Formic Acid Electrooxidation Research Articles

Single-Atom Electrocatalysts.

Recent years have witnessed a dramatic increase in the production of sustainable and renewable energy. However, the electrochemical performances of the various systems are limited, and there is an intensive search for highly efficient electrocatalysts by more rational control over the size, shape, composition, and structure. Of particular interest are the studies on single-atom catalysts (SACs), which have sparked new interests in electrocatalysis because of their high catalytic activity, stability, selectivity, and 100 % atom utilization. In this Review, we introduce innovative syntheses and characterization techniques for SACs, with a focus on their electrochemical applications in the oxygen reduction/evolution reaction, hydrogen evolution reaction, and hydrocarbon conversion reactions for fuel cells (electrooxidation of methanol, ethanol, and formic acid). The electrocatalytic performance is further considered at an atomic level and the underlying mechanisms are discussed. The ultimate goal is the tailoring of single atoms for electrochemical applications.

Effect of OMC and MWNTC support on mass activity of Pd[sbnd]Co catalyst for formic acid electro-oxidation

The electrochemical evaluation of PdCo catalyst supported on Ordered Mesoporous Carbon (PdCo/OMC) synthesized by the polyol method is reported in this work. OMC is synthesized via self-assembly in aqueous solution, as an alternative route to conventional preparation methods. The electrocatalytic performance of PdCo/OMC is evaluated for Formic Acid Oxidation Reaction (FAOR). The results are compared with those of PdCo on Multi-Walled Carbon Nanotubes (PdCo/MWCNT) and Pd/OMC, prepared by the same procedure, with the aim to evaluate the effect of support and the Co contribution during the FAOR. The catalysts are characterized by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS). Cyclic voltammetry (CV) and chronoamperometric studies are carried out in acid media with different formic acid concentrations (0.1, 0.5 and 1 mol L−1). According to TEM results, PdCo/OMC exhibits smaller particle size (∼3.5 nm) and enhanced distribution of nanoparticles compared with PdCo/MWCNT. At 0.5 mol L−1 HCOOH, PdCo/OMC delivers a higher anodic mass current density (185 mA mg−1Pd) of the FAOR than PdCo/MWCNT (94.3 mA mg−1 Pd). Moreover, at 1 mol L−1 HCOOH the jf/jb ratio for PdCo/OMC is 1.09 mA mg−1Pd, higher than the ratios determined for PdCo/MWCNT (0.76) and Pd/OMC (0.85). The improvement of the mass catalytic activity of PdCo/OMC is attributed to the combination of the mesoporous structure of the support, which promotes the catalyst-support interaction facilitating the mass transfer process during the reaction, and to the incorporation of Co in the Pd structure.

Carbon-supported Pd100-XAuX alloy nanoparticles for the electrocatalytic oxidation of formic acid: Influence of metal particles composition on activity enhancement

Two series of carbon-supported Pd100-XAuX catalysts of different Au contents have been synthesized by the “water-in-oil” microemulsion method (Triton X 114, cyclohexane) using precursor solutions of low (series L) and high concentration (series H). The metal nanoparticles are characterized by XRD, electron microscopy (SEM, HRTEM) and XPS to determine their crystal structure, particles size, composition and morphology. The electrochemical measurements provide data concerning surface properties of the PdAu particles, including surface fraction of Pd and Au and the reactivity towards hydrogen sorption/desorption processes. The metal particles of low bulk Au-content (Au/Pd <1) are of high monodispersity and of smaller size than their monometallic Pd and Au counterparts, synthesized by the same method. At higher bulk Au-content (Au/Pd >1), the particle size increases approaching almost the size of Au particles. An enrichment of Pd toward the surface of all the Pd100-XAuX particles occurs, much pronounced in the samples H series. The activity of catalysts (based on the current densities, and TOF values) in the electrooxidation of formic acid (FA) is correlated with the bulk and surface composition of the Pd100-XAuX particles. The high monodispersity of the metal particle size in the studied catalysts made it possible to observe relation between FA electrooxidation activity and surface composition of the PdAu particles. The catalytically active sites formed at the Au surface fraction ca. 0.1–0.12 (Pd 0.9–0.88) display 1.6-times higher activity (TOF) as compared to pure Pd, while higher surface Au fraction strongly reduces activity. The occurrence of electronic modification of Pd by Au in the Pd100-XAuX particles also results in a reduced hydrogen solubility, accompanied by weaker PdH bonds, which might influence the poisoning ability by CO formed through electroreduction of CO2, the product of FA oxidation.

