Rotating Ring Disk Electrode Research Articles

In Situ and Ex Situ Spectroscopic Studies of the Inhibition of Active Sites in Iron-Nitrogen-Carbon Electrocatalyst for Oxygen Reduction Reaction

One of the most promising classes of PGM-free materials for oxygen reduction reaction (ORR) is based on graphene-like carbon containing nitrogen and transition metal (metal-nitrogen-carbon, MNC) 1, which show promise as replacement of Pt in both alkaline exchange membrane (AEM) and proton exchange membrane (PEM) fuel cells. Understanding the specific roles of nitrogen and metal in the activity and durability of MNC-based catalytic materials is a prerequisite for the rational design of ORR electrocatalysts with improved performance. The mechanism of ORR in MNC catalysts has been studied previously by a combination of spectroscopic and theoretical structure-to-activity studies 2-4. Using inhibitors that have unique spectral signatures and have strong binding to the active sites allows elucidating the relationship between the chemistry of active sites and activity. We report results of synchrotron near ambient pressure X-ray photoelectron spectroscopic (NAP-XPS) analysis performed at synchrotron beamlines for series of electrocatalysts belonging to iron-nitrogen-carbon (FeNC) family. In-situ monitoring of oxygen binding to different nitrogen and metal moieties existing at the surface of these materials is performed with and without complexing inhibiting agent based on phosphonate (etidronic acid HEDP). From the spectroscopic analysis of catalyst with and without adsorbed HEDP we observe that major sites blocked by HEDP are hydrogenated and protonated nitrogens. Using near-ambient pressure XPS we have studied how oxygen adsorption in humidified atmosphere is affected by presence of the HEDP potentially blocking N-H/N+ active sites. These spectroscopic observations of inhibited active sites is correlated to an electrocatalytic performance from rotating ring disk electrode (RRDE) tests.

Bifunctional Organic Molecular Electrocatalyst for Hydrogen Evolution Reaction and Hydrogen Peroxide Production

Electrocatalysts are critical for facilitating numerous important electrochemical reactions such as hydrogen evolution reaction (HER) in hydrogen production, oxygen reduction reaction (ORR) in fuel cells,1 and emerging electrochemical production of hydrogen peroxide (H2O2) via a two-electron ORR.2 To date, high-performance electrocatalysts have relied heavily on the use of scarce and expensive precious metals. To lower the cost, researchers have focused on developing platinum group metal-free (PGM-free) electrocatalysts based on earth-abundant elements, such as carbon, nitrogen, and transition metals.3 However, these alternative catalysts often suffer from insufficient stability caused by chemical leaching of transition metal components in harsh acidic environments, which restrict their applications in devices requiring acidic electrolytes, including perfluorosulfonic acids such as Nafion®. In contrast, organic molecules have the advantages of low cost, well-defined active sites and good chemical compatibility with polymer electrolytes. Previous studies have shown the capability of organic molecules to catalyze HER4, 5 and two-electron ORR.6, 7 However, in most cases, these demonstrations were carried out in the electrochemical cell filled with non-aqueous electrolytes and the activities were still very low compared to metal-containing materials. Much work is needed to further improve the activity of organic molecular catalysts, especially in practical polymer electrolyte systems. In this presentation, we will summarize our study of a bifunctional electrocatalyst for HER and two-electron ORR based on a small organic molecule. In rotating disk electrode (RDE) test, the catalyst showed a low onset potential of ca. -0.05 V vs. RHE for HER with an overpotential of ca. -0.35 V at 10 mA/cm2 in 0.5 M H2SO4 at 80 °C. In the MEA-based electrochemical H2 pump test, it reached 10 mA/cm2 at -0.12 V vs. RHE at 80 °C and showed no performance loss in a 60-hour durability test. In addition to the high HER activity, the molecular catalyst also showed high activity and selectivity for two-electron ORR to yield H2O2. The onset potential was ca. 0.6 V vs. RHE with over 80% H2O2 yield in a rotating ring disk electrode (RRDE) test in O2-saturated 0.5 M H2SO4. Density functional theory (DFT) calculations were performed to gain preliminary insight into the reaction mechanisms of HER and two-electron ORR on the organic molecule. The results revealed thermodynamically favorable reaction pathways and structure-activity relationship in this system, which will be presented, too. References Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, (6321).Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E. L.; Rossmeisl, J. Nat Mater 2013, 12, (12), 1137-1143.Hu, C.; Dai, L. Angew. Chem. Int. Ed. 2016, 55, (39), 11736-11758.Cretu, R.; Kellenberger, A.; Medeleanu, M.; Vaszilcsin, N. Int. J. Electrochem. Sci. 2014, 9, (8), 4465-4477.Uchida, T.; Mogami, H.; Yamakata, A.; Sasaki, Y.; Osawa, M. J. Am. Chem. Soc. 2008, 130, (33), 10862-10863.Trojanek, A.; Langmaier, J.; Sebera, J.; Zalis, S.; Barbe, J.-M.; Girault, H. H.; Samec, Z. Chem. Commun. 2011, 47, (19), 5446-5448.Hatay, I.; Su, B.; Méndez, M. A.; Corminboeuf, C.; Khoury, T.; Gros, C. P.; Bourdillon, M.; Meyer, M.; Barbe, J.-M.; Ersoz, M.; Záliš, S.; Samec, Z.; Girault, H. H. J. Am. Chem. Soc. 2010, 132, (39), 13733-13741.

