Bridge-bonded formate: active intermediate or spectator species in formic acid oxidation on a Pt film electrode?

Pt-Coated Ni Layer Supported on Ni Foam for Enhanced Electro-Oxidation of Formic Acid

A Pt-coated Ni layer supported on a Ni foam catalyst (denoted PtNi/Nifoam) was investigated for the electro-oxidation of the formic acid (FAO) in acidic media. The prepared PtNi/Nifoam catalyst was studied as a function of the formic acid (FA) concentration at bare Pt and PtNi/Nifoam catalysts. The catalytic activity of the PtNi/Nifoam catalysts, studied on the basis of the ratio of the direct and indirect current peaks (jd)/(jnd) for the FAO reaction, showed values approximately 10 times higher compared to those on bare Pt, particularly at low FA concentrations, reflecting the superiority of the former catalysts for the electro-oxidation of FA to CO2. Ni foams provide a large surface area for the FAO, while synergistic effects between Pt nanoparticles and Ni-oxy species layer on Ni foams contribute significantly to the enhanced electro-oxidation of FA via the direct pathway, making it almost equal to the indirect pathway, particularly at low FA concentrations.

pH Effects in a Model Electrocatalytic Reaction Disentangled

Varying the solution pH not only changes the reactant concentrations in bulk solution but also the local reaction environment (LRE) that is shaped furthermore by macroscopic mass transport and microscopic electric double layer (EDL) effects. Understanding ubiquitous pH effects in electrocatalysis requires disentangling these interwoven factors, which is a difficult, if not impossible, task without physical modeling. Herein, we demonstrate how a hierarchical model that integrates microkinetics, double-layer charging, and macroscopic mass transport can help understand pH effects of the formic acid oxidation reaction (FAOR). In terms of the relation between the peak activity and the solution pH, intrinsic pH effects without consideration of changes in the LRE would lead to a bell-shaped curve with a peak at pH = 6. Adding only macroscopic mass transport, we can already reproduce qualitatively the experimentally observed trapezoidal shape with a plateau between pH 5 and 10 in perchlorate and sulfate solutions. A quantitative agreement with experimental data requires consideration of EDL effects beyond Frumkin correlations. Specifically, the peculiar nonmonotonic surface charging relation affects the free energies of adsorbed intermediates. We further discuss pH effects of FAOR in phosphate and chloride-containing solutions, for which anion adsorption becomes important. This study underpins the importance of a full consideration of multiple interrelated factors for the interpretation of pH effects in electrocatalysis.

The electrostatic effect and its role in promoting electrocatalytic reactions by specifically adsorbed anions

The adsorption of anions and its impact on electrocatalytic reactions are fundamental topics in electrocatalysis. Previous studies revealed that adsorbed anions display an overall poisoning effect in most cases. However, for a few reactions such as the hydrogen evolution reaction (HER), oxidation of small organic molecules (SOMs), and reduction of CO₂ and O₂, some specifically adsorbed anions can promote their reaction kinetics under certain conditions. The promotion effect is frequently attributed to the adsorbate induced modification of the nature of the active sites, the change of the adsorption configuration and free energy of the key reactive intermediate which consequently change the activation energy, the pre-exponential factor of the rate determining step etc. In this paper, we will give a mini review of the indispensable role of the classical double layer effect in enhancing the kinetics of electrocatalytic reactions by anion adsorption. The ubiquitous electrostatic interactions change both the potential distribution and the concentration distribution of ionic species across the electric double layer (EDL), which alters the electrochemical driving force and effective concentration of the reactants. The contribution to the overall kinetics is highlighted by taking HER, oxidation of SOMs, reduction of CO₂ and O₂, as examples.

