Formic Acid Oxidation Research Articles

Current efficiency and mass transfer effects in electrochemical oxidation of C1 and C2 carboxylic acids on boron doped diamond electrodes

The oxidation of acetic, glycolic, glyoxylic, oxalic and formic acid has been studied on boron doped diamond electrodes. Our voltammetry study on rotating disk electrodes highlights clear differences between the carboxylic acids (formic, oxalic, glyoxylic and glycolic acid) that can be oxidized via a direct electron transfer (DET) and acetic acid which can presumably solely be oxidized by •OH radicals formed in the region where water oxidation takes place. In glycolic and glyoxylic oxidation oxalic acid is the main intermediate formed. Surprisingly, glyoxylic acid could not be detected as an intermediate in the glycolic acid oxidation. Chronoamperometric experiments confirm that all compounds except acetic acid react further to CO2, which could be deduced from the electron balances. It was shown that formic, oxalic, glyoxylic and glycolic acid can be selectively oxidized at 2.3 V vs. Ag/AgCl with high current efficiencies, below or close to mass transfer limiting rates. At higher potentials (2.4 V and 2.5 V) simultaneous water electrolysis results in lower current efficiencies. At these potentials the conversion rates can exceed the limiting rates, which might be attributed to effects related to water oxidation (i.e. O2 evolution and •OH radical formation).

PEDOT-supported Pd nanocatalysts – oxidation of formic acid

Palladium (Pd) nanocatalysts are obtained by electroless deposition of Pd on pre-reduced poly(3,4-ethylenedioxythiophene) (PEDOT) layers doped with either polystyrenesulfonate (PSS) or dodecylsulfate (SDS) ions. The PEDOT-supported Pd catalysts are investigated with respect to formic acid (FA) oxidation in slightly acidic solution under voltammetric and chronoamperometric conditions. Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) studies are carried out before and after exposure to FA. It is established that the dopant used for the synthesis of PEDOT does affect not only the distribution of the Pd nanoparticles on the polymer surface but also the stability of the composites upon electrochemical treatment in the presence of FA. It is found that PEDOT-PSS undergoes structural rearrangement, and significant loss of the Pd catalyst is observed. In comparison, PEDOT-SDS is more resistant to FA exposure since aggregated Pd nanoparticles are providing additional stability of the polymer structure.

Enhancement of anodic oxidation of formic acid on Pd–Fe bimetallic nanoparticles by thermal treatment

The surface composition and catalytic properties of Pd–Fe bimetallic catalysts with identical bulk composition can be continuously tuned by treatment at different temperatures. The activity of these catalysts in formic acid oxidation was related to the treatment temperature. The thermal treatment temperatures ranged from 400 to 600 °C. The Pd–Fe nanoparticles are characterized by an array of analytical techniques including TEM (transmission electron microscopy), XRD (X-ray diffraction), ICP (inductively coupled plasma) and HS-LEIS (low energy ion scattering spectroscopy). The electrocatalytic activity is examined using cyclic voltammetric and chronoamperometric measurements. The Pd–Fe/C catalyst with 500 °C shows the highest electrocatalytic activity for formic acid oxidation, with a current activity 3 times higher than that of before treated Pd–Fe/C catalyst, 5.6 times higher than that of commercial Pt/C catalyst. The migration of Pd to the surface on the nanoparticle catalysts as well as the electrochemical active surface area of the PdFe–H catalysts was shown to play a major role in enhancing the electrocatalytic activity for catalyst. These findings provided important insights into the correlation between the electrocatalytic activity and the treatment temperature of the nanoengineered bimetallic catalysts.

