The “In-cell” and “Ex-cell” Fenton treatment of phenol, 4-chlorophenol and aniline

A comparative study of phenol, 4-chlorophenol and aniline degradation with the electro-generation of H₂O₂ at gas-diffusion electrodes was carried out under three different conditions: electro-Fenton^® treatment in an undivided cell; electro-Fenton treatment in the catholyte of a membrane cell divided by a proton-exchange membrane (in-cell electro-Fenton membrane process); and a treatment of polluted solution in the cathode space of a membrane cell with the generation of H₂O₂, followed by the addition of Fe(II) salt in the other reactor (ex-cell electro-Fenton process).An optimized cell design with no gap between the membrane and the anode, along with the appropriate choice of supporting electrolytes, ensured a voltage reduction with a membrane cell in comparison with that of an undivided cell. The accumulation of hydrogen peroxide in concentrations sufficient for the almost complete destruction (90–98%) of aromatic organic pollutants was achieved in all cases but the ex-cell process with the preparative electrolysis in the pilot scale membrane reactor separated by the proton-exchange membrane MK-40 showed higher treatment efficiency and lower specific energy consumption in comparison with known technologies. Damage of the gas-diffusion layer was observed in some tests which could be caused by alkaline conditions in the pores of the gas-diffusion cathode (GDE). The pH indicator paper showed a color specific for alkaline media in contact with the GDE treated in the solution with pH 3 in the bulk. A possible explanation could be that even in acid media, hydrogen peroxide generation in pores of the gas diffusion layer proceeds with formation of HO ₂ ^? which is common for alkaline media and consecutive protonation occurs at the interface with the acid solution.     

Preparation of a Gas Diffusion Electrode by Ozone Gas Pretreatment and an Ion-exchange Method

Surface oxidation using ozone gas, produced by an electrolytic ozone generator, was applied for preparation of a gas-diffusion electrode (GDE) for an electrochemical energy conversion system. An uncatalyzed carbon sheet containing poly(tetrafluoroethylene) binder was first placed into contact with ozone gas to form active functional groups on the surface of the carbon; then ion-exchange between a weakly bound hydrogen of the functional groups and a platinum cation complex was performed. A GDE having highly dispersed particles of a platinum metal deposited on a porous carbon sheet ws developed by this method. The fuel cell using this GDE showed high performance.     

Macrokinetic analysis of polarisation characteristics of gas-diffusion electrodes in contact with liquid electrolytes, Part II: Oxygen reduction as example for a higher order reaction

Classical partial oxidation processes often suffer from low selectivities. Promising alternatives are electrochemical processes where the oxidation takes place at a packed-bed anode while an oxygen-consuming gas–liquid membrane is used as cathode. As a basis for the reliable design of such a process, the performance of oxygen-consuming gas-diffusion electrodes (GDE) is investigated experimentally and is analysed based on a rigorous model accounting for the reaction microkinetics and all relevant mass and charge transport phenomena. The results indicate that oxygen is transported in the gas-filled pores by Knudsen diffusion and that the cathodic oxygen reduction follows a parallel reaction scheme forming hydrogen peroxide at the carbon black support and water at the applied platinum catalyst particles.     

Dynamic gas-diffusion electrodes for oxygen electroreduction to hydrogen peroxide

Flow-type electrolytic cell is a viable system for many electrochemical processes, in which gas-diffusion electrode (GDE) plays a vital role in providing optimal reacting interfaces for improved performances. Different from traditional GDEs with fixed porosity, here we propose a new type of GDE that features flexible diffusion channels. Particularly, the GDE has been assembled by pairing PTFE substrate with Ag-TCNQ/graphene hydrogels that can be further manipulated through capillary compression. With dynamic adjustment, the average pore size inside GDE ranges from 1750 to 125 nm, and the packing density varies from 56.81 to 446.07 mg cm⁻³. As a consequence, the as-assembled flow-type electrolytic cell can display excellent performances, with a yield rate of 5572 mmol g_cat⁻¹ h⁻¹ and Faradic efficiency of 86.4% at 0.267 V (vs. RHE). In addition, a photovoltaic (PV)-driven flow-electrolytic system has been assembled that achieves a record solar-to-hydrogen peroxide efficiency of 14.8%.     

