Modeling of oxygen reduction reaction in porous carbon materials in alkaline medium. Effect of microporosity

The role of porosity, and more specifically, microporosity, in the performance of carbon materials as Oxygen Reduction Reaction (ORR) catalysts in alkaline medium still has to be clarified. For this purpose, a highly microporous KOH-activated carbon and a microporous char have been prepared and their ORR performance in alkaline media were compared to that of two commercial carbon blacks with low and high surface areas, respectively. Interestingly, all carbon materials show a two-wave electrocatalytic process, where the limiting current and the number of electron transferred increase when going to more negative potentials. The limiting current and onset potential of the second wave is positively related to the amount of microporosity, and H2O2 electrochemical reduction tests have confirmed that the second wave could be related to the catalytic activity towards this reaction. In accordance to these findings, a model is developed that takes into account narrow and wide micropores in both charge transfer reactions and the mass transfer rate of O2 and H2O2. This model successfully reproduces the experimental electrochemical response during ORR of the analyzed porous carbon materials and suggests the important role of narrow micropores in H2O2 reduction.This work was supported by MINECO (CTQ2015-66080-R MINECO/FEDER) and Heiwa Nakajima Foundation

Key factors improving oxygen reduction reaction activity in cobalt nanoparticles modified carbon nanotubes

Multiwall carbon nanotubes (CNTs) decorated with cobalt oxide (CoOx) nanoparticles (NPs) are prepared in various synthesis conditions to investigate their capability as oxygen reduction reaction (ORR) catalysts for fuel cells in alkaline media. The synthesis conditions include the use of protecting, reducing or complexing agents and heat treatment. Higher ORR activity is possible for smaller size of Co NPs catalysts due to the enlarged interfaces between Co species and CNTs. The addition of polyvinylpyrrolidone (PVP) as protecting agent and NaBH4 during the preparation procedure is necessary for obtaining the highest activity since it favors the formation of lower oxidation states for Co species and the incorporation of N groups which improve ORR activity. CNTs loaded with only 1 wt.% of Co NPs prepared by a facile method using PVP, NaBH4 and subsequent heat treatment at 500 °C under N2 atmosphere, demonstrates both similar catalytic activity and stability than Pt/Vulcan (20 wt.% Pt on Vulcan). The synergic chemical coupling effects between CNTs and CoOx NPs and the presence of carbon material with pyridinic N and quaternary N groups formed from the protecting agent decomposition, seem to be the main factors responsible for the remarkable electrocatalytic activity.The authors would like to thank GV and FEDER (PROMETEOII/2014/010), projects CTQ2015-66080-R (MINECO/FEDER), MAT2016-76595-R (MINECO/FEDER), BES-2013-063678 and HEIWA NAKAJIMA FOUNDATION for the financial support

Understanding of Carbon Active Sites for Oxygen Reduction Reaction

Este trabajo de Tesis Doctoral se ha centrado en comprender el comportamiento de electrocatalizadores basados en materiales carbonosos para la reacción de reducción de oxígeno. Con el fin de profundizar en el conocimiento de la naturaleza de los sitios activos de catalizadores basados en materiales carbonosos para esta reacción, se han seleccionado o preparado muestras con diferentes composiciones, texturas porosas y estructuras. De estos resultados se han conseguido importantes avances en el conocimiento del papel que los sitios activos de catalizadores basados en materiales carbonosos desempeñan en dicha reacción. Estos conocimientos y los materiales derivados pueden utilizarse en el desarrollo de cátodos para pilas de combustible en medio alcalino.Heiwa Nakajima Foundatio

Understanding of Carbon Active Sites for Oxygen Reduction Reaction

Este trabajo de Tesis Doctoral se ha centrado en comprender el comportamiento de electrocatalizadores basados en materiales carbonosos para la reacción de reducción de oxígeno. Con el fin de profundizar en el conocimiento de la naturaleza de los sitios activos de catalizadores basados en materiales carbonosos para esta reacción, se han seleccionado o preparado muestras con diferentes composiciones, texturas porosas y estructuras. De estos resultados se han conseguido importantes avances en el conocimiento del papel que los sitios activos de catalizadores basados en materiales carbonosos desempeñan en dicha reacción. Estos conocimientos y los materiales derivados pueden utilizarse en el desarrollo de cátodos para pilas de combustible en medio alcalino.Heiwa Nakajima Foundatio

Understanding of oxygen reduction reaction by examining carbon-oxygen gasification reaction and carbon active sites on metal and heteroatoms free carbon materials of different porosities and structures

Knowledge of the carbon active sites for Oxygen Reduction Reaction (ORR) remains confusing and controversial and thus the detailed mechanism is still not clarified. Considering the nature of the carbon-oxygen interaction in active sites during the gasification reaction, it could be inferred that the sites for this reaction are the same as those participating in the ORR. Herein, the relationship between carbon-oxygen gasification properties and ORR activities was elucidated. Carbon materials including different structures and porosities were selected and extensively characterized in terms of structural and electrochemical properties. Regarding gasification properties, active surface area (ASA) and reactivity for carbon-oxygen reaction were determined. A good linear correlation is found between ORR activity and carbon-oxygen gasification reactivity. Interestingly, similar correlation is found between ORR activity or gasification reactivity and ASA, although two different slopes are observed for the analyzed samples, being higher for the carbon nanotubes (CNT) based samples. The results suggest that the active sites participating in the gasification reaction can be catalytic sites for ORR although the specific catalytic activity is determined by the carbon structure. Thus, active sites in CNT show higher activities toward ORR and carbon gasification reactivities than others.The authors gratefully acknowledge the financial support by MINECO (CTQ2015-66080-R MINECO/FEDER) and HEIWA NAKAJIMA FOUNDATION