A displacement dealloying route to dilute nanoporous PtAu alloys for highly active formic acid electro-oxidation

Functional nanoporous metals and alloys can be fabricated by chemical or electrochemical dealloying routes in corrosive media, followed by customized surface functionalization. The present work describes how a combination of displacement reaction and dealloying can provide a facile one-pot procedure to nanoporous alloys such as PtAu, and the resulted electrocatalysts are characterized by highly dispersed Pt atoms or small clusters embedded within a high surface area Au matrix. These dilute alloy electrocatalysts with low Pt contents exhibit remarkable activity toward formic acid oxidation via a favorable dehydrogenation mechanism. At 0.6 V versus reversible hydrogen electrode, they can demonstrate over 70-fold enhancement in Pt mass activity than that of the commercial Pt/C.

A highly efficient atomically thin curved PdIr bimetallene electrocatalyst.

The multi-metallene with an ultrahigh surface area has great potential in precise tuning of surface heterogeneous d-electronic correlation by surface strain effect for the distinctive surface electronic structure, which is a brand new class of promising 2D electrocatalyst for sustainable energy device application. However, achieving such an atomically thin multi-metallene still presents a great challenge. Herein, we present a new synthetic method for an atomic-level palladium-iridium (PdIr) bimetallene with an average thickness of only ∼1.0 nm for achieving superior catalysis in the hydrogen evolution reaction (HER) and the formic acid oxidation reaction (FAOR). The curved PdIr bimetallene presents a top-ranked high electrochemical active area of 127.5 ± 10.8 m2 gPd+Ir−1 in the reported noble alloy materials, and exhibits a very low overpotential, ultrahigh activity and improved stability for HER and FAOR. DFT calculation reveals that the PdIr bimetallene herein has a unique lattice tangential strain, which can induce surface distortion while concurrently creating a variety of concave-convex featured micro-active regions formed by variously coordinated Pd sites agglomeration. Such a strong strain effect correlates the abnormal on-site active 4d10-t2g-orbital Coulomb correlation potential and directly elevates orbital-electronegativity exposure within these active regions, resulting in a preeminent barrier-free energetic path for significant enhancement of FAOR and HER catalytic performance.

An affordable option to Au single crystals through cathodic corrosion of a wire: Fabrication, electrochemical behavior, and applications in electrocatalysis and spectroscopy

Faceting and nanostructuring of polycrystalline gold electrodes by cathodic corrosion in concentrated potassium hydroxide electrolytes has been systematically studied at different electrode potentials. Current-potential curves for the restructured Au electrodes in 0.1 M H2SO4 show characteristics of Au(111) in the double-layer and oxidation region. Thus, the modified Au electrodes adopt properties typically known for well-defined single crystal surfaces. Besides the preferential surface faceting, the electrochemically active surface area (EASA) is enhanced as a function of potential, concentration and time. Scanning electron micrographs show the formation of triangular pits and nanostructures with a specific orientation confirming the formation of (111)-facets. In this way, the behavior of single crystals is accompanied with the properties of nanoparticles which are of utmost interest in electrocatalysis and surface-enhanced Raman spectroscopy (SERS). The electrocatalytic activity of the newly formed “Au(111)” surface from an Au wire has been tested towards the hydrogen evolution reaction (HER) and for the formic acid oxidation reaction (FAOR). The study of electrocatalytic reactions at these nanostructured electrodes allows to identify active centers, which are absent for extended single crystal surfaces. Adsorbed pyridine on the nanostructured Au electrodes directly shows SERS activity, while untreated polycrystalline Au is SERS-inactive. The use of cathodic corrosion of simple wires is a paradigm of SERS-applications in electrochemistry with clean Au electrodes that provide properties of Au(111) single crystals.

PdAgPt Corner-Satellite Nanocrystals in Well-Controlled Morphologies and the Structure-Related Electrocatalytic Properties.

