Formic Acid Electrooxidation Research Articles

Modulate the metallic Sb state on ultrathin PdSb-based nanosheets for efficient formic acid electrooxidation

Incorporation of oxophilic metals into Pd-based nanostructures has shown great potential in small molecule electrooxidation owing to their superior anti-poisoning capability. However, engineering the electronic structure of oxophilic dopants in Pd-based catalysts remains challenging and their impact on electrooxidation reactions is rarely demonstrated. Herein, we have developed a method for synthesizing PdSb-based nanosheets, enabling the incorporation of the Sb element in a predominantly metallic state despite its high oxophilic nature. Moreover, the Pd90Sb7W3 nanosheet serves as an efficient electrocatalyst for the formic acid oxidation reaction (FAOR), and the underlying promotion mechanism is investigated. Among the as-prepared PdSb-based nanosheets, the Pd90Sb7W3 nanosheet exhibits a remarkable 69.03% metallic state of Sb, surpassing the values observed for the Pd86Sb12W2 (33.01%) and Pd83Sb14W3 (25.41%) nanosheets. X-ray photoelectron spectroscopy (XPS) and CO stripping experiments confirm that the Sb metallic state contributes the synergistic effect of their electronic and oxophilic effect, thus leading to an effective electrooxidation removal of CO and significantly enhanced FAOR electrocatalytic activity (1.47 A mg−1; 2.32 mA cm−1) compared with the oxidated state of Sb. This work highlights the importance of modulating the chemical valence state of oxophilic metals to enhance electrocatalytic performance, offering valuable insights for the design of high-performance electrocatalysts for electrooxidation of small molecules.

Constructing poison-resistant Au-Pd single-atom alloy electrocatalyst on Pd nanocrystals for formic acid electrooxidation

Pd is a promising anodic catalyst for direct formic acid fuel cells, while its performance on formic acid electrooxidation is still suffered from for low catalytic efficiency and durability. In this work, an electrochemical self-catalyzed method was employed to deposit Au single-atoms onto Pd nanocrystals with tuneable compositions. Spherical-corrected Scanning transmission microscope and electrochemical characteristics demonstrated that the Au atoms were atomic-dispersive on Pd nanocrystals, forming Au-Pd single-atom alloy. Systematic studies showed that the activity of Pdx @Auy/C SAA is related to the density of the Au single-atoms on Pd surface. When an optimized balance between active-site exposure and intrinsic activity is achieved, the Pd50 @Au1/C SAA showed superb formic acid electrooxidation activity of 4.98 A mg−1M and unusual high anti-poisoning ability which is distinctly different from most of the Pd based catalysts. The boosted electrochemical performances of Pdx @Auy/C SAA can be attributed to the unique geometric effect that facilitates the direct reaction route and favourable electronic effect that weakens the CO adsorption to Pd sites.

Optimizing dual-intermediates adsorption on Rh-based intermetallics for formic acid electrooxidation

Concurrently optimizing the pathway and electrocatalytic activity for formic acid oxidation reaction (FAOR) is significant but still challenging. Herein, surface and composition engineering strategies are integrated by constructing atomically ordered Rh0.9Pt0.1Fe ternary intermetallic (O-Rh0.9Pt0.1Fe). Supported by the in situ infrared spectra, the carbon monoxide (CO*) mediated indirect FAOR pathway on pure Rh is switched into a direct pathway on RhFe intermetallics (O-RhFe) with isolated Rh atoms, leading to a lower overpotential. Further composition engineering by forming O-Rh0.9Pt0.1Fe contributes to enhanced FAOR activity due to the stabilizing of COOH* intermediate, as indicated by the density functional theory calculations. Besides, O-Rh0.9Pt0.1Fe shows enhanced FAOR activity during potential cycling tests with fewer structural evolutions, while O-RhFe displays a decay. This study provides new insight into tuning reaction pathways and catalytic activity selectively via tailoring adsorption of different intermediates.

