Half-reaction Research Articles

Recent Advances in Electrochemical, Ni‐Catalyzed C−C Bond Formation

AbstractNickel‐catalyzed cross‐electrophile coupling (XEC) is an efficient method to form carbon‐carbon bonds and has become an important tool for building complex molecules. While XEC has most often used stoichiometric metal reductants, these transformations can also be driven electrochemically. Electrochemical XEC (eXEC) is attractive because it can increase the greenness of XEC and this potential has resulted in numerous advances in recent years. The focus of this review is on electrochemical, Ni‐catalyzed carbon‐carbon bond forming reactions reported since 2010 and is categorized by the type of anodic half reaction: sacrificial anode, sacrificial reductant, and convergent paired electrolysis. The key developments are highlighted and the need for more scalable options is discussed.

Low-Pt Octahedral PtCuCo Nanoalloys: "One Stone, Four Birds" for Oxygen Reduction and Methanol Oxidation Reactions.

To find a low-Pt electrocatalyst that is functionally integrated and superior to the state-of-the-art single-Pt electrocatalyst is expectedly a challenge. We have in this study found that the reactivity of the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR), in both acidic and alkaline electrolytes (viz., four half-cell reactions), can be modified and greatly enhanced by the electronic and/or synergistic effects of a low-Pt octahedral PtCuCo alloy. For the ORR, the mass activity (MA) of Pt0.23Cu0.64Co0.13/C in an acidic or alkaline electrolyte was 14.3 or 10.7 times that of the commercial Pt/C. For the MOR, the MA of Pt0.23Cu0.64Co0.13/C in an acidic or alkaline electrolyte was 7.2 or 3.4 times that of the commercial Pt/C. In addition, Pt0.23Cu0.64Co0.13/C exhibited an increased durability and CO tolerance, as compared with the commercial Pt/C. Density functional theory calculations demonstrated that the PtCuCo(111) surface can effectively optimize the O* binding energy. This work has successfully shown an example of how both acidic and alkaline ORR and MOR activities can be significantly synchronously enhanced.

Molecular engineering of carbon nitrides for overall photosynthesis of H2O2

Artificial photocatalysis offers a promising strategy to sustainably produce hydrogen peroxide (H2O2) that is one of the most valuable multifunctional chemicals. Among various photocatalysts, polymeric carbon nitride (pCN) has drawn continuous attention in non-sacrificial H2O2 production. However, the poor activity of half reactions, i.e., the oxygen reduction reaction (ORR) and water oxidation reaction (WOR), greatly restricts the efficiency of photocatalytic H2O2 production. In this highlight, we discuss the significant advances in molecular engineering of carbon nitrides for H2O2 photosynthesis and the importance of the deep understanding of the photocatalysis process for rational design and reaction pathways of organic conjugated polymers to address the growing H2O2 demand. Furthermore, we summarize the emerging applications of photocatalytic H2O2 productions beyond energy and environment.

Metal-organic framework-derived boron-doped iron-cobalt tannate nanoparticles as a high-efficiency electrocatalyst for oxygen evolution reaction

Electrocatalytic oxygen evolution reaction (OER) is an important half reaction for energy conversion and storage, which possesses low OER efficiency due to the multistep proton-coupled electron transfer. Herein, B-doped iron-cobalt tannate nanoparticles (B-doped FeCoTA) are synthesized by one-step hydrothermal method. It is proved that the optimized B-doped FeCoTA exhibits excellent electrocatalytic performance for OER with a low overpotential of 306 mV at a current density of 10 mA cm−2, a small Tafel slope of 28 mV dec−1, a high double-layer capacitance of 6.31 mF cm−2 and excellent OER stability with a low potential of 1.55 V (vs RHE) at 10 mA cm−2, which is ascribed to the synergistic effect between bimetallic and heteroatom doping. The B incorporation can greatly enhance the electrical conductivity between metals, facilitate electron transfer and material exchange, thus greatly reducing reaction dynamics and strengthening OER performance. This work can provide a good strategy for the design and development of new efficient and stable OER electrocatalysts.

