Redox flow batteries (RFBs) can employ various carbon materials as electrodes. A carbon electrode must meet a number of requirements when RFBs are constructed. This short review focuses on carbon electrodes with desirable electrode materials for RFBs. They have attributes for active species that assist the construction of stable RFBs for long-term use, realize much higher energy densities, employ a wide range of active chemical redox materials, and achieve a total cost reduction for each of the different types and differently modified carbon electrodes. Here, we summarize recent approaches employed in the search for carbon materials and the development of methods for modifying carbon surfaces for RFBs.

Voltammetric properties of chlorine dioxide (ClO₂) obtained by using a glassy carbon (GC) electrode fabricated by electrolyzing in ammonium carbamate aqueous solution were first reported. GC electrode surface was covalently modified with nitrogen atoms containing functional groups, which were introduced onto GC surfaces by electrochemical oxidation process in ammonium carbamate aqueous solution. The introduction of nitrogen into the GC surface structure enhanced the electrocatalytic activities of GC electrode towards the electro-oxidation of ClO₂ in an acidic medium more than a bare GC electrode does. A favorable linearity for the peak current signals in cyclic voltammograms was exhibited in the concentration range from 2.0 to 100 ppm.

To achieve an efficient hole injection into triphenylamine derivatives cast on an Au electrode, we focused on the use of Fullerene (C₆₀)-doped triphenylamine derivative as a buffer layer. After measuring the current-voltage properties of a layered device with the Au/C₆₀-doped triphenylamine derivative/triphenylamine derivative/Au, hole injection was improved only at the interface where C₆₀ was introduced. In addition, when 1 mol% of C₆₀ was doped, the energy barrier for the hole injection decreased to 0.06 eV from 0.43 eV. Overall, we successfully developed a device with enhanced rectification properties.

The novel one-pot pyrolysis method with ionic liquid (IL one-pot pyrolysis method) was developed in our previous study to prepare a PtNi alloy nanoparticle-supported multi-walled carbon nanotube (PtNi/MWCNT) composite. Herein, it was found that the PtNi/MWCNT composite could be prepared by IL one-pot pyrolysis method even if an insoluble nickel precursor with crystal water, nickel(II) oxalate dihydrate (NiC₂O₄·2H₂O), was used. The Ni content and crystal structure of the resulting PtNi/MWCNT composites were similar to those of the composites produced under dehydrated and homogeneous conditions. It means that solubility of Ni precursor and existence of crystal water in the precursor had an insignificant effect on the production of the PtNi/MWCNT composite.

The fabrication of low-temperature waste heat power conversion modules will require the development of thermoelectric materials based on mass-produced nanotubes such as super-growth carbon nanotubes (SGCNTs), rather than high-quality nanotubes generated on the laboratory scale. In this work, SGCNT films co-loaded with colloidal ZnO (which has a high Seebeck coefficient) and Ag (which enhances electrical conductivity) were prepared to optimize both carrier concentration and mobility. The resulting carbon-based hybrid films were found to have a p-type power factor of 100.4 µW m⁻¹ K⁻² at 383 K, which represented one of the highest values yet reported for a SGCNT system.

Ni-doped covalent triazine frameworks (Ni-CTF) has been known as an efficient electrocatalysts that achieve conversion of carbon dioxide (CO₂) to carbon monoxide (CO) with the faradaic efficiency (FE) exceeding 90% in neutral electrolytes. Here we report that the FE can reach 60% even in acidic electrolytes, where the hydrogen evolution can proceed competitively, by loading Ni-CTF on gas diffusion electrode (GDE). In the presence of Zn(II) ions, zinc hydroxide analogues that can be formed only in alkaline conditions were deposited on the GDE during the CO₂ reduction in an electrolyte with a bulk pH of 2. On the other hand, when a normal plate electrode (PE) was used, the FE was only 1.2% in an electrolyte with the same pH, and the zinc hydroxide analogues were not formed. These results indicate that the local pH around GDE increased during the cathodic process, which led to an increasing FE of CO₂ reduction even in an acidic electrolyte.