Highly dispersed 1 nm PtPd bimetallic clusters for formic acid electrooxidation through a CO-free mechanism

Highly dispersed 1 nm PtPd bimetallic clusters for formic acid electrooxidation through a CO-free mechanism

Phase Transformation during the Selective Dissolution of a Cu85Pd15 Alloy: Nucleation Kinetics and Contribution to Electrocatalytic Activity

This study determined the critical parameters for the morphological development of the electrode surface (the critical potential and the critical charge) during anodic selective dissolution of a Cu-Pd alloy with a volume concentration of 15 at.% palladium. When the critical values were exceeded, a phase transition occurred with the formation of palladium's own phase. Chronoamperometry aided in the determination of the partial rates of copper ionization and phase transformation of palladium under overcritical selective dissolution conditions. The study determined that the formation of a new palladium phase is controlled by a surface diffusion of the ad-atom to the growing three-dimensional nucleus under instantaneous activation of the nucleation centres. We also identified the role of this process in the formation of the electrocatalytic activity of the anodically modified alloy during electro-oxidation of formic acid. This study demonstrated that HCOOH is only oxidated at a relatively high rate on the surface of the Cu85Pd15 alloy, which is subjected to selective dissolution under overcritical conditions. This can be explained by the fact that during selective dissolution of the alloy, a pure palladium phase is formed on its highly developed surface which has prominent catalytic activity towards the electro-oxidation of formic acid. The rate of electro-oxidation of HCOOH on the surface of the anodically modified alloy increased with the growth of the potential and the charge of selective dissolution, which can be used to obtain an electrode palladium electrocatalyst with a set level of electrocatalytic activity towards the anodic oxidation of formic acid.

Self-catalytic induced interstitial C-doping of Pd nanoalloys for highly selective electrocatalytic dehydrogenation of formic acid

Light-metalloid-atom-doped Pd interstitial nanoalloy is promising candidate for electrocatalysis because of the favorable electronic effect. Herein, an innovative method was developed to synthesize C-doped Pd interstitial nanoalloy using palladium acetate both as metal precursor and C dopant. Elaborate characterizations demonstrated that C atoms were successfully doped into the Pd lattice via self-catalytic decomposition of acetate ions. The as-synthesized C-doped Pd catalysts showed excellent activity and durable stability for formic acid electrooxidation. The mass activity and specific activity at 0.6 V of C-doped Pd were approximately 2.59 A/mg and 3.50 mA cm−2, i.e., 2.4 and 2.6 times of Pd, respectively. DFT calculations revealed that interstitial doping with C atoms induced differentiation of Pd sites. The strong noncovalent interaction between the Pd sites and the key intermediates endowed Pd with high-selectivity to direct routes and enhanced CO tolerance. This work presents a sites-differentiation strategy for metallic catalysts to improve the electrocatalysis.

Sm2O3 promoted Pd/rGO electrocatalyst for formic acid oxidation

Herein, we report the synthesis of Sm2O3 incorporated palladium based electrocatalyst, supported over reduced graphene oxide (rGO). PdxSmy nanoparticles (20 wt %) are distributed over rGO surface by using sodium borohydride reduction technique. Different physicochemical analyses are used to characterize the PdxSmy/rGO electro-catalysts (x = 2,4,6 and y = 8,6,4). The synthesized materials (PdxSmy/rGOs) are investigated for their catalytic capabilities toward electro-oxidation of formic acid. The results show that adding Sm2O3 to the Pd/rGO electrocatalyst boosts electrochemical activities of the materials toward formic acid oxidation. The optimized catalyst (Pd6Sm4/rGO) shows excellent activity towards formic acid oxidation with current density of 46.70 mA/cm2 compared to reference catalysts i.e., Pd/rGO (28.78 mA/cm2) and Pd/C (22.22 mA/cm2). The optimized catalyst also demonstrates high CO tolerance and great stability during formic acid oxidation reaction. The enhanced activity and stability are attributed to the synergistic interaction between Sm2O3, and palladium nanoparticles supported over reduced graphene oxide.

Effect of MoO3 on Pd nanoparticles for efficient formic acid electrooxidation

Palladium is a promising formic acid electro-oxidation (FAO) catalyst due to its higher initial activity than platinum. However, suffering from the adsorption of hydrogen and CO-like species, the activity and stability of Pd are still unsatisfied. Herein, palladium nanoparticles deposited on carbon supported molybdenum trioxide (Pd–MoO3/C) is prepared with MoO3 as the promoter for FAO. X-ray photoelectron spectroscopy analysis proves the close contact between Pd and MoO3, which generates the hydrogen spillover effect and forms the Pd–Mo structure. The hydrogen spillover effect enhances the desorption of hydrogen from Pd and facilitates the FAO activity. Both the spillover effect and Pd–Mo structure contribute to the removal of COad and facilitate the durability and anti-CO poisoning ability of the catalyst. With the optimized ratio of MoO3 to carbon black, the Pd–MoO3/C-20 catalyst owns the best specific activity of 5.86 mA cm−2, which is 1.86 times of Pd/C.