Synthesis and Electrocatalytic Performance of TiO2 Supported on Carbon Nanoparticles Towards Positive Half-Cell Reaction of Hydrogen-1,4p-benzoquinone Flow-Battery

The Hydrogen- 1,4 p-Benzoquinone redox flow battery (H2—BQ RFB) is simple and economic to use with a cell potential of 0.714 V. Carbon-based electrodes are extensively used as electrode materials due to their comprehensive properties but possess poor hydrophilic nature and low electrochemical activity. It is essential to modify carbon-based materials before employing them in battery applications. Because of its low-cost and enhanced catalytic activity, Titanium dioxide (TiO2) is chosen to modify the carbon material. In the present work, TiO2 supported on carbon nanoparticles (TiO2@CNP) electrocatalyst was synthesized by using the ultra-sound assisted sol-gel method for the positive half-cell of H2—BQ RFB. The composition of the sol is optimized by using cyclic voltammetry (CV) analysis. Surface morphology TiO2@CNP coated on carbon paper was carried out using Scanning Electron Microscope (SEM). The TiO2 and carbon interaction bonds were identified using Fourier transform infrared spectroscopy (FTIR). The phase and crystalline nature were identified by using X-ray diffraction (XRD). The activity of the electrode was assessed by CV and Tafel analysis. The electrocatalyst was tested in H2—BQ RFB positive half-cell by galvanostatic charge-discharge and obtained energy efficiency up to 73%.

Revealing promotion of interfacial Pt–O–Fe oxygen bridge structure in PtFe/graphene synthesized by a four-electrode electrochemical system on water splitting

To promote kinetic rate of half reactions in water splitting, we synthesized a bifunctional PtFe/graphene with dual active sites by a facile self-designed four-electrode electrochemical method. Characterization data indicate interfacial oxygen bridge structure of Pt–O–Fe is expectedly formed and electrons transfer from Fe to Pt via this bridge structure. This unique interfacial structure not only regulates electronic structure of Pt and Fe, but also offers interfacial electron-transfer channel to improve overall conductivity, which essentially enhances intrinsic activity towards acidic HER and alkaline OER. For HER, the best Pt–Fe/G shows the mass activity of 4239.3 mA mg−1 at −0.063 V, 3.79 times higher than that of Pt/G (1118.5 mA mg−1) and 5.02 times higher than that of commercial Pt/C (843.1 mA mg−1) catalyst, respectively. Introduction of Fe to Pt decreases the energy barrier of HER process (ΔG∗), and results in conversion of rate-limiting step from H+ adsorption (Volmer step) to H∗ desorption (Tafel step). For OER, the greatest PtFe/G exhibits an overpotential of lower 26 mV (10 mA cm-2) than commercial RuO2 and its mass activity is 16.7 times higher than Fe/G. The formed Pt–O–Fe bond effectively elevates the electrophilic properties and facilitates adsorption of OH−, contributing to the change of rate-limiting step and enormous decrease of reaction activation energy (Ea). This work inspires researchers to construct efficient interfacial oxygen bridge structure to enhance electrocatalytic performance for water splitting and design novel electrochemical method for synthesis of graphene-based composites.

In-situ growth of nonstoichiometric CrO0.87 and Co3O4 hybrid system for the enhanced electrocatalytic water splitting in alkaline media

The development of electrocatalysts for electrochemical water splitting has received considerable attention in response to the growing demand for renewable energy sources and environmental concerns. In this study, a simple hydrothermal growth approach was developed for the in-situ growth of non-stoichiometric CrO0.87 and Co3O4 hybrid materials. It is apparent that the morphology of the prepared material shows a heterogeneous aggregate of irregularly shaped nanoparticles. Both CrO0.87 and Co3O4 have cubic crystal structures. Its chemical composition was governed by the presence of Co, Cr, and O as its main constituents. For understanding the role CrO0.87 plays in the half-cell oxygen evolution reaction (OER) in alkaline conditions, CrO0.87 was optimized into Co3O4 nanostructures. The hybrid material with the highest concentration of CrO0.87 was found to be highly efficient at driving OER reactions at 255 mV and 20 mA cm−2. The optimized material demonstrated excellent durability for 45 h and a Tafel slope of 56 mV dec−1. Several factors may explain the outstanding performance of CrO0.87 and Co3O4 hybrid materials, including multiple metallic oxidation states, tailored surface properties, fast charge transport, and surface defects. An alternative method is proposed for the preparation of new generations of electrocatalysts for the conversion and storage of energy.