Macroscopic analysis of polarization characteristics of gas-diffusion electrodes in contact with liquid electrolytes: Part I: First order reactions

In this paper principles of gas-liquid chemical reaction engineering are applied to analyse the current-potential characteristics of gas-diffusion electrodes (GDE) in contact with liquid electrolytes. A macroscopic electrode model is formulated which accounts for mass transfer in the external diffusion films, in the gas layer and in the flooded layer. The set of model equations accounts for material balances, mass transport kinetics and Butler-Volmer polarization kinetics. Several dimensionless parameter groups are introduced which allow a compact reformulation of the proposed model. For first order reactions its solution can be derived analytically. The introduced parameter groups allow a classification of the different operating modes of a GDE, that is, slow reaction, fast reaction and instantaneous reaction.     

Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chlor-alkali electrolysis

Application of a gas-diffusion type oxygen cathode will contribute to energy saving in chlor-alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80 °C and 1 atm O₂, but loading of 2 mg cm^–2 Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four-electron reduction to produce OH^–, while H₂O₂ is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H₂O₂ has been demonstrated by conducting CV measurements to detect H₂O₂ in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor-alkali electrolysis.     

Effect of diffusion-layer porosity on the performance of polymer electrolyte fuel cells

The gas-diffusion layer (GDL) influences the performance of electrodes employed with polymer electrolyte fuel cells (PEFCs). A simple and effective method for incorporating a porous structure in the electrode GDL using sucrose as the pore former is reported. Optimal (50 w/o) incorporation of a pore former in the electrode GDL facilitates the access of the gaseous reactants to the catalyst sites and improves the fuel cell performance. Data obtained from permeability and porosity measurements, single-cell performance, and impedance spectroscopy suggest that an optimal porosity helps mitigating mass-polarization losses in the fuel cell resulting in a substantially enhanced performance.     

Influence of rib spacing in proton-exchange membrane electrode assemblies

A two-dimensional design analysis of a membrane-electrode assembly for a proton-exchange membrane fuel cell is presented. Specifically, the ribs of the bipolar plates restrict the access of fuel and oxidant gases to the catalyst layer. The expected change in cell performance that results from the partial blocking of the substrate layer is studied by numerical simulation of the oxygen electrode and the membrane separator. The effects of rib sizing and the thickness of the gas-diffusion electrode on the current and water distributions within the cell are presented. For all of the cases considered, the two-dimensional effect only slightly alters the half-cell potential for a given applied current but has a significant influence on water management. Concentrated solution theory with variable transport properties is used in the membrane electrolyte to solve for the electrical potential and local water content. The Stefan-Maxwell equations are used in the gas-diffusion electrode to determine the local mole fractions of nitrogen, oxygen and water vapour. A control-volume formulation is used for the resolution of the coupled nonlinear differential equations. One advantage of the control-volume approach over finite-difference methods is the relative ease in which internal boundary points in fuel-cell and battery models are handled. This and other advantages are briefly discussed.     

A two-compartment cell for using soluble benzoquinone derivatives as active materials in lithium secondary batteries

In this study, soluble redox couples were used as active materials for an electrode using a newly designed two-compartment cell. In this cell, liquid electrolyte was separated by a solid electrolyte diaphragm, which prevents dissolved active materials from reaching the counter electrode. To balance the apparent current density and the apparent energy density, a porous sheet made of carbon paper as a current collector was set on the side of the positive electrode with an active material impregnated into it, and Li foil was set on the side of the negative electrode. Some soluble benzoquinone derivatives were examined by charge/discharge cycling for use as active materials of the positive electrode in lithium secondary batteries. Some of them showed specific capacities close to the theoretical values, assuming two-electron reduction. Among them, 2,5-dipropoxy-1,4-benzoquinone (DPBQ) could be cycled regardless of whether the amount used exceeded the solubility (with precipitate in the electrolyte) or not (all is dissolved). This implies that the active material reacts at the surface of the current collector in the dissolved state, and the precipitated fraction also participates by dissolution into the electrolyte. The results also suggest that a good cycle performance using our two-compartment cell requires an active material with relatively high solubility.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

