Novel electrocatalysts for generating oxygen from alkaline water electrolysis

We have obtained spinel-type Co₃O₄ and La-doped Co₃O₄ in the form of thin film on Ni, using microwave-assisted synthesis, which dramatically exhibit very low overpotentials for the oxygen evolution reaction (OER). Investigations have shown that at the apparent current density of 100 mA cm⁻² in 1 mol dm⁻³ KOH at 25 °C, the new electrodes, Co₃O₄ (oxide loading = 3.4 ± 0.3 mg cm⁻²) and La-doped Co₃O₄ (oxide loading = 2.8 ± 0.4 mg cm⁻²), produce overpotentials, 235 ± 7 and 224 ± 8 mV, respectively. Such low overpotentials for the OER, to our knowledge, have not been found on any mixed oxide electrode material reported in literature till today. Small La addition improved the BET surface area and porosity of the oxide catalyst powder and reduced the charge transfer resistance for the OER on the electrode made of oxide powder.     

Determination of the Tafel slope for oxygen evolution on boron-doped diamond electrodes

It has been reported that the oxygen evolution reaction (OER) on boron-doped diamond (BDD) electrodes appears at high overpotential and results in unusually high Tafel slope. In this work, we have studied the OER in 1 M HClO₄ on BDD macroelectrode and microelectrodes-array (MEA). The correction of the anodic polarization curve for ohmic drop has been performed on BDD macroelectrode taking into account the total uncompensated resistance of the studied system. On BDD MEA, no correction of the polarization curve was necessary due to the small contribution of ohmic drop to the measured potential. At low overpotential (<1.2 V), abnormally high Tafel slopes (340 and 680 mV dec⁻¹ on BDD MEA and BDD, respectively) have been observed. Such high slopes may result from the presence of surface redox couples/functional groups which act as a barrier for OER on BDD. In this potential region, the Tafel slope depends strongly on the state of the electrode surface. In the high overpotential region (>1.2 V), the Tafel slope has been found equal to 120 mV dec⁻¹, which is the theoretical value considering a first or a second electron transfer step as the rate determining step.     

Bioelectrocatalytic oxidation of glucose by hexose oxidase directly wired to graphite

Glucose-oxidizing enzymes are widely used in electrochemical biosensors and biofuel cells; in most applications glucose oxidase, an enzyme with non-covalently bound FAD and low capability of direct electronic communications with electrodes, is used. Here, we show that another glucose-oxidizing enzyme with a covalently bound FAD center, hexose oxidase (HOX), adsorbed on graphite, exhibits a pronounced non-catalytic voltammetric response from its FAD, at − 307 mV vs. Ag/AgCl, pH 7, characterized by the heterogeneous electron transfer (ET) rate constant of 29.2 ± 4.5 s^− 1. Direct bioelectrocatalytic oxidation of glucose by HOX proceeded, although, with a 350 mV overpotential relative to FAD signals, which may be connected with a limiting step in biocatalysis under conditions of the replacement of the natural redox partner, O₂, by the electrode; mediated bioelectrocatalysis was consistent with the potentials of a soluble redox mediator used. The results allow development of HOX-based electrochemical biosensors for sugar monitoring and biofuel cells exploiting direct ET of HOX, and, not the least, fundamental studies of ET non-complicated by the loss of FAD from the protein matrix.     

pH effects on the rate of heterogeneous electron transfer across a fluorine doped tin oxide/monolayer interface

Spontaneously adsorbed monolayers of [Ru(bpy)₂PIC](PF₆)₂ have been formed on fluorine doped tin oxide macro- and microelectrodes, bpy is 2,2′-bipyridyl and PIC is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates. The formal potential of the Ru^2+/3+ process shifts by less than 30 mV upon immobilization suggesting that the monolayers are well solvated. Significantly, chronoamperometry, conducted on a microsecond timescale, reveals that protonation of the PIC bridging ligand modulates the rate of interfacial electron transfer. The heterogeneous electron transfer rate constant, measured at an overpotential of +50 mV, decreases from 7.0 ± 1.1 × 10⁵ to 0.7 ± 0.1 × 10⁵ s⁻¹ as the pH of the supporting electrolyte is increased from 1.7 to 9.3. These observations are consistent with the redox mechanism occurring via a heterogeneous electron transfer process, the rate of PIC which depends on the energy difference between the metal dπ-orbitals and the lowest unoccupied molecular orbital (LUMO) of the bridge. Protonation of the bridging ligand decreases this energy gap, resulting in an overall increase in the rate of the redox reaction. Significantly, despite the close proximity of the luminophores to a conducting surface, the monolayers remain luminescent suggesting that the electronically excited state is only weakly coupled to the electrode surface. This is consistent with bipyridyl as the site of the excited state in the metal complex.     

