation and thus accelerates intrinsic electrocatalytic activity. Expanding on these explorations, a series of transition metal oxide pre-catalysts are obtained using this general design strategy. The work offers a fundamental understanding toward the correlation among surface-structure-activity for the pre-catalyst design.

Carbon-Based Nanomaterials as Sustainable Noble-Metal-Free Electrocatalysts

Nowadays, due to the worldwide growth demand of energy, over consumption of fossil fuel as well as their accompanying increased negative environmental impacts, the development of renewable energy systems, such as fuel cells and water electrolyzers, is becoming one of the "holy grail" for researchers. However, their large-scale applications have been severely limited by precious and unsustainable noble-metal electrocatalysts. Hence, it is highly desirable to develop robust electrocatalysts composed exclusively of low-cost and earth-abundant elements, to reduce or replace expensive and scarce noble-metals. Carbon-based nanomaterials, including heteroatoms-doped carbons and carbon-encapsulated metal materials, have recently attracted great interests because they show remarkable electrocatalytic performance and long-term stability for energy-related reactions, such as oxygen reduction reaction (ORR), hydrogen and oxygen evolution reactions (OER), hydrazine oxidation reaction (HzOR), etc. This review summarizes the recent progress in heteroatoms-doped carbon and carbon-encapsulated metal materials, highlighting the promise as cost-efficient electrocatalysts. Finally, a prospective on the future development of these promising materials is offered.

Carbon Derived from Soft Pyrolysis of a Covalent Organic Framework as a Support for Small-Sized RuO2 Showing Exceptionally Low Overpotential for Oxygen Evolution Reaction

Electrochemical water splitting is the most energy-efficient technique for producing hydrogen and oxygen, the two valuable gases. However, it is limited by the slow kinetics of the anodic oxygen evolution reaction (OER), which can be improved using catalysts. Covalent organic framework (COF)-derived porous carbon can serve as an excellent catalyst support. Here, we report high electrocatalytic activity of two composites, formed by supporting RuO2 on carbon derived from two COFs with closely related structures. These composites catalyze oxygen evolution from alkaline media with overpotentials as low as 210 and 217 mV at 10 mA/cm2, respectively. The Tafel slopes of these catalysts (65 and 67 mV/dec) indicate fast kinetics compared to commercial RuO2. The observed activity is the highest among all RuO2-based heterogeneous OER catalysts-a touted benchmark OER catalyst. The high catalytic activity arises from the extremely small-sized (∼3-4 nm) RuO2 nanoparticles homogeneously dispersed in a micro-mesoporous (BET = 517 m2/g) COF-derived carbon. The porous graphenic carbon favors mass transfer, while its N-rich framework anchors the catalytic nanoparticles, making it highly stable and recyclable. Crucially, the soft pyrolysis of the COF enables the formation of porous carbon and simultaneous growth of small RuO2 particles without aggregation.

Ruthenium Nanoparticles for Catalytic Water Splitting

Both global warming and limited fossil resources make the transition from fossil to solar fuels an urgent matter. In this regard, the splitting of water activated by sunlight is a sustainable and carbon-free new energy conversion scheme able to produce efficient technological devices. The availability of appropriate catalysts is essential for the proper kinetics of the two key processes involved, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). During the last decade, ruthenium nanoparticle derivatives have emerged as true potential substitutes for the state-of-the-art platinum and iridium oxide species for the HER and OER, respectively. Thus, after a summary of the most common methods for catalyst benchmarking, this review covers the most significant developments of ruthenium-based nanoparticles used as catalysts for the water-splitting process. Furthermore, the key factors that govern the catalytic performance of these nanocatalysts are discussed in view of future research directions.