Synthesis of conducting polymer/carbon material composites and their application in electrical energy storage

peer reviewe

Exploring the effect of surface chemistry and particle size of boron-doped diamond powder as catalyst and catalyst support for the oxygen reduction reaction

The interest for the electrodes based on conductive boron-doped diamond powder (BDDP) is increasing in recent years due to their excellent physical and chemical stability, the wide potential window in both aqueous and organic electrolytes, the relatively large specific surface area, and their versatility in comparison to boron-doped diamond thin film electrodes. These materials have been proposed as alternative cathode catalyst support due to their high corrosion resistance when subjected to the highly positive potentials originated during the start-stop operations in fuel cells, especially in automobiles. In this work, we present three BDDP supports with different particle sizes and different surface oxygen contents as supports of different iron species for oxygen reduction reaction in alkaline solution. BDDP supports were modified with carbon nitride (C3N4) or phthalocyanines (Pc) as anchoring points for iron. The different electrocatalytic performance observed confirmed the strong influence of the surface chemistry of the BDDP supports on the activity of the metallic sites for FePc samples while, for Fe-C3N4 samples, the determining effect was the particle size of the BDDP support. Additionally, DFT calculations were used to obtain some insights about the interaction of the FePc with the diamond surface.The authors would like to thank PID2019-105923RB-I00 and PID2021-123079OB-I00 projects funded by MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe”. B. Martínez-Sánchez and G. Alemany-Molina thank Ministerio de Universidades for the FPU18/05127 and FPU20/03969 grants, respectively

Fabrication of Co/P25 coated with thin nitrogen-doped carbon shells (Co/P25/NC) as an efficient electrocatalyst for oxygen reduction reaction (ORR)

Development of electrocatalysts for oxygen reduction reaction (ORR) is important to solve the current problems about energy and fuel. As one of the catalysts with high performance, platina group metals (PGM) based catalysts have been widely known. However, PGM based catalysts are not suitable for large commercial application since the PGM based catalysts are so expensive. In this work, we have developed TiO2 P25 coated with Co and nitrogen-doped carbon layers using commercially available and inexpensive P25 as a support (Co/P25/NC) as one of the alternatives to PGM based catalysts. Co/P25/NC showed an excellent catalytic activity on ORR compared to the other catalysts prepared using SiO2, ZrO2 and Al2O3 as a support. The ORR activity of Co/P25/NC was comparable to Pt based electrocatalysts. In addition, Co/P25/NC showed excellent durability and tolerance toward methanol compared to the Pt based catalyst. This work would provide a new synthetic strategy of electrocatalysts.The financial support by MINECO, Spain (CTQ2015-66080-R (MINECO/FEDER)) and HEIWA NAKAJIMA FOUNDATION is acknowledged

Anchoring a Co/2-methylimidazole complex on ion-exchange resin and its transformation to Co/N-doped carbon as an electrocatalyst for the ORR

Synthesizing high-activity metal/heteroatom-co-doped carbon (M/H–C) electrocatalysts for the oxygen reduction reaction (ORR) is critical for the commercialization of fuel cells, but remains challenging because of the difficulty in making highly dispersed (M/H–C) electrocatalysts. Herein, a commercial ion-exchange resin, Amberlyst 15, is used as a support, and Co/2-methylimidazole is anchored on the ion-exchange site of the support to obtain Co/N/Amb as the precursor of the M/H–C electrocatalyst. After the precursor is pyrolyzed and converted into Co/N/C materials, it is activated with CO2, and the obtained material (act-Co/N/C) exhibits remarkable ORR activity with a high onset potential of 0.943 V (vs. the reversible hydrogen electrode) and an excellent kinetic current density comparable to commercial 5 wt% Pt/C in an alkaline medium. The favorable activity is mainly attributed to the ultra-dispersed CoNx and high porosity of act-Co/N/C. Our synthetic strategy could be applied to the preparation of other well-dispersed M/H–C materials. This work opens a new direction in the synthetic strategy of carbon materials.Financial support from MINECO (CTQ2015-66080-R) and HEIWA NAKAJIMA FOUNDATION is acknowledged

Quantifying Carbon Active Sites Chemisorbing Hydrogen on Oxygen Containing Activated Carbons during Heat Treatment in Hydrogen Atmosphere

Carbon edge sites have been widely studied because of their importance in surface reactivity and electronic properties. The surface chemistry of the carbon edge sites is relevant to various reactions, and carbon active sites are key topics in many applications. Temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) techniques are used to clarify the fate of oxygen atoms present as CO-yielding functional groups on the activated carbon during heat treatment in hydrogen with an argon balance atmosphere. It has been elucidated that CO is decomposed, H2O is released by a reduction reaction with atmospheric H2, and CO2 is evolved by secondary reactions from the CO-yielding functional groups during TPR. Atmospheric H2 consumption during TPR is observed and its rate is characterized. The amounts of carbon active sites are quantified by determining the amount of H2 chemisorbed onto the carbon surfaces. Finally, it is quantitatively determined that the active sites that chemisorb hydrogen are generated after the decomposition of CO and CO2 caused by secondary reactions between ca. 700 and 1100 K from the CO-yielding functional groups. The origin of these CO-yielding functional groups is generally attributed to phenol/ether groups. In addition to these oxygen-containing functional group decompositions, some free sites on the edge sites are activated for H2 chemisorption by heat treatment between ca. 700 and 1100 K