Hydrothermal Treatments of Fe/N/C Based Oxygen Reduction Catalyst for High Durability in NH4F Solution

Introduction N-doped carbon materials (e.g., Fe/N/C and carbon alloy catalysts) with high oxygen reduction reaction (ORR) activity are promising alternatives to conventional Pt-based catalysts for polymer electrolyte fuel cells (PEFCs). However, their durability is insufficient compared with Pt based catalysts. Hence, in this study, a Fe/N/C-type oxygen reduction catalyst was hydrothermally-treated with fluoride ions to improve its durability. The electrochemical properties and the deterioration behavior of the catalysts were examined by a rotating ring disk electrode (RRDE) method and an accelerated durability test (ADT). Experimental Ketjenblack (KB, ECP600JD), 1,10-phenanthroline (phen) methanol solution and iron acetate aqueous solution were added to zinc acetate methanol/water solution, and the obtained suspension was heated at 75°C for 24 h followed by drying and grounding with a mortar to obtain a catalyst precursor. The precursor was heat-treated in a tube furnace at 1050°C for 60 min under Ar flow and at 950°C for 15 min under ammonia gas to obtain a catalyst (denoted as Fe-phen-Zn/KB). The catalyst was dispersed in 100 mM NH4F aqueous solution in a sealed PTFE container, and heated from 20°C to 120°C at a heating rate of ca. 3.3°C min-1 and kept 120°C for 30 min to obtain a catalyst hydrothermally-treated with fluoride ions (denoted as “Fe-phen-Zn/KB(F)”). A catalyst hydrothermally-treated without fluoride ions was also prepared for the comparison (denoted as “Fe-phen-Zn/KB(HyT)”). The catalysts-modified electrode was prepared by casting 2.0 mg mL-1 catalyst / 0.1 wt% Nafion® 2-PrOH ink to a glassy carbon (GC) disk for a Pt ring-GC disk electrode and dried under hot wind. Electrochemical measurements were carried out in 0.1 M HClO4 with the modified working electrode, a reversible hydrogen reference electrode and a GC rod counter electrode. ADTs were carried out for different periods of “ADTs time (t ADT)” using a start/stop protocol, consisting of a triangular waveform (1.0 V to 1.5 V vs. RHE at 500 mVs-1), as proposed for carbon corrosion by the Fuel Cell Commercialization Conference of Japan (FCCJ). X-ray photoelectron spectroscopy (XPS) experiments were carried out using a PHI X-tool (ULVAC-PHI, Inc.) using Al-Kα X-rays. The binding energies were calibrated based on the peak energy positions of Au7/2 (84.0 eV). Results and discussion Fig. 1 shows the dependence of ORR current at 0.7 V vs. RHE on t ADT / min of each catalyst. Whereas the initial ORR currents of the three catalysts were almost the same, the current values of the catalyst after 6 h of ADT is in the order: Fe-phen-Zn/KB(F) > Fe-phen-Zn/KB(HyT) > Fe-phen-Zn/KB, suggesting that the hydrothermal treatments with NH4F are effective for the improvement of durability for Fe/N/C catalysts. The peak energy positions of F1s XPS for Fe-phen-Zn/KB(F) before and after ADTs were ca. 684 eV (fluoride ion environment) and ca. 689 eV (C-F bonding environment), respectively, suggesting that fluoride ions , which adsorbed on and/or intercalated to the catalyst during the hydrothermal treatment with NH4F, formed bond with carbons during ADTs. Based on the N1s XPS spectra, Fe-phen-Zn/KB(F) before ADT involved pyridinic N, Fe-N, graphitic N, NO3 and NO, whereas the catalyst after ADTs only involved pyridonic N, which is probably derived from pyridinic N. Figure 1

Electropolymerization of Metallo-Corroles: Formation of Highly Active Three-Dimensional Catalysts for Oxygen Reduction

Within the class of precious group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR), significant progress has recently been reported for 1st row transition metal complexes of corroles, which generally shows better catalytic activity when compared to other molecular, macrocyclic transition metal complexes such as porphyrins and phthalocyanines. In order to further enhance their catalytic activity and durability we recently reported on electropolymerization of CoIII corroles. The catalytic activity of the polyCorrole and its monomer for oxygen reduction reaction (ORR) was studied using rotating ring disk electrode (RRDE). The polyCorrole exhibits significantly better catalytic activity than its monomer, reflected in lower overpotential as well as higher selectivity to the desired 4e-/4H+ pathway, both attributed to synergistic biomimetic-like effect of the interconnected catalytic centers. In the current work, we further investigated the polymerization process, trying to determine the parameters influencing the catalytic activity and stability of the polymer, such as morphology, polymerization techniques, chemical environment etc. In addition, the stability of the polyCorroles were measured in order to understand the dominating failure mechanisms which will allows further improvement of the catalyst for application in fuel cells.