Oxygen permeation through (Bi2O3)0.74(SrO)0.26–Ag (40% v/o) composite

The dense dual phase composite membrane made from strontium-stabilized bismuth oxide and silver, (Bi₂O₃)_0.74(SrO)_0.26–Ag (40% v/o), was investigated. The composite was found to exhibit very high electrical conductivity at the room temperature, revealing that the silver phase has formed electron-conducting networks in the oxide matrix. The composite shows much improved oxygen permeability compared with the bismuth oxide alone. An oxygen flux of 5×10⁻⁸ mol cm⁻² s⁻¹ was observed for a 1.00 mm thick composite at 700°C with oxygen partial pressures of the feed and permeate side at 0.209, 0.0024 atm, respectively. Combination of electrical conductivity and oxygen permeation measurements reveals that oxygen-ion conduction through the oxide phase of the composite is the rate-limiting step for oxygen permeation.     

Simultaneous determination of ascorbic acid,epinephrine, and uric acid by differential pulse voltammetry using poly(p-xylenolsulfonephthalein) modified glassy carbon electrode

A novel poly(p-xylenolsulfonephthalein) modified glassy carbon electrode was prepared for the simultaneous determination of ascorbic acid (AA), epinephrine (EP) and uric acid (UA). Cyclic voltammetric, chronoamperometric, and differential pulse voltammetric methods were used to investigate the modified electrode for the electrocatalytic oxidation of EP, AA, and UA in aqueous solutions. The separation of the oxidation peak potentials for AA–EP and EP–UA was about 200 and 130 mV, respectively. The calibration curves obtained for AA, EP, and UA were in the ranges of 10–1343, 2–390, and 0.1–560 μmol L⁻¹, respectively. The detection limits (S/N = 3) were 4, 0.1, and 0.08 μmol L⁻¹ for AA, EP and UA, respectively. The diffusion coefficient and the catalytic rate constant for the oxidation of EP at the modified electrode were calculated as 1.40(±0.10) × 10⁻⁴ cm² s⁻¹ and 1.06 × 10³ mol⁻¹ L s⁻¹, respectively. The present method was applied to the determination of EP in pharmaceutical and urine samples, AA in commercially available vitamin C tablet, and EP plus UA in urine samples.     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.     

Mesoporous silica thin films for molecular sieving and electrode surface protection against biofouling

Electrode fouling is a major challenge for the long term use of sensors in real samples as it leads to the decay of the electroanalytical signal and is often caused by the formation of an inhibiting layer formed by biomolecules. We demonstrate here that ordered and vertically aligned mesoporous silica generated at the surface of an indium tin oxide electrode by electrochemically assisted self-assembly act as a molecular sieve and a protective layer for the electrode surface. They indeed prevent the adsorption of size excluded large undesired molecules (e.g. haemoglobin) while allowing the detection of small redox active molecules likely to reach the electrode surface through the film (e.g. propranolol) with almost no loss of sensitivity. At a bare electrode, the oxidation of propranolol is completely inhibited in the presence of 5 μM haemoglobin. At a modified electrode, the sensitivity for propranolol in the absence of haemoglobin is (72.8 ± 2.9) mA mol^− 1 (R² = 0.992, N = 7) and it remains similar in the presence of 5 μM haemoglobin with a value of (67.4 ± 7.2) mA mol^− 1 (R² = 0.992, N = 7).     