(Invited) Direct Formic Acid Fuel Cells Anode Optimization: Flow Fields and Catalysts

Direct formic acid fuel cells (DFAFCs) offer an efficient and compact electrochemical conversion technology to power portable electronic devices ranging from sensors to unmanned surveillance vehicles. Liquid fuel cells are desirable for portable applications due to their ease of liquid refueling and the minimal incorporation of system fail safes for handling, as compared to compressed gasses. Directly compared to other liquid fuels, formic acid has minimal parasitic losses due to (i) efficient anodic overpotentials and (ii) cathodic depolarization due to low fuel crossover.[1]The anodic overpotentials are reduced in the presence of efficient catalysts that enhance the fraction of formic acid that is electro-oxidized via the direct non-strongly adsorbed reaction intermediate pathway,[2] while most liquid fuels are plagued with strongly adsorbed carbon monoxide reaction intermediates that require high parasitic overpotentials. Fig 1 compares the formic acid overpotential on a platinum-ruthenium (PtRu) alloy catalyst through the indirect electro-oxidation pathway and carbon supported platinum catalyst decorated with a submonolayer of bismuth (Pt/C-Bi) promoting the direct electro-oxidation pathway. Two short comings incurred in a DFAFC are (Part 1) durability of the catalyst and (Part 2) efficient removal of the CO2-byproduct.[3] Part 1 – The Bi, at 54% of a monolayer coverage, promotes the direct electro-oxidation pathway by the ‘third-body’ effect and adsorption in the CH-down orientation. Alternative adsorbates to Bi are being sought that are strongly adsorbed to the Pt surface and enhance formic acid electro-oxidation through the direct reaction pathway. Stability of the adsorbate during operation and idle will be a key parameter in down selection. Part 2 – The removal of CO2 is imperative to maintain elevated formic acid concentration both in the anode flow field and at the catalyst interfaces within the 3-dimensional anode catalyst layer. The limited crossover of formic acid to the cathode permits the cell to be operated in the presences of neat fuel concentrations. Previously in the Rice group, it has been shown that DFAFC performance is strongly impacted by catalyst layer porosity via either swelling of the catalyst layer[4] or using a pore former to create larger pore structures in the catalyst layer[5]. As the DFAFC operates under higher loads, additional CO2 must be removed through a typical single serpentine flow field, resulting in non-uniformed fuel distribution at the catalyst interface due to two-phase flow. Recent work on advanced electrically conductive selectively gas permeable anode flow field (SGPFF) designs have resulted in efficient removal of CO2 perpendicular to the active area near the location where it is formed in the catalyst layer.[6] Fig 2 shows the potentiostatic hold performance (0.3V) in a dead-ended flow field operation for a standard flow field vs. the SGPFF using PtRu/C as the anode catalyst. Acknowledgements: We gratefully acknowledge support of this work by the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2 and the Center for Manufacturing Research at Tennessee Tech University.

Pd/C CATALYSTS SYNTHESIZED BY MICROWAVE ASSISTED POLYOL METHOD FOR FORMIC ACID ELECTRO-OXIDATION

Synthesis of carbon supported palladium particles based on a polyol process was investigated. The activity of Pd catalysts, synthesized by microwave assisted polyol method on carbon support, has been examined for formic acid electro-oxidation reaction. The catalysts were characterized with XRD to identify their nanostructure properties. The effect of catalyst preparation parameters on electrocatalytic performance of catalysts was examined. The electrocatalytic properties of Pd/C catalysts for the oxidation of formic acid were investigated by cyclic voltammetry (CV). The electrocatalytic activity of Pd/C synthesized at 30 s and 130 oC are higher than the other Pd/C catalysts synthesized at different temperature and reaction time. It is proposed that the longer reaction time and higher temperature increased the particle size.

PdRu alloy nanoparticles of solid solution in atomic scale: outperformance towards formic acid electro-oxidation in acidic medium

Developing catalyst of high performance and low cost toward the electro-oxidation of formic acid on the anode of fuel cell is critical for the commercialization of direct formic acid fuel cells. Here we reported the synthesis of PdxRu10-x (x=1,3,5,7,9) nanoparticles (NPs) by concurrent reduction of Pd2+ and Ru2+ in polyol solution at 200°C. The particle size of the obtained NPs was confined at 5–15nm in diameter. X-ray diffraction (XRD) analysis revealed face-centered cubic (fcc) crystal structure for PdxRu10-x (x=3,5,7,9), with the lattice parameter proportional to the Pd content. The formation of the solid solution in atomic scale was confirmed for the alloy nanoparticles by XRD and the elemental mapping. Williamson-Hall method revealed that the stacking fault was dependent on the alloying extent of the alloy nanoparticles and reached the minimum for Pd5Ru5, which exhibited the highest activity towards formic acid oxidation among all these prepared samples, with mass activity of 12.6 times higher than that of commercial Pd/C. It was observed that the highest catalytic activity was in agreement with the minimum of the stacking fault of the alloy nanoparticles.

