Anodic Oxidation Reaction Research Articles

Single-metal-atom-anchored RuN2 monolayers: A high-performance electrocatalyst for alkaline hydrogen oxidation reactions

The development of alkaline exchange membrane fuel cells (AEMFCs) is severely hampered by the scarcity of efficient Pt-free catalysts for the anodic hydrogen oxidation reaction (HOR). In this work, we systematically explored the HOR performance of single 3d transition metal atom-doped RuN2 monolayers (TM-RuN2, TM = Ti-Zn) by density functional theory calculations. Formation energy, oxidation potential, and electronic structure analyses indicate that the Ti, V, Fe, Co, and Ni-doped RuN2 not only have a good conductivity, but also possess strong thermodynamic and electrochemical stability. For TM-RuN2, the ortho-N and metal (TM or ortho-Ru) atoms form a dual-active site for the adsorption of H* and OH*, respectively, where the adsorption strength is fine-tunable via the TM-modulated electronic structure. The HOR occurs preferentially through the H2 + OH*-H* + OH* mechanism on the pristine and Co, Ni, Cu, and Zn-doped RuN2, and the Tafel − H* + OH* mechanism on the Ti, V, Cr, Mn, and Fe-doped RuN2. The precisely balanced H* and OH* adsorption strength of Fe-RuN2 results in the ultra-low free energy barriers to HOR. Benefiting from the strong thermodynamic and electrochemical stability, good conductivity, and excellent HOR activity, Fe-RuN2 exhibits a huge potential as HOR electrocatalysts for AEMFCs.

Coupling Electrochemical Sulfion Oxidation with CO2 Reduction over Highly Dispersed p‐Bi Nanosheets and CO2‐Assisted Sulfur Extraction

AbstractWe report herein an electrocatalytic CO2 reduction‐coupled sulfion oxidation system for the co‐productions of valuable formate and sulfur at much enhanced atom utilization. Specifically, an organic ligand‐assisted two‐step reconstruction approach has been developed to fabricate the highly dispersed p‐Bi nanosheets (p‐Bi NSs) for cathodic CO2 reduction reaction (CO2RR), and meanwhile porous Co−S nanosheets (Co−S NSs) was applied for anodic sulfion oxidation reaction (SOR). Significantly high Faradaic Efficiencies of about 90 % for formate production by CO2RR in a wide potential range from −0.6 V to −1.1 V, and excellent SOR performances including an ultra‐low onset potential of about 0.2 V and recycle capacity of S2− in the 0.1 M and 0.5 M S2− solutions, have been simultaneously achieved. In the meantime, both the structure transformation of the catalysts and the reaction pathways are explored and discussed in detail. A two‐electrode CO2RR||SOR electrolyzer equipped with above electrocatalysts has been established, which features as low as about 1.5 V to run the electrolyzer at 100 mA cm−2, manifesting extremely lowered electricity consumption in comparison to conventional CO2RR system. Moreover, a sulfur separation approach has been proposed by using CO2, which is efficient, environmentally friendly and cost effective with value‐added NaHCO3 be obtained as the byproduct.

Coupling Electrochemical Sulfion Oxidation with CO2 Reduction over Highly Dispersed p-Bi Nanosheets and CO2 -Assisted Sulfur Extraction.

We report herein an electrocatalytic CO2 reduction-coupled sulfion oxidation system for the co-productions of valuable formate and sulfur at much enhanced atom utilization. Specifically, an organic ligand-assisted two-step reconstruction approach has been developed to fabricate the highly dispersed p-Bi nanosheets (p-Bi NSs) for cathodic CO2 reduction reaction (CO2 RR), and meanwhile porous Co-S nanosheets (Co-S NSs) was applied for anodic sulfion oxidation reaction (SOR). Significantly high Faradaic Efficiencies of about 90 % for formate production by CO2 RR in a wide potential range from -0.6 V to -1.1 V, and excellent SOR performances including an ultra-low onset potential of about 0.2 V and recycle capacity of S2- in the 0.1 M and 0.5 M S2- solutions, have been simultaneously achieved. In the meantime, both the structure transformation of the catalysts and the reaction pathways are explored and discussed in detail. A two-electrode CO2 RR||SOR electrolyzer equipped with above electrocatalysts has been established, which features as low as about 1.5 V to run the electrolyzer at 100 mA cm-2 , manifesting extremely lowered electricity consumption in comparison to conventional CO2 RR system. Moreover, a sulfur separation approach has been proposed by using CO2 , which is efficient, environmentally friendly and cost effective with value-added NaHCO3 be obtained as the byproduct.