The performance of the graphite anode of lithium-ion batteries is greatly affected by the solid electrolyte interphase (SEI) generated at the first charge. However, there are few studies on the kinetics of the lithium-ion intercalation/de-intercalation reaction in graphite to investigate the effect of SEI. In this study, the correlation between the interfacial lithium-ion transfer resistance (R_ct) and the double layer capacitance (C_dl) of graphite composite electrodes coated with various SEIs was investigated. It was found that the value of 1/R_ctC_dl was different for each SEI, that is, the frequency (or rate) of intercalation and de-intercalation of lithium ions into graphite was different for each SEI. The activation energy of R_ct was almost the same for all the electrolyte solutions. These results indicate that the pre-exponential factor of the Arrhenius equation governing the rate of interfacial ion transfer in a practical graphite anode is dependent on the nature of SEI.

Recently, the single-walled carbon nanotube (SWCNT) has been the focus as a durable electrode with a high rate performance for the electric double layer capacitor (EDLC). However, it has not yet been clarified whether these reported outstanding properties are universal for all SWCNTs. The authors compared various self-standing SWCNT membranes (buckypapers) as the EDLC electrode to evaluate their stability to a floating high-voltage charge. Some SWCNTs exhibited an excellent rate performance even after the floating durability test, but other electrodes were degraded and showed a lower capacitance-retention. This variability in the durability for the SWCNTs can be attributed to the presence of residual metal impurities. Thus, it should be noted that the purity is a significant factor when using SWCNTs as the EDLC electrode, in addition to the nano-structure design from the viewpoint of operation stability.

Three types of widely used carbon materials were examined as scaffolds for the direct-electron-transfer (DET)-type bioelectrocatalysis of bilirubin oxidase (BOD) as an electrocatalyst for a 4-electron reduction of oxygen (O₂). The carbon materials used were: Ketjen Black EC300J (KB) with a primary particle size (ϕ_p) of ca. 40 nm and a hollow structure, Vulcan XC-72R (Vulcan) with ϕ_p of 37 nm and a filled structure, and high purity graphite SP series (JSP) with ϕ_p of 10 µm and well-developed micropore structures. For the three carbon materials, the rotating disk steady-state limiting catalytic current density of the O₂-reduction (|j_c,lim|) increased with the non-Faradaic current (|j_b|) at small |j_b| values and was saturated at large |j_b| values. The |j_c,lim/j_b| ratio in low |j_b| range was in the following order: JSP ≫ Vulcan > KB. Electrochemical and microscopic data suggested that microporous structures of JSP are highly effective for the DET-type reaction of BOD. Gaps between several primary particles in the KB and Vulcan aggregates play important roles as scaffolds for BOD. The inner surface of partially broken KB particles is electrochemically active to give large |j_b| but not effective as BOD scaffolds.

Fe₇S₈@C composite materials are facilely fabricated using the gelatin, FeSO₄·7H₂O and Na₂S·9H₂O by one-step method. The obtained Fe₇S₈@C composite materials show the fabulous rate performances and cycling performances, when controlling the carbon contents in Fe₇S₈@C composite materials. For instance, the Fe₇S₈@C-40 shows the cycling performance at 657.3 mAh/g, after carrying out the charge-discharge cycles 400 times. These electrochemical performances lead us to consider our provided preparation method is the effective way to facilitate the application of Fe₇S₈@C in fabrication of lithium ion batteries (LIBs) as negative electrode materials.

The fouling of electrodes with large molecules and/or electrochemically oxidized biomolecules limits the potential applications of bioelectroanalysis. The present work examined the effects of various plasma treatments when applied to sputtered carbon film electrodes. This was done by assessing the electrode responses to serotonin, a neurotransmitter that can be adsorbed after electrochemical oxidation. Although the oxidation current of serotonin rapidly decreases after five potential cycles with 5-minute intervals at the untreated and H₂O plasma-treated carbon films, the magnitudes of the currents were almost unchanged at the films after NH₃ and NH₃/H₂O gas plasma treatments. Cyclic voltammetric measurements of Fe(CN)₆^3−/4− before and after serotonin oxidation revealed that there are no detectable fouling with oxidized serotonin by either the NH₃ or NH₃/H₂O treatment, unlike the case with untreated and H₂O plasma-treated carbon film electrodes. Similar to the serotonin response, both 5-hydroxyindoleacetic acid and 5-hydroxytryptophol show improved stable responses after the NH₃/H₂O plasma treatment. X-ray photoelectron spectroscopy data suggest that the suppression of fouling might be due to the electrostatic repulsion between the amino group of serotonin and surface charges after the NH₃ and NH₃/H₂O treatments, in addition to the effects of hydrophilic surfaces generated by the plasma treatments.