Ordered and Isolated Pd Sites Endow Antiperovskite‐Type PdFe3N with High CO‐Tolerance for Formic Acid Electrooxidation

AbstractThe fundamental understanding and precise control of catalytic sites are challenging yet essential to explore advanced electrocatalysts for the formic acid oxidation reaction (FAOR). Herein, this work demonstrates a new and promising catalyst prototype of antiperovskite‐type PdFe3N which possesses ordered and isolated Pd sites. The as‐synthesized PdFe3N/N‐rGO exhibits significant enhancement in catalytic activity, robust stability, and Fe anti‐dissolution properties when compared with PdFe3/rGO and Pd/C. Density functional theory (DFT) calculations reveal that isolated and ordered Pd sites are beneficial for high formate coverage and thus suppressing CO formation/poisoning. Moreover, the strong Fe–N covalent bonds improve the vacancy formation energy of Fe, which ensures superior Fe anti‐dissolution properties. Beyond offering a promising candidate for catalyzing FAOR, these findings provide deeper insights into the structure of CO tolerance relationships and benefit the theory‐guided design of highly efficient catalysts.

Boosted formic acid electro-oxidation on platinum nanoparticles and "mixed-valence" iron and nickel oxides.

The modification of Pt nanoparticles (nano-Pt, assembled electrochemically onto a glassy carbon (GC) substrate) with hybrid multivalent nickel (nano-NiOx) and iron (nano-FeOx) oxide nanostructures was intended to steer the mechanism of the formic acid electro-oxidation (FAO) in the desirable dehydrogenation pathway. This binary modification with inexpensive oxides succeeded in mediating the reaction mechanism of FAO by boosting reaction kinetics "electron transfer" and amending the surface geometry of the catalyst against poisoning. The sequence of deposition was optimized where the a-FeOx/NiOx/Pt/GC catalyst (where "a" denotes a post-activation step for the catalyst at -0.5 V in 0.5 mol L-1 NaOH) reserved the best hierarchy. Morphologically, while nano-Pt appeared to be spherical (ca. 100 nm in average diameter), nano-NiOx appeared as flowered nanoaggregates (ca. 56 nm in average diameter) and nano-FeOx (after activation) retained a plate-like nanostructure (ca. 38 nm in average diameter and 167 nm in average length). This a-FeOx/NiOx/Pt/GC catalyst demonstrated a remarkable catalytic efficiency (125 mA mgPt-1) for FAO that was ca. 12.5 times that of the pristine Pt/GC catalyst with up to five times improvement in the catalytic tolerance against poisoning and up to -214 mV shift in the FAO's onset potential. Evidences for equipping the a-FeOx/NiOx/Pt/GC catalyst with the least charge transfer resistance and the highest stability among the whole investigated catalysts are provided and discussed.

Selective edge etching of Pd metallene for enhanced formic acid electrooxidation.

We develop a facile, selective edge etching strategy to create edge sites in Pd metallene using acetic acid. The created edge sites remarkably increase the electrochemically active surface area but reduce the charge transfer resistance, resulting in significant enhancement of catalytic activity and stability toward formic acid oxidation.

The role of coverage effects on the structure–sensitivity of formic acid electrooxidation on Pd surfaces

Direct formic acid (FA) fuel cells (DFAFCs) with Pd anode catalysts are promising for small portable applications. During the electrooxidation of formic acid (FAO), CO accumulates on the Pd electrocatalyst, thereby decreasing its performance. In this work, we use density functional theory (DFT; RPBE) to elucidate the significance of coverage effects on the structure–sensitivity of FAO over four low–index Pd surfaces: Pd(111), Pd(100), Pd(110), and Pd(211). We construct coverage–dependent Gibbs free energy diagrams and in agreement with experiment, predict that Pd(100) and Pd(211) are the most active surfaces. Additionally, we find that the presence of CO spectator species can alter the preferred electrooxidation pathway under different applied potentials. Such pathway transitions appear to be critical for rationalizing experimental trends regarding electrocatalytic activity and are only captured when coverage effects are accounted for. Insights derived from this work may facilitate the design of improved FAO electrocatalysts.