Exploring hybrid seawater electrolysis with anodic oxidation reactions (AORs): recent progress and prospects

The hybrid seawater electrolysis with various anodic oxidation reactions has been reviewed.

Recent progress in energy-saving electrocatalytic hydrogen production via regulating the anodic oxidation reaction.

Hydrogen energy with its advantages of high calorific value, renewable nature, and zero carbon emissions is considered an ideal candidate for clean energy in the future. The electrochemical decomposition of water, powered by renewable and clean energy sources, presents a sustainable and environmentally friendly approach to hydrogen production. However, the traditional electrochemical overall water-splitting reaction (OWSR) is limited by the anodic oxygen evolution reaction (OER) with sluggish kinetics. Although important advances have been made in efficient OER catalysts, the theoretical thermodynamic difficulty predetermines the inevitable large potential (1.23 V vs. RHE for the OER) and high energy consumption for the conventional water electrolysis to obtain H2. Besides, the generation of reactive oxygen species at high oxidation potentials can lead to equipment degradation and increase maintenance costs. Therefore, to address these challenges, thermodynamically favorable anodic oxidation reactions with lower oxidation potentials than the OER are used to couple with the cathodic hydrogen evolution reaction (HER) to construct new coupling hydrogen production systems. Meanwhile, a series of robust catalysts applied in these new coupled systems are exploited to improve the energy conversion efficiency of hydrogen production. Besides, the electrochemical neutralization energy (ENE) of the asymmetric electrolytes with a pH gradient can further promote the decrease in application voltage and energy consumption for hydrogen production. In this review, we aim to provide an overview of the advancements in electrochemical hydrogen production strategies with low energy consumption, including (1) the traditional electrochemical overall water splitting reaction (OWSR, HER-OER); (2) the small molecule sacrificial agent oxidation reaction (SAOR) and (3) the electrochemical oxidation synthesis reaction (EOSR) coupling with the HER (HER-SAOR, HER-EOSR), respectively; (4) regulating the pH gradient of the cathodic and anodic electrolytes. The operating principle, advantages, and the latest progress of these hydrogen production systems are analyzed in detail. In particular, the recent progress in the catalytic materials applied to these coupled systems and the corresponding catalytic mechanism are further discussed. Furthermore, we also provide a perspective on the potential challenges and future directions to foster advancements in electrocatalytic green sustainable hydrogen production.

Critical Aspects to Select Suitable Substrate for Anodic Oxidation Reactions (AORs)

Hybrid water electrolysis (HWE) stands out as a promising avenue for simultaneously producing high-value-added chemicals and clean H2 fuel. In HWE, instead of the oxygen evolution reaction (OER) typical in...

Amorphous-crystalline RuTi nanosheets enhancing OH species adsorption for efficient hydrogen oxidation catalysis.

Anion exchange membrane fuel cells are a potentially cost-effective energy conversion technology, however, the electrocatalyst for the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics under alkaline conditions. Herein, we report that Ru-based nanosheets with amorphous-crystalline heterointerfaces of Ru and Ti-doped RuO2 (a/c-Ru/Ti-RuO2) can serve as a highly efficient HOR catalyst with a mass activity of 4.16 A mgRu-1, which is 19.8-fold higher than that of commercial Pt/C. Detailed characterization studies show that abundant amorphous-crystalline heterointerfaces of a/c-Ru/Ti-RuO2 nanosheets provide oxygen vacancies and unsaturated coordination bonds for balancing adsorption of hydrogen and hydroxyl species on Ru active sites to elevate HOR activity. Moreover, Ti doping can facilitate CO oxidation, leading to enhanced strength to CO poisoning. This work provides a strategy for enhancing alkaline HOR performance over Ru-based catalysts with heteroatom and heterointerface dual-engineering, which will attract immediate interest in chemistry, materials science and beyond.

Photovoltaic-driven Ni(ii)/Ni(iii) redox mediator for the valorization of PET plastic waste with hydrogen production.