Influence of Aluminum Addition to Catalytic Behavior of Bismuth Ruthenate for Oxygen Electrode Reactions

Electrochemical power sources that use oxygen as a reactant may play an essential role in the future energy needs of the society. In particular, fuel cells, aqueous metal-air batteries, and metal hydride (MH)/air batteries hold promise as power sources beyond lithium-ion batteries (LIBs). For such electrochemical power sources, an alkaline solution is often employed as the electrolyte and an oxygen electrode reaction occurs at the cathode. However, the development of highly active catalysts (i.e., with low activation overpotential) for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains challenging. We then have devoted to investigate the electrode catalyst which decreases in the overpotential for both ORR and OER. To the best of our knowledge, a pyrochlore type metal oxide including bismuth ion as the A-site (Bi2Ru2O7, BRO) is one of the most promising bifunctional catalysts and the overpotentials and Tafel slopes were reported in “Electrocatalysis (Vol. 9, 146-152, 2018)”. In order to further decrease in the overpotential and Tafel slopes, we attempted to addition of Al ion to BRO (ABRO) and have evaluated. BRO and ABRO were prepared by a precipitation method. Bi(NO3)3, RuCl3 and NaOH aqueous solutions were used for preparation of BRO. Al(NO3)3 was added to investigate the aluminum addition. Finally, the precipitates were made for calcination under atmospheric air at 500 and 600oC for 3 h. X-ray diffraction (XRD) was used to characterize the products. The weight change during the calcination was investigated by Thermo-gravimetry. The oxygen contents of the products were estimated by using modified temperature-programmed reduction (TPR). The product was put into a quartz tube furnace and was heated up to 700 °C in a hydrogen atmosphere. Water, which was produced by the reaction of hydrogen and oxygen in BRO, was quantified by using Karl Fischer moisture meter. The catalytic behavior for both ORR and OER was investigated by a rotating ring disk electrode (RRDE). The disk and ring electrodes were made by a glassy carbon (GC) and Pt, respectively. The product was dispersed on GC and fixed with a thin film of anion-exchange-membrane (corresponding ionomer solution: AS-4, TOKUYAMA K.K.). Hg|HgO reference electrode and a Ni mesh counter electrode were placed in a perfluoroalkoxyalkane beaker cell. The electrolyte solution was 0.1 mol dm-3 KOH aqueous solution. The XRD pattern of the product was assigned to Bi2Ru2O7 after the calcination at 500oC. When the calcination temperature was at 600oC, it was likely to assign to Bi1.9Ru2O6.92 These results indicated that oxygen and bismuth were made elimination between 500oC and 600oC. The lattice constant of the product obtained at 500oC is very close to that of Bi2Ru2O7; therefore, we reasonably considered that the product was Bi2Ru2O7. In the case of ABRO, there were no peaks for impurity compounds like as the aluminum compounds. Hereafter, we call BRO that obtained at 500oC and 600oC as BRO-500 and BRO-600, respectively. Similar to this, we named ABRO-500 and ABRO-600 for ABRO. TPR results showed that the oxygen contents of BRO and ABRO were depended on the calcination temperature, which corresponds to the XRD results. The oxygen content of BRO decreased with increase in the calcination temperature; whereas opposite tendency was recognized for ABRO. Figure 1 (a) and (b) shows the hydrodynamic voltammograms of BRO for ORR and OER, respectively. The overpotential is around 0.31 V and around 0.16 V for ORR and OER, which shows that overpotential is not related to the calcination temperature in this study. On the other hand, Tafel slope for OER decreased with an increase in the calcination temperature. Fig. 1 (c) and (d) also shows voltammograms for ABRO. The OER overpotential of ABRO-600 was greatly low as around 0.07 V, which suggests that the addition of aluminum during the preparation of BRO gives the influence on the catalytic activity. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). The authors express gratitude to Tokuyama Corporation for supplying AS-4. Figure 1

Graphene Oxide-Supported Cobalt Phthalocyanine As Highly Efficient Electrocatalysts for the Oxygen Reduction Reaction

