Rational Designing of Co–N–C Electrocatalysts for Comprehensive Elucidation of Intrinsic and Extrinsic Activities in the Oxygen Reduction Reaction

Abstract

The catalytic efficacy of an electrocatalyst is mostly apprised by the intrinsic activity of individual active sites and/or by extrinsic activity, which is dependent on the active site density. In oxygen electrochemistry, extrinsic activity can influence the concomitant current density, while intrinsic activity can potentially influence the onset potential, exchange current density, and half-wave potential besides the current density. Modulation of intrinsic activity is mostly limited as it is mainly dependent on the electronic and chemical structure of active sites. The extrinsic activity can be improved by employing the post-synthesis modifications without disturbing the inherent catalytic nature (i.e., intrinsic activity) of individual active sites to ensure the utility of maximum instituted centers. In this work, we have made an attempt to elucidate the intrinsic and extrinsic activities of Co–N–C-based nanostructures to promote the oxygen reduction reaction (ORR) efficiency, which is further elucidated by X-ray absorption spectroscopic studies. The experimental finding suggests that an increase in the N content (∼9 at %) in the host medium, that is, carbon, decisively impacts the intrinsic nature of electrocatalysts with a positive shift in the ORR onset potential of ∼ 131 mV and a nearly 1.5-fold increase in the exchange current density, a matrix that gives a measure of the ability to support catalytic activity inherently. The targeted removal of electrochemically less effective Co nanoparticles (which block/obscure the active centers Co–N x in their vicinity) ensures the structural intactness as the C 1s full width at half maximum of the host matrix along with ORR onset potentials and Tafel slopes remains unchanged. The experimental finding further indicates that the current density can be improved by a factor of 2.6 (i.e., from 2.13 to 5.65 mA/cm2), ensuring the effective utilization of electrocatalysts. Overall, the rational combination of intrinsic and extrinsic activities can be valuable to design a variety of transition metal-carbon based electrocatalysts for diverse electrochemical energy devices.

Stimuli-free Zn/soda-lime glass/CuO-based MIS device for sensing human skin moisture

“…37 Besides, the peaks at 532.1 eV and 533.4 eV correspond to the surface residual of other oxygen species and the superficial adsorbed water, respectively. 35,38,39 Furthermore, the HRXPS spectra of Zn 2p in Fig. 1(d) reveal the presence of Zn 2p 3/2 and Zn 2p 1/2 with peaks at 1021.6 eV and 1044.7 eV with a separation of about 23 eV, respectively.…”

Activity Origin and Catalytic Mechanism of the M–N–C Catalysts for the Oxygen Reduction Reaction

“…Among the different transition metal (Fe, Co, Mn, Ni, Cu, and Zn) active sites, atomically dispersed Fe–N–C and Co–N–C are most considered for ORR due to their higher activity. − The Co–N–C catalysts show alleviated “Fenton-like” process and is considered as an ideal substitute for Fe–N–C electrocatalysts . In many reports, ZIF-67, − cobalt ion-coordinated covalent–organic polymer (Co/COP), ,, cobalt phthalocyanine (CoPc), − etc., have been applied as precursors for the synthesis of Co–N–C catalysts. For instance, Freitas et al used ZIF-67 as single Co- and N-source to synthesize Co(10)–N–C, which possessed high 4e – ORR activity with an E 1/2 of 0.85 V versus RHE under alkaline conditions.…”

“…On the one hand, the Co sites in ZIF-67 are easy to agglomerate into nanoparticles under high temperatures, resulting in a limited number of single-atom Co sites. A considerable fraction of Co–N x sites are buried in the carbon matrix and are difficult to access due to the ultrafine micropores of ZIF-67. , To address this issue, Bisen et al optimized the intrinsic and extrinsic properties of the Co–N–C/900–40 catalyst by adjusting the N content (∼9 at%) and removing Co nanoparticles, resulting in the increase of active sites and the improvement of ORR activity (Figure c and d). In consideration of the similar structures of ZIF-8 and ZIF-67, Zn–Co–ZIF has been extensively studied for the synthesis of Co–N–C electrocatalysts.…”

Green synthesis of a magnesium single-atom catalyst from Spinacia oleracea chlorophyll extracts for sustainable electrocatalytic nitrate reduction to ammonia

“…The lower flat band potential favours easy charge transfer between the cata-lyst and the electrolyte at the interface and hence favours better reaction kinetics. 26,27,37,38 The slope of the inverse capacitance square value (1/C 2 ) vs. potential plot is inversely proportional to the charge carrier density of the MgN x C catalysts. Fig.…”