Complete Electrochemical Oxidation Research Articles

NADP(H)-dependent biocatalysis without adding NADP(H)

Isocitrate dehydrogenase 1 (IDH1) naturally copurifies and crystallizes in a resting state with a molecule of its exchangeable cofactor, NADP+/NADPH, bound in each monomer of the homodimer. We report electrochemical studies with IDH1 that exploit this property to reveal the massive advantage of nanoconfinement to increase the efficiency of multistep enzyme-catalyzed cascade reactions. When coloaded with ferredoxin NADP+ reductase in a nanoporous conducting indium tin oxide film, IDH1 carries out the complete electrochemical oxidation of 6 mM isocitrate (in 4mL) to 2-oxoglutarate (2OG), using only the NADP(H) that copurified with IDH1 and was carried into the electrode pores as cargo-the system remains active for days. The entrapped cofactor, now quantifiable by cyclic voltammetry, undergoes ~160,000 turnovers during the process. The results from a variety of electrocatalysis experiments imply that the local concentrations of the two nanoconfined enzymes lie around the millimolar range. The combination of crowding and entrapment results in a 102 to 103-fold increase in the efficiency of NADP(H) redox cycling. The ability of the method to drive cascade catalysis in either direction (oxidation or reduction) and remove and replace substrates was exploited to study redox-state dependent differences in cofactor binding between wild-type IDH1 and the cancer-linked R132H variant that catalyzes the "gain of function" reduction of 2OG to 2-hydroxyglutarate instead of isocitrate oxidation. The combined results demonstrate the power of nanoconfinement for facilitating multistep enzyme catalysis (in this case energized and verified electrochemically) and reveal insights into the dynamic role of nicotinamide cofactors as redox (hydride) carriers.

Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

The science and technology of proton-exchange membrane (PEM) low-temperature fuel cells is a subject of intense during recent years, particularly with use of hydrogen gas due to its molecular simplicity and fast kinetics of its electrooxidation. However, there are still many fundamental problems to be resolved with respect to generation of pure hydrogen and related pollution side effects, the low density and safety of hydrogen storage and distribution. Thus, there are parallel studies involving such simple organic molecules as methanol, formic acid or ethanol as substitute fuels for a wide range of potential applications. Among important obstacles are slow kinetics of electrocatalytic oxidations of fuels and insufficient durability of operation of catalysts that includes poisoning of active sites by strong adsorption of the reaction intermediate products (e.g. CO-type adsorbates), in addition the fuel cross-over through the membrane, which depolarizes the cathode and decreases its activity. While methanol, which has been the most extensively studied, faces the problem of toxicity and challenges mentioned above, the low electrocatalytic efficiency of the formic acid oxidation (only 2-electron-process) is complicated by corrosion phenomena. When it comes to the electrooxidation of ethanol, extra problems concern difficulty of the strong C-C bond breaking and excessive formation of undesirable intermediate products, acetaldehyde and acetic acid. Dimethyl ether (DME), or methoxymethane having formula CH3OCH3, is the simplest ether, and it can be considered as an isomer of another small organic molecule, ethanol, but with a different functional group. DME is a colorless gas that has been demonstrated in a variety of fuel applications, e.g. as an aerosol propellant or substitute for propane in LPG. DME can be viewed as the synthetic second generation biofuel, which can be produced from lignocellulosic biomass biogas or methane from agricultural waste. With respect to potential applications in low-temperature fuel cells, DME could be an attractive simple fuel due to its high energy density, low toxicity, liquefied easy storage, as well as the molecule’s low dipole moment that should diminish crossover through Nafion membrane, when compared to methanol, ethanol and formic acid. The stoichiometric complete electrochemical oxidation of DME requires three H2O-molecules, produces two CO2-molecules, and yields as many as twelve electrons. Obviously the multi-electron and multi-proton electrode process are complex and composed of distinct and competing reaction pathways and, thus, they are subject to kinetic limitations. The main constraints related to application of common Pt-based catalysts (e.g. Pt and bimetallic PtRu) in direct DME fuel cells concern the system’s low practical electrooxidation efficiency.In the present study, electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys toward electrooxidation of dimethyl ether (DME) has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by intentional and controlled decoration of PtSn/V with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/V and, in particular, PtSn/V admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Despite possible differences in the DME-electrooxidation mechanisms at the hybrid and single component systems, the dissociation of H3C-O-CH3 molecule, which involves C-O bond breaking, formation of the methanol reaction intermediate and CO-type poisoning adsorbates should be operative.