The rapid depletion of oil reserves and the resulting environmental pollution have stimulated the development of clean and renewable alternative energy sources. Fuel cells, which own high efficiency and environmental advantages, have received great attention. 1 The main bottleneck of fuel cells lies in the sluggish kinetics of oxygen reduction reaction (ORR), which is the key process in the cathode. 2 High performance ORR catalysts are urgently desired. In order to reduce the cost of ORR catalyst and to promote the commercialization of fuel cells, many researchers have been exploring alternative low-cost non-precious metal catalysts (NPMCs) with attractive performance for ORR. Among these NPMCs materials, metal phthalocyanines (MPcs) have received a great deal of interest due to highly conjugated structure, chemical and thermal stability as well as low cost of preparation. 3 In addition, the catalytic activity and durability of MPcs can be improved by employing suitable electrocatalyst supports. In this work Cobalt Phthalocyanine (CoPc) was immobilized between layers of graphene oxide (GO) through non-covalent functionalization approach. Briefly, the catalyst ink was prepared using CoPc/GO, water, ethanol and Nafion. Electrodes were prepared by drop-casting over a Rotating Ring-Disc Electrode (RRDE electrode, glassy carbon disk with diameter of 5.61 mm and surface area of 0.2475 cm2 surrounded by a Pt ring with a surface area 0.1866 cm2). The RRDE technique with a three-electrode electrochemical cell was employed to study the ORR activity, using a Pine electrochemical system. A Ag/AgCl (3 M KCl) electrode, a Pt wire and the catalyst coated RRDE were used as reference electrode, counter electrode and working electrode respectively. All electrochemical tests were performed at room temperature in 0.1 M KNO3 solution. Cyclic voltammetry curves (CV) were recorded from 0.2 to -0.8 V until a steady state situation was reached in Ar-saturated and O2-saturated electrolyte. The scan rate was set at 20 mVs−1. For the ORR test, linear sweep voltammetry (LSV) curves were recorded at different electrode rotation speeds (100, 225, 400, 625, 900, 1225, 1600 rpm) in the oxygen saturated 0.1 M KNO3 at the same potential range with a scan rate of 5 mVs-1. The formation of a composite structure was revealed by the appearance of characteristics graphene oxide and phthalocyanine bands in the Raman spectra, as shown in Fig. 1a. The cyclic voltammograms (CVs) of CoPc/GO in Ar and O2-saturated 0.1 M KNO3 solution are shown in Fig. 1b. It can be seen that the composite exhibited only a double-layer capacitive behavior in Ar-saturated KNO3. In contrast, a cathodic peak emerged at -0.15 V for CoPc/GO in O2-Saturated electrolyte. This value is close to related for Pt/C and more positive than that observed for CoPc/3D-G in the literature, 4 indicating the good electrocatalytic abilities of CoPc/GO toward ORR. The polarization curves on CoPc/GO at different rotation speeds are shown in Fig. 1c. The disk current density increased with the increasing rotation rate, from 100 to 1600 rpm, due to the shortened diffusion distance at high speeds, and all reached well-defined diffusion limiting currents. The transferred electron number n of CoPc/GO at various potentials was further calculated by Koutecky-Levich (K-L) equation, as shown in Fig. 1d. The K-L plots have good linearity with a rather consistent slope, implying a similar n on CoPc/GO at different electrode potentials. The average number for n regarding the O2 reduction at CoPc/GO obtained from K-L plots is ∼3.7, indicating the near four-electron pathway during the ORR. The Eonset of CoPc/GO was + 0.02 V (at 1600 rpm). An enhancement in the catalytic activity of CoPc/GO was noticed by using GO as the support, demonstrating that the structure of GO and the strong π-π interaction between GO and CoPc was beneficial. Detailed characterizations of the composite are in progress to correlate its nanostructure with the superior electrochemical performance. T. Shimura, Z. Jiao, and N. Shikazono, J. Electrochem. Soc., 164, F1158 (2017). K. A. Kuttiyiel, Y. Choi, S.-M. Hwang, G.-G. Park, T.-H. Yang, D. Su, K. Sasaki, P. Liu, and R. R. Adzic, Nano Energy, 13, 442 (2015). A. B. Sorokin, Chemical Reviews, 113, 8152-8191 (2013). C. Sun, Z. Li, X. Zhong, S. Wang, X. Yin, and L. Wang, J. Electrochem. Soc., 165 (2), F24-F31 (2018) Figure 1

Rotating Disk Electrode – Online Electrochemical Mass Spectrometry for Oxygen Reduction Reaction on Pt Electrode Surfaces

Introduction Oxygen reduction reaction (ORR) is one of the important electro-catalytic reactions, because of its practical application for the cathode of polymer electrolyte fuel cells (PEFCs). The rotating ring-disk electrode (RRDE) method provides us information on applied-potential-dependent redox phenomena that proceed at the electrode surface and, has been widely used for studying ORR kinetics of the various electrodes. In contrast, few studies have been reported for dynamic behaviors of dissolved O2 molecules in electrolytes during ORR. In this study, we combine the online electrochemical mass spectrometry (OLEMS) [1] with the rotating disk electrode (RDE) method (RDE-OLEMS) to investigate potential-dependent phenomena for the dissolved O2 present in immediate vicinity of the Pt electrode during ORR. Experimental Fig.1(a) presents a schematic representation of the newly developed RDE-OLEMS system to analyze dissolved O2 present in the vicinity of a Pt working disk electrode (ø5mm, t = 4mm). The alumina polished Pt polycrystalline electrode was set into a RDE set-up. Then, the RDE-OLEMS measurements were performed in O2-saturated 0.1 M HClO4 with given disk rotation rates. Dissolved O2 molecules presented in the vicinity of the electrode surface was collected through a porous Teflon to prevent introduction of aqueous components into the vacuum system. During the RDE-OLEMS measurements, the distance between a PEEK Tip and Pt electrode surface was set to be ca. 50 µm by using a micrometer. The collected O2 was introduced into a quadrupole mass spectrometer (Q-mass) and, then, applied-potential-dependent O2 ion current (m/z=32) was recorded and compared with corresponding linear sweep voltammogram (LSV). Results and Discussion Fig.1(b) shows LSV curves of the Pt electrode recorded at anodic-potential-sweep (10 mV/s from 0.0 to 1.1 V vs. RHE) with the PEEK Tip configuration (Fig.1(a)). The LSV curves clearly showed diffusion-limiting currents in the potential region of 0.2 to 0.6 V: the Levich plots (inset) indicates that current densities change linearly with a square-route of the disk rotation rates. The results show that the dissolved O2 molecules can be supplied to the electrode surface in the developed RDE-OLEMS layout (with the PEEK Tip located at ca. 50 µm from the electrode surface). Fig.1(c) shows the RDE-OLEMS results (red) for the collected O2 (m/z=32) recorded at cathodic-potential-sweep from 1.05 to 0.05 V with a scan rate of 0.5 mV/s and a disk rotation rate of 1200 rpm. A simultaneously recorded LSV curve (cathodic-potential-sweep; black) is shown as a reference. It can be clearly seen that the applied-potential-dependent change in O2 ion current (MSCV) well corresponds to the LSV curve. The results clearly indicate that concentration of the dissolved O2 present in the vicinity of the electrode decrease with increasing ORR currents, especially in the diffusion limiting current region. In our poster presentation, we will discuss the results of RDE-OLEMS measurements for Pt and Pt-based alloy single crystal surfaces. Acknowledgement This work was partly supported by a Grant-in-Aid for scientific research (A) from the Japan society for the promotion of science (T. W.).

