We present a theoretical study of CO(ad) electrooxidation on Pt nanoparticles. Effects of size and surface texture of nanoparticles on the interplay of relevant kinetic processes are investigated. Thereby, strong impacts of particle size on electrocatalytic activities, observed in experiments, are rationalized. Our theoretical approach employs the active site concept to account for the heterogeneous surface of nanoparticles. It, moreover, incorporates finite rates of surface mobility of adsorbed CO. As demonstrated, the model generalizes established mean field or nucleation and growth models. We find very good agreement of our model with chronoamperometric current transients at various particle sizes and electrode potentials (Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem., in press, doi:10.1016/j.jelechem.2006.02.024). The full interplay of on-site reactivity at active sites and low surface mobility of CO(ad) unfolds on the smallest nanoparticles ( approximately 2 nm). In this case, the solution of the model requires kinetic Monte Carlo simulations specifically developed for this problem. For larger nanoparticles (>4 nm) the surface mobility of CO(ad) is high compared to the reaction rate constants, and the kinetic equations can be solved in the limiting case of infinite surface mobility. The analysis provides an insight into the prevailing reaction mechanisms and allows for the estimation of relevant kinetic parameters.
Polymer electrolyte membrane water electrolyzers (PEMWE) are promising devices for energy conversion from electricity to chemistry. They are able to deal with important and fast changes in energy production. At the cathode compartment, the hydrogen evolution reaction occurs without kinetic limitation and practically reversibly. Nonetheless, the oxygen evolution reaction (OER) which is the anodic reaction remains a scientific challenge due to kinetic limitations and stability issues. Iridium is currently the best compromise between activity and stability [1]. However, it still undergoes degradation in the harsh oxidative and acidic operating conditions of a PEMWE [2]. The development of optimized OER catalysts is an essential step to practically implement PEMWE. A better understanding of structural effects on the catalysts activity and stability is needed. This study focuses on well-defined electrocatalysts at an atomic scale . The Ir(111) surface was characterized before and after electrooxidation and OER thanks to different electrochemical methods. A Pt(111) single-crystal, widely studied in the literature [3-4], was characterized in the same way and used as a reference. Above 1.4 vs. RHE and 1.3V vs. RHE, respectively for Pt(111) and Ir(111), the initial clean surface undergoes irreversible strucutral changes, related to the formation of defects and leading to an improvement of the oxygen evolution activity. Although classical electrochemical impedance spectroscopy (EIS) is a valuable technique when applied to stable and reversible systems, it fails with irreversible ones. Therefore, we used dynamic electrochemical impedance spectroscopy (DEIS). One can see (fig. 1) that the two methods give the same result in the reversible hydrogen adsorption area (fig.1a) while they differ for oxide formation (fig.1b) and OER (fig.1c). By analyzing the whole set of DEIS spectra, we are able to provide details on mechanism parameters for oxide formation and OER. References [1] Aricò, A. S., et al. (2012). "Polymer electrolyte membrane water electrolysis: status of technologies and potential applications in combination with renewable power sources." Journal of Applied Electrochemistry 43 (2): 107-118. [2] Cherevko, S., et al. (2014). "Dissolution of Noble Metals during Oxygen Evolution in Acidic Media." ChemCatChem 6 (8): 2219-2223. [3] CONWAY, B. E. (1995). "ELECTROCHEMICAL OXIDE FILM FORMATION AT NOBLE METALS AS A SURFACE-CHEMICAL PROCESS." Progress in Surface Science 49 (4): 331-452. [4] Drnec, J., et al. (2017). "Initial stages of Pt(111) electrooxidation: dynamic and structural studies by surface X-ray diffraction." Electrochimica Acta 224 : 220-227. Figure 1
In this paper, we use Fourier transform infrared (FTIR) spectroscopy and stripping voltammetry at saturation and submonolayer CO coverages to shed light on the influence of size on the CO adsorption and electro-oxidation on Pt nanoparticles. Pt nanoparticles supported on low surface area (∼1 m2 g-1) carbon (Sibunit) are used throughout the study. The vibrational spectra of adsorbed CO are dominated by interparticle heterogeneity (contribution of particles of different size in the range from 0.5 to 5 nm) rather than intraparticle heterogeneity (contribution of different adsorption sites). CO stripping voltammetry exhibits two peaks separated by approximately 0.25 V (at 0.02 V s-1), which are attributed to the CO oxidation from "large" (∼3.6 nm) and "small" (∼1.7 nm) Pt nanoparticles. Using stepwise oxidation, we were able to separate the contributions of "large" and "small" nanoparticles and obtain their infrared and voltammetric "fingerprints". Considerable differences are observed between "large" and "small" nanoparticles in terms of (i) the vibrational frequencies of adsorbed CO molecules (ii) their vibrational coupling, and (iii) CO oxidation overpotential.
