Electrochemical Oxidation Of Formic Acid Research Articles

Bimetallic Pd-Based surface alloys promote electrochemical oxidation of formic acid: Mechanism, kinetics and descriptor

Palladium is well noted as anode catalyst for promoting electrochemical oxidation of formic acid (FAO) in direct formic acid fuel cell thanks to its high CO tolerance. Here, using combined approach of density-functional theory calculations and microkinetic modeling, we investigate the fundamental aspects of FAO catalyzed by bimetallic M@Pd(111) single-atom surface alloys (where M = Mo, Fe, Ru, Co, Ni, Cu, Zn, Ag, Au). Our results suggest that M@Pd(111) are highly stable and outperforms Pd(111) for FAO via primarily the direct mechanism: HCOOH → *HCOO (formate) → CO2 + 2H+ + 2e−. It is revealed that the decoordination of *HCOO from bidentate to monodentate adsorption mode (i.e., *bHuCOO → *mHdCOO) followed by the facile carbonyl-H abstraction forming CO2 + (H++e−) could be the potential-determining steps. Moreover, Mo@Pd(111) is predicted to be the most promising bimetallic Pd-based catalyst for FAO based on the weakest *CO binding and the strongest *OH binding with alloyed Mo, which therefore can be used as an effective descriptor for designing FAO catalysts with high activity. The simulated polarization curves of FAO on Pd(111) and Ru@Pd(111) agree reasonably well with the experiment, an indicative of the validity of mechanism and kinetics derived from this study.

Carbon supported palladium-copper bimetallic catalysts for promoting electrochemical oxidation of formic acid and its utilization in direct formic acid fuel cells

Carbon supported palladium-copper (Pd-Cu) bimetallic catalysts (PdxCuy/Cs) are fabricated by modified polyol method to enhance the reaction rate of formic acid oxidation reaction (FAOR) and the performance of direct formic acid fuel cell (DFAFC) through weakening the bond with the intermediate of formic acid. According to the evaluations, when the ratio of Pd and Cu is 3 : 1 (Pd3Cu1/C), catalytic activity is best. Its maximum current density is 1.68-times better than that of commercial Pd/C. Even from the optical and spectroscopic characterizations, such as TEM, EDS, XPS and XRD, Pd3Cu1/C shows an optimal particle size and a higher degree of alloying. This is because in Pd3Cu1/C catalyst, the d-band center that induces the weakening in adsorption of formate anion groups to Pd surface is most positively shifted, and this positive shift promotes the reaction rate of FAOR, which is the rate determining step. When the performance of DFAFCs using the PdxCuy/C catalysts is measured, the maximum power density (MPD) of DFAFC using Pd3Cu1/C catalyst is 158 mW cm−2, and this is the best MPD compared to that of DFAFCs using other PdxCuy/C catalysts. In addition, in a comparison with commercial Pd/C catalyst, when the same amount of catalyst is loaded, MPD of DFAFC using Pd3Cu1/C catalyst is 22.5% higher than that of DFAFC using commercial Pd/C.

Formic Acid Oxidation of Electrodeposited Platinum Nanostructures on Graphene Oxide Modified Electrode in Acid Medium

In this report, we investigated an electrochemical oxidation of formic acid using electrodeposited platinum (Pt) nanostructures on graphene oxide (GO) coated glassy carbon (GC) electrode. The modified electrode is well characterized with the aid of cyclic voltammetry (CV), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman analyses. A crystalline flower-like Pt nanostructures were found on GO modified electrode. The GO-Pt modified electrode displayed improved catalytic current signal in formic acid oxidation than the bare GC and Pt nanastrutures modified electrodes. The loading of GO amount dependent catalytic performance was studied to optimize the GO amount for the efficient electrocatalytic activity. The mass activity of the GO-Pt modified electrode is 2.2 times higher than the Pt modified electrode in formic acid electrooxidation. The GO-Pt nanostructures show good stability in amperometric i-t curve and consequently, show the higher turnover number than the only Pt modified electrode.