Improvement of the Diffusion Coefficient and Concentration of O2 in Nafion® with 2,2’-Bpy

The polymer electrolyte fuel cells (PEFCs) are expected as power sources to reduce the consumption of fossil fuel for a solution to global warming. Because oxygen reduction reaction (ORR) at the cathode of PEFCs is sluggish even if Pt/C catalysts are used, new approach is necessary to improve the cathode reaction, thus smooth gas supply to the surface of cathode catalyst may be effective. We reported that the addition of 2,2'-bpy (concentration in film: 10 M) to Nafion® improved oxygen reduction reaction, suggesting that the diffusion coefficient and the concentration of oxygen are improved by the change of morphology. In this study, the effects of 2,2’-bpy on the diffusion coefficient and concentration of O2 in Nafion® film were investigated based on electrochemical measurements, differential scanning calorimetry (DSC) and ab initio calculations. 20 μL of 5 wt% Nafion® solution was cast on a Pt disk electrode (diameter: 6 mm) and dried under N2. The electrode was soaked in a 2,2’-bpy aqueous solution and rotated at 600 rpm to adsorb 2,2’-bipyridine. The concentration of 2,2’-bpy in the Nafion® was estimated using the change of the absorbance at 281.5 nm of the solution. Electrochemical measurements were performed in 0.05 M sulfuric acid solution with the modified working electrode, a Pt wire counter electrode, and an RHE refence electrode with a rotating ring disk electrode system. First, the potential was cycled between 0.05 V to 1.60 V for 20 cycles under N2 atmosphere, and the rotating ring disk electrode measurements and the potential step measurements from 1.60 V to 0.40 V were carried out under 1 atm O2 to estimate the diffusion coefficients and the concentration of O2 in Nafion. The diffusion coefficient of O2 was calculated as follows: time courses of the amount of charge in the potential step measurements were analyzed by the integral form of Cottrell equation: Q(t)=2nFAD1/2Cfπ-1/2 t 1/2 + Q dl(t) +nFAΓ (1) where t [s] is time, Q(t) [C] is the amount of charge at t, n is the number of electrons, F is the Faraday constant, A [cm2] is the electrode geometric area, D [cm2 s-1] and C f [mol cm–3] are the diffusion coefficient and the concentration of O2 in the Nafion®, respectively, Q dl(t) is the capacitive charge, and Γ [mol cm–2] is the surface concentration of an adsorbate reduced by the potential step. The sum of current flow derived from the second and third terms was eliminated by that measured under nitrogen. D f 1/2 C f can be estimated by the slope of Anson plot (a A) as follows: D f 1/2 C f = a Aπ1/2/(2nFA) (2) The time scale used in the analysis was less than 1.5 s when the change of O2 concentration reached the film/electrolyte interface. In the RRDE measurements, the inverse of limit current of the film-coated electrode is given by the following equation: 1/i l =1/(1.554πFAD f 2/3 ω 1/2 v -1/6Cf) + δ/(nFC f D f) (3) where i l [A cm–2] is diffusion limit current, ω [Hz] is the rotating speed of the electrode, and δ is the thickness of Nafion® film on the electrode. D f C f can be estimated using the intercept of i l -1 vs. ω 1/2 plot (b L) as follows: D f C f=δ/(nFb L) (4) D f and C f values were estimated with eqs. 2 and 4. Fig. 1 shows the dependence of the diffusion coefficient and concentration of O2 in Nafion® film on the concentration of 2,2'-bpy. The diffusion coefficient and the concentration of O2 in the Nafion® film increased drastically until 0.5 M and then decreased and became almost constant above 2 M. DSC measurements suggested that the hydrophilic cluster sizes of the Nafion® film increase with increasing concentration of 2, 2 '-bpy, which induces the increase in the parameters. ORR depends on proton concentration in Nafion® as well as the diffusion coefficient and the concentration of O2, thus, the decrease in the parameters was caused by the inhibition of proton conduction via a Grotthuss-type mechanism. Figure 1

Electrochemical reduction of carbon dioxide on copper-based nanocatalysts using the rotating ring-disc electrode

A continuous hydrothermal flow synthesis method was used to produce copper(I) oxide nanoparticles, which were used as an electrocatalyst for the reduction of CO2. A rotating ring-disc electrode (RRDE) system was used to study the electroreduction processes, including a systematic study (including quantitative NMR analysis) to identify product species formed at the disc and detected at the ring. In 0.5 M KHCO3 electrolyte with a pH of 7.1, carbon dioxide was found to be exclusively reduced to formate. In the potential range −0.5 to −0.9 V vs the reversible hydrogen electrode (RHE), an active material/glassy-carbon disc electrode was shown to produce formate, with a maximum Faradaic efficiency of 66% (at −0.8 V vs RHE).

