The catalytic activity of Pt catalysts towards the oxygen reduction reaction (ORR) was investigated on a catalyst system developed by thermally induced chemical deposition of Pt on carbon. The use of this deposition method made it possible to prepare a practical catalyst system with various Pt loadings on the support. Increasing the Pt loading caused a change in the Pt surface morphology which was confirmed by transmission electron microscopy (TEM) and CO stripping voltammetry measurements. The occurrence of a low and high-potential CO oxidation peak suggested the presence of Pt agglomerates and Pt nanoparticles, respectively. An increase in Pt loading lead to a subsequent decrease in the electrochemical surface area (ECSA, m 2 Pt /g Pt ) as the platinum surface transitioned from isolated platinum nanoparticles to platinum agglomerates. The specific activity was found to increase with increasing Pt loadings, while the mass activity decreased with loading. The mass and specific activity data from this study was found to follow a 'master curve' obtained by the comparison of normalised activities from various different studies in the literature. Pt selectivity was also affected by Pt loading and hence Pt surface morphology. At low Pt loadings, i.e. large interparticle distances, the amount of H 2 O 2 produced was significantly higher than for high Pt loadings. This confirms the presence of a 'series reaction pathway' and highlights the importance of the H 2 O 2 desorptionreadsorption mechanism on Pt nanoparticles and the ultimate role of Pt interparticle distance on the ORR mechanism.

The catalytic activity and selectivity of Pt nanoparticles towards the oxygen reduction reaction (ORR) were investigated as a function of the Pt catalyst distribution. By means of the sputtering deposition technique, it was possible to fabricate Pt catalysts with different loadings that consisted of dispersed 2–3 nm particles, nanoparticle agglomerates and extended particulate layers. The transition from dispersed nanoparticles to extended layers led to a decrease in the electrochemical surface area (ECSA, m2Pt gPt−1) and to a shift of the platinum oxide reduction peak to more positive potentials, which indicates a decrease in the adsorption energy for oxygenated species. The latter finding was correlated to the observed decrease in specific activity with the increasing ECSA, that is, in the case of isolated nanoparticles, the higher adsorption energy for oxygenated species causes a reduction in the specific activity towards the ORR as larger amounts of active sites are blocked compared to extended surfaces. The presented data of specific and mass activity versus ECSA were found to follow a “master curve” obtained by comparing normalised Pt activities from different studies. The transition from dispersed Pt nanoparticles to extended layers also influences the Pt selectivity. At a decreased interparticle distance, a significant increase in the H2O2 production was observed below 0.6 V versus the reversible hydrogen electrode, which indicates the important role of a H2O2 desorption–readsorption reaction mechanism during the ORR on Pt nanoparticles.

This work focuses on the performance and stability of selected commercial carbon electrode materials before and after heat-treatment in an operating all-vanadium redox flow battery (VRB). Heat treatment results in improved cell performance for all tested materials, with SGL 39 AA carbon papers and SIGRACELL GFD4.6 EA carbon felt showing the best performance. Further investigation of these two materials by in situ reference electrode measurements reveal improvements after heat-treatment that originate mainly from the negative electrode or V 2+ /V 3+ side of the cell. Upon extended cycling, carbon felt is found to be stable. Carbon papers however, show significant performance losses originating from the negative electrode side. The potential limit during charging and the exposure to very negative potentials appears to be a critical issue at the negative electrode in the VRB. Analysis of both materials after cycling by scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy reveal significant differences in their surface chemistry, structure and morphology. These differences give valuable insights into the behavior and degradation of different carbon materials used in VRBs. Redox flow batteries (RFBs) are a promising technology for efficient energy storage and grid stabilization.1,2 The all-vanadium redox flow battery (VRB), which uses vanadium ions in different oxidation states at the positive and negative electrodes, is the most advanced RFB to date.3 The electrodes are a crucial component of the VRB, as they provide the surface on which the respective electrochemical reactions occur. Thus, catalytic activity, wettability and mass transport properties of the electrodes strongly affect VRB performance. Ideal electrodes for the VRB should provide both: long term durability and stable catalytic activity. Various materials have been considered as electrodes for the use in VRBs including non-carbon based dimensionally stable anode electrodes and carbon based electrodes such as carbon felt, carbon paper, carbon nanotubes, carbon nanofibers or graphene oxides. 4 To enhance electrochemical activity and wettability of carbon based materials in VRBs, different surface modification methods have been used. Carbon electrodes have been coated with metals such as iridium, 5 doped with nitrogen 6 or decorated with nanomaterials such as graphene-nanowalls 7 or graphite carbon nanotubes. Most of these surface treatment approaches introduce functional groups, commonly oxygen onto the carbon electrode surface. This leads to increased wettability and redox activity, which is generally attributed to the increased concentration of surface-active oxygen functional groups.11 Among the various surface modifications, heattreatment is still regarded as the most common and facile approach to incorporate oxygen groups onto the surface of carbon materials. 4 In an effort to better understand the role of oxygen functional groups on electrode performance, Fink et al. studied pristine and heat-treated Rayon (a regene...

This paper provides valuable insights into the kinetics of the vanadium (V) reduction reaction occurring at a glassy carbon (GC) model electrode surface treated by different oxidative and mechanical methods. Oxidative treatments were applied by thermal, acid and electrochemical means. Mechanical polishing on an abrasive sandpaper surface was used to prepare a rough GC electrode, this surface was also further electrochemically oxidised. The resulting surfaces were studied by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Activity is defined in terms of peak potential separation (∆Ep) and this descriptor was verified by EIS. No correlation between activity and oxygen to carbon ratio (O/C) or any specific oxygen functional group was found in this study. Raman spectroscopy revealed significant structural changes for GC electrodes treated by electrochemical oxidation and abrasive polishing. A correlation between structural disorder in GC and improvements in activity was observed. However, a limit of structural disorder exists, beyond which no substantial improvements in activity can be achieved.

The V 2+ /V 3+ reaction occurring at the negative electrode in the all-vanadium redox flow battery has been identified as performance limiting in the system. Given the complexity of the commercial carbon electrodes typically used for this application, a model electrode approach is adopted in this work to study electrode activity and stability. This was done using edge and basal plane pyrolytic graphite electrodes modified by electrochemical oxidation. Differential electrochemical mass spectrometry was used for the first time in this work to investigate the parasitic hydrogen evolution reaction on the oxidized carbon surfaces. The basal plane surface showed a higher faradaic efficiency for V 3+ reduction compared to that for the edge plane surface. The oxidized surfaces were subject to extended cycling, after which the basal electrode showed a dramatic loss in activity compared to the edge surface which was relatively stable. This activity loss was related to the poor mechanical stability of the basal plane surface. The electrodes were analyzed before and after cycling by different techniques including X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy. The influences of the type of carbon and its related properties on electrode activity and stability are discussed.