Copper iodide nanoparticles within the organic matrix: an efficient catalyst for the electro-oxidation of formic acid

The synthesis of ultrafine copper (I) iodide (CuI) nanoparticles has been reported using a modified route of ‘in situ polymerization and composite formation (IPCF)’ method (Mallick et al 2007 J. Phys. Condens. Matter 19 196225, Mallick et al 2012 J. Macromol. Sci. B 51 134–141). The CuI nanoparticles with the average size of 10 nm are dispersed within the polyaniline matrix. The as-synthesized composite material was characterized by optical, surface and microscopic techniques. The polymer stabilized CuI nanoparticles has also been applied as a catalyst for the electrochemical detection of formic acid. The cyclic voltammetry and the square wave voltammetry techniques were used in this study for the efficient electro-oxidation of the analyte. The result shows the gradual increase of current intensity due to the increase of formic acid concentration. A moderately good stability and performance has shown by the CuI nanoparticles based device for the electrochemical oxidation of formic acid with the sensitivity and limit of detection values of 0.0197 μA. mM−1. cm−2 and 0.048 mM, respectively.

Improving the electrochemical oxidation of formic acid by tuning the electronic properties of Pd-based bimetallic nanoparticles

Pd-based bimetallic nanoparticles have superior electrochemical activity and stability for formic acid oxidation (FAO) than pure Pd. Previous DFT-based calculations show that the catalytic properties of the Pd surface could be altered by modifying its electronic properties. However, only a few experimental studies investigate how the electronic properties of Pd are modified by introducing various metals and how the resulting electronic perturbation affects its electrochemical activity and stability for FAO. Here, we demonstrate a correlation between electrochemistry and electronic properties for Pd-M bimetallic nanoparticles (M = Ru, Pt, Cu, Au, and Ag). The volcano shape relationship obtained between activity and d-band center values suggests that the electronic effects play a major role in modifying the surface electrochemical properties of Pd-M bimetallic nanoparticles for FAO. Among all the bimetallic catalysts investigated in this study, Pd-Pt/C and Pd-Cu/C with d-band center values of 2.58 eV and 2.85 eV, respectively, demonstrate the highest activities and Pd-Cu/C exhibits the highest stability for FAO.

Electrocatalytic Activities towards the Electrochemical Oxidation of Formic Acid and Oxygen Reduction Reactions over Bimetallic, Trimetallic and Core–Shell-Structured Pd-Based Materials

The structural design of nanosized electrocatalysts is extremely important for cathodic oxygen reduction reactions (ORR) and anodic oxidation reactions in small organic compounds in direct fuel cells. While Pt is still the most commonly used electrode material for ORR, the Pd electrocatalyst is a promising alternative to Pt, because it exhibits much higher electrocatalytic activity towards formic acid electrooxidation, and the electrocatalytic activity of ORR on the Pd electrode is the higher than that of all other precious metals, except for Pt. In addition, the mass activity of Pt in a core–shell structure for ORR can be improved significantly by using Pd and Pd-based materials as core materials. Herein, we review various nanoscale Pd-based bimetallic, trimetallic and core–shell electrocatalysts for formic acid oxidation and ORR of polymer electrolyte fuel cells (PEFCs). This review paper is separated into two major topics: the electrocatalytic activity towards formic acid oxidation over various Pd-based electrocatalysts, and the activity of ORR on Pd-based materials and Pd core–Pt shell structures.

PdAg@Pd core-shell nanotubes: Superior catalytic performance towards electrochemical oxidation of formic acid and methanol

The catalytic activity of alloy nanomaterials can be well modulated through the electrochemical dissolution of the more active component from alloy nanomaterials. In this paper, PdAg alloy nanotubes of diverse Pd to Ag ratio are prepared by galvanic displacement electrochemical reaction. The Ag-leached PdAg nanotubes are characterized by cyclic voltammetric scans, high-angle annular dark-field scanning transmission electron spectroscopy, energy dispersive X-ray elemental mapping and X-ray photoelectron spectroscopy measurements and demonstrate the formation of PdAg@Pd core-shell nanotubes structure with roughed surface and largely enhanced electrochemical surface area. The PdAg@Pd core-shell nanotubes show significantly enhanced catalytic activities and durability towards electro-oxidation of formic acid and methanol than the as-prepared PdAg nanotubes, with the mass activity and specific activity of 1.93 A mg−1 and 2.67 mA cm−2 towards formic acid respectively, which are 7.8 and 4.8 times that of Pd/C. The increased catalytic performance is ascribed to the unique surface and electronic structure of PdAg@Pd core-shell nanotubes.