For hydrogen production by water electrolyzers, iridium dioxide (IrO₂) works as a catalyst for oxygen evolution reaction (OER) at an anode. In this report, we aim to study the formation mechanism of IrO₂ nanoparticles on graphene by inducing nanoscale defects artificially. The defects on graphene grown on a copper foil by chemical vapor deposition were created by UV-ozone treatment, and IrO₂ nanoparticles were deposited by hydrothermal synthesis method. We investigated the amount of defects and oxygen-functional groups on graphene by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The size and distribution of defects and IrO₂ nanoparticles on graphene were analyzed by atomic force microscopy (AFM). Raman spectroscopy and XPS measurement showed that defects and oxygen-functional groups increased with the UV-ozone treatment time. The size of IrO₂ nanoparticles was reduced to ca. 4.5 nm on defective graphene, whereas the nanoparticles deposited on pristine graphene is ca. 8.8 nm in diameter. It is found that the IrO₂ nanoparticles were deposited and anchored on the edge of hole-like defects on graphene. In addition, the size of deposited nanoparticles can be controlled by the extent of modification in graphene.

Novel method of fabricating nitrogen-doped carbon-rich amorphous silicon-carbon alloy nanoparticles was successfully established using radio-frequency (r. f.) plasma-enhanced vapor deposition (CVD) system with a porous aluminum plate set between a cathode and an anode. Nanoparticles (Nps) were fabricated in high-density plasma regions generated in pores of the porous aluminum plate. Sizes of Nps were dependent on the transit time required for nuclei to pass through high density plasma regions during CVD synthesis. The average diameter of nitrogen-doped (N-doped) carbon-rich amorphous silicon-carbon alloys (C-rich a-SiC) Nps was successfully controlled within the range from 271.2 to 14.8 nm by changing the conditions of CVD synthesis: transit time, r. f. power, chamber pressure, and plate thickness. The optical gaps of C-rich a-SiC Nps were satisfactorily controlled by changing Si/C ratio at amorphous Si-C network in C-rich a-SiC (changing Si/C ratio of the source material used in CVD synthesis). The optical gaps of C-rich a-SiC Nps were controllable from 2.04 to 1.19 eV. The Nps showed photon-to-current conversion functionality in the photoelectrochemical measurement under 360 nm irradiation. In other words, C-rich a-SiC Nps are applicable to devices using photon-to-electron conversion in nano-meter size.

In the present study, we aim to synthesize heteroatom (nitrogen or boron) doped-reduced graphene oxide (N-rGO or B-rGO) as a catalyst for the electro-oxidation of hydroquinones, used as a candidate of fuel (hydrogen carrier molecule) for direct-type fuel cells (DFCs), and evaluate the doping effect on its catalytic activity. N-rGO and B-rGO were prepared from a mixture of graphene oxide (GO) and urea or boron trioxide by pyrolysis method. We characterized the morphology and crystal structure of the prepared materials by transmission electron microscopy, and X-ray diffraction, respectively. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy show the loading amount of the heteroatoms, 10.4 wt% N and 2.9 wt% B, as well as their chemical nature. The electrochemical analysis of the prepared materials by rotating disk electrode system reveals high activity of B-rGO, 15 and 85 mV lower overvoltage compared with rGO at the half-wave potential of diffusion-limited current, for the electro-oxidation of hydroquinone and methyl-hydroquinone, respectively, because of its electron-accepting nature. We demonstrate that thus modified carbons exhibit high activity, B-rGO > N-rGO > rGO, for the oxidation of hydroquinone derivatives as non-metallic anodes of DFCs.

Recent developments of low-power electronic devices have triggered interests towards small-scale energy storage. One promising approach is to fabricate micro-supercapaciors (MSCs). In this work, we demonstrate a facile fabrication of MSCs by laser reduction and patterning of graphene oxide nanoribbon (GONR) thin films coated on poly(ethylene terephthalate) substrate through LightScribe technique. We fabricated the in-plane geometry electrodes consisting of reduced GONRs (i.e., graphene nanoribbons, GNRs) with the lateral spatial resolution of approximately 20 µm in addition to the stacked geometry electrodes for comparison. The fabricated in-plane-GNR electrode device showed superior electrochemical properties compared with the stacked-GNR electrode ones. Our impedance measurements supported that this is due to high lateral ion diffusivity along the basal plane of GNRs. In addition, GNR-based in-plane-electrode device also showed a higher capacitance than the graphene-based one, which is due to the more efficient edge effects of GNRs.