The functions of heterogeneous metallic nanocrystals (HMNCs) can be undoubtedly tuned by controlling their morphologies and compositions. As a less-studied kind of HMNCs, corner-satellite multi-metallic nanocrystals (CSMNCs) have great research value in structure-related electrocatalytic performance. In this work, PdAgPt corner-satellite nanocrystals with well-controlled morphologies and compositions have been developed by temperature regulation of a seed-mediated growth process. Through the seed-mediated growth, the morphology of PdAgPt products evolves from Pd@Ag cubes to PdAgPt corner-satellite cubes, and eventually to truncated hollow octahedra, as a result of the expansion of {111} facets in AgPt satellites. The growth of AgPt satellites exclusively on the corners of central cubes is realized with the joint help of Ag shell and moderate bromide, and hollow structures form only at higher reaction temperatures on account of galvanic displacement promoted by the Pd core. In view of the different performances of Pd and Pt toward formic acid oxidation (FAO), this structure-sensitive reaction is chosen to measure electrocatalytic properties of PdAgPt HMNCs. It is proven that PdAgPt CSMNCs display greatly improved activity toward FAO in direct oxidation pathway. In addition, with the help of AgPt heterogeneous shells, all PdAgPt HMNCs exhibit better durability than Pd cubes and commercial Pt.

Carbon-Supported Single-Atom Catalysts for Formic Acid Oxidation and Oxygen Reduction Reactions.

The commercialization of fuel cells, especially for direct formic acid fuel cells (DFAFCs) and proton-exchange membrane fuel cells (PEMFCs), is significantly restrained by the high cost, poor stability, and sluggish kinetics of platinum group metals (PGM) catalysts for both the anodic formic acid oxidation reaction (FAOR) and the cathodic oxygen reduction reaction (ORR). Currently, it has confronted with challenges, including exploring highly active, cost-effective, and stable catalysts to replace PGM for DFAFCs and PEMFCs. Recently, the increasing investigation has been focused on the single-atom catalysts (SACs) to enhance the catalytic performance owing to the maximum atom utilization and highly exposed active sites. The aim of this review is to present the recent research activities on carbon supported SACs. At the beginning of the review, metal-based SACs supported on different carbon supports, and the typical characterization methods are introduced. Subsequently, recent advances in metal-based SACs for FAOR and ORR catalysis are scientifically summarized. Particularly, some representative metal-based SACs for ORR activity are further exemplified with a deeper understanding of structure-activity relationships. Finally, the challenges and opportunities of SACs are prospected, such as the mechanism understanding and commercial applications.

Bimetallic Pd–Fe Supported on Nitrogen-Doped Reduced Graphene Oxide as Electrocatalyst for Formic Acid Oxidation

This study was conducted to exploit the properties of nitrogen-doped reduced graphene oxide (N-rGO) as support material for formic acid fuel cell. Nitrogen-doped reduced graphene oxide was synthesized by the hydrothermal synthesis method using graphene oxide (GO) flakes and urea as a nitrogen source. Palladium and iron with controllable atomic ratio were used as the active metals. Graphene oxide and carbon nanotube-supported PdFe nanoparticles were synthesized for comparison. The structure, morphology, and chemical composition of the synthesized catalysts were ascertained by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The average particle sizes for Pd3Fe/N-rGO and Pd/N-rGO were 4.65 and 3.95 nm, respectively. The electrochemical characterizations (CO stripping, cyclic voltammetry, and chronoamperometry) showed that the Pd3Fe/N-rGO electrocatalyst had higher electrocatalytic activity and stability compared with that of Pd3Fe/rGO and Pd3Fe/CNT. The mass activity of Pd3Fe/N-rGO in 0.5 M of HCOOH and 0.5 M of H2SO4 solutions was 1463.9 mAmg−1 Pd, which was 3.3 and 1.35 times that of the activity obtained with graphene oxide and carbon nanotubes with the same composition, respectively. The superior performance of the Pd3Fe/N-rGO catalyst was ascribed to the presence of nitrogen functionalities in the nitrogen-doped reduced GO and the synergistic interaction between Pd and Fe nanoparticles. Nitrogen-doped reduced GO promoted the formation of smaller and narrowly distributed nanoparticles and exerted favorable electronic effects because of electron transfer from N to Pd. Therefore, Pd3Fe/N-rGO can serve as a potential electrocatalyst for the oxidation of formic acid.