PdO/Pd-CeO2 hollow spheres with fresh Pd surface for enhancing formic acid oxidation

Palladium (Pd) is an excellent catalyst for the electrochemical oxidation of formic acid. However, it is difficult for palladium to show true catalytic activity because its highly active surface is susceptible to oxidation and poisoning. Herein, we describe a facile strategy to prepare high activity PdO/Pd-CeO2 hollow spheres with available fresh Pd surface from PdO-CeO2 hollow spheres through an in situ electrochemical reduction process. Benefit from the fresh active surface, the PdO/Pd-CeO2 hollow spheres catalyst exhibit an enhanced mass activity and durability for formic acid oxidation compared to the Pd-CeO2 hollow spheres that are prepared by heat reduction. Moreover, the incorporated ceria can promote the oxidation of the intermediates adsorbed on the Pd surface through its facile adsorption of reactive oxygen species, resulting in an improved resistance to poisoning for the PdO/Pd-CeO2 hollow spheres in an acidic medium.

Dehydration Pathway for the Dissociation of Gas-Phase Formic Acid on Pt(111) Surface Observed via Ambient-Pressure XPS

While model studies of surface science under ultrahigh vacuum (UHV) have made significant contributions to understanding electrochemistry, many issues related to electrochemical phenomena still remain unanswered due to the extreme environmental differences between UHV and liquid conditions. Electrochemical formic acid (HCOOH) oxidation is one such example. While the dehydration step in the indirect oxidation pathway (HCOOH → H2O + COad → 2H+ + 2e– + CO2) is observed in the electrochemical oxidation of formic acid on Pt(111) surface, the surface science studies conducted in UHV condition reported the complete HCOOH dissociation to H2 and CO2 on Pt(111) surface with no adsorbed CO at room temperature. A dehydration mechanism may also exist in gas-phase HCOOH dissociation in some conditions different from UHV, but it has not been demonstrated with a surface science method due to pressure limitations. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we observed the dehydration mechanism of ga...

Stabilizing CuPd Nanoparticles via CuPd Coupling to WO2.72 Nanorods in Electrochemical Oxidation of Formic Acid.

Stabilizing a 3d-transition metal component M from an MPd alloy structure in an acidic environment is key to the enhancement of MPd catalysis for various reactions. Here we demonstrate a strategy to stabilize Cu in 5 nm CuPd nanoparticles (NPs) by coupling the CuPd NPs with perovskite-type WO2.72 nanorods (NRs). The CuPd NPs are prepared by controlled diffusion of Cu into Pd NPs, and the coupled CuPd/WO2.72 are synthesized by growing WO2.72 NRs in the presence of CuPd NPs. The CuPd/WO2.72 can stabilize Cu in 0.1 M HClO4 solution and, as a result, they show Cu, Pd composition dependent activity for the electrochemical oxidation of formic acid in 0.1 M HClO4 + 0.1 M HCOOH. Among three different CuPd/WO2.72 studied, the Cu48Pd52/WO2.72 is the most efficient catalyst, with its mass activity reaching 2086 mA/mgPd in a broad potential range of 0.40 to 0.80 V (vs RHE) and staying at this value after the 12 h chronoamperometry test at 0.40 V. The synthesis can be extended to obtain other MPd/WO2.72 (M = Fe, Co, Ni), making it possible to study MPd-WO2.72 interactions and MPd stabilization on enhancing MPd catalysis for various chemical reactions.

Electrochemical oxidation of formic acid at carbon supported Pt coated rotating disk electrodes

The effect of electrode rotation on the oxidation of formic acid in aqueous sulphuric acid has been investigated at a glassy carbon electrode coated with a carbon supported Pt catalyst. Substantial mass transport effects were observed in cyclic voltammetry, steady-state measurements at constant potential, and chronoamperometry. However, a purely mass transport limited current was not observed under any conditions because of a decrease in the kinetic current at high potentials due to Pt oxide formation. Steady-state measurements, and currents from the cathodic scans in cyclic voltammetry, gave linear Koutecky–Levich plots with slopes in agreement with the literature diffusion coefficient. However, non-linearity and inaccurate slopes were observed for anodic scans and chronoamperometry. This has been shown to be due to small increases in the kinetic current with increasing rotation rate. Accurate kinetic currents can be obtained by applying the Koutecky–Levich equation at each rotation rate and use of the known mass transport limited current.