Nitrogen-doped carbon black supported Pd nanoparticles as an effective catalyst for formic acid electro-oxidation reaction

Pd nanoparticles supported on nitrogen doped carbon black (Vulcan XC-72R) with two different levels of doping were prepared by the microwave-assisted ethylene glycol reduction process and used as catalyst for the formic acid electro-oxidation (FAEO). The results indicate that the different nitrogen doping contents in Pd/N-C catalysts have a significant effect on the performance of FAEO. A higher N content facilitates the uniform dispersion of Pd nanoparticles on carbon black with narrow particle size distribution. Furthermore, the electrochemical results show that the catalyst with a higher N-doping content possesses a higher catalytic activity and a long-term stability for FAEO. The peak current density of the Pd/N-C (high) catalyst is 1.27 and 2.31 times that of the Pd/N-C (low) and homemade Pd/C-H catalyst. The present paper may provide a simple method for preparation of high-performance anode catalyst for direct formic acid fuel cells (DFAFCs).

Graphene oxide-assisted facile synthesis of platinum–tellurium nanocubes with enhanced catalytic activity for formic acid electro-oxidation

In order to obtain a loaded Pt-based catalyst with enhanced high activity and stability towards formic acid electro-oxidation (FAO), PtTe nanoparticles loaded on graphene oxide (GO) were fabricated by a facile and scalable method. XRD and HRTEM results show that the morphology of PtTe particles could be affected by the additive amount of GO and Te. It is observed that the supported PtTe particles are cubic. The XPS results show the change in the Pt electronic structure after the incorporation of Te, which impedes the chemisorption of the CO intermediate and promotes the dehydrogenation pathway of FAO. By electrochemical analysis, the performance towards FAO is greatly enhanced. The mass activity of PtTe/GO-67 is at 0.45 V (versus SCE), which is 11.5 times as high as that of Pt/C The incorporation of Te atoms and the content of GO are two major parameters for tuning the crystal structure and morphology and enhancing catalytic activity.

Facile Synthesis of a Porous Pd/Cu Alloy and its Enhanced Performance toward Methanol and Formic Acid Electrooxidation.

Reported is a porous Pd/Cu alloy catalyst synthesized by a one-pot method, in which the Pd/Cu alloy is formed by using l-ascorbic acid as reducing agent to simultaneously reduce the copper precursor and the palladium precursor. The copper incorporated with Pd can reduce the cost of the catalyst and enhance the catalytic performance. The morphology of the Pd/Cu alloy catalyst can be controlled by altering the ratios of Pd to Cu. Electrochemical characterizations indicate that Pd/Cu alloy catalysts possess good activity and long-term stability for the electrooxidation of methanol and formic acid. Compared with commercial Pd/C, the as-prepared Pd65 Cu35 shows enhanced activities of electrooxidation of methanol and formic acid. This study highlights an easy strategy to obtain the shape-controlled Pd/Cu alloyed catalysts and their potential application as electrocatalysts in fuel cells or other fields.

Galvanic displacement of silver deposited on carbon nanowalls by palladium and the electrocatalytic behavior of the resulting composite

The composites Pd0(Ag), which are highly active in formic acid electrooxidation, were synthesized by galvanic displacement and electrochemical leaching.

Bimetallic Au@Pt nanoparticles dispersed in conducting polymer—A catalyst of enhanced activity towards formic acid electrooxidation

Organic-inorganic hybrid system composed of bimetallic Au@Pt nanoparticles with diameter of a few nanometers uniformly dispersed in the conducting polymer matrix was synthesized by a simple electrochemical/chemical method on glassy carbon electrode. The elaborated catalyst demonstrates an enhanced specific catalytic activity (9.7Amg−1 Pt) in electrooxidation of formic acid. The reason of this improved performance is discussed in terms of the polymer structure used as the matrix for bimetallic catalyst and mechanism of formation of Au@Pt nanoparticles. Electrocatalytic activity of the hybrid systems obtained by means of two different procedures are compared to understand better the influence of the synthesis conditions on the properties of the catalyst. The composites are characterized by means HR-TEM, X-ray diffraction and X-ray fluorescence methods. Arrangement of nanoparticles in Au@Pt composite is determined by EDX elemental mapping. A new method of preparation of the samples embedded in thin polymer film for XRD and XRF measurements is proposed.