Reduction of carbon dioxide in 3-dimensional gas diffusion electrodes

Experiments on the electrochemical reduction of CO₂ were carried out by using a Cu/PTFE-bonded gas diffusion electrode (GDE) to investigate the effect of solvents, Cu/C ratio and electrolyte concentration on the characteristics of reduction products The experimental conditions were a voltage range from -2 0 to -3 5 V vs saturated calomel electrode (SCE) and electrolyte concentration from 0 1 M to 0 5 M Significant performance differences were found between water and organic solvent (isobutanol + EtOH) The GDE was more active with water than that with organic solvent And then, in the case of the Cu/PTFE-bonded gas diffusion electrode using organic solvent, the maximum faradaic efficiencies of reduction products were achieved in a Cu/C ratio of 0 5 The faradaic efficiency of C₂H₄ among the reduction products decreased as the Cu/C ratio increased, whereas, those of CO and alcohol increased Since the difference in pH at the electrode influences the variation of product selectivity, the faradaic efficiency of C₂H₄ might increase due to the pH increase caused by electrolyte concentration difference     

Gas diffusion electrodes for high temperature PEM-type fuel cells: role of a polymer binder and method of the catalyst layer deposition

The topic of this study is the optimization of the preparation procedure and chemical composition of a gas diffusion electrode (GDE) for utilization in high-temperature PEM fuel cells. A phosphoric acid-doped polybenzimidazole derivative membrane was used as a polymer electrolyte. The following parameters were studied: nature and content of the polymeric binder (PTFE—hydrophobic, PBI—hydrophilic) in the catalytic layer (CL) and concentration of platinum in catalytic powder (affecting the thickness of the CL). Brushing and spraying were selected as the most suitable techniques of CL deposition. Surprisingly, both polymeric binders investigated in the framework of this study were found to provide a similar GDE performance for CL deposited on the gas diffusion layer surface by spraying.     

杂质离子对固体聚合物电解质水电解槽性能的影响

研究了Ni2+、Cu2+、Ca2+、Mg2+等杂质离子对固体聚合物水电解槽性能的影响.在电解槽阳极室中分别加入这些杂质离子的硫酸盐,分析槽电压、阳极和阴极电势随时间的变化关系.结果显示杂质离子对电解槽有极大的损害.槽电压的迅速上升主要是由于阴极超电势的增加而造成.其原因是Ni2+、Cu2+在阴极催化剂与Nafion膜的界面上沉积为金属,覆盖阴极催化剂,减小了反应活性位;而Ca2+、Mg2+则形成一层导电性差的氢氧化物膜,不但覆盖催化剂,减小了反应活性位,而且增加了阴极催化剂与Nafion膜界面的接触电阻,导致阴极超电势的迅速增高.     

Mass transport in a hydrogen gas diffusion electrode

Experimental data are presented concerning the diffusion-limited current density for hydrogen oxidation in a gas diffusion electrode (GDE) under various conditions. These current densities were obtained using mixtures of hydrogen and inert gases. To elucidate the dependence of the overall mass transport coefficient on the gas phase diffusion coefficient and the liquid phase diffusion coefficient of the hydrogen, a simplified model was derived to describe the transport of hydrogen in a GDE based on literature models. The GDE consists of a hydrophobic and a hydrophilic layer, namely a porous backing and a reaction layer. The model involves gas diffusion through the porous backing of the GDE combined with gas diffusion, gas dissolution and reaction in the reaction layer of the electrode. It was found that the transport rate of hydrogen under the experimental circumstances is determined by hydrogen gas diffusion in the pores of the porous backing, as well as in the macropores of the reaction layer. Diffusion of dissolved hydrogen in the micropores of the reaction layer, through the liquid, is shown to be of little significance.     

The value of mixed conduction for oxygen electroreduction on graphene–chitosan composites

Graphene-based electrocatalysts have been widely investigated for their excellent performance in electrocatalytic oxygen reduction. The surface chemistry of graphene-based electrocatalysts is important for developing more efficient fuel cells and metal-air batteries. In addition, the nanostructured gas-diffusion electrode (GDE) on which the electrocatalysts are loaded needs to be carefully tailored to facilitate mass transport (reactants and products). A polymer binder is often used to fabricate the GDE which means there is a need to optimize the ratio of binder to electrocatalyst. Herein we demonstrate the impacts of graphene-based GDE nanostructures on the efficiency of oxygen electroreduction by comparing a series of graphene/chitosan composites with varying compositions. In these nanostructured GDEs graphene acts as the electrocatalyst and chitosan as the binder. Our results illustrate a critical ratio of graphene to chitosan for enhanced electrocatalytic surface area and facilitated mass transport, while a continuous network for electron conduction is effectively established. We believe this work is an important piece of the puzzle to better understanding the electrode behavior of electrocatalysts consisting of graphene-like two-dimensional materials in oxygen reduction reaction.     

