Controlled Potential Electrolysis Research Articles

Heterometallic Transition Metal Oxides Containing Lewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide to Carbon Monoxide with Bimodal Activity.

Electrocatalytic CO2 reduction (e-CO2RR) to CO is replete with challenges including the need to carry out e-CO2RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO2 to CO with a very high faradaic efficiency albeit at -2.5 V versus Fc/Fc+. It is now demonstrated that introducing a nonredox metal Lewis acid, preferably GaIII, as a binding site for CO2 in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCuIIFeIIIGaIII(H2O)3W9O37]8-, leads to bimodal activity optimal both at -2.5 and -1.5 V versus Fc/Fc+; reactivity at -1.5 V being at an overpotential of ∼150 mV. These results were observed by cyclic voltammetry and quantitative controlled potential electrolysis where high faradaic efficiency and chemoselectivity were obtained at -2.5 and -1.5 V. A reaction with 13CO2 revealed that CO2 disproportionation did not occur at -1.5 V. EPR spectroscopy showed reduction, first of CuII to CuI and FeIII to FeII and then reduction of a tungsten atom (WVI to WV) in the polyoxometalate framework. IR spectroscopy showed that CO2 binds to [SiCuIIFeIIIGaIII(H2O)3W9O37]8- before reduction. In situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with pulsed potential modulated excitation revealed different observable intermediate species at -2.5 and -1.5 V. DFT calculations explained the CV, the formation of possible activated CO2 species at both -2.5 and -1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO2 binding steps, the active site for reduction, and the role of protons in facilitating the reactions.

Diiron Complexes with Rigid and Conjugated S-to-S Bridges for Electrocatalytic Reduction of CO2.

Electroreduction of CO2 to value-added low-carbon chemicals is a promising way for carbon neutrality and CO2 utilization. It was found that the diiron complex [(μ-bdt)Fe2(CO)6] (bdt = benzene-1,2-dithiolate) has high catalytic activity for electrocatalytic CO2 reduction. To further study the effect of the S-to-S bridge on the catalytic performances of diiron complexes for electrochemical CO2 reduction, four diiron complexes 1-4 with different rigid and conjugated S-to-S bridges were either selected or designed. The electrocatalytic studies showed that under optimal conditions, 2 with a 2,3-naphthalenedithiolato bridge exhibited the lowest catalytic onset potential (Eonset = -1.75 V vs Fc+/0), while 4 with a diphenyl-1,2-vinylidene bridge displayed the highest catalytic activity (TOFmax = 295 s-1), which is 1.5 times that of [(μ-bdt)Fe2(CO)6]. The controlled potential electrolysis experiments of 4 in 0.1 M MeOH/MeCN at -2.35 V vs Fc+/0 gave a total faradaic yield close to 100%, with selectivities of 77%, 9%, and 14% for HCOOH, CO, and H2, respectively. The mechanism for CO2 reduction was studied using density functional theory, IR spectroelectrochemistry, and electrochemical methods. The results indicate that modifying the structure of the S-to-S bridge is an effective strategy to improve the catalytic performance of diiron complexes for electrocatalytic CO2 reduction.

Bio-Inspired histidine-based copper complex: An efficient and robust electrocatalyst for electrochemical hydrogen evolution

The quest for an efficient and sustainable electrocatalyst for hydrogen evolution reaction (HER) using earth-abundant elements is crucial for cost-effective and environmentally friendly hydrogen production. While many catalysts show good activity and stability in strongly basic conditions, fewer materials are effective under neutral conditions, which are more practical for electrolyzer operations. This study introduces a dinuclear copper catalyst, [Cu2(μ-OH) (μ-NO3) (L)2], where L is the Schiff base ligand derived from salicylaldehyde and histidine amino acid (Complex 1), for HER in both aqueous and non-aqueous environments. In non-aqueous conditions, with acetic acid as a proton source, the catalyst exhibits outstanding HER performance, achieving an overpotential of 170 mV. At an acid concentration of 0.086 M, it achieves a catalytic current of ∼17.5 mA cm−2 at an overpotential of 0.52 V. Remarkably, in fully aqueous neutral conditions, Complex 1 reaches a significant current density with an ic/ip ratio of ∼82 at a scan rate of 1.0 V s−1, and an apparent rate constant (kobs) for electrocatalytic HER of ∼13,200 s−1. The catalyst also shows excellent activity in 0.1 M H3PO4, with low overpotentials of ∼150 mV at 2.0 mA cm⁻2 and 270 mV at 10.0 mA cm⁻2. In controlled potential electrolysis (CPE), the catalyst maintains stable catalytic currents of 25 mA cm⁻2 and 75 mA cm⁻2 for 18–28 h at a potential of 0.590 V vs. RHE in neutral and acidic conditions, respectively, without noticeable decrease in catalytic current. Detailed characterizations confirm the molecular integrity of the catalyst and its predominantly homogeneous catalytic mechanism.

