The effect of pH on the electrocatalytic oxidation of formic acid/formate on platinum: A mechanistic study by surface-enhanced infrared spectroscopy coupled with cyclic voltammetry

Propitious Dendritic Cu2O-Pt Nanostructured Anodes for Direct Formic Acid Fuel Cells

This study introduces a novel competent dendritic copper oxide-platinum nanocatalyst (nano-Cu₂O-Pt) immobilized onto a glassy carbon (GC) substrate for formic acid (FA) electro-oxidation (FAO); the prime reaction in the anodic compartment of direct formic acid fuel cells (DFAFCs). Interestingly, the proposed catalyst exhibited an outstanding improvement for FAO compared to the traditional platinum nanoparticles (nano-Pt) modified GC (nano-Pt/GC) catalyst. This was evaluated from steering the reaction mechanism toward the desired direct route producing carbon dioxide (CO₂); consistently with mitigating the other undesired indirect pathway producing carbon monoxide (CO); the potential poison deteriorating the catalytic activity of typical Pt-based catalysts. Moreover, the developed catalyst showed a reasonable long-term catalytic stability along with a significant lowering in onset potential of direct FAO that ultimately reduces the polarization and amplifies the fuel cell's voltage. The observed catalytic enhancement was believed to originate bifunctionally; while nano-Pt represented the base for the FA adsorption, nanostructured copper oxide (nano-Cu₂O) behaved as a catalytic mediator facilitating the charge transfer during FAO and providing the oxygen atmosphere inspiring the poison's (CO) oxidation at relatively lower potential. Surprisingly, moreover, nano-Cu₂O induced a surface retrieval of nano-Pt active sites by capturing the poisoning CO via "a spillover mechanism" to renovate the Pt surface for the direct FAO. Finally, the catalytic tolerance of the developed catalyst toward halides' poisoning was discussed.

Revealing the Effect of Protein Weak Adsorption to Nanoparticles on the Interaction between the Desorbed Protein and its Binding Partner by Surface-Enhanced Infrared Spectroelectrochemistry

In recent years, the properties of protein corona have attracted intense interest in the field of nanobio interface, but a long-ignored research issue is how the desorbed proteins suffering from conformational change upon weak association with nanoparticles affect their functional properties when further interacting with their downstream protein partners. In this Article, surface-enhanced infrared absorption spectroscopy (SEIRAS) and electrochemical cyclic voltammetry were used to study the adsorption and redox properties of the soluble cytochrome c (cyt c) on 11-mercaptoundecanoic acid (MUA) self-assembled monolayer (SAM) after weakly binding to and then desorbed from nano-TiO₂. For the first time, our study reveals that the weak interaction between cyt c and nano-TiO₂ induces the protein to undergo a heterogeneous conformational change. More importantly, the cyt c with a largely unfolded conformation exhibits a weaker interaction with its binding partner mimics than the native-like cyt c but a faster adsorption rate even at a concentration that is much lower than that of native-like cyt c. Correspondingly, the cyt c with a large unfolding shows a greatly positive-shifted formal potential (E_f) relative to the native-like protein possibly due to the disruption of the pocket structure of heme in the vicinity of Met80. These properties could enable the largely unfolded cyt c to undergo a favorable binding but an unavailable electron transfer to cytochrome c oxidase even in the presence of high-concentration native cyt c, probably causing the disruption of electron flow.

Formate adsorption on Pt nanoparticles during formic acid electro-oxidation: insights from in situ infrared spectroscopy

Adsorbed formate is observed on a supported Pt nanoparticle for the first time during formic acid electro-oxidation. Bands assigned to OCO stretching and CH bending reveal some OCO but little CH bond weakening on adsorption compared to the free anion. The formate potential dependence is similar to polycrystalline electrodes while adsorbed CO persists up to +1.2 V, 0.5 V higher than on polycrystalline Pt.

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.

The Dilemma of Supporting Electrolytes for Electroorganic Synthesis: A Case Study on Kolbe Electrolysis

Remarkably, coulombic efficiency (CE, about 50 % at 1 Farad equivalent), and product composition resulting from aqueous Kolbe electrolysis are independent of reactor temperature and initial pH value. Although numerous studies on Kolbe electrolysis are available, the interrelations of different reaction parameters (e.g., acid concentration, pH, and especially electrolytic conductivity) are not addressed. A systematic analysis based on cyclic voltammetry reveals that solely the electrolytic conductivity impacts the current-voltage behavior. When using supporting electrolytes, not only their concentration, but also the type is decisive. We show that higher concentrations of KNO3 result in reduced CE and thus in significant increase in electric energy demand per converted molecule, whereas Na2 SO4 allows improved space-time yields. Pros and cons of adding supporting electrolytes are discussed in a final cost assessment.