A molecular wire modified glassy carbon electrode for achieving direct electron transfer to native glucose oxidase

Here we describe a strategy for achieving direct electron transfer to native glucose oxidase (GOx), an enzyme in which the redox active centre is buried deep within the glycoprotein. To achieve this a glassy carbon electrode is modified with a mixed monolayer of 4-carboxyphenyl and a 20 Å long oligo(phenylethynyl) molecular wire (MW), assembled from the respective aryl diazonium salts. Subsequently GOx is adsorbed to the interface, followed by covalent attachment. The redox chemistry of the active centre of glucose oxidase, flavin adenine dinucleotide, was observed at an E_1/2 of –443 mV (vs. Ag|AgCl). The enzyme was shown to retain its activity. Most importantly, in the absence of oxygen the electrode was still able to biocatalytically turn over glucose at −400 mV, thereby demonstrating that the enzyme was being recycled back to its catalytically active oxidized form from its inactive reduced form. The rate of enzyme turnover was 1.1 s⁻¹.     

Microwave-assisted synthesis of multimetal oxygen-evolving catalysts

Oxygen evolution reaction (OER) plays a pivotal role in water-splitting. Here, we report a facile method to synthesize multimetal supported on commercial carbon black via a time-saving microwave process. Crystalline FeNi₃ nanoparticles homogeneously doped with Mo are formed via a microwave treatment and activated to metal oxyhydroxide in-situ during cyclic voltammetry test with overpotential of only 280 mV at 10 mA cm^− 2 for OER in alkaline electrolyte, outperforming RuO₂. Our synthesis methodology is a promising alternative for large-scale production, delivering a valuable contribution to catalyst preparation and electrocatalytic water oxidation research.     

Electrodeposition of cobalt-sulfide nanosheets film as an efficient electrocatalyst for oxygen evolution reaction

In this communication, we report the development of cobalt-sulfide nanosheets film on Ti mesh (Co-S/Ti mesh) via electrodeposition as a novel oxygen evolution anode in basic media. Electrochemical experiments suggest that this Co-S/Ti mesh electrode exhibits high catalytic activity and good stability. It needs overpotential of 361 mV to drive current density of 10 mA cm^− 2 and its catalytic activity is maintained for at least 20 h.     

Determination of formal potential of NADH/NAD+ redox couple and catalytic oxidation of NADH using poly(phenosafranin)-modified carbon electrodes

The electrochemical regeneration of NADH/NAD⁺ redox couple has been studied using poly(phenosafranin) (PPS)-modified carbon electrodes to evaluate the formal potential and catalytic rate constant for the oxidation of NADH. The PPS-modified electrodes were prepared by electropolymerization of phenosafranin onto different carbon substrates (glassy carbon (GC) and basal-plane pyrolytic graphite (BPPG)) in different electrolytic solutions. The formal potential was estimated to be ? 0.365 ± 0.002 V vs. SHE at pH 7.0. As for the bare carbon electrodes, the oxidation of NADH at the BPPG electrode was found to be enhanced compared with the GC electrode. For the PPS-modified electrodes, it was found that the electrocatalysis of PPS-modified electrodes for the oxidation of NADH largely depends on the carbon substrate and electrolyte solution employed for their preparation, i.e., the PPS-modified BPPG electrode prepared in 0.2 M NaClO₄/acetonitrile solution exhibits an excellent and persistent electrocatalytic property toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 740 and 670 mV compared with those at the bare GC electrode and the PPS-modified GC electrode prepared in 0.2 M H₂SO₄ solution, respectively. A quantitative analysis of the electrocatalytic reaction based on rotating disk voltammetry gave the electrocatalytic reaction rate constants of the order of 10³–10⁴ M^?1 s^? 1 depending on the preparation conditions of the PPS-modified electrodes.     

Preparation and characterization of Al and Mn doped ZnO (ZnO: (Al,Mn)) transparent conducting oxide films

This paper presents the electro-optical, chemical and structural properties of doped-ZnO films deposited by DC-reactive magnetron sputtering at room temperature using the bi-dopant Al and Mn. A minimum resistivity of 3.46×10⁻⁴ Ω cm, exceeding 75.0% average transmittance (380–800 nm), and fundamental band gap of 3.48±0.01 eV have been obtained. XPS analyses show that Zn uniformly remains in the valence state of Zn²⁺; all of the Al and a little amount of Mn with valence state of Mn⁴⁺ are supposed to have donor effect, while dominant Mn²⁺ will induce to form more oxygen vacancies and this proposal has been verified by O 1s XPS results. It has been concluded that the presence of more oxygen vacancies will attenuate the effect of hybridization of p–d orbitals in the matrix of ZnO. It has been found that all the as-deposited films have c-axis preferred orientation with flat and smooth surface (RMS surface roughness is of the order of ∼3 nm over 5×5 μm² area).     