Enhanced electrochemical oxidation of ethanol using a hybrid catalyst cascade architecture containing pyrene-TEMPO, oxalate decarboxylase and carboxylated multi-walled carbon nanotube

The work presented herein demonstrates a hybrid bi-catalytic architecture for the complete electrochemical oxidation of ethanol. The new catalytic system contains pyrene-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-N-oxyl) immobilized on the surface of carboxylated multi-walled carbon nanotubes (MWCNT-COOH), and oxalate decarboxylase enzyme (OxDc) immobilized onto a carbon cloth electrode. Electrolysis revealed a stable amperometric curve and an excellent current density value over a duration of 10 h. In addition, the hybrid system immobilized on the carbon electrode exhibits outstanding stability after electrolysis. Nuclear magnetic resonance (NMR) and gas chromatography (GC) demonstrate that the hybrid electrode system is able to oxidize ethanol to CO2 after 10 h of electrolysis. Overall, this study illustrates the enhancement of an enzymatic biofuel cell through the hybrid multi-catalytic systems, which exhibit high oxidation rates for all substrates involved in complete ethanol oxidation, enabling the collection of up to 12 electrons per molecule of ethanol.

Anodic Reaction Mechanisms in Direct Carbon Fuel Cells Using Carbon Particles Dispersed in Molten Carbonate

Highly efficient technologies for the conversion of solid fuels, such as coal and biomass, to electricity are required for clean energy production and sustainable energy supply. Direct carbon fuel cells (DCFCs), which convert the chemical energy in solid carbon directly into electricity without gasification, have attracted much attention because of their theoretical ability to achieve 100 % efficiency. However, solid carbon particles have disadvantages in terms of fluidity when compared to gaseous fuels. Thus, carbon particles are dispersed into molten carbonate to form a carbon/carbonate slurry in which the carbon particles can move. Then, the anode is inserted into the carbon/carbonate slurry for discharge in the DCFC [1-5]. Carbon dispersed into the molten carbonate is easily transported to the anode, leading to an improvement of the formation of triple phase boundary (carbon, electrolyte, and anode). During discharge, carbon is supposed to be consumed through the complete electrochemical oxidation (C+2CO3 2-→3CO2+4e- (R1)) which releases four electrons per carbon. However, several studies have suggested a possible formation of CO from partial electrochemical oxidation (C+1/2CO3 2-→3/2CO+e- (R2)) which releases one electron per carbon, or even from the Boudouard reaction (C+CO2→2CO (R3)) where CO2 produced during discharge reacts with carbon particles [3,6]. Although the partial oxidation (R2) and the Boudouard reaction (R3) reduce the DCFC efficiency, little effort has been made to study the reaction mechanisms in the DCFC. The aim of this study is to investigate the anodic reaction mechanisms in the DCFC using carbon/carbonate slurry experimentally and theoretically. Figure shows setup of the DCFC using carbon/carbonate slurry stirred by gas bubbling. The working electrode (WE), counter electrode (CE), and reference electrode (RE) were made from gold sheets. Cell discharge was controlled with a potentio/galvano-stat. The reactor was heated using an electric furnace to 1073 K. Activated carbon particle with an average diameter of 44.1 μm was used as fuel. Ternary molten carbonate (Li2CO3/Na2CO3/K2CO3 (= 16.9/25/58.1 mol%)) was used as electrolyte. The carbon content in the carbonate was set to 1.0 wt% or 2.0 wt%. The carbon/carbonate slurry was contained in the porous alumina tube having an average pore diameter of 120 nm. This allowed the separation between the carbon/carbonate slurry and the molten carbonate electrolyte. In the setup, the carbon/carbonate slurry was able to be stirred by gas bubbling. CO and CO2 concentrations in off-gases were measured by FID with methanizer, and CO and CO2 production rates were determined at the open circuit mode or constant discharge mode at 20 mA/cm2. Moreover, to study the impact of Boudouard reaction (R3) on CO formation, CO2 bubbling with a flow rate of 1.0 ml/min, which was approximately 5 times of theoretical CO2 produced during discharge at 20 mA/cm2, was used in the carbon/carbonate slurry at the open circuit condition, and CO concentration in off gases was measured. As a result, an amount of CO was formed when the carbon/carbonate slurry was not stirred by Ar gas bubbling during the constant discharge at both the carbon contents. At the open circuit condition, CO was not formed by CO2 bubbling (1.0 ml/min) simulated CO2 produced during discharge. This indicated that the impact of Boudouard reaction (R3) was insignificant in the DCFC. Moreover, measured CO and CO2 production rates were well balanced on the calculation based on two electrochemical oxidations (R1 and R2). And, the partial electrochemical oxidation accounted for approximately 80 % in the carbon consumption at the carbon content of 2.0 wt%, whereas that accounted for approximately 40 % at the carbon content of 1.0 wt%. It was shown that the ratio of partial electrochemical oxidation depended on the carbon concentration near the anode. On the other hand, when the carbon/carbonate slurry was stirred by Ar bubbling with a flow rate of 50 ml/min, only CO2 was formed and the production rate of CO2 corresponded to the theoretical rate based on R1 at both the carbon contents. This indicated the carbon particles were consumed only by the complete electrochemical oxidation (R1). It was shown that stirring the carbon/carbonate slurry was effective to enhance the complete electrochemical oxidation. Reference [1] N. J. Cherepy et al., J. Electrochem. Soc., 152 (2005) A80-A87. [2] X. Li et al., Ind. Eng. Chem. Res., 47 (2008) 9670-9677. [3] H. Watanabe et al., J. Power Sources, 273 (2015) 340-350. [4] H. Watanabe et al., Energy and Fuels, 30 (2016) 1835-1840 [5] H. Watanabe et al., J. Chem. Eng. Jpn., 49 (2016) 237-242 [6] T.M. Gür, J. Electrochem. Soc., 157 (2010) B751-B759 Figure 1