Pincer shaped N-Mesoionic Pyridylidene Amide (PYA) for palladium catalyzed CO2 reduction

Palladium complexes (4-6) bearing mesoionic N-donor pyridylidene amide ligands (PYAs) with different pendant groups of varying size and electronic properties were successfully synthesized from their respective pro-ligands (1-3). 4-6 were proved effective catalysts for CO2 reduction to synthesis gas. The first principle quantum studies of CO2 reduction provided better insight into the mechanism of these active catalysts. The highest selectivity of 71 % (Faradaic efficiency) and minimum overpotential of 0.390 V was observed for CO. Controlled potential electrolysis (CPE) and post-CPE cyclic voltammogram manifested that all catalysts were robust.

Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis.

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFeII(MeCN)2][PF6]2, where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors. Using triarylaminium radical cations in MeCN, up to 182 turnovers of N2 per Fe were obtained from chemical catalysis with an extremely low loading of the Fe catalyst (0.043 mM, 0.004 mol % catalyst). This chemical catalysis was successfully transitioned to mediated electrocatalysis for the oxidation of ammonia. Molecular electrocatalysis by the Fe catalyst and the mediator (p-MeOC6H4)3N exhibited a catalytic half-wave potential (Ecat/2) of 0.18 V vs [Cp2Fe]+/0 in MeCN, and achieved 9.3 turnovers of N2 at an applied potential of 0.20 V vs [Cp2Fe]+/0 at -20 °C in controlled-potential electrolysis, with a Faradaic efficiency of 75%. Based on computational results, the catalyst undergoes sequential oxidation and deprotonation steps to form [LFeIV(NH2)2]2+, and thereafter bimetallic coupling to form an N-N bond.

Electrocatalytic Properties of meso‐ Perfluorinated Metallo Corroles for the Reduction of CO2

Corrole, a contracted tetrapyrrolic macrocycle has been also used to design molecular for the CO2 reduction. The electrochemical activity towards CO2 reduction can rival those of their porphyrin analogues. However, the catalytic activity of the metallocorrole is initiated at the corresponding MII/I couple. Accordingly, a catalytic current in presence of CO2 with cobalt corrole appears when the CoI species is generated. We have designed an electron deficient A2B corrole holding two ‐CF3 groups and a benzonitrile in the meso positions and its cobalt complex (1). We reasoned that these groups could shuffle the redox potentials to reach the M(I) oxidation states at more positive values thereby lowering the overpotential for the catalytic CO2 reduction. Our results clearly show that catalyst 1 when adsorbed on a carbon electrode, shows the most favourable catalytic performance for CO production, achieving an efficiency of 85% with a current density of ‐1.5 mA cm‐2 at ‐1.0 V vs NHE. The current densities of controlled potential electrolysis with increasing amount of KHCO3, were found to increase more than one order of magnitude with the formation of MeOH.

One step to direct extracted gold and silver from industrial Cu-Ag-Sb-Au alloy by a convenient bimetallic electrolytic separation process

Cu-Ag-Sb-Au alloy produced from lead anode slime reduction smelting through twice vacuum distillation is a high value-added precious metal resource. Currently, using pyrometallurgical process, the antimony and copper elements can be removed respectively by multistage oxidation refining, and then the precious metals are recycled by electrolytic refining. This process has a number of shortcomings such as high energy consumption, high pollution and low direct yield. In this study, a highly efficient new process for extracting precious metals from Cu-Ag-Sb-Au alloy by one-step bimetallic potential-controlled electrolysis is proposed. The electrolytic process of Cu-Ag-Sb alloy is studied by linear scanning voltammetry and electrochemical impedance spectroscopy, and the experimental verification is carried out under the guidance of Cu-Ag-Sb alloy experiments. The results show that gold and silver are enriched in the electrolytic anode slime, while copper and antimony are reduced and deposited on the cathode, which realizes the efficient separation of gold and silver from copper and antimony. The direct yield of precious metals and the anode treatment rate are as high as 99.97 % and 16.02 kg/day·A·m2, respectively, but the energy consumption is 453.90 kW·h/t only. This process strongly supports the treatment of Cu-Ag-Sb-Au alloy with low cost and high efficiency. At the same time, in combination with the vacuum distillation process, the oxidation refining is successfully removed from the lead and copper smelting system, thereby achieving efficient recovery and effective reuse of the precious metals.

