Rotating Ring Disk Electrode Voltammetry Research Articles

Rotating Ring-Disk Electrode Voltammetry for Determining the COR–OER Selectivity in Seawater Electrolysis under Neutral to Alkaline Conditions

Hydrogen production through the direct electrolysis of seawater is gaining attention as a method for achieving carbon neutrality. The Cl⁻ anion oxidation reaction (COR) in seawater, which occurs at the counter electrode (anode) of hydrogen production and generates active chlorine species with a corrosive nature, should be suppressed as much as possible for hydrogen production and replaced with the oxidation of water, i.e., oxygen evolution reaction (OER). The COR and OER proceed competitively at the anode; however, no studies have separately investigated their behavior in a pH region close to that of seawater. This study uses a simple electrochemical method with a rotating ring-disk electrode (RRDE) to determine the independent behavior of COR and OER with respect to potential and pH in an aqueous NaCl solution, where IrO2 (COR dominant) and MnO2 (OER dominant) are used as catalysts. The results obtained based on the RRDE voltammetry in aqueous NaCl solutions at various pH values were consistent with the COR selectivity determined from product analysis. Thus, the described method will be a valid electrochemical protocol.

A rotating ring disc electrode study of photo(electro)catalyst for nitrogen fixation.

There are numerous reports of photo(electro)catalysts demonstrating activity for nitrogen reduction to ammonia and a few reports of photo(electro)catalysts demonstrating activity for nitrogen oxidation to nitric acid. However, progress in advancing solar-to-fertilizer applications is slow, due in part to the pace of catalyst screening. Most evaluations of photo(electro)catalysts activity occur using batch reactors. This is because common product analyses require accumulation of ammonia or nitric acid in the reactor to overcome instrument detection limits. The primary aim here is to examine the use of an electroanalytical method, rotating ring disk electrode voltammetry (RRDE), to detect ammonia produced by a nitrogen fixing photo(electro)catalyst. To examine the potential for RRDE, we investigated a photo(electro)catalyst known to reduce nitrogen to ammonia (titania), while varying the applied electrochemical potential and degree of illumination on the disk. We show that the observed ammonia oxidation at the ring electrode corresponds strongly with ammonia measurements obtained from the bulk electrolyte. Indicating that RRDE may be effective for catalyst screening. The chief limitation of this approach is the need for an alkaline electrolyte. In addition, this approach does not rule out the presence of adventitious ammonia.

(Invited) Oxygen Reduction at Low-Pt-Content-Catalysts in Acid Media: Development of Systems and Electroanalytical Diagnostic Methodology

The proton-exchange membrane fuel cell (PEMFC) technology is one of the most promising approaches for energy conversion in automotive applications. Despite the use of expensive noble metal electrocatalysts, the sluggish kinetics of the oxygen reduction reaction (ORR) in acid media, and formation of the undesirable hydrogen peroxide intermediate are still the main drawbacks, on top of the stability problems, when it comes to the widespread commercialization of PEMFC devices. The progress in this subject is greatly hindered by the high cost and scarcity of the state-of-the-art platinum-based materials which are regarded as the most effective cathode catalysts. Therefore, most of the current research studies have been devoted to the optimization of active centers and maximization of their utilization, which should allow the lowering of the cathode Pt loadings without loss of performance and durability. Unfortunately, the problem of electrochemical stability and the danger of generation of higher quantities of the undesirable hydrogen peroxide intermediate may become even more serious in the case of systems utilizing lower amounts of the Pt catalystAmong important strategies is hybridization, activation, and stabilization of carbon-supported Pt catalysts by functionalization through admixing with certain nanostructured and typically substoichiometric metal oxides. Special attention is paid to application of the bi- or multi-metallic Pt-based alloys in their various forms and structures. Additionally, doping of carbon carriers with heteroatoms would produce surface functional groups and active centers capable of not only improving the catalysts’ performance, but also affecting their stability. Among other important issues are such features as porosity, hydrophilicity, and degree of graphitization of carbon components, in addition to the existence of metal–support interactions, high electrochemical active surface area, electronic structure of interfacial Pt, and the feasibility of adsorptive or activating interactions with oxygen molecules.Voltammetric approaches, particularly the rotating ring-disk electrode (RRDE) methodology, are typically applied to diagnose mechanisms and dynamics of ORR. A tempting alternative for recording the electrochemical responses is to integrate the current and to report charge passed as a function of time. Chronocoulometry offers important advantages including good signal-to-noise ratio because the act of integration smooths random noise on the current transients. Furthermore, by integrating the responses, it is possible to separate surface phenomena (e.g. interfacial oxidation of carbon or platinum) more readily from bulk electrochemical responses (ORR). The double-potential-step chronocoulometry permits, in principle, estimation of the number of electrons (n) involved in the oxygen reduction and the percent of formation of the H2O2 intermediate. The problem lies in understanding the reaction kinetics and mechanisms in order to define properly experimental parameters (potential steps and pulse times). Comparative measurements are performed using RRDE voltammetry. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, or ZrO2) have been demonstrated to stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction of oxygen. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627) and under auspices of the European Union EIT Raw Materials ALPE 19247 Project (Specific Grant Agreement No. EIT/RAW MATERIALS/SGA2020/1).

