(Invited) Oxygen Reduction Reaction in Alkaline Media: The Effect of Carbon Support Crystallinity, Conductivity, and Doping

Abstract

The rapid development of energy consumption combined with growing environmental awareness is conducive to the wide-ranging search for ecological energy sources. In the recent decade, a significant amount of research has focused on the development of anion-exchange membrane (AEM) fuel cells (AEMFCs), as such approach exhibits faster oxygen reduction reaction (ORR) kinetics in the alkaline environment of the cell [1]. However, to become a serious alternative to the current mainstream acidic fuel cells, alkaline systems should overcome current limitations of performance and stability [2]. The design and synthesis of high-performance electrocatalysts facilitating the slow oxygen reduction reaction (ORR) is, therefore, a key issue in the context of the full commercialization of fuel cells. Transition metal oxides with spinel structure constitute an interesting group of potential catalysts that can replace the widely used materials based on Pt [3, 4]. However, due to the low electrical conductivity and the tendency to undesirable aggregation of spinel nanoparticles, it is important to support them on appropriate carbon carriers. Since the nature of the carbon carrier plays a significant role in the ORR activity, we aimed to comprehensively investigate the influence of the electric conductivity of carbon supports, the role of the surface functional oxygen-bearing groups, and doping with heteroatoms on the performance of the bare and cobalt-manganese spinels deposited on the functionalized carbon supports, acting as ORR electrocatalysts in the alkaline media [5].We investigated commercially available Vulcan XC-72, Printex85, MWCNT, amorphous carbon (C-am.), and carbons synthesized by structural nano-replication using the mesoporous molecular sieves MCM-48 (carbon denoted CMK-1) and the spherical silica calcinated at temperature range 650-1050 oC (SPH-650-1050). To investigate the effect of doping on the ORR catalytical activity the MWCNT was modified by nitrogen and sulfur heteroatoms. Additionally, the doped supports were then treated by plasma to introduce surface oxygen groups. On all investigated carbon supports the nanoparticles of manganese-cobalt spinel were dispersed. They were prepared at 150oC by a microwave-assisted hydrothermal route. Comprehensive physicochemical characterization of carbon support was performed using Raman and UV-Vis spectroscopies, XPS, TPD, XRD, TGA techniques, electron microscopy (TEM), and N2 adsorption (BET) (Figure 1). The relative electric conductivity measurements were performed using the contactless microwave technique. The electrochemical properties of the obtained materials were determined by cyclic voltammetry (CV), whereas the electrocatalytic activity of the obtained materials towards ORR was determined using the RDE and RRDE techniques.It was found that the nature of the carbon carrier plays a triple role in the ORR activity, determining dispersion and faceting of the spinel nanoparticles, as well as the extent of the undesired 2e– reduction pathway, controlled by the fraction of an amorphous component. In the case of SPH samples, it was shown that with the increasing carbonization temperature of the carbon support the electric conductivity increase in parallel to the crystallization extent. The electrocatalytic performance of the carbon carriers and the supported cobalt manganese spinels exhibits a volcano-type shape reaching the maximum for carbon materials obtained at 850°C. Such dependence results from two descriptors changing oppositely with the carbonization temperature: an increasing electric conductivity and a decreasing amount of the carbonyl and quinone groups in the sample. The electric conductivity multiplied by the amount of the carboxyl and quinone groups exhibits the same variation with the carbon calcination temperature as the number of electrons exchanged in ORR, confirming that optimal conjunction of these both properties plays a decisive role in this process. The dispersed spinel active phase favors the direct 4e− pathway of ORR and improves the value of the E onset potential distinctly. To the best of our knowledge, the role of electric conductivity and surface oxygen functionalities in the ORR activity over carbon materials and supported spinel electrocatalysts was disentangled for the first time in this study. Acknowledgments: This work was supported by the Polish National Science Centre (NCN) project OPUS-14, No. 2017/27/B/ST5/01004.