CO2 and Formate Pathway of Methanol Electrooxidation at Rhodium Electrodes in Alkaline Media: An In Situ Electrochemical Attenuated Total Refection Surface-Enhanced Infrared Absorption Spectroscopy and Infrared Reflection Absorption Spectroscopy Investigation

Rh catalysts exhibit unexpected high activity for the methanol oxidation reaction (MOR) in alkaline conditions, making them potential anodic catalysts for direct methanol fuel cells (DMFCs). Nevertheless, the MOR mechanism on Rh electrodes has not been clarified thus far, which impedes the development of high-efficiency Rh-based MOR catalysts. To investigate it, a combination of in situ electrochemical techniques called attenuated total refection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and infrared reflection absorption spectroscopy (IRAS) is used. Cyclic voltammograms of MOR at Rh electrodes show considerable activity in alkaline media rather than acidic media, although the real-time ATR-SEIRA spectral results demonstrate that methanol can rarely self-decompose on Rh at open-circuit conditions. Meanwhile, in combination of ATR-SEIRAS and IRAS results, CO₂ and formate are thought to be MOR products, suggesting a dual-pathway mechanism ("CO₂ pathway" and "formate pathway"). Specifically, CO_ad species, which are the major intermediates in the CO₂ pathway, can produce at lower potentials and be oxidized into CO₂ at a potential of 0.5-0.75 V. Concurrently, the formate can be produced from 0.5 V and diffuse into the bulk electrolyte to become one of the MOR products, while the further electrochemical conversion of formate to CO₂ is essentially negligible. More directly, the apparent selectivity (r) of the CO₂ pathway is estimated to reach ca. 0.63 at 0.9 V, confirming the potential-dependent selectivity of MOR at Rh surfaces. This study might provide fresh insights into the design and fabrication of effective Rh-based catalysts for MOR.

The Electrochemical Oxidation and Mass Transfer Mechanism of Formic Acid on the Catalyst Electrode Surface

The organic small molecule fuel battery has attracted wild attention in recent years. Unfortunately, the inherent catalyst poisoning phenomenon hinders its commercialization. Exploring the anodic catalytic reaction mechanism is urgent. This article investigates the nucleation mechanism of HCOOH on the catalyst electrode surface. The electrochemical results indicate that the HCOOH oxidation conforms to the two-dimensional instantaneous nucleation process. The corresponding adsorption model of CO on the catalyst surface was finally established.

Enhanced electrocatalytic activity of Cu-modified, high-index single Pt NPs for formic acid oxidation

A key goal of nanoparticle-based catalysis research is to correlate the structure of nanoparticles (NPs) to their catalytic function. The most common approach for achieving this goal is to synthesize ensembles of NPs, characterize the ensemble, and then evaluate its catalytic properties. This approach is effective, but it excludes the certainty of structural heterogeneity in the NP ensemble. One means of addressing this shortcoming is to carry out analyses on individual NPs. This approach makes it possible to establish direct correlations between structures of single NPs and, in the case reported here, their electrocatalytic properties. Accordingly, we report on enhanced electrocatalytic formic acid oxidation (FAO) activity using individual Cu-modified, high-indexed Pt NPs. The results show that the Cu-modified Pt NPs exhibit significantly higher currents for FAO than the Pt-only analogs. The increased activity is enabled by the Cu submonolayer on the highly stepped Pt surface, which enhances the direct FAO pathway but not the indirect pathway which proceeds via surface-absorbed CO*.

Single-crystal Pt nanoparticles with a diameter of ∼200 nm were electrosynthesized, covered with a single monolayer of Cu, and then fully characterized. The resulting materials exhibit excellent electrocatalytic properties for formic acid oxidation.

Accelerated interfacial proton transfer for promoting the electrocatalytic activity

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pK_a of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H⁺ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.

Adsorbing 4MPy on Pt surface promotes proton transfer at the interfacial layer, maintaining an optimal interfacial pH and promotes electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes.

Benchmarking Catalysts for Formic Acid/Formate Electrooxidation

Energy production and consumption without the use of fossil fuels are amongst the biggest challenges currently facing humankind and the scientific community. Huge efforts have been invested in creating technologies that enable closed carbon or carbon neutral fuel cycles, limiting CO2 emissions into the atmosphere. Formic acid/formate (FA) has attracted intense interest as a liquid fuel over the last half century, giving rise to a plethora of studies on catalysts for its efficient electrocatalytic oxidation for usage in fuel cells. However, new catalysts and catalytic systems are often difficult to compare because of the variability in conditions and catalyst parameters examined. In this review, we discuss the extensive literature on FA electrooxidation using platinum, palladium and non-platinum group metal-based catalysts, the conditions typically employed in formate electrooxidation and the main electrochemical parameters for the comparison of anodic electrocatalysts to be applied in a FA fuel cell. We focused on the electrocatalytic performance in terms of onset potential and peak current density obtained during cyclic voltammetry measurements and on catalyst stability. Moreover, we handpicked a list of the most relevant examples that can be used for benchmarking and referencing future developments in the field.