Electrodes composited by bisbenzimidazole silver(I) coordination polymers as electrocatalysts for hydrogen evolution and H2O2-sensing

Two new silver(Ⅰ) coordination polymers (CPs) {[Ag2(p-HMOPhIDC)(BBP)](H2O)}n (1), {[Ag2(m-HMOPhIDC)(BBP)](H2O)}n (2) (BBP = 1,3-[bis(2,2′-benzimidazole)] propane, p-H3MOPHIDC = 2-(4-methoxyphenyl)–2H-imidazole-4,5-dicarboxylic acid, m-H3MOPHIDC = 2-(3-methoxyphenyl)–2H-imidazole-4,5-dicarboxylic acid), were prepared under hydrothermal condition and structurally characterized. The CPs 1–2 both exhibit binuclear 1D “square waveform” chain structures. Structural analysis shows that the two silver ions in CP 1 are crystallographically independent but chemically identical; whereas the two silver ions in CP 2 are crystallographically independent and chemically different. Electrocatalytic hydrogen evolution reaction (HER) of carbon paste composite electrodes (CPCEs 1–2) prepared by mixing graphite powder with coordination polymers was investigated by variable temperature linear sweep voltammetry (VT-LSV), controlled potential electrolysis (CPE) and electrochemical impedance spectroscopy (EIS) in 0.5 M H2SO4 electrolyte. It was found that the overpotential η10293K of CPCEs 1–2 positively shifted 449 and 172 mV compared with sCPCE (−966 mV), and the Tafel slope of sCPCE and CPCE 1–2 were 276, 160 and 226 mV dec-1, respectively. The results proved that CPCEs 1–2 could effectively catalyze and accelerate the HER behavior. The higher HER activity of the CPCE 1 than CPCE 2 can be attributed to the fact that CP 1 contains more three-coordinated silver ions. Moreover, the recognition performance of CPCEs 1–2 for H2O2 was further studied by chronoamperometry in 0.2 M phosphate buffer solution (PBS, pH = 6). The two sensors can detect H2O2 in a linear range from 0.5 μM to 4 mM with a detection limit of 128 µM for CPCE 1 and 27.1 µM for CPCE 2 at a signal-to-noise ratio of 3, and also revealed long-term stability and good selectivity.

Electrocatalytic facets of a newly designed cobalt(III) complex towards sustainable hydrogen evolution

A hybrid cobalt complex (C7H10ON)[Co(L)2].CH3OH (Co-complex) bearing an organic cation and anionic cobalt complex has been designed, synthesized, and structurally characterized. The tridentate chelator, 2-((2-hydroxybenzylidene)amino)-4-methylphenol (H2L), was prepared by the condensation reaction between salicylaldehyde and 4-methyl-2-amino phenol. Crystal structure analysis reveals a slight distortion in the octahedral coordination geometry of the cobalt(III) centre. This complex was evaluated in the hydrogen evolution reaction (HER) using a glassy carbon working electrode in acetonitrile (ACN), using tBu4NClO4 as the supporting electrolyte. The catalyst exhibited heightened catalytic activity when acetic acid was introduced, leading to a significant elevation in the catalytic wave at −1.6 V (V vs. Ag wire). The most pronounced activity was observed by adding 15 mM acetic acid, with a turnover number (TON) of 10.1. The catalyst’s durability was confirmed through controlled potential electrolysis (CPE) at −1.6 V for 2 h, displaying a steady current for the initial 20 min, followed by a gradual increase over the subsequent 2 h. A negligible amount of deposited metallic cobalt on the surface of the glassy carbon electrode through a ligand dissociation was evident during the electrocatalytic studies, as ensured by X-ray Photoelectron Spectroscopy (XPS) analysis. Further, the presence of ligand-centric reduction wave in the complex signifies the synergistic effect of the M−L cooperation and offers a plausible ECEC mechanistic route for the release of molecular hydrogen.