Electrocatalytic valorization of PET plastic waste provides an appealing route by converting intermittent renewable energy into valuable chemicals and high-energy fuels. Normally, anodic PET hydrolysate oxidation and cathodic water reduction reactions occur simultaneously in the same time and space, which increases the challenges for product separation and operational conditions. Although these problems can be addressed by utilizing membranes or diaphragms, the parasitic cell resistance and high overall cost severely restrict their future application. Herein, we introduce a Ni(ii)/Ni(iii) redox mediator to decouple these reactions into two independent processes: an electrochemical process for water reduction to produce hydrogen fuel assisted by the oxidation of the Ni(OH)2 electrode into the NiOOH counterpart, followed subsequently by a spontaneous chemical process for the valorization of PET hydrolysate to produce formic acid with a high faradaic efficiency of ∼96% by the oxidized NiOOH electrode. This decoupling strategy enables the electrochemical valorization of PET plastic waste in a membrane-free system to produce high-value formic acid and high-purity hydrogen production. This study provides an appealing route to facilitate the transformation process of PET plastic waste into high-value products with high efficiency, low cost and high purity.

Electrocatalytic functional group conversion-based carbon resource upgrading.

The conversions of carbon resources, such as alcohols, aldehydes/ketones, and ethers, have been being one of the hottest topics most recently for the goal of carbon neutralization. The emerging electrocatalytic upgrading has been regarded as a promising strategy aiming to convert carbon resources into value-added chemicals. Although exciting progress has been made and reviewed recently in this area by mostly focusing on the explorations of valuable anodic oxidation or cathodic reduction reactions individually, however, the reaction rules of these reactions are still missing, and how to purposely find or rationally design novel but efficient reactions in batches is still challenging. The properties and transformations of key functional groups in substrate molecules play critically important roles in carbon resources conversion reactions, which have been paid more attention to and may offer hidden keys to achieve the above goal. In this review, the properties of functional groups are addressed and discussed in detail, and the reported electrocatalytic upgrading reactions are summarized in four categories based on the types of functional groups of carbon resources. Possible reaction pathways closely related to functional groups will be summarized from the aspects of activation, cleavage and formation of chemical bonds. The current challenges and future opportunities of electrocatalytic upgrading of carbon resources are discussed at the end of this review.

Evaluating the impact of anodic oxidation reactions on water splitting using Prussian blue analog-derived metal-(oxy)hydroxides

Prussian blue analog (PBA)-derived Fe–Co(O)OH showed improved anodic oxidation of a series of organic and inorganic compounds without the interference from the oxygen evolution reaction.

Coupling electrochemical CO2 reduction with value-added anodic oxidation reactions: progress and challenges

This review presents recent advances and future challenges of emerging and potential alternative anodic reactions with high energy-efficiency and value-added products in overall CO2 electrolysis.

Synergistic effects of CuS/TiO2 heterointerfaces: Enhanced cathodic CO2 reduction and anodic CH3OH oxidation for paired electrosynthesis of formate

The electrochemical reduction of carbon dioxide into energy-carrying compounds or value-added chemicals is of great significance for diminishing the greenhouse effect. However, it is still imperative to replace the less-value anodic oxygen evolution reaction (OER) to improve the technical economy. Herein, we firstly reported a bifunctional CuS/TiO2 catalyst for both anodic methanol oxidation reaction (MOR) and cathodic carbon dioxide reduction (CO2R). The in-built abundant CuS/TiO2 heterointerfaces are found to boost the CO2R and MOR to produce formate. Based on the unique bifunctionality of CuS/TiO2, a paired electrosynthesis of formate was performed with a total Faradaic efficiency (FE) of about 170 %, in which the cathodic CO2R achieved a formate FE of about 70 %, and the anodic MOR exhibited an almost 100 % formate FE.

Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst.

By substituting the oxygen evolution reaction (OER) with the anodic urea oxidation reaction (UOR), it not only reduces energy consumption for green hydrogen generation but also allows purification of urea-rich wastewater. Spin engineering of the d orbital and oxygen-containing adsorbates has been recognized as an effective pathway for enhancing the performance of electrocatalysts. In this work, we report the fabrication of a bifunctional electrocatalyst composed of amorphous RuO2-coated NiO ultrathin nanosheets (a-RuO2/NiO) with abundant amorphous/crystalline interfaces for hydrogen evolution reaction (HER) and UOR. Impressively, only 1.372 V of voltage is required to attain a current density of 10 mA cm-2 over a urea electrolyzer. The increased oxygen vacancies in a-RuO2/NiO by incorporation of amorphous RuO2 enhance the total magnetization and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics. The density functional theory study reveals that the amorphous/crystalline interfaces promote charge-carrier transfer, and the tailored d-band center endows the optimized adsorption of oxygen-generated intermediates. This kind of oxygen vacancy induced spin-polarized electrons toward boosting HER and UOR kinetics and provides a reliable reference for exploration of advanced electrocatalysts.