Robust and Efficient Pd–Cu Bimetallic Catalysts with Porous Structure for Formic Acid Oxidation and a Mechanistic Study of Electrochemical Dealloying

To obtain highly efficient and robust electrocatalysts for formic acid oxidation, PdxCuy/C binary catalysts with porous structures were successfully prepared via electrochemical dealloying. Catalysts with different Pd/Cu atomic ratios were characterized through transmission electron microscopy, inductively coupled plasma atomic emission spectroscopy, and X-ray diffraction. The optimized dealloyed Pd1Cu4/C catalyst with a porous structure displayed a catalytic activity of 2611 A g−1 and high stability (30.5% activity retention under repeated cyclic voltammetric (CV) patrol), whereas a commercial Pd/C-Aldrich benchmark showed a catalytic activity of 785 A g−1 and retained 16.5% activity. A detailed mechanistic study of electrochemical dealloying was performed. Under repeated CV patrol, Pd-enriched porous architectures evolved from Pd-poor surfaces, accompanied by successive Cu dissolution.

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy.

Traditional electrochemical measurements based on either current or potential responses only present the average contribution of an entire electrode's surface. Here, we present an electrochemical photothermal reflectance microscope (EPRM) in which a potential-dependent nonlinear photothermal signal is exploited to map an electrochemical process with sub-micron spatial resolution. By using EPRM, we are able to monitor the photothermal signal of a Pt electrode during the electrochemical reaction at an imaging speed of 0.3 s per frame. The potential-dependent photothermal signal, which is sensitive to the free electron density, clearly revealed the evolution of surface species on the Pt surface. Our results agreed well with the reported spectroelectrochemical techniques under similar conditions but with a much faster imaging speed. We further mapped the potential oscillation during the oxidation of formic acid on the Pt surface. The photothermal images from the Pt electrode well matched the potential change. This technique opens new prospects for real-time imaging of surface chemical reaction to reveal the heterogeneity of electrochemical reactivity, which enables broad applications to the study of catalysis, energy storage, and light harvest systems.

Mechanistic effects of blending formic acid with ethanol on Pd activity towards formic acid oxidation in acidic media.

The direct formic acid fuel cell (DFAFC) is one of the most promising direct liquid fuel cells. Pd is the most active catalyst towards formic oxidation, however, it suffers from CO-like poisoning and instability in acidic media. Blending formic acid with ethanol is known to synergistically enhance the Pt catalytic activity of Pt. However, it has not been studied in the case of Pd. In this study, ethanol/formic acid blends were tested, aiming at understanding the effect of ethanol on the formic acid oxidation mechanism at Pd and how the direct and indirect pathways could be affected. The blends consisted of different formic acid (up to 4 M) and ethanol (up to 0.5 M) concentrations. The catalytic activity of a 40% Pd/C catalyst was tested in 0.1 M H2SO4 + XFA + YEtOH using cyclic voltammetry, while the catalyst resistance to poisoning in the presence and absence of ethanol was tested using chronopotentiometry. The use of these blends is found to not only eliminate the indirect pathway but also slowly decrease the direct pathway activity too. That is believed to be due to the different ethanol adsorption orientations at different potentials. This study should open the door for further studying the oxidation of FA/ethanol blends using different pHs and different Pd-based catalysts.

Carbon Nanotube-Copper Oxide-Supported Palladium Anode Catalysts for Electrocatalytic Enhancement in Formic Acid Oxidation

Carbon Nanotube-Copper Oxide-Supported Palladium Anode Catalysts for Electrocatalytic Enhancement in Formic Acid Oxidation

Engineering porous Pd-Cu nanocrystals with tailored three-dimensional catalytic facets for highly efficient formic acid oxidation.