Urea treatment of nitrogen-doped carbon leads to enhanced performance for the oxygen reduction reaction

Abstract

A family of platinum group metal-free catalysts for oxygen reduction in alkaline media

In this work a series of platinum group metal-free catalysts based on iron, cobalt, copper, nickel, silver and their bimetallic combinations are synthesized and their behavior for oxygen reduction in alkaline media is tested. A phthalocyanine-like ligand supported on carbon black serves as the nitrogen source and carbon support, respectively, for the catalysts. The as-prepared materials are thermally activated by pyrolysis at temperatures ranging from 600 °C to 1000 °C. Oxygen reduction reaction (ORR) activity and stability in 0.1 M KOH are tested by rotating ring disk electrode (RRDE) experiments. Several of the catalysts show ORR activities in the RRDE experiment that are superior to Pt, with Co, Co/Fe and Ag/Co being the best. All catalysts show remarkable stability, even when they are tested under air for 5000 RRDE cycles. RRDE experiments are also used to study the effect of catalyst loading and to determine reaction order. A thorough study of the catalyst performance in a single cell fuel cell test is conducted; exploring different methods to prepare the electrodes and the effects of catalyst loading, the catalyst-to-binder ratio and the removal of excess metal on the cell performance. Different methods of preparation of membrane electrode assemblies are also studied.

Effect of carbon precursor and initial pH on cobalt-doped carbon xerogel for oxygen reduction

In this study, cobalt-doped carbon xerogel (Co-CX) was synthesised via sol-gel polymerisation of phenolic compounds (i.e., resorcinol, phenol and m-cresol) and formaldehyde, and this polymerisation was catalysed by cobalt nitrate and followed by a carbonisation process. The effect of the initial pH value (5.5, 6.5 and 7.5) as well as the type of carbon precursors on the structural properties of Co-CX was investigated via field emission scanning electron microscope (FESEM), Brunauer-Emmett-Teller (BET) and X-ray diffractometry (XRD). The catalytic activity of Co-CX for the oxygen reduction reaction (ORR) in 0.1 M KOH was studied using a rotating ring-disk electrode (RRDE) technique. The structural properties and ORR activities were affected by different initial pH values as well as the type of carbon precursor. A carbon precursor consisting of resorcinol-formaldehyde with an initial pH value of 7.5 exhibited the best catalytic activity. The initial pH plays an important role in promoting micro/mesopores. The FESEM and BET results revealed that Co doping promotes the formation of additional pores. The RRDE result indicated that Co-CX exhibited good catalytic activity that tends to favour a four-electron pathway.

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.

Use of Rotating Ring‐Disk Electrodes to Investigate Graphene Nanoribbon Loadings for the Oxygen Reduction Reaction in Alkaline Medium

AbstractThe oxygen reduction reaction (ORR) is typically slow. Its kinetics, however, are influenced not only by the structure, nature, and doping of electrocatalysts, but also by the loadings of these materials, where all of these factors influence ORR selectivity to produce H2O and/or H2O2. The loadings employed for graphene nanoribbon (GNR)‐modified glassy carbon (GC) electrodes and GC disk modified with commercial Pt (20 wt.%) on carbon (PtC) at 150 μg cm−2 (also resulting in electrode roughness) produced turbulence in the electrolyte flow, significantly changing the geometry of the rotating ring‐disk electrode (RRDE) and collection efficiency (N), as well as causing N to change with the rotation rate of the electrode. This effect toward the ORR was investigated with two analytical methods derived by Wu et al. and Zhou et al. A current deconvolution method for a better‐resolved Tafel analysis separating formation and reduction reactions resulted in more insightful understanding of the ORR responses provided by the RRDE for different GNR and PtC loadings.

Oxygen Reduction on Catalysts Prepared by Pyrolysis of Electrospun Styrene–Acrylonitrile Copolymer and Multi-walled Carbon Nanotube Composite Fibres

For the first time, the oxygen reduction reaction (ORR) is studied on pyrolysed electrospun multi-walled carbon nanotube (MWCNT) and styrene–acrylonitrile (SAN) composite fibre catalysts in alkaline medium. Scanning electron microscopy images revealed that the prepared catalysts mainly consist of MWCNTs, while nitrogen doping of the catalysts is confirmed by X-ray photoelectron spectroscopy. This indicates that SAN can be used as a nitrogen precursor. The ORR studies carried out by rotating disc electrode (RDE) and rotating ring-disc electrode (RRDE) methods showed that the prepared catalysts were considerably more active towards the ORR than the pristine MWCNTs.

