Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes

Oxygen plasma-etched nano-honeycomb diamond thin film electrodes were examined for electrochemical capacitor applications in non-aqueous electrolytes. As-deposited and nano-honeycomb diamond electrodes in 0.5 M TEABF₄/PC both exhibited a wide potential window (approx. 7.3 V), similar to that of glassy carbon electrodes. For as-deposited diamond, the impedance behavior was found to be similar for non-aqueous and aqueous electrolytes, and the double-layer capacitance was found to be 21.8 μF cm⁻², almost the same as that obtained in aqueous electrolytes. For the honeycomb diamond electrodes, however, the impedance behavior observed in non-aqueous electrolytes was significantly different from that in aqueous electrolyte and indicated that the ac signal cannot penetrate to the bottom of the honeycomb pores in the non-aqueous electrolytes due to low conductivity, and that not all the surface may contribute to the double-layer capacitance. This result was verified by mathematical simulation.     

TiO2 photocatalysts and diamond electrodes

Photocatalysis and electroanalysis are two seemingly disparate research areas, but they are linked by the fact that both involve the use of well-known materials, TiO₂ and diamond, respectively, in new ways in the service of both environmental and medical sciences. In the present article, recent developments in the area of TiO₂ photocatalysis and diamond electrochemistry are summarized, with emphasis on our findings at the University of Tokyo. In the photocatalysis section, we present the fundamental aspects of TiO₂ photocatalysis and its practical applications, including air purification, self-cleaning surfaces and transparent superhydrophilic coatings. The diamond electrochemistry section deals with the electrochemical characterization and applications of diamond electrodes, which exhibit high sensitivity and excellent stability for electroanalysis, in contrast to conventional electrode materials. A particularly interesting environmental application of diamond electrodes has been developed; this involves the trace analysis of lead without the use of mercury.     

Fabrication and characterization of planar reference electrode for on-chip electroanalysis

A new method for the fabrication of a planar silver/silver chloride reference electrode for electrochemical detection is described. The film microelectrode was directly fabricated on the surface of a glass slide through a combination of electroless deposition and electroplating. Thickness of the electrode film could be easily controlled by setting the repetition times of the deposition. The whole process can be operated in ordinary chemistry laboratory at a low cost, which makes it possible for the development of disposable devices. The electrode potential showed extreme stability in 3 mol/L KCl and water during the tested period of 14 and 10 days, respectively. pH does not have a significant effect on the performance of the electrode. In both potentiometric and amperometric applications, the film Ag/AgCl electrode demonstrated to be a good reference for electroanalysis with its performance comparable to that of commercial reference electrode and the electrodes fabricated from different batches showed good reproducibility.     

Boosting the electrochemical properties of diamond electrodes using carbon nanotube scaffolds

Diamond is a very attractive electrode material for analytical measurements including for instance bio-sensing. However, it suffers from a relatively low double layer capacitance and high impedance when it comes to the development of supercapacitors or neural interfaces, applications for which it could also be extremely promising. One way to increase the double layer capacitance of the material is to increase its specific surface area. Here we propose here to use vertically aligned carbon nanotubes (VACNTs) with high surface areas as a template onto which boron doped diamond is grown. The resulting composite was found to exhibit a double layer capacitance as high as 0.58 mF cm⁻² and very low impedance when compared to planar diamond electrodes in phosphate buffer saline solution. The influence of the VACNT length as well as of the thickness of the diamond coatings on the electrode performances were also investigated and are discussed in this paper.     

Ordered array of diamond ultramicroband electrodes for electrochemical analysis

Boron-doped diamond ultramicroband arrays with different array densities and interelement spacings were fabricated using silicon technology and selective diamond deposition (SAD) technique to yield microvoltammetric electrodes. The electroactive ultramicroband elements were designed with one microscopic critical dimension to impart microelectrode behavior while the other dimension was made larger to yield an increase in signal current. Cyclic voltammetry studies in this work showed that with sufficient interelement separation, the ultramicroband arrays display sigmoidal pseudo-steady-state cyclic voltammograms characteristic of microband electrodes. The ultramicroband arrays yielded higher faradaic current per unit area, than either square ultramicroelectrode array or conventional planar diamond electrode from earlier reported work. This is due to enhanced mass transport to the ultramicroband elements at slow scan rates. Larger current density and higher signal-to-noise (S/N) ratio leads to better limits of detection, making it possible to fabricate a more sensitive electrode for applications such as electroanalysis, electrocatalysis, trace element analysis, mechanistic and fast transfer kinetics studies, electrochemistry in highly resistive media, as well as sensors in flow and biological system.     

