Long-Lasting Oscillations in the Electro-Oxidation of Formic Acid on PtSn Intermetallic Surfaces

The oscillatory electro-oxidation of 2-propanol on platinum: the effect of temperature and addition of methanol

The oscillatory electro-oxidation of 2-propanol on platinum and platinum-based catalysts has attracted growing attention in recent years due to its importance in the interconversion between chemical and electrical energies. This reaction might proceed with a very high selectivity to acetone, nearly without the formation of carbon dioxide, and the reversibility of the 2-propanol/acetone pair is very appropriate for hydrogen transfer. An important aspect of this system is the ubiquitous emergence of potential oscillations under current control, and it has been pointed out as a problem to be avoided and a primary cause of limitations to the use of 2-propanol in practical devices. Herein, we present an experimental study of the electrochemical instabilities in the electro-oxidation of 2-propanol on platinum. The system was studied using polycrystalline platinum, in acidic media and at different temperatures. Besides the extensive characterization of the potential oscillations, we have also discussed possible venues for engineering the dynamics to benefit from the potential oscillations. In this sense, we have also characterized the instabilities in the system containing a mixture of 2-propanol and methanol. The efficiency of a hypothetical fuel cell operated under different conditions is also presented.

Recent Advances in Anode Electrocatalysts for Direct Formic Acid Fuel Cell-II-Platinum-Based Catalysts

Platinum-based catalysts have a long history of application in formic acid oxidation (FAO). The single metal Pt is active in FAO but expensive, scarce, and rapidly deactivates. Understanding the mechanism of FAO over Pt important for the rational design of catalysts. Pt nanomaterials rapidly deactivate because of the CO poisoning of Pt active sites via the dehydration pathway. Alloying with another transition metal improves the performance of Pt-based catalysts through bifunctional, ensemble, and steric effects. Supporting Pt catalysts on a high-surface-area support material is another technique to improve their overall catalytic activity. This review summarizes recent findings on the mechanism of FAO over Pt and Pt-based alloy catalysts. It also summarizes and analyzes binary and ternary Pt-based catalysts to understand their catalytic activity and structure relationship.

Single-Nanoparticle Coulometry Method with High Sensitivity and High Throughput to Study the Electrochemical Activity and Oscillation of Single Nanocatalysts

To explore nanocatalysts with high electro-catalytic performance and less loading of precious metals, efforts have been made to develop electrochemical methods with high spatial resolution at the single nanoparticle level. Herein, a highly sensitive single-nanoparticle coulometry method is successfully developed to study the electrochemical activity and oscillation of single PtTe nanocatalysts. Based on microbattery reactions involving the formic acid electro-oxidation and the deposition of Ag on the single PtTe nanocatalyst surface, this method enables the transition from the undetectable sub-fA electric signal of the formic acid electro-oxidation into strong localized surface plasmon resonance scattering signal of Ag detected by dark-field microscopy. The lowest limiting current for a single nanocatalyst is found to be as low as 25.8 aA. Different trends of activity versus the formic acid concentration and types of activity of the single nanocatalyst have been discovered. Unveiled frequency-amplitude graph shows that the two electrochemical oscillation modes of low frequency with high amplitude and vice versa coexist in a single PtTe nanocatalyst, indicating the abundantly smooth surfaces and defects of nanocatalysts. This conducted study will open up the new avenue for further behavioral and mechanistic investigation of more types of nanocatalysts in the electrochemistry community.

Probing the surface fine structure through electrochemical oscillations

In the course of (electro)catalytic reactions, reversible and irreversible changes, namely the formation of adsorbed poisons, catalyst degradation, surface roughening, etc., take place at distinct time-scales. Reading the transformations on the catalyst surface from the measurement of the reaction rates is greatly desirable but generally not feasible. Herein, we study the effect of random surface defects on Pt(100) electrodes toward the electro-oxidation of methanol in acidic media. The surface defects are gently generated in situ and their relative magnitudes are reproducibly controlled. The system was characterized under conventional conditions and investigated under an oscillatory regime. Oscillatory patterns were selected according to the presence of surface defects, and a continuous transition from large amplitude/low frequency oscillations (type L) on smooth surfaces to small amplitude/high frequency oscillations (type S) on disordered surfaces was observed. Importantly, self-organized potential oscillations were found to be much more sensitive to the surface structure than conventional electrochemical signatures or even other in situ characterization methods. As a consequence, we proved the possibility of following the surface fine structure in situ and in a non-invasive manner by monitoring the temporal evolution of oscillatory patterns. From a mechanistic point of view, we describe the role played by surface defects and of the adsorbed and partially oxidized, dissolved species on the oscillations of type S and L.

1/f 2 noise in bistable electrocatalytic reactions on mesoscale electrodes

The formation of a self-organized spatial domain during current-controlled CO oxidation, a kinetically bistable reaction, is investigated experimentally and by deterministic simulations as a function of the electrode size and of the supporting electrolyte concentration. Decreasing the microelectrode size leads to the suppression of the spatial instability at the electrode and thus stabilizes the S-NDR branch of the reaction. The critical microelectrode size capable of supporting sustained domain formation is shown to be strongly affected by the sulfuric acid concentration, the characteristic time of the positive feedback loop increasing with the sulfate concentration. Furthermore, we demonstrate that for microelectrode diameters close to the instability threshold, small amplitude electrochemical potential fluctuations appear in the system. These potential fluctuations cannot be captured by deterministic mathematical models and are attributed to a strong enhancement of molecular fluctuations or intrinsic noise in the vicinity of the spatial instability. Analysis of the electrochemical noise revealed a 1/f ² frequency dependence and several common features with neuronal shot noise.

The effect of temperature on the coupled slow and fast dynamics of an electrochemical oscillator

The coupling among disparate time-scales is ubiquitous in many chemical and biological systems. We have recently investigated the effect of fast and, long-term, slow dynamics in surface processes underlying some electrocatalytic reactions. Herein we report on the effect of temperature on the coupled slow and fast dynamics of a model system, namely the electro-oxidation of formic acid on platinum studied at five temperatures between 5 and 45 °C. The main result was a turning point found at 25 °C, which clearly defines two regions for the temperature dependency on the overall kinetics. In addition, the long-term evolution allowed us to compare reaction steps related to fast and slow evolutions. Results were discussed in terms of the key role of PtO species, which chemically couple slow and fast dynamics. In summary we were able to: (a) identify the competition between two reaction steps as responsible for the occurrence of two temperature domains; (b) compare the relative activation energies of these two steps; and (c) suggest the role of a given reaction step on the period-increasing set of reactions involved in the oscillatory dynamics. The introduced methodology could be applied to other systems to uncover the temperature dependence of complex chemical networks.