The Complete Oxidation of Glycerol to CO2: A Hybrid Enzymatic and Organic Electrocatalytic Cascade

There has been a significant interest lately in the complete oxidation of biofuels to CO2. The use of multiple biocatalysts allows for deeper biofuel oxidation and thus maximizes the energy density that can be extracted per molecule of biofuel. However, there are still many biofuels for which a suitable biocatalyst is not known. We have recently taken an alternative approach to combine the high catalytic activity of enzymatic biocatalysts with the promiscuous nature of small molecule organic catalysts. We demonstrate the complete electrochemical oxidation of the biofuel glycerol to CO2 using a hybrid enzymatic and small-molecule catalytic system. Combining an enzyme, oxalate oxidase, and an organic oxidation catalyst, 4-amino-TEMPO, we are able to electrochemically oxidize glycerol at a carbon electrode, while collecting as many as 16 e-s per molecule of fuel. Additionally, we investigate the anomalous electrocatalytic properties that allow 4-amino-TEMPO to be active under the acidic conditions required for oxalate oxidase to function.

Hybrid enzymatic and organic electrocatalytic cascade for the complete oxidation of glycerol.

We demonstrate the complete electrochemical oxidation of the biofuel glycerol to CO2 using a hybrid enzymatic and small-molecule catalytic system. Combining an enzyme, oxalate oxidase, and an organic oxidation catalyst, 4-amino-TEMPO, we are able to electrochemically oxidize glycerol at a carbon electrode, while collecting up to as many as 16 electrons per molecule of fuel. Additionally, we investigate the anomalous electrocatalytic properties that allow 4-amino-TEMPO to be active under the acidic conditions that are required for oxalate oxidase to function.

Enhancing the efficiency of direct carbon fuel cells by bubbling Ar gas in carbon/carbonate slurry