Exploring hydrogen production using platinum catalysts

Recent developments illustrate the potential for platinum to improve the half-cell hydrogen evolution reaction.

Revealing the Efficient and Constant Photon Utilization of Photocatalytic Fixed Bed Reaction.

Because of the very high potential barrier, semiconductor-co-catalyst interfacial electron transfer occurring on a second time scale severely slows the photocatalytic reaction. Besides, the undesired deprivation of electrons from the co-catalyst by photogenerated oxidative intermediates in a photocatalytic slurry suspension further lowers the light-intensity-dependent photon utilization. We demonstrate here that photocatalyst immobilization can flatten the potential barrier and increase the selectivity of electrons for the target reaction. This is because the induced spatial separation of half reactions in the formed fixed bed reactors can suppress the loss of photogenerated charge carriers and increase the density of electrons in the semiconductor. The photocatalytic fixed bed reaction thus exhibits efficient and constant photon utilization.

Highly active macrocyclic nickel(II) complex for hydrogen evolution reaction in neutral aqueous conditions

Hydrogen (H2) production via water splitting under completely aqueous conditions particularly using an earth-abundant component for both half reactions is very appealing and may reduce our dependence on fossil fuels. In this work, we present a water-soluble molecular nickel(II) complex bearing a macrocyclic N‒heterocyclic carbene (NHC)/pyridine hybrid ligand, ([NiL](PF6)2 (L = bis(2‒pyridyl-methylimidazolylidene)methane as a catalyst for hydrogen evolution reaction (HER) under completely aqueous conditions. The catalyst displayed an outstanding catalytic activity for HER in a phosphate electrolyte of neutral pH, as established by a series of techniques. Long-term (11 h) bulk electrolysis at 0.88 V overpotential provides a maximum of 173.5C charge and total ca. 165.08 μmol (∼15 μmol per hour) H2 with Faradaic efficiency of 98%, while without a catalyst less than 3C was produced within the same time. To the best of our knowledge, the investigation of water reduction with Ni–NHC type catalyst under aqueous conditions has not been reported to date.

Development of interdigitated electrode based enzyme sensor for triglyceride monitoring using engineered oxygen insensitive glycerol 3-phosphate oxidase

Triglycerides (TG) are an important biomarker of various diseases including hyper-lipidemia. Current emergent requirements for TG sensing include the development of electrochemical sensors for easy, cost-effective, disposable and single use triglyceride sensors. We propose a TG sensor using an engineered glycerol 3-phosphate oxidase (GlpO) and electrochemical detection. In this study, we aimed to develop an electrochemical TG sensor based on engineering the oxidative half reaction of GlpO to be oxygen insensitive, with the employment of an interdigitated electrode (IDE). To construct an oxygen insensitive mutant GlpO, we constructed a double mutant, Met104Phe/Asp169Glu, which revealed an engineered GlpO exhibiting repressed oxidase activity, while its dye-mediated dehydrogenase activity enhanced. The electrochemical enzyme sensor was constructed for the measurement of TG based on engineered GlpO using disposable screen-printed carbon electrode (SPCE) or disposable IDE using Ru complex as the electron mediator. By using the disposable IDE sensor, a highly sensitive electrochemical sensor for Glp was achieved, with the sensitivity of 0.4 µA/mM. The improved sensitivity was 2.4 times higher than that of SPCE (0.17 µA/mM), with LOD of 0.022 mM. Furthermore, the dry sensor strip, where the double mutant GlpO and Ru complex were dropcasted and dried on the IDE, was prepared and the Glp concentration was successfully measured in the delipidized serum solution. Therefore, by the employment of the established pretreatment method for TG to release Glp, a highly sensitive, accurate, easy and rapid electrochemical measurement method for TG will be developed using constructed M104F/D169E mutant GlpO with the combination of IDE.