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC₍₁₎ and WC₍₂₎, which differ considerably in their specific surface area (0.5 and 6 m² g^–1), was carried out. The H₂–air gas mixture (H₂ 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (i _d(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC₍₁₎ catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC₍₂₎ and Pt) but their overall stability is much better.     

Capacitive performance of an ultralong aligned carbon nanotube electrode in an ionic liquid at 60 °C

An ultralong (1.0 mm) aligned carbon nanotube (ACNT) electrode was fabricated by a cut/paste method. The electrode retains the intrinsic properties, including robust mechanical property, high surface area, and regular pore structure, of individual nanotubes. Electrochemical properties of the ACNT electrode in an ionic liquid (IL) electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance spectroscopy. The ACNT electrode achieved a specific capacitance of 27 F/g, had excellent rate capability, and a long cycle life at 60 °C, indicating that an ACNT electrode/IL electrolyte electrochemical double layer capacitor is promising for high temperature (60 °C) applications. The capacitive performance of ACNT electrode is excellent, because it possesses large pores and regular pore structures, which is revealed by N₂ adsorption and scanning electron microscopy.     

Electrochemical behaviour of the solid electrolyte Ag6I4WO4

Electrochemical investigation of the solid electrolyte Ag₆I₄WO₄ (0.8 AgI + 0.2 Ag₂WO₄ mixture) was performed. By means of cyclic voltammetry, normal pulse polarography and ac polarography methods the electrode processes and the capacity of electric double layer on electrode/electrolyte interface were investigated at several temperature levels (room temperature, 373 K, 386 K and 423 K). It was shown that on the electrode, either a Pt or Ag one, at direct and reverse cathodic polarization, Ag⁺ ? Ag redox process occurs, and for it a substantial exchange current value was found. Hence it is concluded that the redox process at all the observed temperatures is very fast and that the rate increases with a rise in temperature. Also the passivation of the electrode at anodic polarization affecting the electrolyte decomposition voltage level considerably was pointed out. The electric double-layer capacity values found show that the structure of the layer changes with temperature and dc potential, which is manifested on the capacity.     

Structural optimization of gas diffusion electrodes loaded with LaMnO3 electrocatalysts

In this study, gas diffusion electrodes (GDEs) with two catalyst layers were fabricated and tested for their electrode performance for oxygen reduction in an alkaline solution. The LaMnO₃/carbon black catalyst layers were fabricated using a reverse micelle method to finely disperse the LaMnO₃ particles onto the carbon matrices, for which commercial Ketjen Black (KB) (1270 m² g⁻¹) and Vulcan XC-72R (VX) (254 m² g⁻¹) were used. The three-layer-structured GDE with the two LaMnO₃/KB and LaMnO₃/VX catalyst layers exhibited a superior oxygen reduction activity when compared to that of a conventional GDE with only one LaMnO₃/KB catalyst layer. Pore size distribution and gas permeability measurements revealed that the LaMnO₃/VX layer was more porous and had higher gas permeability than the LaMnO₃/KB layer. These results suggest that the intermediate layer of LaMnO₃/VX can efficiently supply oxygen to reaction sites dispersed in the LaMnO₃/KB and LaMnO₃/VX catalyst layers, which consequently leads to an improvement in the electrode performance.     

Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC

A kind of composite cathode, La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ-Ce_0.8Sm_0.2O_2−δ (LSCF-SDC), was presented in this paper. The electrochemical performance of the cathode on the electrolyte of SDC and YSZ coated with a thin SDC (YSZ/SDC) layer was studied by electrochemical impedance spectroscopy (EIS) and cathodic polarization techniques for their potential utilization in the intermediate temperature solid oxide fuel cell (IT-SOFC). Also studied was the relationship between the electro-catalytic characteristics and the electrode microstructure. Results showed that the LSCF-SDC composite electrode performed better on the SDC electrolyte than on the electrolyte of YSZ/SDC. The polarization resistance, R_p, of the cathode on the SDC electrolyte was 0.23 Ω cm² at 700 °C and 0.067 Ω cm² at 750 °C, much lower than the corresponding R_p of the same cathode on the YSZ/SDC electrolyte. At 750 °C, the cathodic overpotential of the composite cathode on the SDC electrolyte was 99.7 mV at the current density of 1.0 A cm⁻².