Microstructural and gas separation properties of CVD modified mesoporous γ-alumina membranes

Chemical vapor deposition (CVD) was used to modify 4 nm pore, sol–gel derived, γ-alumina membranes supported on macroporous α-alumina. Aluminum oxide was deposited in the pores of the γ-alumina membrane by alternating additions of trimethylaluminum (TMA) and water vapor. By reducing the pore size, the permeance of non-condensable gasses was reduced much more than the permeance of condensable gasses due to capillary condensation or preference adsorption of water vapor. The modified membrane that exhibited the best separation properties had a water vapor permeance ranging from 1.5×10⁻⁶ to 3.0×10⁻⁷ mol/m² s Pa, an oxygen permeance ranging from 1.7×10⁻⁷ to 1.5×10⁻⁹ mol/m² s Pa, and a separation factor as high as 140 at room temperature. The microstructure of the pores contained some irregularities which were attributed to an atomic layer CVD (ALCVD) mechanism modified by homogeneous reactions. The effect of the modified ALCVD was higher permeances than would be expected. P-type zeolite membranes were also made and found to have similar separation properties to the more heavily modified γ-alumina membranes.     

Direct electrochemistry of hemoglobin and glucose oxidase in electrodeposited sol–gel silica thin films on glassy carbon

This work points out that electrogeneration of silica gel (SG) films on glassy carbon electrodes (GCEs) can be applied to immobilize biomolecules – hemoglobin (Hb) or glucose oxidase (GOD) or both of them in mixture – without preventing their activity. These proteins were physically entrapped in the sol–gel material in the course of the electro-assisted deposition process applied to form the thin films onto the electrode surface. SG films were prepared from a precursor solution by applying a suitable cathodic potential likely to induce a local pH increase at the electrode/solution interface, accelerating thereby polycondensation of the silica precursors with concomitant film formation. Successful immobilization of proteins was checked by various physico-chemical techniques. Both Hb and GOD were found to undergo direct electron transfer, as demonstrated by cyclic voltammetry. GCE–SG–Hb gave rise to well-defined peaks at potentials E_c = −0.29 V and E_a = −0.17 V in acetate buffer, corresponding to the Fe^III/Fe^II redox system of heme group of the protein, while GCE–SG–GOD was characterized by the typical signals of FAD group at E_c = −0.41 V and E_a = −0.33 V in phosphate buffer. These two redox processes were also evidenced on a single voltammogram when both Hb and GOD were present together in the same SG film. Hb entrapped in the silica thin film displayed an electrocatalytic behavior towards O₂ and H₂O₂ in solution, respectively in the mM and μM concentration ranges. Immobilized GOD kept its biocatalytic properties towards glucose. Combined use of these two proteins in mixture has proven to be promising for detection of glucose in solution via the electrochemical monitoring of oxygen consumption (decrease of the oxygen electrocatalytic signal).     

Three colour electrochromic metallopolymer based on a ruthenium phenolate complex bound to poly(4-vinyl)pyridine

The photonic and electrochemical properties of a novel Ru–phenolate based metallopolymer are reported. The complex undergoes a ruthenium based reversible oxidation at approximately +0.400 V and irreversible box ligand oxidation at +0.800 V vs. Ag/AgCl. Oxidation of thin films in aqueous electrolyte at +0.500 V reversibly switches the colour from wine red to light green and a red orange colour is observed for mixed redox composition. In contrast, oxidation at potentials more positive than +1.500 V shows no visible colour change but produces a change in the near infra-red region. To determine the electrochromic switching rate and to identify the rate determining step of the, scan rate dependent cyclic voltammetry was performed under semi-infinite linear diffusion conditions in aqueous lithium perchlorate. These data reveal that the homogeneous charge transport diffusion coefficient, D_CT, is 3.6 ± 0.2 × 10⁻¹³ cm² s⁻¹, i.e., under these conditions it takes approximately 90 s to fully oxidise a 100 nm thick film.     