CuCo2O4@ACoNi-LDH (A=Fe, Cu, Zn, Cr) as efficient electrocatalyst for freshwater, seawater and urea oxidation reaction

Water electrolysis has a promising prospect in the production of green hydrogen energy, but its oxygen evolution reaction kinetics is relatively slow. Therefore, in this experiment, CuCo2O4@ACoNi-LDH (A = Fe, Cu, Zn, Cr) with heterogeneous core-shell catalytic material was firstly prepared by means of hydrothermal and calcination process, and its anodic oxidation reaction in freshwater, seawater and urea was investigated. In the oxygen evolution reaction (OER) of electrolytic water, CuCo2O4@CuCoNi-LDH displays the best catalytic activity. When the current density is 10 mA cm−2, only the overpotential of 199 mV is needed (potential of 1.429 V). The competitive oxidation and corrosion of chloride ions in seawater result in the decrease of catalytic activity. For urea oxidation reaction (UOR), CuCo2O4@FeCoNi-LDH displays the best catalytic performance, only potential of 1.292 V was required with the same current density, and the catalyst had good stability during the reaction for 12 h. Density functional theory calculation shows that CuCo2O4 plays an important role in the catalytic process. The synergistic catalysis of FeCoNi-LDH and CuCo2O4 results in the improvement of catalyst activity and stability. The CuCo2O4 material plays a key role in catalyzing urea splitting and the FeCoNi-LDH material protects CuCo2O4 from corrosion according to experimental results and density functional theory calculations. This experiment provides a new idea for the hydrogen production, exploration and the treatment of urea wastewater for transition metal oxides and layered hydroxides electrocatalyst with core-shell heterostructure.

A Universal Approach for Controllable Synthesis of Homogeneously Alloyed PtM Nanoflowers toward Enhanced Methanol Oxidation.

Platinum (Pt)-based alloys have received considerable attention due to their compositional variability and unique electrochemical properties. However, homogeneous element distribution at the nanoscale, which is beneficial to various electrocatalytic reactions, is still a great challenge. Herein, a universal approach is proposed to synthesize homogeneously alloyed and size-tunable Pt-based nanoflowers utilizing high gravity technology. Owing to the significant intensification of micro-mixing and mass transfer in unique high gravity shearing surroundings, five typical binary/ternary Pt-based nanoflowers are instantaneously achieved at room temperature. As a proof-of-concept, as-synthesized Platinum-Silvernanoflowers (PtAg NFs) demonstrate excellent catalytic performance and anti-CO poisoning ability for anodic methanol oxidation reaction with high mass activity of 1830mA mgPt -1, 3.5 and 3.2 times higher than those of conventional beaker products and commercial Pt/C, respectively. The experiment in combination with theory calculations suggest that the enhanced performance is due to additional electronic transmission and optimized d-band center of Pt caused by high alloying degree.

Recent Advancements in Electrochemical Hydrogen Production via Hybrid Water Splitting.

As one of the most promising approaches to producing high-purity hydrogen (H2 ), electrochemical water splitting powered by the renewable energy sources such as solar, wind, and hydroelectric power has attracted considerable interest over the past decade. However, the water electrolysis process is seriously hampered by the sluggish electrode reaction kinetics, especially the four-electron oxygen evolution reaction at the anode side, which induces a high reaction overpotential. Currently, the emerging hybrid electrochemical water splitting strategy is proposed by integrating thermodynamically favorable electro-oxidation reactions with hydrogen evolution reaction at the cathode, providing a new opportunity for energy-efficient H2 production. To achieve highly efficient and cost-effective hybrid water splitting toward large-scale practical H2 production, much work has been continuously done to exploit the alternative anodic oxidation reactions and cutting-edge electrocatalysts. This review will focus on recent developments on electrochemical H2 production coupled with alternative oxidation reactions, including the choice of anodic substrates, the investigation on electrocatalytic materials, and the deep understanding of the underlying reaction mechanisms. Finally, some insights into the scientific challenges now standing in the way of future advancement of the hybrid water electrolysis technique are shared, in the hope of inspiring further innovative efforts in this rapidly growing field.