Alternative Me-N-C Based Carbon Foams As Fe-Free and Non-PGM Electrocatalysts

There is a demand for reducing the environmental load of automobiles to solve the current environmental problems. Fuel cell electric vehicles (FCEVs) which use hydrogen as fuel emit no carbon dioxide, and have already been commercialized. platinum-based materials are widely used as oxygen reduction reaction (ORR) catalysts. However, their high cost and limited durability impedes large-scale implementation. Until now, non-platinum group metal (non-PGM) Fe and N doped carbon (Fe-N-C) catalysts have achieved promising ORR activity [1]. However, these Fe-N-C catalysts still suffer from poor durability, due to the formation of reactive peroxide radicals as a side reaction during the ORR. Meanwhile, the use of Co species avoids the production of such radicals, potentially preserving the electrocatalyst [2]. Therefore, in this study we investigate the dependence of ORR activity on a variety of different metal ions by synthesizing alternative Me-N-C carbon foam electrocatalysts (Me = Co, Ni, Sn).Pt-free electrocatalysts were synthesized from nitrogen-doped carbon foams (N-C) and various metal salt and small molecule precursors. Porous nitrogen-doped carbon foams with large surface area were made in-house. These were adsorbed with metal acetate and then heated in inert atmosphere, followed by acid washing to remove metallic phases. Finally, heat treatment under ammonia was performed. The effect of heat treatment temperature and metal loading on the microstructure and porosity was evaluated. The resulting catalysts were evaluated for catalytic activity using rotating-ring disk electrode voltammetry (RRDE) in acid.For Co-N-C electrocatalysts it was found that heat treatment of the nitrogen-doped carbon foam support has a significant effect on catalyst performance, largely due to changes in surface area and porosity. The metal loading also has a strong effect on the performance, with an optimum Co loading of 0.1 wt% resulting in an onset potential of 0.8 VRHE and a half-wave potential of around 0.65 VRHE. Different metal ions were also systematically investigated, and it was found that Sn is highly promising as an alternative to Fe in non-PGM electrocatalysts.

Trimetallic Mn‐Fe‐Ni Oxide Nanoparticles Supported on Multi‐Walled Carbon Nanotubes as High‐Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media

AbstractDiscovering precious metal‐free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen‐functionalized multi‐walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube‐supported trimetallic (Mn‐Ni‐Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth‐abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four‐electrode configuration cell assembly comprising an integrated two‐layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single‐layer bifunctional ORR/OER electrodes after OER polarization.

A Solvent-Controlled Oxidation Mechanism of Li2O2 in Lithium-Oxygen Batteries

Summary Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries.