Variations in the time courses of the activation of fully-printable carbon-based multi-porous-layered-electrode perovskite solar cells (MPLE-PSCs) can lead to differences between the photocurrent density (J_sc) values obtained from one-sun photocurrent density-voltage (J–V) measurements and from incident-photon-to-current efficiency (IPCE) integration when using monochromic light. In the present work, the J_sc calculated from IPCE data was initially equal to half that obtained from one-sun J–V measurements. However, equivalent values were obtained when the J–V measurements were performed after 10 min of irradiation by a white light LED from the side of the device. This finding will be very important with regard to permitting accurate photovoltaic evaluation of MPLE-PSCs in the future.

New porous carbon materials are proposed in which washi, the Japanese paper as well as cotton and other cellulose materials are carbonized by new method, retaining their original fibrous structures. Porous carbon sheets thus fabricated are used as gas diffusion layer in fuel cells. Fuel cell testing is conducted and the optimum conditions of fabrication are investigated. Low cost and superior characteristics of new carbon structure are demonstrated based on morphological study and fuel cell polarization data.

The metal air battery using an expanded natural graphite sheet as the cathode and galvanized plate as the anode was investigated as a power units for emergency. The discharge energy increased to 8 mW/cm³ from 5 mW/cm³ by optimizing the thickness of 0.8 mm and density of 0.5 g/cm³ for the expanded natural graphite sheets. The DC voltage proportionally and theoretically increased by increasing the number of battery cells. The capacity of this battery also increased by increasing the cell size for a case of less than 200 cm². We hope that this battery can be used in many applications, such as a backup power supply, emergency battery, etc., because a DC 12 V output can be obtained for 15 cells connected in series, and the cathode and anode materials can be stored for a long time in the atmosphere.

Relaxation analysis has been carried out on fresh and charge-discharge cycled graphites to evaluate the effect of cycle process on the relaxation behavior after lithium insertion. While the formed stage I at charging partly transforms into stage II during relaxation, the cycled samples exhibit smaller fraction of transformation. The charge-discharge cycles also restrict the relaxation variation in c-axis of stage II, even though lithium occupation ordering similarly occurs between the fresh and cycled samples. Variation in c-length of stage II would be the results of stage transformation from I into II, and charge-discharge process enables to follow the equilibrium stages rapidly at charging.

Graphene-like graphite prepared by heating graphite oxide under vacuum at 800 °C was fluorinated by elemental fluorine in the presence of HF at room temperature. The interlayer spacing of the resulting material was 0.639 nm and it showed C_xF type characteristics. The fluorine content of it (x = 1.7) was higher than that obtained from natural graphite (x = 2.3). The discharge capacity of it as a cathode of lithium primary battery reached 940 mAh g⁻¹ at a low current density, which was 50% larger than the theoretical capacity based on the 100% discharge of fluorine.

This study reports a novel electrochemical sensing technique based on electro-deposited Pt particles on glassy carbon electrodes modified with nitrogen atoms containing functional groups (PtNF-GC) electrode for detecting sulfite in aqueous solution. PtNF-GC electrodes have a favorable electrocatalytic activity of sulfite oxidation, which moves the oxidation peak potential to the negative direction of potential unlike Pt disk, a bare glassy carbon electrode and electro-deposited Pt particles on glassy carbon (Pt-GC) electrode. The linear relationship between the oxidation current peak and sulfite concentration is over the range up to 10 mM with a detection limit of 100 µM. Our prepared PtNF-GC electrode was applied for the sulfite detection in sample solution with favorite recovery of sulfite. Additionally, a possible reaction mechanism of sulfite was discussed.

Nanoporous gold fabricated via anodization in a buffer solution comprising potassium chloride served as an effective scaffold for the direct electron transfer-type bioelectrocatalysis of bilirubin oxidase from Myrothecium verrucaria (BOD). BOD adsorbed on a porous gold electrode showed an increased population of effective orientations required for direct electron transfer reactions. The catalytic current of BOD reached an oxygen-transfer-limited value at the nanoporous gold electrodes despite neutral conditions.