Remarkable catalytic activity of electrochemically dealloyed platinum–tellurium nanoparticles towards formic acid electro-oxidation

Aiming to improve the activity of Pt-based catalysts for formic acid electro-oxidation, dealloyed PtxTey/C catalysts were prepared via electrochemical leaching process. The dealloyed-Pt3Te/C catalyst presented superior activity and stability for formic acid electro-oxidation. The mass activities of dealloyed-Pt3Te/C at 0.25 V and 0.4 V vs. SCE were about 10.6 and 16.5 times as high as that of commercial Pt/C, respectively. It is found that the FAEO on D-Pt3Te/C is mainly through dehydrogenation pathway. The weak CO adsorption, increased electrochemical specific area and ensemble effect are suggested as reasons for the remarkable enhancement.

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.

Dealloyed platinum-copper with isolated Pt atom surface: Facile synthesis and promoted dehydrogenation pathway of formic acid electro-oxidation

In order to obtain the catalyst with Pt isolated-atom surface and high performance towards formic acid electro-oxidation (FAEO), cyclic potential dealloying method was employed for preparing dealloyed PtCu/C (D-PtCu/C) catalysts, and the effect of dealloying parameters on the activity of D-PtCu/C catalysts also were investigated. XRD results show that both of PtCu and D-PtCu catalysts are face-centered cubic (fcc) structure. TEM, XPS and ICP results demonstrate the dissolution of Cu atoms and the formation of core-shell structure. And HRTEM results further exhibit the etched surface of D-PtCu/C catalyst which should be made up of isolated atom and atomic clusters of Pt. By electro-chemical analysis, the performance towards FAEO is extremely enhanced. The onset potential of FAEO on D-PtCu/C is at −0.1V (vs. SCE) and only one peak is confirmed at 0.5V which negatively shifts about 0.19V compared with the peak of commercial Pt/C (at 0.69V). The mass activity of optimized D-PtCu/C reaches up to 1253.89mAmg−1 Pt at 0.5V, which is 6.20 times as high as that of Pt/C (202.40mAmg−1 Pt). And the specific activity (2.36mAcm−2) is 8.43 times of Pt/C (0.28mAcm−2). The increased activity should be ascribed to the electronic effect and ensemble effect, which reduce the chemisorption of CO and promote the dehydrogenation process.

Facile synthesis of well dispersed Pd nanoparticles on reduced graphene oxide for electrocatalytic oxidation of formic acid

In present study, we report a facile synthesis of crystalline, small size Pd nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Pd/RGO for electrocatalytic oxidation of formic acid (FA). Here, first graphene oxide (GO) was reduced by the green method using l-ascorbic acid and citric acid and further Pd NPs were decorated on RGO by a facile method without using any reducing agents. The reduction of GO to RGO and synthesis of Pd NPs was confirmed by the X-ray diffraction (XRD) and X-ray photoelectrons (XPS) techniques. Surface morphology of Pd/RGO nanocomposite was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The electrocatalytic behavior of Pd/RGO nanocomposite was tested by using of cyclic voltammetric (CV) technique for electro-oxidation of FA in mixed solution of 0.5 M HCOOH + 0.5 H2SO4 at RT. Results shows that the higher electrocatalytic activity of Pd/RGO nanocomposite compare to Pd NPs.

Light-induced deposition of Pd-based nanoalloy on TiO 2 nanotubes for formic acid electrooxidation

In this paper, we reported a facile UV-light induced method for depositing the Pd nanoparticles and Pd–Pt nanoalloys onto vertically oriented TiO2 nanotube (TNT) arrays with anodization. As-prepared TNT-supported Pd nanoparticles and Pd–Pt nanoalloys were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). EDS and XRD analysis verified that Pd and Pt particles had been successfully reduced. SEM images showed that Pd and Pd–Pt elements were evenly distributed on the surface of the TiO2 nanotube, and XPS measurement suggested electronic effects of the alloying elements on TiO2 nanotube. The electrocatalytic activities of TNTs-supported Pd-based catalysts had been investigated by the conjunction of cyclic voltammetry, linear sweeping voltammetry, chronoamperometry and CO stripping voltammograms. The results indicated that Pd0.97Pt0.03/TNTs catalysts exhibited the best electrocatalytic performance for formic acid electrooxidation, making it very attractive for practical applications in direct formic acid fuel cells.