Recent advances in formic acid electro-oxidation: from the fundamental mechanism to electrocatalysts

Direct formic acid fuel cells have attracted significant attention because of their low fuel crossover, high safety, and high theoretical power density among all the proton-exchange membrane fuel cells. Much effort has been devoted to the study of formic acid oxidation, including the reaction processes and electrocatalysts. However, as a model reaction, the anodic electro-oxidation process of formic acid is still not very clear, especially regarding the confirmation of the intermediates, which is not helpful for the design and synthesis of high-performance electrocatalysts for formic acid oxidation or conducive to understanding the reaction mechanisms of other small fuel molecules. Herein, we briefly review the recent advances in investigating the mechanism of formic acid electro-oxidation and the basic design concepts of formic acid oxidation electrocatalysts. Rather than an exhaustive overview of all aspects of this topic, this mini-review mainly outlines the progress of this field in recent years.

Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation

To meet the requirements of potential applications, it is of great importance to explore new catalysts for formic acid oxidation that have both ultra-high mass activity and CO resistance. Here, we successfully synthesize atomically dispersed Rh on N-doped carbon (SA-Rh/CN) and discover that SA-Rh/CN exhibits promising electrocatalytic properties for formic acid oxidation. The mass activity shows 28- and 67-fold enhancements compared with state-of-the-art Pd/C and Pt/C, respectively, despite the low activity of Rh/C. Interestingly, SA-Rh/CN exhibits greatly enhanced tolerance to CO poisoning, and Rh atoms in SA-Rh/CN resist sintering after long-term testing, resulting in excellent catalytic stability. Density functional theory calculations suggest that the formate route is more favourable on SA-Rh/CN. According to calculations, the high barrier to produce CO, together with the relatively unfavourable binding with CO, contribute to its CO tolerance.

Phase diagrams and dynamical evolution of the triple-pathway electro-oxidation of formic acid on platinum

A detailed numerical study including stability phase diagrams for the dynamical evolution of the electro-oxidation of formic acid on platinum was reported. The study evidences the existence of intertwined stability phases and the absence of chaos.

Conformal Solution Deposition of Pt-Pd Titania Nanocomposite Coatings for Light-Assisted Formic Acid Electro-Oxidation

Many nanofabrication processes require sophisticated equipment, elevated temperature, vacuum or specific atmospheric conditions, templates, and exotic chemicals, which severely hamper their implementation in real-world applications. In this study, we outline a fully wet-chemical procedure for equipping a 3D carbon felt (CF) substrate with a multifunctional, titania nanospike-supported Pt-Pd nanoparticle (Pt-Pd-TiO₂@CF) layer in a facile and scalable manner. The nanostructure, composition, chemical speciation, and formation of the material was meticulously investigated, evidencing the conformal coating of the substrate with a roughened layer of nanocrystalline rutile spikes by chemical bath deposition from Ti³⁺ solutions. The spikes are densely covered by bimetallic nanoparticles of 4.4 ± 1.1 nm in size, which were produced by autocatalytic Pt deposition onto Pd seeds introduced by Sn²⁺ ionic layer adsorption and reaction. The as-synthesized nanocomposite was applied to the (photo)electro-oxidation of formic acid (FA), exhibiting a superior performance compared to Pt-plated, Pd-seeded CF (Pt-Pd@CF) and commercial Pt-C, indicating the promoting electrocatalytic role of the TiO₂ support. Upon UV-Vis illumination, the performance of the Pt-Pd-TiO₂@CF electrode is remarkably increased (22-fold), generating a current density of 110 mA cm^-2, distinctly outperforming titania-free Pt-Pd@CF (5 mA cm^-2) and commercial Pt-C (6 mA cm^-2) reference catalysts. In addition, the Pt-Pd-TiO₂@CF showed a much better stability, characterized by a very high poisoning tolerance for in situ-generated CO intermediates, whose formation is hindered in the presence of TiO₂. This overall performance boost is attributed to a dual enhancement mechanism (∼30% electrocatalytic and ∼70% photoelectrocatalytic). The photogenerated electrons from the TiO₂ conduction band enrich the electron density of the Pt nanoparticles, promoting the generation of active oxygen species on their surfaces from adsorbed oxygen and water molecules, which facilitate the direct FA electro-oxidation into CO₂.