An oxo‐acetato‐bridged trinuclear cobalt(III) cluster‐persuaded cobalt oxide active electrocatalyst for efficient hydrogen evolution activity

This study presents the synthesis and electrocatalytic performance of the cobalt(III) complex [Co3(μ1,1,1‐O)(μ1,3‐CH3CO2)(L)3]2(DMF)3 (Cotri), featuring a double deprotonated (N,O)‐donor ligand (L2‐), where the protonated form of it (H2L) is 2,2′‐(hydrazine‐1,2‐diylidenebis(ethan‐1‐yl‐1‐ylidene))diphenol. The X‐ray structure reveals a trinuclear cobalt cluster formed with three cobalt(III) centers interlinking through oxido (μ1,1,1‐O) and acetato (μ1,3‐O2CCH3) bridges in coupling with bis‐congregator‐type chelating ligands. Cotri adopts cobalt centers of octahedral and trigonal bipyramidal coordination geometries and bears a dominant Co(2)…H(DMF) agostic interactions appraised by lattice dimethyl formamide (DMF) molecules. Further, Cotri shows promising heterogeneous electrocatalytic hydrogen evolution activity in 1 M KOH, with an overpotential of 441 mV to achieve 10 mA/cm2 current density. Tafel slope analysis suggests the Volmer–Heyrovsky step as the rate‐determining step. The number of active sites and TOF for Cotri were estimated at 4.010 × 10−8 mol and 0.2467 s−1, respectively, ensuring its excellent hydrogen generation activity. Stability assessments through controlled potential electrolysis (CPE) and cyclic voltammetry (CV) for multiple cycles addressed the transformation of Cotri to Co2O3 nanoparticles, which turned out to be a more active Co2O3 electrocatalyst. The morphology and particle size of the in situ developed Co2O3 active catalyst under the electrochemical conditions attributes to the superior hydrogen evolution activity.

Facile Preparation of Stable NiII‐ and CoII‐tetraaminophthalocyanine Electropolymers for Highly Efficient Heterogeneous Carbon Dioxide Reduction

This study focused on preparation of stable polymer films of NiII‐ and CoII‐tetraaminophthalocyanines, p‐NiTAPc and p‐CoTAPc, respectively, for highly efficient heterogeneous electrochemical carbon dioxide (CO2) reduction in a flow electrolysis cell. Major development represented in this work was fabrication of p‐NiTAPc and p‐CoTAPc films via electropolymerization of their corresponding monomers on carbon‐based substrates without using binder or conducting additive materials to obtain efficient gas diffusion electrodes (GDEs) for scalable, productive and selective CO2‐to‐CO conversion. The target polymers were characterized by UV‐visible spectrophotometry, attenuated total reflection‐Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X‐ray spectroscopy and cyclic voltammetry. According to controlled potential electrolysis and gas chromatography, p‐NiTAPc‐catalyzed CO2 reduction at –0.99V vs. reversible hydrogen electrode (RHE) gave 953 mL of CO in a period of 16 hours with current density and Faradaic efficiency (FE) of 109±1 mA·cm‐2 and 99±2%, respectively. A p‐CoTAPc‐modified GDE exhibited superior catalytic performance to the case of p‐NiTAPc in terms of catalyst stability and CO productivity by performing the continuous CO2 reduction at the potential of –1.10 V vs. RHE for up to 41 hours and affording almost 3 times higher amount of CO with the current density of 161±5 mA/cm2 and 95±2% FE.

Dinuclear Copper Complex for High‐Rate Hydrogen Evolution Under Neutral Aqueous Conditions

AbstractThe development of an efficient and stable electrocatalyst for the hydrogen evolution reaction (HER), based on earth‐abundant components, represents a crucial step toward cost‐effective and environmentally friendly hydrogen production. This study presents the utilization of a dinuclear copper catalyst, denoted as [Cu‐Gly‐SB] (Complex 1), for HER under both aqueous and non‐aqueous conditions. In non‐aqueous settings, the catalyst achieves excellent HER performance, requiring only a 270 mV overpotential when acetic acid is used as the proton donor. Notably, in fully aqueous conditions, complex 1 attains a remarkable current density of 18.8 mA ⋅ cm−2 at −0.7 V vs. RHE in cyclic voltammetry. The kobs value of ≈2.7×104 s−1 in aqueous solution at pH 7.0 further underlines the superior catalytic performance of 1, outperforming most non‐noble‐metal molecular catalysts functioning in fully aqueous solutions. The robust stability of 1 is demonstrated through controlled potential electrolysis (CPE) over a span of 48 hours, achieving an impressive catalytic current of 11.0 mA ⋅ cm−2 at −0.39 V. Moreover, the catalytic current gradually increases with higher reduction potentials, reaching a substantial 100 mA ⋅ cm−2 at an overpotential of 590 mV during CPE >48 hours. Thorough characterizations further confirm the molecular nature of the catalyst.