The use of RRDE voltammetry to study acid-base reactions in unbuffered solutions

Electrochemical mechanisms in unbuffered solutions are difficult to analyze because of the presence of second order reactions. If the protonation reaction is irreversible, then the second order reverse reaction is not kinetically relevant. For protonation on N- or O-atoms, the reaction is often reversible, and leads to an EC reversible mechanism. Rotating ring-disk electrode (RRDE) voltammetry is ideal to investigate this reaction because it is possible to focus on the limiting currents. The limiting currents are neither sensitive to the solution resistance, nor to electron transfer kinetics, but they are sensitive to reaction rates and equilibrium constants in solution. In this report, the theory for the EC reversible mechanism was developed for RRDE voltammetry. Two cases were examined. The first case (Case 1) is the situation where A is reduced to A−, and is then reversibly converted to a new product. An example of this is the reduction of A in a buffered media, where the reduction product is converted to its conjugate acid (HB + A− ⇆ B− + HA). In a buffered media, both the forward and reverse reactions are pseudo-first order. The second case (Case 2) is the situation where the forward reaction is generally (pseudo)first-order, and reverse reaction is second order. The theory was then applied to the reaction of Fe(OEP) (NO)- with 2,6-dichlorophenol (2,6-dcp) in THF in buffered and unbuffered media.

Low temperature synthesis of copper manganese oxide – polyaniline composite: Electrochemical characterizations for oxygen reduction reaction in acid media

ABSTRACTCopper manganese mixed oxide with polyaniline based composite is electrodeposited by chronoamperometric method on Vulcan XC72 carbon paper used as a backing layer in proton exchange membrane fuel cell. The electro deposited catalyst characterized by XRD and SEM shows the formation of Cu1.5Mn1.5O4 with polyaniline coating during the process of polymerization under constant potential in acidic medium. Oxygen reduction reaction (ORR) activity of the catalyst evaluated by linear sweep voltammetry, rotating ring disk electrode voltammetry and scanning electrochemical microscopy measurements established ∼2 e transfer process and significant catalytic surface roughness with promising activity towards ORR.

Electrochemical dioxygen reduction catalyzed by a (nitro)cobalt(perfluorophthalocyanine) complex and the possibility of a peroxynitro complex intermediate

The (nitro)([Formula: see text],[Formula: see text]-dimethyl-4-aminopyridine) complex of perfluorinated cobalt(III) phthalocyanine Co(III)F16Pc(Me2Npy)(NO[Formula: see text] catalyzes the electrochemical oxygen reduction reaction (ORR) in pH 4.0, 7.0, and 10.0 buffer and 0.05 M sulfuric acid solution when deposited on a glassy carbon electrode. Cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE), and rotating ring-disk electrode voltammetry (RRDE) have been used to determine the reduction product as hydrogen peroxide although in concentrations too small to observe by qualitative methods such as oxidation of NaI in solution. The dependence of the values of the peak potentials for the reduction on the pH of the solution and the -log[Me2Npy] are consistent with protonation up to pH 7.6 and pyridine ligand loss during the reduction. The addition of nitrite at 0.1 and 1 M to pH 7.0 solutions in contact with films of CoF16Pc on the glassy carbon electrode decreases the ORR current and shifts the peak potential of the ORR from -0.21 V vs. NHE to -0.19 V vs. NHE. The addition of nitrite at 0.1 and 1 M to films of Co(III)F16Pc(Me2Npy)(NO[Formula: see text] on glassy carbon, however, has no effect on either the current or the potential. While the electrochemical evidence for this proposal is not definitive, modeling has been used to examine the center of reduction in the alternative mechanisms by evaluation of the LUMOs of the hypothetical intermediates in both closed and open shell cases. The formation of five-coordinate Co(II)F16Pc(NO) is proposed to occur initially in the reduction mechanism. It is also possible that O2 reduction takes place at the NO ligand center by way of a nitrogen-bound peroxynitrite intermediate. The [Formula: see text] ligand appears to remain bound during the ORR. Direct coordination of O2 to the metal center requiring a six-coordinate species, Co(III)F16Pc(O[Formula: see text](NO[Formula: see text], Co(II)F16Pc(O[Formula: see text](NO) or [Co(II)F16Pc(O[Formula: see text](NO[Formula: see text]][Formula: see text] and has been considered in DFT modeling studies. The instability of the two-electron reduced, protonation species, [Co(I)F16Pc(NO2OH)][Formula: see text] in its loss of peroxynitrous acid suggests that the reduction of O2 may occur by two one-electron reduction steps rather than a two-electron step.

Lindqvist Polyoxoniobate Ion-Assisted Electrodeposition of Cobalt and Nickel Water Oxidation Catalysts.