Amendment of palladium nanocubes with iron oxide nanowires for boosted formic acid electro−oxidation

To quickly move the formic acid (FA) fuel cells closer to a real commercialization, an inexpensive, efficient, and durable electrocatalyst for the direct FA electro-oxidation (FAEO) was developed. This involved a sequential modification of a glassy carbon (GC) substrate with palladium nanocubes (ca. 70 nm, nano-Pd) and iron oxide nanowires (nano-FeOx, ca. 40 nm and 150 nm in average diameter and length, respectively). The deposition sequence and loading level of nano-FeOx in the catalyst were optimized to minimize the catalyst's poisoning with CO that might probably release from a parallel dehydration of FA or from CO2 reduction. Surprisingly, the FeOx/Pd/GC catalyst exhibited a high (21.6 mA cm−2) specific activity for FAEO, which denoted ca. 7 times that of the “pristine” Pd/GC catalyst. This was synchronized with a better (up to fivefold increase in turnover frequency) “long-termed” stability that extended for 90 min of continuous electrolysis at room temperature. A successful effort was dedicated to improving more the catalyst's stability by activating the catalyst electrochemically at –0.5 V vs Ag/AgCl/KCl (sat.) in 0.2 mol L−1 NaOH. The CO stripping agreed perfectly with the impedance analysis in appending the observed enhancement in the catalytic efficiency of FAEO to a favorable electronic modulation at the Pd surface that boosted the oxidative desorption of poisoning CO species at a lower potential.

Co-electrodeposited PtPd anodic catalyst for the direct formic acid fuel cells

This investigation displayed a superb catalytic performance toward the formic acid electro-oxidation reaction (FAOR) in an alkaline medium at a binary catalyst composed of Pt and Pd that were simultaneously electrodeposited onto a glassy carbon (GC) surface. Interestingly, the proposed PtPd/GC catalyst displayed a significant enhancement in the catalytic activity (by ca. 12 times higher direct (Ipd) to indirect (Ipind) current ratio concurrently with ca. -91 mV shift in the onset potential (Eonset) and stability (ca. 2.5 times higher current density after 3600 s of continuous electrolysis) toward FAOR if compared to the Pt/GC catalyst. The catalytic enhancement was thought to arise mostly from minimizing the CO adsorption, i.e., third body effect, at the Pt surface during FAOR.

Interfacial engineering of holey platinum nanotubes for formic acid electrooxidation boosted water splitting

Both structure and interface engineering are highly effective strategies for enhancing the catalytic activity and selectivity of precious metal nanostructures. In this work, we develop a facile pyrolysis strategy to synthesize the high-quality holey platinum nanotubes (Pt-H-NTs) using nanorods-like PtII-phenanthroline (PT) coordination compound as self-template and self-reduction precursor. Then, an up-bottom strategy is used to further synthesize polyallylamine (PA) modified Pt-H-NTs (Pt-H-NTs@PA). PA modification sharply promotes the catalytic activity of Pt-H-NTs for the formic acid electrooxidation reaction (FAEOR) by the direct reaction pathway. Meanwhile, PA modification also elevates the catalytic activity of Pt-H-NTs for the hydrogen evolution reaction (HER) by the proton enrichment at electrolyte/electrode interface. Benefiting from the high catalytic activity of Pt-H-NTs@PA for both FAEOR and HER, a two-electrode FAEOR boosted water electrolysis system is fabricated by using Pt-H-NTs@PA as bifunctional electrocatalysts. Such FAEOR boosted water electrolysis system only requires the operational voltage of 0.47 V to achieve the high-purity hydrogen production, showing an energy-saving hydrogen production strategy compared to traditional water electrolysis system.

Pulsed laser-driven green synthesis of trimetallic AuPtCu nanoalloys for formic acid electro-oxidation in acidic environment

The process of hydrogen energy storage via water electrolysis is challenging because of the highly explosive nature of hydrogen gas. To circumvent this issue, the formic acid aqueous solution is being intensively investigated in the field of fuel cells as of its excellent stability and no hazard of combustion or explosion. In formic acid oxidation (FAO), a formic acid solution is decomposed into CO2, thus generating hydrogen ions and electrons. This process can occur via two pathways, i.e., with or without CO generation. In this study, we prepared AuPtCu alloys as a catalyst for formic acid fuel cell (FAFC) usage employing an ecofriendly pulsed laser without requiring a reducing agent or surfactant. The FAO process catalyzed by the AuPtCu alloys was found to ensue via a direct path, i.e., dehydrogenation in the existence of Pt and Au. Formic acid was directly oxidized to CO2 while CO was removed by Cu. The optimal Au1Pt1Cu0.5 catalyst exhibited the best electrochemical FAO activity, outperforming traditional Pt/C, most likely because of the Cu content enhancing the amount of active surface sites. This research demonstrates the potential of AuPtCu for application in direct FAFCs as a power source for transportable devices.