Preparation of double-shell Co9S8/Fe3O4 embedded in S/N co-decorated hollow carbon nanoellipsoid derived from Bi-Metal organic frameworks for oxygen evolution reaction

Metal organic frameworks (MOFs) based catalysts are the most promising candidates for electrocatalytic reaction. Herein, we describe a fast, cost-effective and efficient method to construct an oxygen evolution electrocatalyst with a “bivalve hollow nanoellipsoid” structure, namely three dimensional (3D) bi-shells Co9S8/Fe3O4 embedded in S/N co-decorated hollow carbon nanoellipsoid (H-Co9S8/Fe3O4@SNC). The bivalve metallic compounds hollow structure decreases the proton-electron transfer kinetics on carbon nanoellipsoid, and promoting the oxygen evolution reaction (OER). Specifically, the H- Co9S8/Fe3O4@SNC reveals an excellent catalytic activity with a low overpotential of 280 mV at the current density of 10 mA cm−2 and a small Tafel slope of 87 mV dec−1. Moreover, this H-Co9S8/Fe3O4@SNC electrode is applied to rotating ring-disk electrode (RRDE) and show remarkable catalytic stability and give ∼97.5% Faradic efficiency toward OER via chronoamperometric measurement at a rotation rate of 1600 rpm. Following that, the mechanism is investigated and reveal the excellent catalytic activity is mainly owed to the hollow structure and the catalytic synergistic effects of the double shell Co9S8/Fe3O4, simultaneously, the introduction of oxygen vacancy during the pyrolysis process to the formation of Fe3O4.

Nitrogen/Phosphorus Co-Doped Side-Hole-Rich Carbon Nanotubes As Efficient Metal-Free Catalysts for Oxygen Reduction Reaction

Nitrogen/phosphorus co-doped side-hole-rich carbon nanotubes (N,P-HCNT) were fabricated as metal-free electrocatalysts toward oxygen reduction reaction. The multi-walled carbon nanotubes (MWCNT) were simply reacted with hydrogen peroxide, and numerous of holes were formed at the MWCNT surface during the reaction. The resulting side-hole-rich carbon nanotubes (HCNT) were mixed with urea and triphenyl phosphine to incorporate nitrogen and phosphorus into the HCNT. The formation of N,P-HCNT was characterized by some analyses including X-ray photoelectron spectroscopy, field emission scanning electron microscopy and high resolution transmission electron microscopy. The Brunauer-Emmett-Teller analyses were also conducted to confirm the changes in surface properties of the N,P-HCNTs. Rotating ring disk electrode (RRDE) results indicated that the N,P-HCNTs exhibits excellent ORR activity in both acidic and alkaline conditions. In addition, the N,P-HCNT showed good long-term stability and resistance to methanol crossover effect than commercial platinum-based catalysts. Thus, the N,P-HCNT can be a new promising catalysts for oxygen reduction reaction.

(Invited) Oxygen Reduction on Polycrystalline Gold in Alkaline Electrolytes: Experimental and Theoretical Aspects

Despite decades of research, no consensus has been reached regarding basic aspects of the mechanism of the oxygen reduction reaction (ORR) on Au electrodes in aqueous neutral and alkaline solutions1, 2., Most of the problems from an experimental viewpoint stem from the lack of techniques capable of identifying species believed to be involved as intermediates in the reaction, including both adsorbed and solution-phase superoxide and peroxide. Recently, we developed a rotating ring-disk technique incorporating a judiciously functionalized Au ring displaying remarkably high specificity toward solution-phase superoxide in neutral solutions which made it possible to detect such species generated by the reduction of dioxygen at a bare Au disk electrode as a function of the applied potential3. Our contribution illustrates the use of in situ differential reflectance spectroscopy to monitor the adsorption of hydroxyl ion on a Au rotating disk electrode during dioxygen reduction, as well as that of on line mass spectrometry under forced convection to measure the rates of heterogeneous peroxide disproportionation on the same electrode surface using a jet impinging electrolyte arrangement4. In basic solution, reduction of oxygen on Au electrodes is thought to proceed at least in part through a hydrogen peroxide intermediate. Many authors have speculated that subsequent disproportionation of the peroxide intermediate to oxygen and water plays an important mechanistic role, but measurements of disproportionation rates have not been attempted1, 2. This is most likely due to the difficulty of deconvoluting this reaction rate from the rates of oxygen / peroxide reduction. To this end, we measured both the consumption rate of peroxide using a rotating ring disk electrode (RRDE) and the production rate of oxygen by on-line mass spectrometry on a Au electrode at open circuit in 0.1 M NaOH. Shown in Figure 1 are plots of the partial pressure of dioxygen monitored with the mass spectrometer at two fixed disk potentials, i.e. 0.2 and -1.2 V, at which peroxide is oxidized and reduced respectively under diffusion limited conditions, yielding maximum and minimum O2 signal. The cell was then disconnected and the open circuit potential, after which the O2 signal settled at an intermediate value 1/3 of the way between that seen at 0.2 and -1.2 V. This spontaneous production of O2 from H2O2 without net current through the disk can only be explained by invoking H2O2 disproportionation. Furthermore, since only 1 O2 is produced per 2 H2O2 via disproportionation, the O2 signal at OCP represents consumption of 2/3 of the total H2O2 reaching the Au electrode. Fig. 1: A Au disk electrode surrounded by a porous, gas-permeable Teflon ring was mounted in the wall-jet configuration (flow rate 0.45 mL/s), and the O2 pressure passing through the Teflon ring was monitored by mass spec while the following electrochemical experiment was conducted: the Au electrode potential was held at either 0.2 (black) or -1.2 V vs Ag/AgCl (red) for 40 s, and then the circuit was opened. The solution was 0.1 M NaOH with 1 mM H2O2 (solid lines) or without H2O2 (dotted lines). The selected potentials of 0.2 and -1.2 V induce mass transport limited oxidation and reduction of H2O2, respectively, and so will yield the maximum and minimum O2 signal. Other authors have invoked superoxide as yet another intermediate in the reduction of oxygen on Au in alkaline solutions. Also to be discussed in this presentation is a theoretical analysis of the mechanism shown in Scheme I, which accounts for the contributions associated with all three species to the measured currents.