Half-Electrolysis of Water with the Aid of a Supercapacitor Electrode.

Half-electrolysis runs one desirable half-cell reaction with the aid of a counter supercapacitor electrode which replaces the other unwanted half-cell reaction occurred inevitably in conventional electrolysis. Herein, it is developed to complete the whole cell reaction of water electrolysis, in alternative steps, with a capacitive activated carbon (AC) electrode and an electrolysis Pt electrode. When positively charging the AC electrode, a hydrogen evolution reaction occurs at the Pt electrode. By reversing the current, the charge stored in the AC electrode is discharged to assist the oxygen evolution reaction on the same Pt electrode. Consecutive completion of the two processes realizes the overall reaction of water electrolysis. This strategy leads to stepwise production of H2 and O2 without the need of a diaphragm in the cell and hence results in a lower energy consumption compared with the practical conventional electrolysis.

Critical Role of Interface Design in Acceleration of Overall Water Splitting and Hybrid Electrolysis Process: State of the Art and Perspectives

Electrocatalytic water splitting to hydrogen (H2) is an ideal approach to generate renewable energy. One of the major drawbacks is the tightly coupled kinetically sluggish and energy inefficient anodic oxygen evolution reaction (OER) with hydrogen forming a cathodic half-cell reaction which leads to a significant reduction in overall cell efficiency. In this context, before reviewing the literature, we have first briefly analyzed the energetics of overall water splitting, problems, and challenges under different pH conditions which can be useful for the further understanding of the process. Replacement of the anodic OER by a thermodynamically favorable substrate oxidation offers flexibility, value addition, and energy efficiency in the case of hybrid or assisted water electrolysis to afford hydrogen. Recent progress in terms of sacrificial oxidants in hybrid water electrolysis are discussed in this context, where the sacrificial oxidants are so chosen that the oxidation often leads to its value addition. Also here, we have offered insights into interface designing in heterostructures by modulating chemical and electronic environments for the enhancement of the intrinsic catalytic activity and stability. The effect of incorporation of such materials into the overall water splitting reaction, their catalytic active sites, and interactions with intermediates are thoroughly explored. This review can be a good complement for better understanding of the elucidation of the interface role in hybrid water electrolysis for future commercial applications.

Uranium Phthalocyanine-Anchored Acid-Functionalized Multiwalled Carbon Nanotubes for Water Electrolysis

One of the major challenges in the commercial production of hydrogen and oxygen from the electrochemical water splitting reaction is the nonavailability of potential and low-cost electrocatalysts for the enhancement of both the half-cell reactions. The bifunctional catalyst capable of demonstrating lower overpotentials for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) eliminates the usage of a membrane, simplifies the overall system design, reduces the cost, and enhances the electrochemical water splitting activity. Here, a bifunctional hybrid composite catalyst comprising an organic N4 macrocycle and acid-functionalized multiwalled carbon nanotubes (MWCNTs) is fabricated and tested for the water splitting reaction to produce H2 and O2. The uranium tetra[4-(2-{(E)-[(4-bromophenyl)imino]methyl}phenoxy)]phthalocyanine (UTBrImPc) is synthesized via a two-step process of imine and oxy-bridge linkage formation. The synthesized ligands and phthalocyanine macrocycle are characterized using various spectroscopic and analytical techniques. The glassy carbon electrode (GCE) and Ni foam are used as the conducting substrate for the fabrication of the UTBrImPc/MWCNT bifunctional electrode to generate H2 and O2. Surprisingly, the fabricated bifunctional hybrid catalyst on the GCE exhibited a lesser overpotential of 15 mV for the HER, which is close to that of the benchmark Pt/C catalyst. Furthermore, the Ni/UTBrImPc/MWCNT displayed a lower overpotential of 368 (±2) mV at a current density of 10 mA·cm–2 for the OER, which is close to the overpotential shown by the precious benchmark catalyst IrO2. In addition, the fabricated electrodes showed remarkable long-term stability for more than 20 h by a chronoamperometric method. The superior results for both the HER and OER at the composite organic hybrid catalyst may be due to the collusive effect of the acid-functionalized MWCNTs with UTBrImPc, which enhances the electronic conductivity and surface area. The developed state-of-the-art catalyst can be employed as a bifunctional catalyst in water electrolyzers.