Anodic Oxidation of Aliphatic and Aromatic Amides, Bisamides and Related Derivatives

AbstractThe current comprehensive review covers four main parts, of which some are presented in the form of a personal account. 1. Anodic oxidation of a) N‐alkyl and N,N‐diallyl carbonyl moieties, b) Azacycloalkyl amides and the effect of ring‐size on the outcome, c) Lactams, d) Sulfonamides, and e) The effect of a substituent R attached to carbonyl (N−CO−R) or sulfonamides (N−SO2R) on their oxidation potentials. 2. Anodic oxidation of aromatic amides, in particular of type Ph2CHCONHAr, lacking hydrogen(s) at the α‐position to nitrogen. They undergo three types of bond‐cleavage, yielding a variety of fragmentation products. 3. a) Anodic oxidation of symmetrical bisamides of type ZCONH(CH2)nNHCOZ (Z=Me, Ph, Ar; n=2–4) under constant current electrolysis, in methanol. For n=3, 4 they afford mostly mono‐ and dimethoxylation products. For n=2 they undergo ′CH2−CH2′ bond cleavage to yield fragmentation products. b) Anodic oxidation of symmetrical bisamides of type ArCONH(CH2)2NHCOAr under controlled potential electrolysis, in acetonitrile, leads to gem‐unsymmetrical bisamides of type ArCONHCH2NHCOMe as the major product, in fair yields. 4. Utilization of the anodic process in organic synthesis to form C−C bonds by both intra‐ and inetrmolecular processes; as well as C−O and C−N bonds for generating heterocycles.

Electrocatalytic Water Oxidation by Mononuclear Copper Complexes of Bis-amide Ligands with N4 Donor: Experimental and Theoretical Investigation.

The present work describes electrocatalytic water oxidation of three monomeric copper complexes [CuII(L1)] (1), [CuII(L2)(H2O)] (2), and [CuII(L3)] (3) with bis-amide tetradentate ligands: L1 = N,N'-(1,2-phenylene)dipicolinamide, L2 = N,N'-(4,5-dimethyl-1,2-phenylene)bis(pyrazine-2-carboxamide), L3 = N,N'-(1,2-phenylene)bis(pyrazine-2-carboxamide), for the production of molecular oxygen by the oxidation of water at pH 13.0. Ligands and all complexes have been synthesized and characterized by single crystal XRD, analytical, and spectroscopic techniques. X-ray crystallographic data show that the ligand coordinates to copper in a dianionic fashion through deprotonation of two -NH protons. Cyclic voltammetry study shows a reversible copper-centered redox couple with one ligand-based oxidation event. The electrocatalytic water oxidation occurs at an onset potential of 1.16 (overpotential, η ≈ 697 mV), 1.2 (η ≈ 737 mV), and 1.23 V (η ≈ 767 mV) for 1, 2, and 3 respectively. A systematic variation of the ligand scaffold has been found to display a profound effect on the rate of electrocatalytic oxygen evolution. The results of the theoretical (density functional theory) studies show the stepwise ligand-centered oxidation process and the formation of the O-O bond during water oxidation passes through the water nucleophilic attack for all the copper complexes. At pH = 13, the turnover frequencies have been experimentally obtained as 88, 1462, and 10 s-1 (peak current measurements) for complexes 1, 2, and 3, respectively. Production of oxygen gas during controlled potential electrolysis was detected by gas chromatography.