This study aims to enhance the direct carbon fuel cell (DCFC) efficiency by bubbling Ar gas in a carbon/carbonate slurry. The current discharge curve and CO and CO2 production rates were measured during discharge at 20 mA cm−2. When carbon content in carbonates (WC/carbonate) was 1.0 wt%, Ar bubbling decreased the CO/CO2 production ratio from 0.16 to 0.051, and increased the CO2 production rate, which approached the theoretical value for complete electrochemical oxidation of carbon. Moreover, bubbling increased the cell voltage by about 0.1 V at steady state. At WC/carbonate = 3.0 wt%, the CO/CO2 production ratio without bubbling increased significantly to 8.5, indicating that partial electrochemical oxidation became dominant. The C/O2− ratio is key for explaining the electrochemical oxidation of the carbon. When the C/O2− ratio was low, such as at WC/carbonate = 1.0 wt%, complete electrochemical oxidation became dominant, and bubbling enhanced the complete electrochemical oxidation. When the C/O2− ratio was high, such as at WC/carbonate = 3.0 wt%, partial electrochemical oxidation became dominant, and bubbling did not inhibit it. For WC/carbonate = 1.0 wt%, bubbling increased both coulombic and voltage efficiencies, resulting in an increase in total efficiency from 52% to 64%.

Comparative DEMS study on the electrochemical oxidation of carbon blacks

The intention of the study presented here is to compare the electrochemical oxidation tendencies of a pristine Ketjen Black EC300 high surface area (HSA) carbon black, and four graphitised counterparts heat-treated between 2100 and 3200 °C, such as those typically used as corrosion resistant carbon (CRC) supports for polymer electrolyte membrane fuel cell (PEMFC) catalysts. A methodology combining cyclic voltammetry (CV) and differential electrochemical mass spectrometry (DEMS) is used, which allows the characterisation and comparison of the complete electrochemical oxidation rates and behaviours of the various carbon blacks. It is observed that the behaviour of the carbon black towards electrochemical oxidation is highly dynamic, and dependent on the properties of the pristine carbon back, the degree of electrochemical oxidation and the potential pre-history. The behaviour of the non-graphitised and CRC samples graphitised ≤2450 °C suggests that electrochemical oxidation leads to the passivation of the carbon, which when reduced in a potential excursion to ≤0.24 VRHE is lost allowing subsequent oxidation; however, CRC samples graphitised ≥2800 °C did not exhibit this same behaviour.

Hydrogen production by electrochemical decomposition of formic acid via solid polymer electrolyte

The aim of this work is to investigate the feasibility of simultaneous hydrogen production by electrochemical decomposition of formic acid via solid polymer electrolyte (SPE) in an electrochemical reactor. Titanium oxide coated with iridium oxide as anode and carbon fibre with Pt catalyst as cathode were used in the experiments. Effects of applied current density, flow rates and temperature of formic acid solution, concentration of supporting electrolyte and pH of the solution on performance of the process have been investigated. The effect of membrane thickness has also been examined. The results suggest that electrolysis using SPE is a promising method for the treatment of organic pollutants. Hydrogen with purity of 99.999% at ambient temperature by using carbon fibre cathode with Pt catalyst can be produced simultaneously and COD removal efficiency of 95% has been achieved not requiring any chemical addition and temperature increase. Also complete electrochemical oxidation of formic acid at the original pH to CO 2 and H 2O without production of intermediate has been proved by HPLC analysis.

A simple and high efficient direct borohydride fuel cell with MnO 2-catalyzed cathode

A simple direct borohydride fuel cell (DBFC) was constructed by use of an MnO 2-catalyzed air cathode and an Au-catalyzed anode. It is found that the MnO 2 cathode not only exhibits considerable electrocatalytic activity for oxygen reduction in the BH 4 - solutions, but also shows indiscernible catalytic activities for the electrooxidation and chemical hydrolysis of BH 4 - ions. As a result, these DBFC cells can avoid not only the performance degradation of the cathode arising from “ BH 4 - crossover” but also do not need to use expensive ion exchange membranes and Pt catalyst in the cathode. Based on this new configuration of DBFC, we used nanosized Au particles as an anodic electrocatalyst to avoid the chemical hydrolysis of BH 4 - ions, thereby to realize the complete electrochemical oxidation of BH 4 - ions.

In Situ XAS Investigation of Transformation of Co Monolayer on Carbon-Supported Platinum Clusters Underpotential Control

A complete electrochemical oxidation−reduction cycle for a Co monolayer on carbon-supported Pt clusters in alkaline solution has been monitored in situ by X-ray absorption spectroscopy (XAS). The n...