A method has been developed for the efficient electrodeposition of cobalt and nickel nanostructures with the assistance of the Lindqvist ion [Nb6O19](8-). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry, and a range of electrochemical techniques have been used to characterize the morphology, composition, catalytic water oxidation activity and stability of the films in alkaline solution. SEM images show that films consisting of nanoparticles with diameters of ca. 30 to 40 nm are formed after 40-50 potential cycles of deposition. Nb and Co/Ni are detected in the films by EDX. ICP-MS results show an elemental ratio of 1:1 for Co:Nb and 1:3 for Ni:Nb, respectively. Raman spectra reveal the presence of both [Nb6O19](8-) and Co(OH)2/Ni(OH)2. The films exhibit excellent stability and efficiency for electrocatalytic water oxidation in alkaline solution. Turnover frequencies of 12.9 and 13.2 s(-1) were determined by rotating ring disk electrode voltammetry at an overpotential of 480 mV for Co and Ni films, respectively. Fourier transformed large amplitude alternating current (FTAC) voltammetry reveals an additional underlying oxidation process for Co under catalytic turnover conditions, which indicates that a Co(IV) species is involved in the efficient catalytic water oxidation reactions. FTAC voltammetric data also suggest that the Ni films undergoes a clear phase transformation upon aging in aqueous 1 M NaOH and the electrogenerated higher oxidation state Ni from β-NiOOH is the more active form of the catalyst.

Sulfur-doped carbon nanotubes as catalysts for the oxygen reduction reaction in alkaline medium

Based on the unique electronic properties and high surface area of carbon nanotubes as well as the similar electronegativity of sulfur and carbon, a novel electrocatalyst for the oxygen reduction reaction (ORR) was fabricated by directly annealing oxidized carbon nanotubes and p-benzenedithiol in nitrogen. The structural and chemical properties of the resulting sulfur-doped carbon nanotubes (pSCNTs) were investigated using transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The catalytic activity of the pSCNTs towards ORR in alkaline medium was evaluated using rotating ring disk electrode voltammetry. The as-synthesized pSCNT-900 (annealed at 900°C) exhibits excellent electrochemical performance towards ORR with an onset potential of –0.082V (vs Ag/AgCl), a high kinetic current density of 34.6mAcm−2 at –0.35V), a dominant four-electron transfer mechanism (n=3.71 at –0.35V), as well as excellent methanol tolerance and durability. The results obtained are significant for the development of S-doped carbon-based catalysts for alkaline fuel cells.

Solvate Ionic Liquids as Electrolyte for Li-Oxygen Battery

Rechargeable Li-air (Li-O2) batteries have recently attracted enormous attention due to their prospective high energy density. To realize reversible and stable operation of Li-air batteries, however, many serious problems, such as volatilization of electrolyte and electrolyte decomposition by O2 - radical, have to be solved.[1] Our group recently reported that equimolar mixtures of glymes and Li salts behave as ionic liquids (ILs) rather than as ordinary concentrated electrolyte solutions.[2] For instance, as shown in Fig. 1, triglyme (G3) can coordinate to a Li+ ion, forming a 1:1 complex cation, [Li(G3)1]+; it can also behave as an independent cation similar to the cations of typical ILs. Thus, the glyme-Li salt complex has been categorized as a new subclass of ILs called “solvate ILs”.[3] The solvate ILs have many desirable properties as lithium conducting electrolyte[4]-[5] as well as low volatility and thermal- and electrochemical stabilities, which are crucial for Li-air batteries. In the present study, we applied solvate ILs to electrolyte of Li-air battery.[6] The solvate IL, [Li(G3)1][TFSA], was prepared by simply mixing purified triglyme (G3) and lithiumbis(trifluoromethanesulfonyl)amide (LiTFSA) in a 1:1 molar ratio. The electrochemical properties of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the solvate ILs were investigated by cyclic voltammetry (CV), rotating ring-disk electrode voltammetry (RRDE) and galvanostatic charge-discharge measurement. Fig. 2 shows CVs for a glassy carbon (GC) electrode in [Li(G3)1][TFSA] saturated with O2 or Ar (as background). A large reduction peak emerges when the electrode is polarized to negative potential under the O2 atmosphere, and a clear anodic peak can be seen on reversing the sweep from 1.5 to 4.2 V, although no appreciable current is observed under the Ar atmosphere. It suggests that reversible ORR-OER takes place on the GC electrode in [Li(G3)1][TFSA]. The results of RRDE and charge-discharge test in this solvate ILs will also be reported. Acknowledgement This study was partly supported by RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for which the authors are grateful.

Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions

Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800°C for 2h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650°C for 2h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH− in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst.

Palladium Content Effect on the Electrocatalytic Activity of Palladium–Polypyrrole Nanocomposite for Cathodic Reduction of Oxygen

Application of a catalyst preparation method known as ‘water-in-oil’ microemulsion allowed to obtain a series of palladium/polypyrrole (Pd/PPY) electrocatalysts with various Pd contents (2–20 wt%) and Pd particle size within a narrow and similar range of 6–8 nm. The textural features of the samples (surface area, porosity) as well as the palladium phase (metal particle size, shape) were characterized by a number of techniques. Electrocatalytic activity of the prepared materials towards oxygen reduction reaction (ORR) was studied in 0.5 M H2SO4 using rotating ring disc electrode voltammetry (RRDE) and cyclic voltammetry techniques. All prepared Pd/PPY samples exhibited electrocatalytic activity towards ORR. At a constant palladium loading on the electrode, the polymer to Pd weight ratio was found to play a decisive role in the electrocatalytic performance of the samples. Their catalytic performance in terms of the increased reduction current and positively shifted half-wave potential (RRDE measurements) was improved when Pd content in polymer was grown from 2 to 12 wt% Pd. At the optimum Pd content, 10–12 wt%, the analysis of the RRDE data confirmed that molecular oxygen underwent mainly a desired four-electron reduction. By further increasing the Pd content and thus decreasing the PPY to Pd ratio, the selectivity to water continuously decreased while the fraction of undesired H2O2 product significantly increased. Therefore, a role of PPY to Pd weight ratio in the electrocatalytic performance of the investigated catalysts is discussed here.

Oxygen Reduction on TiO2-Coated Carbon Nanofibers Decorated with Graphene Platelets

The oxygen reduction reaction (ORR) was studied on heat-treated electrospun polyacrylonitrile (PAN) nanofibers with and without iron (II) phthalocyanine using rotating ring disk electrode voltammetry. In an effort to mitigate loss of the active sites and increase durability, atomic layer deposition was used to coat the nanofibers with TiO2 thin films. The results show that Fe-containing heat-treated nanofibers and TiO2-coated Fe-containing nanofibers exhibit a ~4-electron reduction of O2 with low % H2O2 yields (~1-1.5 %).

Using Rotating Ring Disc Electrode Voltammetry to Quantify the Superoxide Radical Stability of Aprotic Li–Air Battery Electrolytes

Despite the promising high specific energy density of lithium–air batteries, their commercialization remains hindered by numerous issues, including the poor stability of the electrolyte due to its reaction with the superoxide radical (O2•–) produced upon discharge at the battery’s cathode. In this work, we have used rotating ring disc electrode (RRDE) voltammetry to study this reaction and to quantify the stability of the electrolyte against O2•– by its pseudo-first-order reaction constant, k. Our results confirm the recently reported reactivity of propylene carbonate (PC, which was used in many of the initial works on Li–air batteries), while unveiling the enhanced stability of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI), with a k value at least 3 orders of magnitude lower than that estimated in PC. Moreover, our RRDE-transient measurements indicate that the diffusion of O2•– in this ionic liquid is ≈70 times slower than that in PC, which could partially explain the poor discharge capacities observed in Li–air battery tests using Pyr14TFSI.

Nitrogen doped carbon nanotubes and their impact on the oxygen reduction reaction in fuel cells

Nitrogen doped carbon nanotubes (N-CNTs) with different nitrogen contents were synthesized as non-precious catalysts for oxygen reduction reaction (ORR) through varying the relative amount of pyridine and ethanol in the stock solution. The structural and chemical properties of the N-CNTs were investigated using scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The characterizations indicated that an increase in the pyridine to ethanol ratio of the stock solution produced N-CNTs with higher nitrogen contents. The electrocatalytic activity of N-CNTs towards ORR in alkaline conditions was evaluated using rotating ring disc electrode voltammetry which showed a positive correlation between nitrogen content and ORR activities. By combining the results of ORR activity and material characterization, it is concluded that an increase in the nitrogen content of N-CNTs can effectively improve the ORR activities.