Cobalt-based CoSe/CoO heterostructure: A catalyst for efficient oxygen evolution reaction

Hydrogen is known as the fuel for 21st century, the best and clean method to produce hydrogen is electrolysis of water. The oxygen evolution reaction (OER) is of vital position in water splitting, enabling the production of clean hydrogen fuel and contributing to renewable energy solutions. However, the inherently sluggish nature of the OER presents a major challenge for practical water electrolysis but at the same time electrocatalysts play a pivotal role by overcoming this challenge through accelerating the reaction kinetics and reducing the energy requirements. In the realm of electrocatalysts for OER, cobalt-based materials have gained significant attention due to their low overpotential and cost-effectiveness. Among prepared catalysts, the CoSe/CoO heterostructure has emerged as a standout performer, offering an optimal balance of efficiency and minimal overpotential. This electrocatalyst exhibits exceptional electrochemical performance, featuring a low 1.31 V onset potential (vs. RHE) and a mere 170 mV overpotential at a current density of 40 mA/cm2 for OER, setting a new standard for Co-based catalysts. Moreover, the CoSe/CoO heterostructure surpasses Co based bench mark electrocatalysts by displaying the lowest overpotential. Moreover, when subjected to controlled potential electrolysis at 1.65 V (vs. RHE) with a stable current density of 60 mA/cm2, it showcases remarkable electrochemical stability, affirming its practicality for real-world applications. The hydrothermal synthesis of CoSe, CoO, and their heterostructure (CoSe/CoO) represents a straightforward and accessible approach, rendering them as promising candidates for water splitting, specifically in the context of oxygen evolution. These outstanding results achieved with non-precious metal electrocatalysts open new avenues for practical OER applications, providing a platform for the scientific community to advance the development of effective electrocatalysts using non-noble metals in the field of electrochemical water splitting.

Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(II) derivative anchored with a 'scorpionate' precursor as a potential electrocatalyst for heterogeneous H2 evolution.

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]2 (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, bdtbpza {bis-(3,5-di-tert-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N3O2. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian H = -J(S1·S2) for the Si = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H2 evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H2O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H2-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.

Copper-based electrocatalyst for hydrogen evolution in water.

In aqueous pH 7 phosphate buffer, during controlled potential electrolysis (CPE) at -1.10 V vs. Ag|AgCl the literature square planar copper complex, [CuIILEt]BF4 (1), forms a heterogeneous deposit on the glassy carbon working electrode (GCWE) that is a stable and effective hydrogen evolution reaction (HER) electrocatalyst. Specifically, CPE for 20 hours using a small GCWE (A = 0.071 cm2) gave a turnover number (TON) of 364, with ongoing activity. During CPE the brownish-yellow colour of the working solution fades, and a deposit is observed on the small GCWE. Repeating this CPE experiment in a larger cell with a larger GCWE (A = 2.7 cm2), connected to a gas chromatograph, resulted in a TON of 2628 after 2.6 days, with FE = 93%, and with activity ongoing. After this CPE, the working solution had faded to nearly colourless, and visual inspection of the large GCWE showed a material had deposited on the surface. In a 'rinse and repeat test', this heterogeneous deposit was used for further CPE, in a freshly prepared working solution minus fresh catalyst, which resulted in similar ongoing HER activity to before, consistent with the surface deposited material being the active HER catalyst. EDS, PXRD and SEM analysis of this deposit shows that copper and oxygen are the main components present, most likely comprising copper and copper(I) oxide ((Cu2O)n) formed from 1. The use of 1 leads to a deposit that is more catalytically active than that formed when starting with a simple copper salt (control), likely due to it forming a more robustly attached deposit, which also enables the observed long-lived catalytic activity.

Exploration of anode candidacy of Ni0.2Co2.8O4 and integrated Ni0.2Co2.8O4/MWCNTs in supercapacitor and oxygen evolution reaction

In the current research work, Ni0.2Co2.8O4 and Ni0.2Co2.8/MWCNTs have been synthesized via facile sol-gel and wet impregnation method. The synthesized materials attained the crystalline structures as evident from X-ray diffraction analysis (XRD). The uniform morphology and well dispersion of Ni0.2Co2.8O4 onto MWCNTs was observed via scanning electron microscopy (SEM). The electrochemical investigations for supercapacitor application by cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS) revealed that, among both materials, Ni0.2Co2.8O4/MWCNTs has high specific capacitance (CV; 505.8 Fg-1 at 5 mV/s, GCD; 1598 Fg-1 at 0.5 A/g), greater capacitance retention (85 %) at 1000 cycles and has lower charge transfer resistance (Rct; 3.48 Ω cm2). These findings reflected the potential candidacy of Ni0.2Co2.8O4/MWCNTs to be used as anode material in supercapacitor. Further investigations by CV and linear sweep voltammetry (LSV) for oxygen evolution reaction (OER) activity in 1.0 M KOH showed comparatively low over potential of 340 mV @100 mA/cm2 for the same integrated material. Additionally, the lower Tafel slope (47 mV/dec) and solution resistance authenticated it as an appropriate electrocatalyst for OER in water splitting. The CPE (controlled potential electrolysis) revealed the stability of both materials for OER in water oxidation.