Pd-catalysts for DFAFC prepared by magnetron sputtering

Samples of a palladium catalyst for direct formic acid fuel cell (DFAFC) applications were prepared on the Elat® carbon cloth by magnetron sputtering. The quantity of Pd was equal to 3.6, 120 and 720μg/cm2. The samples were tested in a fuel cell for electro-oxidation of formic acid, and were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).The XPS measurements revealed a high contribution of PdCx phase formed at the Pd/Elat® surface interface, with carbon concentration in PdCx from x=9.9–14.6at.%, resulting from the C substrate and CO residual gases. Oxygen groups, e.g. hydroxyl (–OH), carbonyl (CO) and carboxyl (COOH), resulted from the synthesis conditions due to the presence of residual gases, electro-oxidation during the reaction and oxidation in the atmosphere. Because of the formation of CO and CO2 on the catalysts during the reaction, or because of poisoning by impurities containing the –CH3 group, together with the risk of Pd losses due to dissolution in formic acid, there was a negative effect of catalyst degradation on the active area surface. The effect of different loadings of Pd layers led to increasing catalyst efficiency. Current–voltage curves showed that different amounts of catalyst did not increase the DFAFC power to a great extent. One reason for this was the catalyst structure formed on the carbon cloth. AFM and SEM measurements showed a layer-by-layer growth with no significant variations in morphology.The results for electric power recalculated for the Pd loading per 1mg of catalyst layers in comparison to carbon substrates decorated by Pd nanoparticles showed that there is potential for applying anodes for formic acid fuel cells prepared by magnetron sputtering.

Enhancement of the formic acid electrooxidation activity of palladium using graphene/carbon black binary carbon supports

Combinations of graphene (Gr) and carbon black (C) were employed as binary carbon supports to fabricate Pd-based electrocatalysts via one-pot co-reduction with Pd2+. The electrocatalytic performance of the resulting Pd/Gr-C catalysts during the electrooxidation of formic acid was assessed. A Pd/Gr0.3C0.7 (Gr oxide:C = 3:7, based on the precursor mass ratio) electrocatalyst exhibited better catalytic performance than both Pd/C and Pd/Gr catalysts. The current density generated by the Pd/Gr0.3C0.7 catalyst was as high as 102.14 mA mgPd−1, a value that is approximately 3 times that obtained from the Pd/C (34.40 mA mgPd−1) and 2.6 times that of the Pd/Gr material (38.50 mA mgPd−1). The anodic peak potential of the Pd/Gr0.3C0.7 was 120 mV more negative than that of the Pd/C and 70 mV more negative than that of the Pd/Gr. Scanning electron microscopy images indicated that the spherical C particles accumulated on the wrinkled graphene surfaces to form C cluster/Gr hybrids having three-dimensional nanostructures. X-ray photoelectron spectroscopy data confirmed the interaction between the Pd metal and the binary Gr-C support. The Pd/Gr0.3C0.7 also exhibited high stability, and so is a promising candidate for the fabrication of anodes for direct formic acid fuel cells. This work demonstrates a simple and cost-effective method for improving the performance of Pd-based electrocatalysts, which should have potential industrial applications.

Surfactant‐Free Synthesis of Graphene‐Supported PdCu Nanocrystals with High Alloying Degree as Highly Active Catalyst for Formic Acid Electrooxidation

In recent years, graphene/metal nanocrystal hybrids have attracted tremendous attention in catalysis and electrocatalysis. This work demonstrates a surfactant‐free coprecipitation/coreduction method to synthesize graphene‐supported nanocrystalline PdCu (PdCu/GN) hybrids with high alloying degree. The strong interaction between graphene oxide and metal hydrates results in uniform precipitation on the graphene oxide surface, which plays a critical role in excellent dispersion and high alloying degree of PdCu nanocrystals. Cyclic voltammetry and chronoamperometry measurements show that nanocrystalline PdCu/GN hybrids have higher activity and durability than commercial Pd/C catalyst for the formic acid oxidation reaction.

N-doped graphene-supported binary PdBi networks for formic acid oxidation

As advanced electrodes for direct formic acid cells, nitrogen-doped graphene (NG) supported palladium-bismuth nanoparticles have been successfully fabricated through typical wet-chemical method. In studying the effects of NG support on PdBi nanoparticles for the electrooxidation of formic acid, we find that the as-prepared Pd1Bi1/NG network-like electrocatalysts exhibit much higher electrocatalytic activities than the Pd1Bi1/RGO, Pd1Bi1 and commercially available Pd/C catalysts in term of mass activity (1.69, 4.33 and 15.5times higher, respectively). The remarkably enhanced performances are associated with the electron transport between Bi and N, bi-functional effect between Pd, Bi and NG hybrids as well as the well-dispersed network-like structure on the surface of NG. The investigations of PdBi/NG in this work for promoting the electrocatalytic performances and the electron effect between Bi and N will accelerate the development for the field of direct formic acid fuel cells.