Rational synthesis of bi- or multi-metallic nanomaterials with both dendritic and porous features is appealing yet challenging. Herein, with the cubic Cu2O nanoparticles composed of ultrafine Cu2O nanocrystals as a self-template, a series of Pd-Cu nanocrystals with different morphologies (e.g., aggregates, porous nanodendrites, meshy nanochains and porous nanoboxes) are synthesized through simply regulating the molar ratio of the Pd precursor to the cubic Cu2O, indicating that the galvanic replacement and Kirkendall effect across the alloying process are well controlled. Among the as-developed various Pd-Cu nanocrystals, the porous nanodendrites with both dendritic and hollow features show superior electrocatalytic activity toward formic acid oxidation. Comprehensive characterizations including three-dimensional simulated reconstruction of a single particle and high-resolution transmission electron microscopy reveal that the surface steps, defects, three-dimensional architecture, and the electronic/strain effects between Cu and Pd are responsible for the outstanding catalytic activity and excellent stability of the Pd-Cu porous nanodendrites.

Nanocomposite Micropower Alternative Power Sources for Electronic Technology

Modern process methods and approaches to production of useful electrode materials make it possible to design alternative power sources, in particular, fuel cells with high specific characteristics. Owing to their unique properties, carbon nanotubes and porous silicon are the most promising materials for developing micropower current sources. Substrates based on porous silicon have high corrosion and mechanical resistance, and they also feature a large pore surface area. Hence, the search for the best composition of matrix carriers with a large active surface and modifier metals with their minimum load is becoming an urgent process problem in power and electronic engineering. The use of metal nanoparticles in a catalyst can increase the catalytic activity in electrochemical reactions occurring in fuel cells and their corrosion resistance. Investigations into electrochemical oxidation of hydrogen and formic acid in fuel cells seem to be the most important for practice due to a wide application range of these types of fuels. This work deals with the formation of nanocomposite electrodes with nanoparticles of platinum group metals for fuel cells to be used as self-contained micropower current sources in electronic engineering. A process is described for forming effective electrode materials based on nanotubes with nanoparticles of platinum (Pt) and palladium (Pd) for low-temperature self-contained power sources. Parameters of nanocomposite electrodes in matrix-carriers are investigated by the methods of atomic-force, high-resolution transmission electron microscopy. The results from tests of formic acid–air and hydrogen–air fuel cell models are presented. The optimal loading of platinum group metals and dimensions of electrode nanocomposites giving maximum specific power and current density are found.

Construction of AuPdPt spherical nanodendrites with a multilayered structure by manipulating etching and regrowth in seeded growth

Size-tunable gold–palladium–platinum nanodendrites with a multilayered structure are successfully prepared via etching–regrowth-involved seeded growth, exhibiting superb electrochemical activity for formic acid oxidation.

PdNi/N-doped graphene aerogel with over wide potential activity for formic acid electrooxidation

Anti-CO poisoning ability is significant in formic acid oxidation in the fuel cell technique. Herein, PdNi alloy supported on N-doped graphene aerogel (PdNi/GA-N) was found to have catalytic ability toward formic acid electrooxidation over a wide potential range because of the improved anti-CO poisoning ability. This catalyst was fabricated by simple freeze-drying of mixture solution of graphene aerogel, polyvinylpyrrolidone, Pd2+ and Ni2+ and the subsequent thermal annealing reduction approach in the N2/H2 atmosphere. Pd–Ni alloy particles anchored over the folding N-doped graphene surface with a porous hierarchical architecture structure in the 3D directions. It showed the catalytic performance of its maximum mass activity of 836 mA mg−1 in a broad potential range (0.2–0.6 V) for formic acid oxidation. The CO stripping experiment demonstrated its largely improved anti-CO poisoning ability with the peak potential of 0.67 V, approximately 60 and 40 mV less compared to those of Pd/GA-N and Pd/C samples. The high anti-CO poisoning ability and strong electronic effect resulting from the interaction between the 3D GA-N support and the Pd–Ni alloy makes it a promising catalyst for application in direct formic acid fuel cells.