Adsorption of Acetate on Au(111): An in-situ Scanning Tunnelling Microscopy Study and Implications on Formic Acid Electrooxidation

The adsorption of acetate on an Au(111) electrode surface in contact with acetic acid at pH 2.7 was imaged in-situ using scanning tunnelling microscopy (STM). Two different ordered structures were imaged for acetate adsorbed in the bidentate configuration on the unreconstructed 1 × 1 surface at 0.95 V (vs. the saturated calomel electrode, SCE). The first structure, ( 19 × 19 ) R 23 . 45 ∘ , is metastable and transforms at constant potential within 20 minutes to a ( 2 × 2 ) structure, which is thermodynamically more favourable. The ( 2 × 2 ) acetate adlayer starts to form at step edges and propagates via nucleation and growth onto terraces. The findings from in-situ STM are in agreement with the electrochemical behaviour of acetate on Au(111) characterized by voltammetry. A comparison is made with formate adsorption on Au(111). While acetate is not reactive, in contrast to formate, it can act as a spectator species in formic acid electrooxidation.

Rapid synthesis of ultrasmall platinum nanoparticles supported on macroporous cellulose fibers for catalysis

Herein, we report a facile method for the synthesis of platinum nanoparticles (PtNPs) about 2.25 nm in size by heating a solution of chloroplatinic acid and sodium rhodizonate. The PtNPs were synthesized in about 5 min. The PtNPs were supported on macroporous cellulose fibers that were obtained from Kimwipe paper (KWP). The cellulose fiber-supported PtNPs (PtNPs@KWP) exhibited excellent catalytic activity towards the reduction of organic pollutants [e.g. methyl orange (MO)] in the presence of hydrogen (H₂) gas and formic acid (FA). FA and H₂ gas were utilized as clean and alternative reducing agents. The reduction of MO was performed in two different types of water matrices viz. deionized water (DIW) and simulated fresh drinking water (FDW). In both water matrices, the FA mediated reduction of MO was found to be faster than the H₂ gas-bubbled one. The PtNPs@KWP demonstrated excellent cycling stability without leaching the PtNPs or platinum ions into the solution for at least five cycles.

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.

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the "active sites" lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works.

Tailored dendritic platinum nanostructures as a robust and efficient direct formic acid fuel cell anode

Engineering of platinum structures with precisely controlled morphology provides an excellent opportunity to efficiently tailor their catalytic performance, greatly improving their durability and activity.

Silsesquioxane stabilized platinum-palladium alloy nanoparticles with morphology evolution and enhanced electrocatalytic oxidation of formic acid

Bimetallic catalysts have attracted enormous attention with their enhanced electrocatalytic properties in fuel cells. Herein a series of silsesquioxane (POSS) stabilized platinum-palladium (PtPd) alloy nanoparticles (NPs) with morphology evolution were facilely synthesized with the co-chemical reduction using formaldehyde as the reductant. By varying the ratio of Pt to Pd, the PtPd alloy NPs evolved from truncated octahedrons to octahedrons, and triangular nanoplates. The mechanism of morphology evolution is that Pt and Pd could self-assemble on POSS to form Pt_xPd_1-x intermediates with different Pt/Pd ratios. In addition, formaldehyde could selectively bind to the {1 1 1} facets of Pd to control the growth rates of different facets and help Pt_xPd_1-x intermediates with different Pt/Pd ratio grow into different morphology of Pt_xPd_1-x alloys. The morphology tuning endowed the PtPd alloy NPs superior performance for formic acid electrooxidation. Compared with Pt, Pd NPs, and commercial Pt/C catalyst, the PtPd alloy NPs displayed larger electrochemically active surface area, enhanced electrocatalytic activity and durability toward oxidation of formic acid, and increased CO tolerance. This work suggested that modification of catalytic activity through morphology tuning with composition adjustment might provide some new pathways for the design of promising catalysts with advanced performance.