Chemically controlled surface compositions of Ag–Pt octahedral catalysts

Abstract

A Photochemical, Room-Temperature, and Aqueous Route to the Synthesis of Pd Nanocubes Enriched with Atomic Steps and Terraces on the Side Faces

Palladium nanocubes enriched with atomic steps and terraces on the side faces were synthesized at room temperature in an aqueous solution containing a salt precursor, ascorbic acid, KBr, and poly(vinylpyrrolidone) under UV–vis irradiation. The key to the success of this synthesis was the use of PdBr42– as a precursor with strong absorption in the region of 300–600 nm. Single-crystal seeds were formed in the reaction system upon exposure to a medium pressure Hg arc lamp, and the seeds then grew into cubic nanocrystals under extended irradiation. Because the adatoms inherently diffuse slowly across the surface at room temperature, the nanocrystals favored an asymmetric growth mode, generating side faces covered by a series of atomic steps and terraces rather than smooth {100} facets. The steps and terraces would disappear to generate smooth side faces if the sample was subjected to postsynthesis annealing at 130 °C. When applied as catalysts toward the electrochemical oxidation of formic acid, the low-coordination surface atoms such as those presented on the atomic steps resulted in significant reduction of activity relative to the smooth {100} facets. This trend could be rationalized through the overbinding of reactive species to the low-coordination sites.

Pd Nanoparticles Coupled to WO2.72 Nanorods for Enhanced Electrochemical Oxidation of Formic Acid.

We synthesize a new type of hybrid Pd/WO2.72 structure with 5 nm Pd nanoparticles (NPs) anchored on 50 × 5 nm WO2.72 nanorods. The strong Pd/WO2.72 coupling results in the lattice expansion of Pd from 0.23 to 0.27 nm and the decrease of Pd surface electron density. As a result, the Pd/WO2.72 shows much enhanced catalysis toward electrochemical oxidation of formic acid in 0.1 M HClO4; it has a mass activity of ∼1600 mA/mgPd in a broad potential range of 0.4-0.85 V (vs RHE) and shows no obvious activity loss after a 12 h chronoamperometry test at 0.4 V. Our work demonstrates an important strategy to enhance Pd NP catalyst efficiency for energy conversion reactions.

Chemically designed CeO2 nanoboxes boost the catalytic activity of Pt nanoparticles toward electro-oxidation of formic acid

CeO2 nanoboxes designed by controlling various chemical parameters enhance both the efficiency and stability of Pt nanoparticles towards the electrochemical oxidation of formic acid.

Highly efficient electrocatalytic oxidation of formic acid on palladium nanoparticles-graphitic carbon nitride composite

The electrochemical formic acid oxidation reaction (FAO) is an anodic process of direct formic acid fuel cells. Carbon supported platinum (Pt/C) and Palladium (Pd/C) catalysts are widely used as anode materials for FAO. The CO poisoning of Pt/C and poor stability of Pd/C are main obstacles for its commercialization. We report here superior electrocatalytic activity of Palladium nanoparticles-carbon nitride composite (PdNPs/CNx) towards formic acid oxidation (FAO) with high mass activity (1640 mAmgPd−1) in acid media (0.5 M H2SO4) which is 7.2 times higher than that of commercial Pd/C catalyst. It also exhibited an outstanding stability for FAO (mass activity of this catalyst was 23 times higher than commercial Pd/C after 1000 cycles).

Ag–Pt Compositional Intermetallics Made from Alloy Nanoparticles

Intermetallics are compounds with long-range structural order that often lies in a state of thermodynamic minimum. They are usually considered as favorable structures for catalysis due to their high activity and robust stability. However, formation of intermetallic compounds is often regarded as element specific. For instance, Ag and Pt do not form alloy in bulk phase through the conventional metallurgy approach in almost the entire range of composition. Herein, we demonstrate a bottom-up approach to create a new Ag-Pt compositional intermetallic phase from nanoparticles. By thermally treating the corresponding alloy nanoparticles in inert atmosphere, we obtained an intermetallic material that has an exceptionally narrow Ag/Pt ratio around 52/48 to 53/47, and a structure of interchangeable closely packed Ag and Pt layers with 85% on tetrahedral and 15% on octahedral sites. This rather unique stacking results in wavy patterns of Ag and Pt planes revealed by scanning transmission electron microscope (STEM). This Ag-Pt compositional intermetallic phase is highly active for electrochemical oxidation of formic acid at low anodic potentials, 5 times higher than its alloy nanoparticles, and 29 times higher than the reference Pt/C at 0.4 V (vs RHE) in current density.