(Invited) Influence of the Crystal Structure of Manganese Oxides on the ORR Kinetics: A Combined Experimental and Computational Study

Recently, while studying the oxygen reduction reaction (ORR) in alkaline medium on Mn oxides with various crystal structures, we documented [1] an unparalleled activity of Mn2O3 oxide with bixbyite structure. Then we also observed a clear correlation of the electrocatalytic activity with the formal potential of Mn(IV/III) interfacial redox transition and suggested the ORR mechanism involving this transition as a mediator step. To extend our understanding of this complex mechanism, we also compared the behavior of various oxides in peroxide oxidation/reduction reactions [2] and in respect to the peroxide yield during the ORR. All materials were studied in the form of composites with Sibunit carbon (weight ratio 1:1). This carbon binder provides a high utilization of the oxide surface in electrocatalysis [3]. The voltammetry in deairated 1M NaOH solution and rotating disc electrode (RDE) measurements in the same but O2-saturated solution were used to determine the total charge and the ORR kinetic currents. Rotating ring disc electrode (RRDE) was applied to compare the peroxide yields on various Mn oxides. All the values were compared after normalizing them to the BET surface area. The number of electrons involved in the oxygen reduction was close to 4 and 3 for more and less active oxides, respectively. Less active oxides demonstrated significantly higher peroxide yields and slower peroxide reduction, with a convection-independent limiting current which documents a slow chemical step in a wide potential region. The independent experimental facts which can be considered for comparison with simulations are as follows: surface concentration of the Mn active centers (estimated from the total charge in Mn(IV/III) transition region);specific activity of pure carbon generating additional peroxide, which further reacts on the oxide component in carbon-oxide composites (see in Ref.[4] about the dual role of carbon);ORR polarization curves;polarization curves for peroxide reactions;peroxide yield during the ORR. This wide set of data provides a valuable input for multi-parametric simulations, which significantly decreases the number of possible solutions. The modeling involved the rate constants for five reaction steps as well as the parameter for interactions of the adsorbed oxygen species involved in the Mn(IV/III) transition (in terms of Frumkin isotherm). We found that the complete set of data can be interpreted only assuming the structural dependence of at least two rate constants: one at the initial stage (peroxide formation) and another at the final stage (peroxide reduction). Another intriguing possibility to explain the very high Mn2O3 activity in the ORR is to assume a direct ORR pathway for this oxide, and to consider a series pathway for all other oxides. To elucidate whether the latter hypothesis is reasonable, we addressed the O-O bond break on Mn2O3 and MnOOH (typical examples of high and low activities, respectively) in the framework of a quantum chemical approach resting on the cluster model of the oxide surface and the DFT level of theory. The computational results were used to map the effective reaction path. We found that for both model surfaces the direct bond cleavage at the oxide surface is hardly feasable both for the O2 molecule and most likely for peroxide intermediates. This finding makes it possible to exclude the direct pathway and to focus on molecular reasons of the structural effects on the rate constant for other steps such as the reduction of the adsorbed peroxide intermediate. The work is supported by EraNetRus program, project (#270 NANO-Morf). [1] A.S. Ryabova et al, Electrochimica Acta 187 (2016) 161–172. [2] A.S. Ryabova et al, ChemElectroChem 3 (2016) 1667 – 1677. [3] A.S. Ryabova et al, Electrochimica Acta 246 (2017) 643–653. [4] T. Poux et al, Catalysis Today 189 (2012) 83– 92.

Reduced-Graphene-Oxide with Transition Metal Hexacyanometallates As Active Support for Traces of Platinum Catalyst at Low Loading during Oxygen Electroreduction

Reduced graphene-oxide-supported the various metals analogue of polynuclear Prussian Blue are considered here together with traces of platinum as well as low loadings of platinum (10-15 mg cm-2) as catalytic materials for reduction of oxygen in acid media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of oxygen in acid solution (the highest currents and the most positive electroreduction potentials) using cyclic and rotating ring-disk electrode (RDE) voltammetric experiments. Among important issues are possible activating interactions between analogue of Prussian Blue, platinum nanoparticles and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified reduced graphene-oxide-supported is also characterized here by additional diagnostic experiments, namely galvanodynamic measurements with use of so called gas diffusion electrode (GDE) operating under conditions resembling those characteristic of the cathode in a real fuel cell.