Novel fuel blends facilitating the electro-oxidation of formic acid at a nano-Pt/GC electrode

This paper addresses the promoting effect of the electrooxidation of formic acid (FAO) at a nano-Pt/GC electrode in the presence of selected low molecular weight alcohols (R–OH) as blending components.

A highly active Pd–P nanoparticle electrocatalyst for enhanced formic acid oxidation synthesized via stepwise electroless deposition

A highly active Pd–P nanoparticle electrocatalyst for formic acid oxidation was synthesized using NaH₂PO₂ as the reducing agent.

Optimal Electrocatalytic Pd/MWNTs Nanocatalysts toward Formic Acid Oxidation

The operating conditions such as composition of electrolyte and temperature can greatly influence the formic acid (HCOOH) oxidation reaction (FAOR). Palladium decorated multi-walled carbon nanotubes (Pd/MWNTs) were successfully synthesized and employed as nanocatalysts to explore the effects of formic acid, sulfuric acid (H₂SO₄) concentration and temperature on FAOR. Both the hydrogen adsorption in low potential range and the oxidation of poisoning species during the high potential range in cyclic voltammetry were demonstrated to contribute to the enhanced electroactivity of Pd/MWNTs. The as-synthesized Pd/MWNTs gave the best performance under a condition with balanced adsorptions of HCOOH and H₂SO₄ molecules. The dominant dehydrogenation pathway on Pd/MWNTs can be largely depressed by the increased dehydration pathway, leading to an increased charge transfer resistance (R_ct ). Increasing HCOOH concentration could directly increase the dehydration process proportion and cause the production of CO_ads species. H₂SO₄ as donor of H⁺ greatly facilitated the onset oxidation of HCOOH in the beginning process but it largely depressed the HCOOH oxidation with excess amount of H⁺. Enhanced ion mobility with increasing the temperature was mainly responsible for the increased current densities, improved tolerance stabilities and reduced R_ct values, while dehydration process was also increased simultaneously.

Factors Influencing the Growth of Pt Nanowires via Chemical Self-Assembly and their Fuel Cell Performance

This work reports a detailed investigation of the template-free synthesis of Pt nanowires via the chemical reduction of Pt salt precursors with formic-acid. The results indicate that both the oxidation state of Pt in the salt and the pH value of the aqueous solution comprising the platinum salt and formic acid are critical factors for the formation of Pt nanowires. Nanowires are obtained from platinum atoms in a +IV oxidation state, with ligating chloride anions (H2 PtCl6 and K2 PtCl6 ) or nonligating chloride anions (PtCl4 ). Increasing the pH of the aqueous Pt salt and HCOOH solution leads to a drastic reduction of the nanowires' length between pH 3 and 4.5. A mechanism involving formate as a reducing agent and formic acid as a structure directing agent explains these results. The Pt nanowires are stable up to 200 °C; therefore, these nanowires are suitable for use as catalysts in proton-exchange-membrane fuel cell. The optimized synthesis conditions are then selected for investigating the kinetics of the oxygen reduction reaction (ORR) of such nanowires in a fuel cell. The ORR mass activity of the Pt nanowires is 130 A g(-1) Pt at 0.9 V iR-free potential; significantly higher than that of two commercial Pt/C catalysts tested in the same conditions. The higher mass activity is explained based on a higher surface specific activity. Accelerated degradation tests indicate that Pt nanowires supported on carbon are as stable as Pt nanoparticles supported on carbon.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Formic acid oxidation on platinum: a simple mechanistic study

Formic acid oxidation on Pt(111) under electrocatalytic conditions occurs when a formate anion approaches the Pt(111) surface in the CH-down orientation, and barrierlessly releases carbon dioxide as the H binds to the surface.

Non-covalent interactions at electrochemical interfaces: one model fits all?

The shift with increasing concentration of alkali-metal cations of the potentials of both the spike and the hump observed in the cyclic voltammograms of Pt(111) electrodes in sulfuric acid solutions is shown to obey the simple model recently developed by us to explain the effect of non-covalent interactions at the electrical double layer. The results suggest that the model, originally developed to describe the effect of alkali-metal cations on the cyclic voltammogram of cyanide-modified Pt(111) electrodes, is of general applicability and can explain quantitatively the effect of cations on the properties of the electrical double layer.

Electrocatalysis of formic acid on palladium and platinum surfaces: from fundamental mechanisms to fuel cell applications

A brief overview is presented on recent progress in mechanistic studies of formic acid oxidation, synthesis of novel Pd- and Pt-based nanocatalysts and their practical applications in direct formic acid fuel cells.