Assembly of two trinuclear-cluster-based metal-organic frameworks: Structures and methanol electro-oxidation properties

The direct methanol fuel cell (DMFC) is an efficient and non-polluting green energy conversion device. However, the electrocatalysts for the anodic methanol oxidation reaction (MOR) in DMFC are often precious metal catalysts, which suffer from high cost, poor stability, and easy deactivation. In this work, two isostructural metal-organic frameworks (MOFs) [M3(μ3-OH)(TATB)(Nic)2(H2O)2(DMA)] (M = Co, Ni) were successfully synthesized under solvothermal conditions. Both frameworks possess a unique trinuclear cluster [M3(μ3-OH)(COO)5] secondary building unit (SBU) and a rare binodal (3,7)-connected network. Due to the high density of metal sites and prominent pore system within CTGU-36-Ni, it can serve as an effective molecular electrocatalyst for MOR, while CTGU-36-Co is an inert material. Further experimental results showed that the introduction of conductive substances could lead to a significant increase in the MOR area activity of CTGU-36-Ni. This work not only provides a non-noble metal molecular MOR electrocatalyst but also opens a new vision for the design and application of crystalline materials in various energy conversion processes.

Improved DMFC water management via superwetting, high-performance catalyst synthesized by nanocapsule method

In this paper, an effective water management structure for direct methanol fuel cell (DMFC) is designed based on the super-hydrophilicity of nitrogen-doped carbon aerogel which functions as the support of Pt catalyst. Considering the case where Pt nanoparticles deposited into the nanopores of the support are difficult to contact with the ionomer, the catalyst is synthesized by nanocapsule method, and the experimental results prove that a favorable improved oxygen reduction reaction activity of the prepared catalyst has been achieved. The produced hydrophilic catalyst is used to construct a two-layered cathode catalyst layer (CCL) with hydrophilic inner layer and hydrophobic outer layer as water management structure, since a single-layered hydrophilic CCL shows poor performance due to serious water flooding. A mathematical model is developed to study the influence of two-layered CCL, and its effect on cell performance is also analyzed by polarization tests. For the improved liquid-feed DMFCs, high hydraulic pressure is formed in CCL to promote water recovery and inhibit methanol crossover. For the improved vapor-feed DMFC, the enhanced water recovery secures the water required for the anodic oxidation reaction, which increases the output performance and stability greatly. This study provides a new method for the optimization of DMFC.

Impact of Anodic Oxidation Reactions in the Performance Evaluation of High-Rate CO2 /CO Electrolysis.

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO2 /CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, several impacts of anodic oxidation of liquid products in terms of performance evaluation are investigated. First, the crossover behavior of several typical liquid products through an anion-exchange membrane is analyzed. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the Faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, a theoretical guideline with hierarchical indicators is further developed to predict and regulate the possible AORs in an electrolyzer. The perspective concludes by giving some suggestions on rigorous performance evaluations for high-rate CO2 /CO electrolysis in an MEA-based setup.

Amino-doping regulates the surface of nickel-based catalysts to promote the formation of the real active states

The generation of the real active states (NiOOH) on the nickel-based catalysts’ surface is a crucial step for anodic oxidation reactions. However, achieving a sufficient number of active states on nickel-based electrocatalysts surface remains a significant challenge. In this study, we propose an effective surface modification strategy using N-contained species to regulate the surface of nickel-based catalysts, and successfully construct a stable and efficient bifunctional electrocatalyst namely, N-doped-Ni(OH)2/NF (noted as NDN/NF). The NDN/NF catalyst shows an outstanding performance, with a current density of 10 mA cm−2 achievable at a voltage of only 1.35 V (vs. RHE) in ethanol oxidation reaction (EOR), outperforming most nickel-based catalysts. The catalyst also exhibits exceptional stability with a constant overpotential during the 20 h chronopotentiometry (CP) tests. The formation of active states is promoted by the shortened Ni–O bond resulting from the coordination deficiency of nickel, as confirmed by density functional theory (DFT) calculations.