Synthesis and Magnetic, Optical, and Electrocatalytic Properties of High-Entropy Mixed-Metal Tungsten and Molybdenum Oxides

High-entropy oxides (HEOs) are of interest for their unique physical and chemical properties. Significant lattice distortions, strain, and tolerance for high-vacancy concentrations set HEOs apart from single-metal or mixed-metal oxides. Herein, we synthesized and characterized the structures and compositions, along with the optical, magnetic, and electrocatalytic properties, of two families of high-entropy mixed-metal tungsten and molybdenum oxides, AWO4 and B2Mo3O8, where A and B are 3d transition metals. The HEOs A6WO4 (A = Mn, Fe, Co, Ni, Cu, and Zn) and B25Mo3O8 (B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO4 and B2Mo3O8 parent compounds, were synthesized using high-temperature solid-state methods. X-ray photoelectron spectroscopy analysis of the surfaces revealed that the HEOs largely had the metal oxidation states expected from the bulk chemical formulas, but in some cases they were different than in the parent compounds. A6WO4 exhibited antiferromagnetic (AFM) ordering with a Néel temperature of 30 K, which is less than the average of its AFM parent compounds, and had a narrow band gap of 0.24 eV, which is much lower than all of its parent compounds. B25Mo3O8 was paramagnetic, despite the existence of AFM and ferromagnetic ordering in several of its parent compounds and had no observable band gap, which is analogous to its parent compounds. Both A6WO4 and B25Mo3O8 exhibited superior catalytic activity relative to the parent compounds for the oxygen evolution reaction, the oxidation half reaction of overall water splitting, under alkaline conditions, based on the overpotential required to reach the benchmark surface area normalized current density. Consistent with literature predictions of OER durability for ternary tungsten and molybdenum oxides, A6WO4 and B25Mo3O8 also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.

Cobalt decorated graphitic carbon nitride photoanode for electrochemical ethanol oxidation: A sustainable way towards clean energy

Energy conversion devices based on liquid fuels have gained considerable attention in recent times to meet the increasing global demand for energy. In this work, graphitic carbon nitride (gCN) nanosheets have been synthesized by pyrolysis of urea and Co has been decorated in different molar ratios over its surface by the solution phase method. The prepared catalysts have been utilized for photo electrooxidation of ethanol, an anodic half-cell reaction in direct ethanol fuel cells. Electrochemical studies show that the catalyst containing 3 mol % of Co shows the best activity with a peak current density of 6.91 mA/cm2 obtained at a peak potential of 0.28 V with maximum current density of 40 mA/cm2. The effect of light on the catalytic activity has also been studied. On illuminating the surface of the electrode with light, an increment of 85% in current density is observed which indicates higher charge transfer that enhanced the photoactivity of the catalyst. This study confirms the practical applicability of the non-expensive carbonaceous material Co–C3N4 utilization as a photoanode in future energy systems.

Simultaneous Anodic and Cathodic Formate Production in a Paired Electrolyzer by CO2 Reduction and Glycerol Oxidation.

Electrochemical CO2 conversion is a key technology to promote the production of carbon-containing molecules, alongside reducing CO2 emissions leading to a closed carbon cycle economy. Over the past decade, the interest to develop selective and active electrochemical devices for electrochemical CO2 reduction emerged. However, most reports employ oxygen evolution reaction as an anodic half-cell reaction causing the system to suffer from sluggish kinetics with no production of value-added chemicals. Therefore, this study reports a conceptualized paired electrolyzer for simultaneous anodic and cathodic formate production at high currents. To achieve this, CO2 reduction was coupled with glycerol oxidation: a BiOBr-modified gas-diffusion cathode and a Nix B on Ni foam anode keep their selectivity for formate in the paired electrolyzer compared to the half-cell measurements. The paired reactor here reaches a combined Faradaic efficiency for formate of 141 % (45 % anode and 96 % cathode) at a current density of 200 mA cm-2 .