Validation of Mixed Potential Theory Using Formic Acid and Ferric Ion As a Redox Couple

The present work explores the possibility of validating mixed potential theory using the redox reaction between formic acid with ferric sulphate (HCOOH + 2Fe3+ ⇋ CO2 + 2H+ + 2Fe2+) on platinum catalyst. It is hypothesized that the catalytic oxidation of formic acid by ferric sulphate can be viewed as two distinct electrochemical reactions proceeding simultaneously on the electrode, one being the electrochemical oxidation of formic acid and the other, electrochemical reduction of ferric ions. Moreover, for electroneutrality, the rate of oxidation of formic acid must match exactly with that of the reduction of ferric sulphate. The manifestation of this phenomenon is the unique electrode potential generally referred as the mixed potential. The value of the mixed potential can be determined easily by measuring the open circuit potential (OCP) of the electrode which is immersed in a solution containing both formic acid and ferric sulphate. Further, in the present work an attempt has been made to investigate the correlation between catalytic chemical reaction and the individual electrochemical halves in terms of conversion of ferric ions.The catalytic oxidation of formic acid by ferric sulphate was studied at rotating disk electrode on which platinum black catalyst was drop casted. The reaction was studied through open circuit potential (OCP) measurement (where no external electromotive force is applied) at different rotational rates of the electrode. The number of moles of ferric ions was determined spectroscopically over a period of five hours. It was observed that OCP increased with time due to decrease in the concentration of ferric ions. These experiments have been designated as chemical experiments. Simultaneously, the two electrochemical halves of the above redox reaction were studied independently at rotating disk electrode. Formic acid oxidation studies were performed through chronoamperometry and that of ferric ion reduction was studied via cyclic voltammetry technique. These sets of experiment are refereed as electrochemical experiments. From the electrochemical data obtained for reduction of ferric ions, key kinetic and transport parameter were determined and these were used further to model the current-voltage behavior for reduction of ferric ions to ferrous ions over time, assuming a first order reaction. Using this model in conjunction with OCP time data it was possible to predict the number of moles of ferric ion consumed as function of time during the OCP experiment. These predictions were compared with the number of moles of ferric ions actually consumed during OCP experiment. These comparisons were made at different rotation rates of the electrode and very good agreement between the measured and predicted values was observed at all rates of rotation. This validates the hypothesis that the kinetics of a catalytic chemical reaction can be obtained from the individual current potential relationship for the two electrochemical reactions.The present work suggests that catalytic chemical reactions follow an electron transfer mechanism. This approach would enable prediction of chemical kinetics for primarily catalytic reactions through few electrochemical parameters. Further, it opens up possibility of estimation of key electrochemical parameters for several catalytic systems and then the kinetics can be predicted by selecting the required parameters for corresponding oxidation as well as reduction reaction. Figure 1

Influence of spontaneous decomposition on the electrochemical formic acid oxidation on a nanostructured palladium electrode

The formic acid electrooxidation reaction was studied in a flow cell in 1M HCOOH–0.5M H2SO4 solution on a palladium film electrode. The evaluation of the open circuit potential (OCP) decay demonstrated that the spontaneous dehydrogenation of formic acid is quickly verified, leading to hydrogen adsorption and absorption. Moreover, the absence of carbon containing adsorbed species was verified by the application of voltammetric stripping after OCP and chronoamperometry. The results obtained demonstrate that the electrocatalytic oxidation of formic acid on Pd takes place through the spontaneous HCOOH dissociative adsorption with CO2 evolution and the adsorbed hydrogen electrooxidation.

Novel platinum-macrocycle composite catalysts for direct formic acid fuel cells

A highly active catalyst for the electrochemical oxidation of formic acid for direct formic acid fuel cell is synthesized using a macrocycle as platinum’s co-catalyst. The resulting carbon-supported Pt–Ni(N,N′-mono-8-quinolyl-o-phenylenediamine) (hereafter referred to as Pt–Ni(mqph)), with a 50/50 ratio and heat-treated at 350 °C in Ar atmosphere, shows a high catalytic performance exceeding that of Pt-only catalysts in terms of oxidation onset potential and current density. The performance of such an anode composite catalyst in single direct formic acid fuel cell can reach up to 166 mW mg (Pt)−1, greatly surpassing that of 85 mW mg (Pt)−1 for Pt anode.