NiCo2O4@quinone-rich N–C core–shell nanowires as composite electrode for electric double layer capacitor

The bind-free carbon cloth-supported electrodes hold the promises for high-performance electrochemical capacitors with high specific capacitance and good cyclic stability. Considering the close connection between their performance and the amount of carbon material loaded on the electrodes, in this work, NiCo₂O₄ nanowires were firstly grown on the substrate of active carbon cloth to provide the necessary surface area in the longitudinal direction. Then, the quinone-rich nitrogen-doped carbon shell structure was formed around NiCo₂O₄ nanowires, and the obtained composite was used as electrode for electric double layer capacitor. The results showed that the composite electrode displayed an area-specific capacitance of 1794 mF∙cm^–2 at the current density of 1 mA∙cm^–2. The assembled symmetric electric double layer capacitor achieved a high energy density of 6.55 mW∙h∙cm^–3 at a power density of 180 mW∙cm^–3. The assembled symmetric capacitor exhibited a capacitance retention of 88.96% after 10000 charge/discharge cycles at the current density of 20 mA∙cm^–2. These results indicated the potentials in the preparation of the carbon electrode materials with high energy density and good cycling stability.     

铁基触媒合成金刚石形成的金属包膜成分的研究

利用电子探针(EPMA)和X射线光电子能谱(XPS)研究了包围金刚石单晶的铁基金属包膜和触媒的成分分布。结果表明,在金刚石生长过程中,接近金刚石单晶的包膜内层中的碳含量是变化的,但均高于接近金刚石的触媒层。然而,与包围金刚石单晶的触媒表面相比,包膜表面碳含量低、铁含量高。分析认为,高温高压下,金刚石生长的碳源主要来自于包膜,但碳并非均匀地在包膜熔体内层向金刚石扩散。结合前期研究发现的“包膜内层无石墨和无定形碳结构”的事实分析,金刚石生长所需的碳极有可能来源于包膜内层铁碳化物的瞬间分解,结果导致包膜表面瞬间碳含量低、铁含量高。     

Preparation and characterization of boron doped diamond electrodes on diamond anvil for in situ electrical measurements under high pressure

A layer of boron doped diamond (BDD) film was deposited selectively on a diamond anvil and employed as electrodes for measuring the electrical resistivity of matter under high pressure. Both heavily doped and lightly doped electrodes were characterized by Raman spectroscopy and scanning electron microscopy. Though the BDD film electrodes contain sp² carbon, it is still suitable for in situ high pressure electrical measurements. The dependability of diamond film electrodes was tested at high pressure up to 36 GPa, by measuring the electric resistance of C60 fullerene powder, and no damage of the electrodes was observed.     

Synthesis of nanocrystalline diamond films using microwave plasma CVD

Diamond is one of the most valuable materials for the industrial applications because of its excellent properties including high hardness, with good electrical insulation and thermal conductivity. Mechanical polishing processes of diamond are difficult and very costly. To limit those costs, it is reasonable to think that the surface roughness of the as-grown diamond film should be as small as possible. In this study, a nanocrystalline diamond film was synthesized on a 4-inch Si wafer at 923 K and methane concentration of 10 vol.%, (H₂/CH₄=100/10 sccm) using a microwave plasma CVD system. In order to increase the nucleation density, the substrate was pretreated by dry scratch method with diamond powder of two sizes (250 nm and 5 nm). The nucleation density was approximately 1×10¹¹ cm⁻². The grown diamond films were analyzed by Raman spectroscopy and X-ray diffraction (XRD). The grain size was observed to be approximately 10 nm by FE-SEM observation. Surface roughness was measured as Rms=8.4 nm by atomic force microscope (AFM). The as-grown properties of those nanocrystalline diamond films were almost efficient for tribological and the optical applications.     

Electric double layer capacitors with gelled polymer electrolytes based on poly(ethylene oxide) cured with poly(propylene oxide) diamines