A time-resolved study of the multiphase chemistry of excited carbonyls: Imidazole-2-carboxaldehyde and halides

Imidazole-2-carboxaldehyde (IC) reactivity in the presence of halide anions (Cl^–, Br^–, I^–) has been studied by laser flash photolysis in aqueous solution at room temperature. The absorption spectrum of the triplet state of IC has been measured with a maximum absorption at 330 nm and a weaker absorption band around 650 nm. Iodide anions proved to be efficient quenchers of the triplet state IC, with a rate coefficient k_q of (5.33 ± 0.25) × 10⁹ M⁻¹ s⁻¹. Quenching by bromide and chloride anions was less efficient, with k_q values of (6.27 ± 0.53) × 10⁶ M⁻¹ s⁻¹ and (1.31 ± 0.16) × 10⁵ M⁻¹ s⁻¹, respectively. The halide (X^–) quenches the triplet state; the resulting transient absorption feature matches that of the corresponding radical anion (X₂^–). We suggest that this type of quenching reactions is a driving force of oxidation reactions in the oceanic surface microlayer (SML) and a source of halogen atoms in the atmosphere.     

Noncovalently functionalized water-soluble multiwall-nanotubes through azocarmine B and their application in nitric oxide sensor

A new noncovalent approach for the dissolution of MWNTs in water by azocarmine B (ACB) is reported. Through a simple electro-polymerization procedure, a novel electrochemical NO sensor based on water-soluble MWNTs and polyazocarmine B (PACB) nanofilm electrode was prepared, which showed excellent electrocatalytic activity towards the oxidation of nitric oxide (NO). The oxidation current linearly increased with the nitric oxide concentration in the range of 2.2 × 10⁻⁷–1.2 × 10⁻⁴ mol L⁻¹ with a low detection limit of 2.8 × 10⁻⁸ mol L⁻¹. The sensor has the merit of good stability, reproducibility, high sensitivity and selectivity, and it can be used to monitor NO released from rat liver cells effectively.     

Pulse radiolysis study of the formation and the reactivity of baicalin radical anion,and in comparison with rutin,quercetin and acyrlate ester radical anions in ethanol

The reaction of solvated electrons with baicalin in N₂-saturated ethanol has been studied by pulse radiolysis. The results show that a solvated electron can add to baicalin and generate a baicalin radical anion with a maximum UV absorbance peak at 360 nm. Its molar extinction coefficient at this wavelength is 1.3×10⁴ M⁻¹ cm⁻¹. The rate constant for the build-up of the baicalin radical anion is 1.3(±0.4)×10¹⁰ M⁻¹ s⁻¹. Decay of the radical anion is induced by a proton transfer reaction and a recombination reaction, which involves a pseudo-first-order reaction with rate constant 2.6(±0.4)×10³ s⁻¹ and a second-order reaction with rate constant 1.3(±0.2)×10⁹ M⁻¹ s⁻¹. The effect of acetaldehyde on the decay of the baicalin radical anion was also investigated. Electron transfer between the baicalin radical anion and acetaldehyde was not observed, probably due to the low rate of electron transfer between the baicalin radical anion and acetaldehyde. Reactivity of the rutin, quercetin, baicalin and ethyl acrylate radical anions are also compared.     

Gelatin-functionalized carbon nanotubes for the bioelectrochemistry of hemoglobin

In this paper, we compared the use of gelatin-functionalized carbon nanotubes (CNTs) as substrates for Hemoglobin (Hb) immobilization and as electrodes for electrochemical reduction of the absorbed Hb. The non-covalently gelatin-functionalized CNTs possessed an improved solubility in aqueous solution and may have an enhanced biocompatibility with Hb. The characteristics of Hb/gelatin-CNTs composite films were studied by using UV–vis spectroscopy, FTIR spectroscopy and electrochemical methods. The immobilized Hb showed a couple of quasi-reversible redox peaks with a formal potential of −0.35 V (vs. SCE) in 0.10 M pH 7.0 phosphate buffer solution (PBS). The surface concentration of electroactive Hb immobilized on gelatin-CNT/GC electrode was about 4.34 × 10⁻¹⁰ mol cm⁻².