Electrocatalytic activity of nitrogen doped carbon nanotubes with different morphologies for oxygen reduction reaction

Nitrogen doped carbon nanotubes (NCNTs) were synthesized by a single step chemical vapor deposition technique using either ferrocene or iron(II) phthalocyanine as catalyst and pyridine as the carbon and nitrogen precursor. Variations in surface morphology and electrocatalytic activity for oxygen reduction reaction (ORR) were observed between the NCNTs synthesized using different catalysts. The structural and chemical characterizations were carried out using transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical activity of NCNTs was evaluated with rotating ring disc electrode (RRDE) voltammetry. Structural characterization suggested more defects formed on the NCNTs synthesized from ferrocene (Fc-NCNTs) which led to a rugged surface morphology compared to the NCNTs synthesized from iron(II) phthalocyanine (FePc-NCNTs). Based on the RRDE voltammetry study, Fc-NCNTs demonstrated much higher activity for ORR than FePc-NCNT. Evidences from the structural and chemical characterizations illustrate the potential impact of catalyst structure in shaping the surface structure of NCNTs and the positive effect of surface defects on ORR activity. These results showed that potential improvements on ORR activity of NCNTs could be achieved by tailoring the surface structure of NCNTs by using catalysts with different structures.

Long-Term Performance of Pt-Decorated Carbon Nanotube Cathodes in Phosphoric Acid Fuel Cells

We report the electrochemical properties and performance of Pt-decorated carbon nanotube (Pt-CNT) catalysts for long-term operation in phosphoric acid fuel cells (PAFCs). Electrochemical measurements of Pt-CNT catalysts including cyclic voltammetry, rotating ring disk electrode voltammetry, and electrochemical impedance spectroscopy were conducted to evaluate the catalyzed oxygen reduction reaction (ORR). Furthermore, we tested the long-term performance of the Pt-CNT catalysts in 2′′ × 2′′ PAFC cathodes operating at 190 °C in 85% H3PO4 for extended periods (up to 240 days). Lifetime studies show that electrodes containing the Pt-CNT catalysts were approximately 20 times more stable than conventional Pt-C catalyst materials, even with a substantially thinner catalyst layer. This finding of the enhanced Pt-CNT catalyst stability bodes well for possible personal electronics or automotive applications, where catalyst longevity is an essential requirement.

Electrocatalytic Dioxygen Reduction at Glassy Carbon Electrode Modified with Polyaniline Grafted Multiwall Carbon Nanotube Film

AbstractThe work details the electrocatalysis of oxygen reduction reaction (ORR) in 0.5 M H2SO4 medium on a modified electrode containing a film of polyaniline (PANI) grafted multi‐wall carbon nanotube (MWNT) over the surface of glassy carbon electrode. We have fabricated a novel modified electrode in which conducting polymer is present as connected unit to MWNT. The GC/PANI‐g‐MWNT modified electrode (ME) is fabricated by electrochemical polymerization of a mixture of amine functionalized MWNT and aniline with GC as working electrode. Cyclic voltammetry and amperometry are used to demonstrate the electrocatalytic activity of the GC/PANI‐g‐MWNT‐ME. The GC/PANI‐g‐MWNT‐ME exhibits remarkable electrocatalytic activity for ORR. A more positive onset potential and higher catalytic current for ORR are striking features of GC/PANI‐g‐MWNT‐ME. Rapid and high sensitivity of GC/PANI‐g‐MWNT‐ME to ORR are evident from the higher rate constant (7.92×102 M−1 s−1) value for the reduction process. Double potential chronoamperometry and rotating disk and rotating ring‐disk electrode (RRDE) experiments are employed to investigate the kinetic parameters of ORR at this electrode. Results from RDE and RRDE voltammetry demonstrate the involvement of two electron transfer in oxygen reduction to form hydrogen peroxide in acidic media.