Using a gel electrolyte for electric double layer capacitors usually encountered a drawback of poor contact between the electrolyte and the electrode surface. A gel electrolyte consisting of poly(ethylene oxide) crosslinked with poly(propylene oxide) as a host, propylene carbonate (PC) as a plasticizer, and LiClO₄ as a electrolytic salt was synthesized for double layer capacitors. Diglycidyl ether of bisphenol-A was blended with the polymer precursors to enhance the mechanical properties and increase the internal free volume. This gel electrolyte showed an ionic conductivity as high as 2 × 10⁻³ S cm⁻¹ at 25 °C and was electrochemically stable over a wide potential range (ca. 5 V). By sandwiching this gel-electrolyte film with two activated carbon cloth electrodes (1100 m² g⁻¹ in surface area), we obtained a capacitor with a specific capacitance of 86 F g⁻¹ discharged at 0.5 mA cm⁻², while the capacitance was 82 F g⁻¹ for a capacitor equipped with a liquid electrolyte of 1 M LiClO₄/PC. The capacitance decrease with the current density was less significant for the gel-electrolyte capacitor. We found that the less restricted ion diffusion near the electrolyte/electrode interface led to the smaller overall resistance of the gel-electrolyte capacitor. The high performance of the gel-electrolyte capacitor has demonstrated that the developed polymer network not only facilitated ion motion in the electrolyte bulk phase but also gave an intimate contact with the carbon surface. The side chains of the polymer in the amorphous phase could stretch across the boundary layer at the electrolyte/electrode interface to come into contact with the carbon surface, thus improving transport of Li⁺ ions by the segmental mobility in polymer.     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Application of diamond electrodes to electrochemical processes

Conducting diamond thin film is a new electrode material that has received great attention recently because it possesses several technologically important characteristics such as an inert surface with low adsorption properties, remarkable corrosion stability, even in strong acidic media, and an extremely wide potential window in aqueous and non-aqueous electrolytes. Thanks to these properties diamond electrodes meet the requirements for a wide range of electrochemical applications. The object of this article is to summarise and discuss the recent results available in the literature concerning the application of diamond electrodes to electrochemical processes such as water treatment and electro-synthesis of organic and inorganic compounds.     

Electron Transfer Kinetics on Boron-Doped Diamond Part I: Influence of Anodic Treatment

Boron-doped diamond electrodes, both as-grown and polarized anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the role played by the surface-state density. Many different treatments were employed: as-grown (BDD_ag), mildly polarized (BDD_mild), strongly polarized in perchloric acid (BDD_severerpar;, and strongly polarized in a sulphuric acid-acetic acid mixture (BDD_AcOHrpar;. Charge transfer processes at the electrode surface were studied by cyclic voltammetry. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe^III/II;(CN)₆rpar; and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H₂Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The active surface area and the reaction rate constants decreased significantly upon anodic polarization. Important drops in the charge carrier concentration on the surface and in true surface area led to hindrance of electron transfer at the electrode.     

Sand erosion of freestanding diamond films prepared by DC arcjet

Sand erosion behavior of freestanding diamond films prepared by high power dc arcjet operating at gas recycling mode was investigated by a laboratory designed blast type apparatus. Four different erodent: glass bids, SiO₂, Al₂O₃, and SiC were used. The speed of the erodent particles was measured by the double disk method. It was found that the erosion behavior for the as-grown and the polished diamond films was quite different. At the initial stage, the erosion rate was rather high for the as-grown diamond film, which rapidly decreased with the increase in erosion time, and eventually reached the steady state erosion stage on prolonged erosion. Whilst for the polished diamond film, the erosion rate was quite low at the initial stage, which rapidly increased with the increase in erosion time, and finally reached the steady state erosion stage after a long erosion time. Surface damage was investigated by SEM observations and Raman spectroscopy. Possible mechanism responsible for the difference in erosion behavior was briefly explained. Comparison of the erosion behavior of freestanding diamond films was also made with the other commonly used IR window materials (Ge, MgF₂, MgAl₂O₄, ZnS and quartz glass). Detailed results were discussed.     

One-step fabrication and capacitive behavior of electrochemical double layer capacitor electrodes using vertically-oriented graphene directly grown on metal

We report on a one-step binder-free fabrication method for electrochemical double layer (EDL) capacitor electrodes consisting of vertically-oriented graphene uniformly grown on a metallic current collector. The double-layer capacitive behavior of the resulting electrode is studied in both aqueous and organic electrolytes. Compared with conventional graphene-based EDL capacitor electrode fabrication methods, this method offers the following advantages: (a) no need to use a binder, (b) open channels for better ion access, and (c) exposed edge planes for improved material wettability. These unique features lead to excellent capacitive behavior in organic electrolytes, including a specific capacitance slightly higher than that in aqueous electrolytes at the same potential scan rate and a high knee frequency (～3174 Hz in the current work).     