Abstract Designing electrocatalysts with optimal activity and selectivity relies on a thorough understanding of the surface structure under reaction conditions. In this study, experimental and computational approaches are combined to elucidate reconstruction processes on low‐index Pd surfaces during H‐insertion following proton electroreduction. While electrochemical scanning tunneling microscopy clearly reveals pronounced surface roughening and morphological changes on Pd(111), Pd(110), and Pd(100) surfaces during cyclic voltammetry, a complementary analysis using inductively coupled plasma mass spectrometry excludes Pd dissolution as the primary cause of the observed restructuring. Large‐scale molecular dynamics simulations further show that these surface alterations are related to the creation and propagation of structural defects as well as phase transformations that take place during hydride formation.
Proton exchange membrane fuel cells (PEMFC), are energy converters being developed for transport as well as for stationary and portable applications. Their distinguishing features include lower temperature/pressure ranges without emission of pollutants (gas or particles). They are a leading candidate to replace the aging alkaline fuel cell technology, which was used in the space shuttle [1]. The PEMFC technology is now mature and starts to be deployed on the field, but some drawbacks must be overcome ; in particular, the durability must be increased. The insufficient durability is strongly linked to the corrosion of the carbon support in the cathode electrocatalyst. It is particularly observed under high potential and especially during start/stop phases of the cell. It leads to the detachment and agglomeration of the catalyst nanoparticles, the decrease of the carbon hydrophobicity that adversely affects the water management and the collapse of the carbon structure, phenomena that increase mass-transport losses. This study evaluates the effect of fluorination on textures and structures of different carbons and gives first insights into their durability when used as platinum electrocatalyst substrate for proton exchange membrane fuel cell (PEMFC) cathodes. Different types of carbonaceous materials such as carbon nanotubes (graphitized but with a low specific surface area), carbon blacks (currently used in the best commercial electrocatalysts, with either a large specific surface area and medium degree of organization for the one and a large degree of graphitization but a low specific surface area for the second)) and carbon aerogels (controlled texture but low degree of organization) are chosen. These textures induce a trade-off in terms of properties: graphitic carbons are more resistant to oxidation, whereas greater specific surface areas are more favorable to the dispersion of a large amount of catalyst nanoparticles per unit volume. These model materials were modified by surface treatment in order to increase their durability by increasing their hydrophobicity through controlled fluorination [2]. The objective is to limit the corrosion induced by the surface oxygen content and the electrolyte, by saturating dangling bonds with fluorine atoms. Fluorination is carried out using dynamic process (under a flow of pure molecular fluorine F2(g) [3]. Fluorination conditions were drastically controlled in order to obtain a composition i.e F/C molar ratio not greater than 0.2, to avoid decomposition of carbon into gaseous species such as CF4 and C2F6(g) [4]. All fluorinated samples were texturally, morphologically and chemically characterized by XRD, TEM, nitrogen sorption, FTIR, TGA and solid state NMR (13C and 19F nuclei). The catalytic activity of these electrocatalysts towards the oxygen reduction reaction was determined by linear sweep voltammetry (rotating disk electrode technique). Accelerated stress tests, load cycle (0.6-1 V) and start-up/shutdown (1-1.5 V) protocols, conducted at 80°C in a four-electrode cell, were performed to investigate the robustness of the bare and fluorinated Pt electrocatalysts. The results and the impact of the fluorination are discussed and compared to those for a 40 wt% commercial state-of the-art electrocatalyst. 1.Loyselle, P.P., Kevin., Teledyne Energy Systems, Inc., Proton Exchange Member (PEM) Fuel Cell Engineering Model Powerplant. Test Report: Initial Benchmark Tests in the Original Orientation. NASA. Glenn Research Center. Retrieved 15 September 2011. 2.Berthon-Fabry, S., et al., First Insight into Fluorinated Pt/Carbon Aerogels as More Corrosion-Resistant Electrocatalysts for Proton Exchange Membrane Fuel Cell Cathodes. ELECTROCATALYSIS, 2015. 6(6): p. 521-533. 3.Ahmad, Y., et al., Structure control at the nanoscale in fluorinated graphitized carbon blacks through the fluorination route. Journal of Fluorine Chemistry, 2014. 168: p. 163-172. 4.Ahmad, Y., et al., The synthesis of multilayer graphene materials by the fluorination of carbon nanodiscs/nanocones. Carbon, 2012. 50(10): p. 3897-3908.