In Situ Observation of the Potential-Dependent Chemical State and Structure of a Cu Monolayer Deposited on the Surface of Carbon-Supported Platinum Clusters

A complete electrochemical oxidation−reduction cycle for a Cu monolayer on carbon-supported Pt clusters in alkaline solution has been monitored in situ by X-ray absorption spectroscopy. No desorption of Cu was observed upon oxidation. Near-range order and oxidation state of the Cu layer as a function of potential was obtained by in situ X-ray absorption spectroscopy. In contrast to bulk copper, the adsorbed copper monolayer transforms directly from the reduced state to Cu2+ and from the oxidized form to Cu0. Adsorption of OH- ions on the Cu layer was observed for potentials below those for the Cu0 → Cu2+ and Cu2+ → Cu0 transitions.

Molecular chemistry at well-defined surfaces

Surface chemistry studies are described in which molecular layers chemisorbed at well-defined Pt or Ag electrode surfaces from aqueous solutions were characterized by means of ex–situ UHV surface analysis techniques such as Auger electron spectroscopy (AES), electron energy–loss spectroscopy (EELS), and low-energy electron diffraction (LEED), and in–situ electrode–solution interfacial characterization methods such as cyclic voltammetry (CV) and coulometry. Types of compounds studied include alcohols, ethers, aldehydes, ketones, carboxylic acids, amines, amino–acids, amides, cyanides, thiocyanides, halides, mercaptans, thiols, sulfones, sulfonic acids, benzenes, five– and six– membered heterocyclic aromatics, multinitrogen heteroaromatics, fused–ring aromatics, and deuterated molecules. Molecules chemisorbed at electrode surfaces are often not removed by rinsing, facilitating investigation by electrochemical methods, and arc stable also in vacuum, permitting identification and characterization of the adsorbed species by AES, EELS and LEED and related methods. Molecular stoichiometry (elemental composition) and packing density (surface concentration, molecules per unit area) are measured by means of AES. Surface molecular orientation can be deduced from molecular packing densities, supplemented with adsorbate vibrational spectra, electrochemical reactivity, LEED structure and other related observations. Studies of series of related adsorbates offer the advantage that additional insight can be gained from intercomparison of molecular surface behavior as a function of systematic structural and chemical differences. Electrochemical reactivity is found to be a function of molecular orientation: horizontally oriented adsorbates at Pt surfaces in most cases undergo complete electrochemical oxidation to CO2 and other oxides, while vertically oriented adsorbates undergo less extensive oxidation; reversible redox centers such as the hydroquinone and catechol moieties retain their electrochemical reactivity and reversibility at electrode surfaces only when the redox center is pendant, and direct attachment of a redox center at surfaces results in loss of redox electro- activity.Close similarity between adsorbed layer EELS spectra and the IR spectra of the unadsorbed liquid or vapor is usually found for vertically oriented adsorbates for which adsorption docs not substantially perturb the main–frame molecular structure. Surface chemical phenomena involving molecular adsorbates at electrode surfaces, including molecular Packing density, mode of attachment, surface orientation, long–range surface order, molecular structure, spectroscopic behavior, and chemical and electrochemical reactivity, are greatly influenced by factors such as adsorbate molecular structure, electrode material, crystallographic structure of electrode surface, electrode potential, solution pH, adsorbate concentration, and temperature.

A Methanol Fuel Cell with an Invariant Alkaline Electrolyte

The inconvenience and expense of using hydrogen as a fuel for fuel cells have prompted interest in inexpensive liquid fuels such as methanol. The chief difficulties encountered by workers in the field of methanol cells have been lack of electrolyte invariance, incomplete fuel oxidation, and deactivation of the cathode by dissolved fuel in the electrolyte.Feasibility has been shown for an intermediate temperature methanol fuel cell using a cesium carbonate electrolyte and Teflon‐bonded platinum electrodes at an optimum temperature of 115°–130°C, which is free from all of the above‐mentioned problems. Performance has been studied as a function of temperature, electrolyte composition, fuel composition, and other variables. Material balances have been performed showing complete electrochemical oxidation of the fuel to and . At an operating point of 0.55v at 20 ma/cm2, the comparative thermal efficiency is 42%. The conversion of the fuel to electricity in one pass through the cell is 75 to 85%. Power densities of 40–45 milliwatt/cm2 can be drawn from the cell. An endurance test of over 560 hr has shown a gradual increase in performance with time, and no change in the electrolyte. Performance on carbon monoxide is also reported.