Insights into the Effect of Pb on Formic Acid Electro-Oxidation on Pt

Formic acid is a promising energy carrier and electrochemical formic acid oxidation can serve as an important reaction within fuel cells. Although Pt remains the best-known catalyst, but the major currents contributing pathways is indirect oxidation pathways, which leads to poor activities at low potentials. The surface adatoms like Pb have been known to promote the direct oxidation pathways significantly. We have carried out theoretical calculations, and experiments to probe the reason for this enhancement. Theoretical results show an upshift of Pt energy levels above the Fermi energy upon adsorption of Pb on the Pt surface, which influences the binding energy of the adsorbates. Reaction free energy calculations of typical intermediates from water (H2O) and formic acid (HCOOH) molecule indicate that Pb adsorption suppresses the adsorption of Hydrogen atoms at potentials typical of underpotential deposition on Pt electrode. This results in larger number of vacant sites on the Pt surface which is now able to carry out formic acid oxidation to CO2. The calculations were supported by electrochemical experiments of underpotentially deposited hydrogen and potential dependent apparent activation energies. Figure 1

Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid

The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphology, size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidation reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphology with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in solution, which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochemical impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions.

Pt Nanoelectrodes Sealed in Quartz Capillaries Modified with Underpotential‐Deposited Bismuth for Formic Acid Electrooxidation

AbstractPlatinum disk nano‐ and micro‐electrodes with radii in the range 40 nm–12.5 μm were fabricated using quartz capillaries and the laser‐puller assisted approach. The hydrogen underpotential deposition (H‐UPD), performed in a 0.5 M H2SO4 aqueous solution, revealed that the nanoelectrodes, with radii of 160 nm and less, displayed extraordinarily large surface areas, which in terms of roughness factors (RFs, i. e., the ratio of the real surface areas to the geometric surface areas) were in the range 1030–3600. This finding was attributed to diffusion of adsorbed species at the Pt/quartz interface along portions of the Pt wires sealed within the glass. RFs between 2 and 3 were instead found at the microelectrodes. Similar results were also obtained with underpotential‐deposited metallic bismuth. In this case, diffusion of Bi adatoms onto the Pt surface was hindered to some extent, providing a RF value of 172. Bi‐modified Pt nano‐ and micro‐electrodes were employed to study the electrooxidation of HCOOH, which is of interest in the field of fuel cells. It was found that the nanoelectrode displayed higher activity towards the electrooxidation of HCOOH and tolerance to CO poisoning, compared to the microelectrode.

Promoting formic acid and ethylene glycol electrooxidation activity on Ga modified Pd based catalysts

Herein, carbon nanotube (CNT)-supported Ga@PdAgCo catalysts were synthesized by sodium borohydride (SBH) sequential reduction method. These catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma-mass spectrometry (ICP-MS). Characterization results revealed that these catalysts were succesfully preared at desired loading and atomic ratios. From the XRD pattern, the crystallite size of 0.5% Ga@PdAgCo(80:10:10)/CNT catalysts was found as 6.95 nm by utilizing the Scherrer equation. From TEM measurements, the average particle sizes of Pd/CNT, PdAgCo(80:10:10)/CNT, and 0.5% Ga@PdAgCo(80:10:10)/CNT catalysts were found to be 54 nm, 25 nm, and 7 nm, respectively. It is clear that particle sizes obtained from TEM and XRD were close to eachother. Electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and cyclic voltammetry (CV) measurements were realized to examine the formic acid and ethylene glycol electrooxidation performances of catalysts. 0.5% Ga@PdAgCo(80:10:10/CNT) and 7% Ga@PdAgCo(80:10:10/CNT) catalysts had the best specific activity and mass activity as 3.37 mA/cm2 (297.61 mA/mg Pd) and 4.95 mA/cm2 (462.59 mA/mg Pd) for ethylene glycol and formic acid electrooxidation, respectively. In addition, EIS results showed that Ga@PdAgCo(80:10:10/CNT) catalyst had a faster electron transfer rate via low charge transfer resistance. As a result, 0.5% Ga@PdAgCo(80:10:10/CNT) catalyst is a promising new anode catalyst for direct ethylene glycol fuel cells.