Electrochemical water oxidation reaction by dinuclear Re(V) oxo complexes with a 1,4-benzoquinone core via the redox induced electron transfer (RIET) process.

We report two dinuclear rhenium(V) oxo complexes 1 and 2 types, [ReV(O)(Cl)3(L2-)ReV(O)(Cl)3][NBu4]2 (1, L2- = dianionic 2,5-dihydroxy 1,4-benzoquinone (DBQ2-)) and (2, L2- = dianionic chloranilic acid (CA2-) ligands), as a homogeneous electrocatalyst for water oxidation reactions in the acetonitrile-water mixture. The evolution of dioxygen gas at the anode was confirmed by a GC-TCD study. In controlled potential electrolysis (CPE), oxidation at 1.30 V (vs. Ag/AgCl) at neutral pH, 1 and 2 afforded 1+ [ReVI(O)(Cl)3(DBQ˙3-)ReVI(O)(Cl)3]- and 2+ [ReVI(O)(Cl)3(CA˙3-)ReVI(O)(Cl)3]- ions, respectively, via the redox induced electron transfer (RIET) process. Electrochemically generated species of 1+ and 2+ could be isolated in dry acetonitrile. 1+ and 2+ ions give strong EPR signals in fluid solution as well as under frozen glass conditions due to the [ReVI(O)(Cl)3(L˙3-)ReVI(O)(Cl)3]- ↔ [ReVI(O)(Cl)3(L2-)ReV(O)(Cl)3]- (where L2- = DBQ2- and CA2-) equilibrium. However, the continuation of the CPE study (1.30 V vs. Ag/AgCl) in the presence of acetonitrile-water mixture oxidised the in situ generated species of 1+ and 2+ to higher valent ReVIO species. These species (1+ and 2+) bound water through the water nucleophilic attack (WNA) to produce peroxide intermediate species of [ReV(OOH)(Cl)3(DBQ2-)ReV(OOH)(Cl)3] (A1) and [ReV(OOH)(Cl)3(CA2-)ReV(OOH)(Cl)3] (A2) for catalysts 1 and 2, respectively. Interestingly, A1 and A2 were authenticated and analysed by ESI mass spectrometry and infrared spectroscopy and were the active precursors of this water oxidation process. The extent of current generation under similar conditions suggested that complex 1 is superior to complex 2 for the water oxidation reaction. Notably, the maximum turnover frequency (TOFmax) of catalysts 1 and 2 were 2.1 and 1.6 s-1 at 0.27 V and 0.24 V over potential, respectively, which is very significant in WOR.

An efficient Fe2Se3/Fe2O3 heterostructure electrocatalyst for oxygen evolution reaction

In this study, novel iron-based electrocatalysts, including Fe2Se3, Fe2O3, and Fe2Se3/Fe2O3 heterostructures, have been developed to bring down overpotential in the oxygen evolution reaction (OER). Among these, the Fe2Se3/Fe2O3 heterostructure emerges as a highly promising electrocatalyst for OER, offering exceptional efficiency, cost-effectiveness, and minimal overpotential. The Fe2Se3/Fe2O3 electrocatalyst exhibits outstanding electrochemical performance, with a remarkable onset potential of 1.31 V (vs. RHE) and a minimum 160 mV overpotential at 20 mA/cm2 for OER. Notably, this prepared electrocatalyst demonstrates the lowest overpotential when compared to benchmark Fe-based OER electrocatalysts. Furthermore, during controlled potential electrolysis at 1.65 V (vs. RHE), the electrocatalyst maintains a stable current density of 65 mA/cm2, highlighting its impressive electrochemical robustness. The hydrothermal synthesis method used for the synthesis of Fe2Se3, Fe2O3, and their heterostructure is particularly very simple and straightforward. Which making them excellent candidates for water splitting applications, particularly in OER. These significant achievements accomplished through the utilization of non-precious metal electrocatalysts, open up new possibilities for practical OER implementations. Undoubtedly, these findings will greatly benefit the scientific community in the development of efficient electrocatalysts using non-noble metals for the electrochemical water splitting process.