竹炭基高比表面积活性炭电极材料的研究

以竹节为原料，在隔绝空气的条件下，经不同温度炭化处理后与KOH混合，制取竹炭基高比表面积活性炭。考察了炭化温度、KOH与竹炭的质量比、活化温度和活化时间等工艺因素对活性炭收率、微孔结构和吸附性能的影响，探讨了竹炭基高比表面积活性炭作双电层电容器电极时的充放电特性及其比电容与各种因素的关系。研究结果表明，控制适宜的炭化、活化工艺条件可制得双电极比电容达55F／g的竹炭基高比表面积活性炭，由它组装的双电层电容器具有良好的充放电性能和循环性能，但内阻过高，大电流下充放电时电容量下降过大。     

Effect of electronic structures on electrochemical behaviors of surface-terminated boron-doped diamond film electrodes

In order to analyze the electrochemical behaviors of hydrogen-terminated and oxygen-terminated boron-doped diamond film electrodes, experiments of the cyclic voltammetry and AC impedance spectroscopy have been performed. For the purpose of clarifying the detailed electronic structures of these diamond films, current–voltage spectroscopy curves and XPS spectra have been investigated by scanning probe microscopy and X-ray photoelectron spectroscopy, respectively. For the hydrogen-terminated boron-doped diamond film electrode, its potential window is narrower than that of the oxygen-terminated boron-doped diamond. The impedance results indicate that the diamond film resistances and capacitances corresponding to different surface-terminated boron-doped diamond electrodes vary significantly. The surface band gap of hydrogen-terminated diamond film is narrow with empty surface states in it. In contrast, the surface band gap of oxygen-terminated diamond film is wide and clean. Compared with hydrogen-terminated diamond film, the surface energy bands of oxygen-terminated diamond film bend downwards. Based on their electronic structures, the electrochemical behaviors of the two boron-doped diamond film electrodes have been discussed.     

New directions in structuring and electrochemical applications of boron-doped diamond thin films

Conductive boron-doped diamond electrodes have been shown to be highly suitable as electrochemical detectors in flow injection analysis and high performance liquid chromatographic analysis, achieving high sensitivity and stability for certain species that cannot be detected at other electrodes due to electrode deactivation or high electrochemical oxidation potential. The use of this electrode material for the detection of chlorophenols and theophylline is demonstrated. Apart from the electrochemistry of diamond, various methods have been developed to fabricate well-aligned nanocylindrical diamond films and periodic bulb-like structures, which may be useful for sensors and electronic devices such as field emission displays.     

Porous diamond with high electrochemical performance

Synthetic diamond materials are currently attracting attention for applications such as thin films supercapacitors or medical implantable electrodes where chemically stable materials featuring high double layer capacitance as well as low electrochemical impedance are sought. Those properties may be reached with high aspect ratio diamond provided that current collection is done efficiently through the diamond layer. In this paper, we introduce a very novel material, namely SPDia™, based on boron-doped diamond grown on a highly porous polypyrrole scaffold prepared by chemical vapour deposition. This composite was first characterised by SEM and Raman spectroscopy to cheque the diamond crystallinity and the structural evolution of the polypyrrole during the CVD process. Then cyclic voltammetry and electrochemical impedance spectroscopy were performed to assess its electrochemical reactivity. It was found to exhibit remarkable properties, that include a large double layer capacitance with values reaching up to 3 mF cm⁻² in aqueous LiClO₄ and a low electrochemical impedance, thus highly competitive with respect to other nanostructured diamond materials as recently reported.     

Fabrication and application of Diachem® electrodes

Conductive diamond coated electrodes have undergone intensive investigation over the past number of years. The unique electrochemical properties of diamond electrodes such as their extreme chemical resistance, even at high doping concentrations, and high overvoltage for water electrolysis opens the door to numerous applications. For the industrial use of extremely promising electrode material sufficient availability, also for large area electrodes is necessary. The development of large area DIACHEM^® electrodes therefore has been performed on a range of base materials of numerous material geometries. For the production of these electrodes a large area HFCVD process is used which allows for reproducible coatings of substrates up to 50×100 cm. A doping with boron allows the reproducible setting of the resistivity of the diamond coating in the range of 5 to 100 m Ω cm. The extent to which DIACHEM^® electrodes have been developed by now means that they can presently be applied in various industrial applications. The most important applications include: industrial waste water treatment, in particular the mineralization of toxic organic compounds; the disinfection of water; circulation systems with water which are only possible through use of an electrochemical recycling process; electrochemical synthesis, in particular from strong oxidising solutions and galvanic processes such as the recycling of chrome baths. Previous investigations have shown that the use of DIACHEM^® electrodes results in either a significant improvement in the effectiveness or that the process was only possible through use of this particular electrode material. The user gains a combination of the typical advantages of the electrochemical process with the high efficiency of the DIACHEM^® electrode which, in turn, has generated enormous interest in this particular material. In order to satisfy this interest, CONDIAS GmbH offers the DIACHEM^® electrode commercially.