Role of ZSM-5/AC hybrid support on the catalytic activity of Pd-Ag electrocatalysts towards ethanol oxidation: An experimental and kinetic study

Ethanol is a promising future fuel for Direct Alcohol Fuel Cells (DAFCs) and one of the significant half-cell reactions of this type of FC is ethanol oxidation reaction (EOR). However, the high cost, low selectivity of the catalyst towards complete oxidation of ethanol to CO2, and poor electrode kinetics are the key challenges in commercializing this technology. The way to mitigate these issues is to design affordable, stable, and highly efficient electrocatalysts. In this prospect, we developed a zeolite-carbon (ZSM-5/AC) supported Pd-Ag binary alloy catalysts via a simple co-impregnation followed by co-reduction method. The influence of ZSM-5/AC composite support and its composition on the activity of bimetallic Pd-Ag catalyst has been studied in detail. The PdAg/ZSM-5/AC electrocatalyst with 1:3 wt ratio of ZSM-5:AC exhibited better catalytic performance (10.9 mA cm−2) and tolerance towards intermediate species than PdAg/AC and Pd/AC electrocatalysts. The larger jf/jb (forward to backward current density ratio) of 2.8 for PdAg/ZSM-5/AC indicates lower carbonaceous species accumulation on the zeolite-modified electrode. Additionally, the material possessed a good long-term cyclic stability up to 500 cycles. The high activity observed for PdAg/ZSM-5/AC could be attributed to synergism between bimetallic catalysts and high surface area zeolite support. Overall, the study reveals that the Pd-Ag bimetallic and ZSM-5/AC are potentially efficient electrocatalyst and support, respectively, for the ethanol oxidation reaction in DEFC application.

Self-sacrifice Stainless Steel Mesh derived FeNi-MOF Materials as Water Splitting Electrocatalyst

The oxygen evolution on the anode electrode, the most prominent half-cell reaction all through the water-splitting procedure, is primarily hampered and limited by sluggish dynamics and low reaction efficiency. This work provides an interesting research idea. By utilizing recycled resources as precursors, followed by in-situ hydrothermal method, micron-sized single Metal-organic frameworks (MOF) crystals were rapidly prepared. The as-obtained MOF crystals FeNiBDC exhibited an improving electrocatalytic OER performance with a small overpotential of 334 mV (η10: 10 mA cm−2), a Tafel slope of 100.2 mV dec−1, and a relatively low resistance of 8.13 Ω, compared with the precursor of stainless-steel mesh.

Lattice-disordered high-entropy metal hydroxide nanosheets as efficient precatalysts for bifunctional electro-oxidation

Electro-oxidation reactions (EORs) are important half reactions in overall and assisted water electrolysis, which are crucial in achieving economic and sustainable hydrogen production and realizing simultaneous wastewater treatment. Current studies indicate that the high-valence metal ions that are locally enriched in the catalysts or generated in situ during the anodic preoxidation process are active species for EORs. Hence, designing (pre)catalysts with enriched local active sites and boosted preoxidation is of great importance. In this work, with a focus on improving the EOR performance toward the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR), we fabricated a lattice-disordered high-entropy FeCuCoNiZn hydroxide nanoarray catalyst that exhibits robust bifunctional OER and UOR behavior. The high-entropy feature could bring in a unique catalytic ensemble effect and remarkably improve the intrinsic OER/UOR activity. The lattice-disordered structure could not only enrich the local high-valence metal ions as active sites but also provide abundant reactive surface sites to accelerate the preoxidation process, thus leading to enriched active sites for the OER and UOR. Benefitting from the structural merits, the lattice-disordered high-entropy catalyst exhibits excellent OER and UOR activity with low overpotential, large current density and enhanced intrinsic activity, and no performance degradation but dramatic 35.3% and 88.7% enhancement in activity can be achieved during the long-term OER and UOR tests, respectively. The robust OER and UOR performance makes the lattice-disordered high-entropy catalyst a promising candidate for overall and urea-assisted water electrolysis from industrial, agricultural and sanitary wastewater.