Solvent effects on the structural and catalytic properties of Pt nano- and microstructures synthesised in solvothermal system

Platinum (Pt) nano- and microstructures were synthesised in different types of solvent (e.g. water (H2O), dimethylformamide (DMF), monoethanolamine (MEA) and a mixture of H2O and DMF) by using solvothermal technique. Spherical particles of diameter ranging from 80 nm to 230 nm were formed when prepared in H2O, whereas particles with irregular shapes were produced from the H2O/DMF media. Particles with flower-like structures of diameter ranging from 140 nm to 200 nm were produced by using DMF solvent, while cubic microstructures with sizes of about 0.7 μm−1.0 μm were formed in the MEA media. The crystallinity of Pt particles prepared in different media increased, following the trend of H2O < H2O/DMF < MEA < DMF. The flower-like Pt structures exhibited a high catalytic performance towards the oxidation of formic acid, as demonstrated by the high electroactive surface area of 0.4058 m2, which was better than Pt black in terms of resistance towards CO poisoning, and showed high stability with current density of 0.41 mA/cm2 for 1000 s. The Pt particles synthesised in MEA exhibited small electroactive area, but more resistance towards CO poisoning as compared to sample prepared in H2O. Therefore, this showed that the catalytic properties of Pt are highly dependent on the faceted structures, whereby large number of edges and corners increased the surface-to-volume ratio, and thus the absorption sites for electrooxidation of formic acid.

PdAg Nanoparticles with Different Sizes: Facile One-Step Synthesis and High Electrocatalytic Activity for Formic Acid Oxidation.

Recently, direct formic acid fuel cells (DFAFCs) which possess superior advantages such as a low operating temperature, light environmental pollution and high energy density, have been considered as one of the power generation technologies with a bright prospect. Herein, bimetallic PdAg nanoparticles (NPs) with different particle sizes were successfully produced via an easy one-pot solvothermal co-reduction synthetic route and their electrocatalytic performance for formic acid oxidation (FAO) were further investigated. In our strategy, the size of PdAg NPs can be easily controlled by only varying the concentration of precursors. The larger sized PdAg alloy (9.5 nm, noted as PdAg-L) was obtained at a low concentration of precursors, while the smaller PdAg alloy (3.7 nm, named as PdAg-S) was separated from the reaction system with higher solubility by centrifugation. The electrocatalytic activity and stability of the obtained PdAg NPs could be well optimized when incorporated with carbon (C), which is owing to a synergetic effect. The PdAg-S/C exhibits the highest mass activity with around 1.6 times that of PdAg-L/C and 2 times that of commercial Pd/C, which can be attributed to its larger ECSA and lower adsorption energy of the intermediate to facilitate the direct oxidation of HCOOH molecule.

Smart selection of fuel blends: Robust oxidation of formic acid in its blend with urea at NiOx/Pd nanoparticles-based binary anodes

Enhanced oxidation of formic acid (FAO) is observed at NiOx/Pd/GC anodes in the presence of urea as a blending fuel. FAO is noticeably enhanced (in the presence of urea as a blending fuel) as demonstrated by CV measurements where a five-fold increase of the direct FAO peak (Ipd), and a favorable negative shift of its onset potential (Eonset) are observed. Additionally, a six-fold increase of the fuel utilization (FU), expressed in the amount of charge consumed during FAO is markedly enhanced in FA-urea fuel blend (0.3 M FA and 0.2 M urea) compared to that obtained in the absence of urea (i.e., 0.3 M FA only) at the NiOx/Pd/GC electrode. Additionally, capacitance measurements of the electrode/electrolyte interface increases in line with FAO activity. Urea molecules are thought to form a bridged H-bonding with formate anions thus facilitating a favorable adsorption geometry allowing for a facile C–H scissoring, thus promoting the oxidative dehydrogenation pathway of FA to CO2. A plausible explanation assumes the formation of 6- and/or 8-membered rings between formate anion and urea (via hydrogen bonding). These findings emphasize the importance of the proper design of the catalyst together with the smart selection of the blending fuel with FA.