Electron Transfer Number For Oxygen Reduction Reaction Research Articles

Highly active electrocatalysts for oxygen reduction from carbon-supported copper-phthalocyanine synthesized by high temperature treatment

The active, carbon-supported copper phthalocyanine (CuPc/C) nano-catalyst, as a novel cathode catalyst for oxygen reduction reaction, is synthesized via a combined solvent-impregnation along with the high temperature treatment. The catalytic activities of both pyrolyzed and unpyrolyzed catalysts are screened by linear sweep voltammetry (LSV) employing a rotating disk electrode (RDE) technique to quantitatively obtain the oxygen reduction reaction (ORR) kinetic constants and the reaction mechanisms. The results show that heat-treatment can significantly improve the ORR activity of the catalyst, and the optimal heat-treated temperature is around 800 °C, under which, an onset potential of 0.10 V and a half-wave potential of −0.05 V are achieved in alkaline electrolyte. Besides the ORR kinetic rate is increased, the ORR electron transfer number is also increased from 2.5 to 3.6 with increasing heat-treatment temperature from 600 to 800 °C. Also, the effect of catalyst loading in the catalyst layer on the corresponding ORR activity is also studied, and finds that increasing the catalyst loading, the catalyzed ORR kinetic current density can be significantly increased. For a fully understanding of this heat-treatment temperature effect, XRD, TEM, SEM–EDX, TG and XPS are used to identify the catalyst structure and composition. TG results demonstrated that the presence of Cu prevents phthalocyanine from thermal decomposition, thus contribute to higher nitrogen content which can form more Cu–Nx activity sites for the ORR. XPS analysis indicates that both pyridinic-N and graphitic-N may be responsible for the enhanced ORR activity.

Synthesis of carbon-supported binary FeCo–N non-noble metal electrocatalysts for the oxygen reduction reaction

In this paper, a carbon-supported binary FeCo–N/C catalyst using tripyridyl triazine (TPTZ) as the complex ligand was successfully synthesized. The FeCo–TPTZ complex was then heat-treated at 600 °C, 700 °C, 800 °C, and 900 °C to optimize its oxygen reduction reaction (ORR) activity. It was found that the 700 °C heat-treatment yielded the most active FeCo–N/C catalyst for the ORR. XRD, EDX, TEM, XPS, and cyclic voltammetry techniques were used to characterize the structural changes in these catalysts after heat-treatment, including the total metal loading and the mole ratio of Fe to Co in the catalyst, the possible structures of the surface active sites, and the electrochemical activity. XPS analysis revealed that Co–N x , Fe–N x , and C–N were present on the catalyst particle surface. To assess catalyst ORR activity, quantitative evaluations using both RDE and RRDE techniques were carried out, and several kinetic parameters were obtained, including overall ORR electron transfer number, electron transfer coefficient in the rate-determining step (RDS), electron transfer rate constant in the RDS, exchange current density, and mole percentage of H 2O 2 produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was ∼3.88, with H 2O 2 production under 10%, suggesting that the ORR catalyzed by FeCo–N/C catalyst is dominated by a 4-electron transfer pathway that produces H 2O. The stability of the binary FeCo–N/C catalyst was also tested using single Fe–N/C and Co–N/C catalysts as baselines. The experimental results clearly indicated that the binary FeCo–N/C catalyst had enhanced activity and stability towards the ORR. Based on the experimental results, a possible mechanism for ORR performance enhancement using a binary FeCo–N/C catalyst is proposed and discussed.

Heat-treated cobalt–tripyridyl triazine (Co–TPTZ) electrocatalysts for oxygen reduction reaction in acidic medium

In this paper, carbon-supported cobalt–tripyridyl triazine (Co–TPTZ) complexes were synthesized by a simple chemical method, then heat-treated at 600, 700, 800, and 900 °C to optimize their activity for the oxygen reduction reaction (ORR). The resulting catalysts (Co–N/C) all showed strong catalytic activity toward the ORR, but the catalyst heat-treated at 700 °C yielded the best ORR activity. Co–N/C catalysts with several Co loadings – 0.64, 2.0, 2.96, 3.33, 5.28, and 7.18 wt% – were also synthesized and tested for ORR activity. X-ray diffraction and energy dispersive X-ray analysis were used to characterize these catalysts in terms of their structure and composition. To achieve further quantitative evaluation of the catalysts in terms of their ORR kinetics and mechanism, rotating disk electrode and rotating ring-disk electrode techniques were used with the Koutecky–Levich theory to obtain several important kinetic parameters: overall ORR electron transfer number, electron transfer coefficiency in the rate-determining step (RDS), chemical reaction rate constant, electron transfer rate constant in the RDS, exchange current density, and mole percentage of H 2O 2 produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.5 with 14% H 2O 2 production, suggesting that the ORR catalyzed by Co–N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process; based on these measurements, an ORR mechanism is proposed based on the literature and our understanding, to facilitate further investigation. The stability of a Co–N/C catalyst was also tested by fixing a current density to record the change in electrode potential with time. For comparison, two other catalysts, Fe–N/C and TPTZ/C, were also tested for stability under the same conditions as the Co–N/C catalyst. Among these three, the 5 wt% Co–N/C was most stable.

Carbon-supported Co1.67Te2 nanoparticles as electrocatalysts for oxygen reduction reaction in alkaline electrolyte

Well-dispersed Co1.67Te2 nanoparticles supported on carbon black have been synthesized via a solid-state reaction using Co and Te precursors in an autoclave at elevated temperatures. Their oxygen reduction reaction (ORR) activity and selectivity as a function of heating temperatures in catalyst synthesis were evaluated by rotating disk (RDE) and ring-disk electrodes (RRDE). It was found that the best performing catalyst (CoTe/C-900) was synthesized at a temperature of 900 °C, with regard to the most positive RDE onset (−0.18 V vs Ag/AgCl) and half-wave potentials (−0.35 V vs Ag/AgCl) as well as the lowest peroxide yield (ca. 5%) in alkaline solution (0.1 M KOH, pH = 13). Meanwhile, well-defined limiting currents were reached in the mass transfer-controlled potential range at various rotating speeds, attesting to the high density and uniform distribution of ORR active sites on the catalyst. The average electron transfer number of ORR was determined to be 3.5 for the CoTe/C-900 catalyst by using a modified Koutecky–Levich equation, nearly providing a four-electron pathway for the ORR. A transition of the Tafel slope from ca. −60 mV/dec to ca. −120 mV/dec with overpotential is directly associated with oxide formation and their coverage variation onto catalysts, suggesting a change of the rate-determining step in the ORR mechanism from intermediate-migration to charge-transfer. Extensive physical characterizations including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and X-ray photoelectron spectroscopy (XPS) were conducted for each CoTe/C sample prepared at various heating temperatures to provide insights into the origins of active sites, and Co1.67Te2 chalcogenide nanoparticles supported on carbon were found to be highly active toward ORR in alkaline electrolytes.