This report presents simple, rapid and an efficient electropolishing of Ti metal in choline chloride-based ionic liquid (so called Ethaline) at 20 °C. This electrolyte is relatively benign and environmentally friendly which is desirable for electropolishing. Potentiostatic method was applied to Ti electropolishing in this electrolyte. Under an electrochemical condition of 6–10 V for 30 min. and at 20 °C, a promising electropolishing process was performed without obvious gas evolution. To characterize the electropolished surface, atomic force microscope (AFM) and scanning electron microscope (SEM) were used. The achieved an apparent mirror-like finished surface with an average surface roughness (Ra) was 5.7 nm which is considerably different from that of unpolished one (Ra = 118.8 nm). The microscopic results showed leveling and brightening of the surface of Ti by carrying out the current procedure. This electrolyte provides sufficient environment to dominate the mass transport mechanism which is responsible for reduction in the surface roughness.

Electrochemical conversion of CO₂ gas emitted to the atmosphere to useful chemicals has been expected to suppress the global greenhouse effect and to conserve the natural resources. For the electrochemical reduction of CO₂ on an Ag electrode, the effect of the addition of 1-ethyl-3-methylimidazolium ethyl sulfate (EMISE) ionic liquid to the aqueous solution of 0.1 mol dm⁻³ K₂CO₃, and the effect of the polarization methods, i.e., the potentiostatic polarization and the potential-pulse polarization, on the reduction efficiency were investigated. From the gas chromatography measurement, CO as the main product of CO₂ reduction and H₂ as a by-product of water decomposition were obtained. Faraday efficiency of CO formation obtained by the potential-pulse polarization was considerably higher than that obtained by the potentiostatic polarization. The addition of EMISE could add further improvement in the efficiency of CO formation higher than 70%. Such improvement provided by the potential-pulse polarization method and the addition of EMISE was interpreted by the depletion and recovery of the reactants on the electrode in the pulse cycle and promotion of CO₂ molecular formation from HCO₃⁻ ions in the dense adsorption layer of EMIM cations on the electrode surface.

NASICON-type Na₃V₂(PO₄)₃ is a promising cathode material for Na-ion batteries. Although it is well known that two Na⁺ can be extracted from Na₃V₂(PO₄)₃ by charging the cathode material, an electrochemical three-Na⁺ extraction has not been reported yet, to the best of our knowledge. In this work, we studied factors that limit the three-Na⁺ extraction from Na₃V₂(PO₄)₃. In DFT calculations, the voltage of the third-Na⁺ extraction is predicted to be more than 4.5 V vs. Na⁺/Na⁰, which is above the potential windows of the conventional organic electrolytes. Our study of Na₃V_1.5Al_0.5(PO₄)₃ reveals that such a high voltage is required when Na ions are extracted from Na1 sites in the NASICON structure. From NEB calculations, the activation energy of the Na⁺ extraction from the Na1 site is predicted to be 753 meV for NaV₂(PO₄)₃. Ab-initio molecular dynamics simulations also suggest that the Na ions which remain in NaV₂(PO₄)₃ are kinetically locked up in Na1 sites. Our results indicate that the three-Na⁺ extraction is limited due to the high voltage and the large activation energy. We also compare Na₃V₂(PO₄)₃ with Li₃V₂(PO₄)₃, in which the three-Li⁺ extraction has been reported.

Metallic lithium is considered to be the ultimate negative electrode for a battery with high energy density due to its high theoretical capacity. In the present study, to construct a battery with high energy density using metallic lithium as a negative electrode, charge/discharge tests were performed using cells composed of LiFePO₄ and metallic lithium at various lithium utilization values. A relationship was observed between utilization and cycle performance, and the degradation behavior of metallic lithium was evaluated by cross-sectional FE-SEM observations. We also investigated the effects of additives in the electrolyte and found that FEC and VC effectively improved cycle performance.

In order to investigate the corrosion behavior of SUS 304L steel in alkaline solutions, linear sweep voltammogram measurements were performed in NaOH solutions at pH 12 and 13 at 333 K. Constant potential electrolysis was carried out in an NaOH solution at 333 K at pH 13 from 5 to 20 hours. The surface analysis of the samples before and after the constant potential electrolysis was performed by atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The XPS measurements of the samples before the electrolysis showed that Fe and Cr oxide films were formed on the SUS 304L stainless steel surface, and the presence of FeOOH, CrOOH and Ni(OH)₂ on the surface was confirmed after the electrolysis. It is suggested that the dissolved metal ions from the SUS 304L surface reacted with OH⁻ and formed a precipitated film on the surface in the alkaline environment.