Ubiquitous Strategy for Probing ATR Surface-Enhanced Infrared Absorption at Platinum Group Metal−Electrolyte Interfaces

Atomic Layer Deposition of TiO2 on Si Window Enables In Situ ATR-SEIRAS Measurements in Strong Alkaline Electrolytes

The Si window is the most widely used internal reflection element (IRE) for electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), yet local chemical etching on Si by concentrated OH- anions bottlenecks the reliable application of this method in strong alkaline electrolytes. In this report, atomic layer deposition of a 25 nm nonconductive TiO2 barrier layer on the reflecting plane of a Si prism is demonstrated to address this challenge. In situ ATR-SEIRAS measurement on a Au film electrode with the Si/TiO2 composite IRE in 1 M NaOH reveals reversible global spectral features without spectral distortion at 1000-1300 cm-1, in stark contrast to those obtained with a bare Si window. By applying this structured ATR-SEIRAS, ethanol electrooxidation on a Pt/C catalyst in 1 and 5 M NaOH is explored, manifesting that such high pH values prevent the adsorption of as-formed acetate in the C2 pathway but not that of CO intermediate in the C1 pathway.

Insights into Electrochemical CO2 Reduction on Metallic and Oxidized Tin Using Grand-Canonical DFT and In Situ ATR-SEIRA Spectroscopy

Electrochemical CO2 reduction (CO2R) to formate is an attractive carbon emissions mitigation strategy due to the existing market and attractive price for formic acid. Tin is an effective electrocatalyst for CO2R to formate, but the underlying reaction mechanism and whether the active phase of tin is metallic or oxidized during reduction is openly debated. In this report, we used grand-canonical density functional theory and attenuated total reflection surface-enhanced infrared absorption spectroscopy to identify differences in the vibrational signatures of surface species during CO2R on fully metallic and oxidized tin surfaces. Our results show that CO2R is feasible on both metallic and oxidized tin. We propose that the key difference between each surface termination is that CO2R catalyzed by metallic tin surfaces is limited by the electrochemical activation of CO2, whereas CO2R catalyzed by oxidized tin surfaces is limited by the slow reductive desorption of formate. While the exact degree of oxidation of tin surfaces during CO2R is unlikely to be either fully metallic or fully oxidized, this study highlights the limiting behavior of these two surfaces and lays out the key features of each that our results predict will promote rapid CO2R catalysis. Additionally, we highlight the power of integrating high-fidelity quantum mechanical modeling and spectroscopic measurements to elucidate intricate electrocatalytic reaction pathways.

Elucidating the structure-stability relationship of Cu single-atom catalysts using operando surface-enhanced infrared absorption spectroscopy

Understanding the structure-stability relationship of catalysts is imperative for the development of high-performance electrocatalytic devices. Herein, we utilize operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to quantitatively monitor the evolution of Cu single-atom catalysts (SACs) during the electrochemical reduction of CO2 (CO2RR). Cu SACs are converted into 2-nm Cu nanoparticles through a reconstruction process during CO2RR. The evolution rate of Cu SACs is highly dependent on the substrates of the catalysts due to the coordination difference. Density functional theory calculations demonstrate that the stability of Cu SACs is highly dependent on their formation energy, which can be manipulated by controlling the affinity between Cu sites and substrates. This work highlights the use of operando ATR-SEIRAS to achieve mechanistic understanding of structure-stability relationship for long-term applications.

Promoting CO2 Electroreduction to Multi-Carbon Products by Hydrophobicity-Induced Electro-Kinetic Retardation

Advancing the performance of the Cu-catalyzed electrochemical CO2 reduction reaction (CO2 RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO2 , reducing the Faradaic efficiencies (FEs) and current densities for multi-carbon (C2+ ) products. Recent studies have proposed that increasing surface availability for CO2 by cation-exchange ionomers can enhance the C2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C2+ selectivity, and thus the resulting C2+ FEs remain low. Herein, we discover that the electro-kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group-functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro-kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm-2 , more than twice of bare or hydrophilic Cu surfaces.

Constructing asymmetric double-atomic sites for synergistic catalysis of electrochemical CO2 reduction

Elucidating the synergistic catalytic mechanism between multiple active centers is of great significance for heterogeneous catalysis; however, finding the corresponding experimental evidence remains challenging owing to the complexity of catalyst structures and interface environment. Here we construct an asymmetric TeN2-CuN3 double-atomic site catalyst, which is analyzed via full-range synchrotron pair distribution function. In electrochemical CO2 reduction, the catalyst features a synergistic mechanism with the double-atomic site activating two key molecules: operando spectroscopy confirms that the Te center activates CO2, and the Cu center helps to dissociate H2O. The experimental and theoretical results reveal that the TeN2-CuN3 could cooperatively lower the energy barriers for the rate-determining step, promoting proton transfer kinetics. Therefore, the TeN2-CuN3 displays a broad potential range with high CO selectivity, improved kinetics and good stability. This work presents synthesis and characterization strategies for double-atomic site catalysts, and experimentally unveils the underpinning mechanism of synergistic catalysis.

Identifying Adsorbed OH Species on Pt and Ru Electrodes with Surface-Enhanced Infrared Absorption Spectroscopy through CO Displacement

OH adspecies are involved in numerous electrocatalytic reactions, such as CO, H2, methanol, and ethanol oxidation and oxygen reduction reactions, as a reaction intermediate and/or reactant. In this work, we have, for the first time, identified the OH stretching band of OH adspecies on Pt, Ru, and Pt/Ru electrodes with surface-enhanced infrared absorption spectroscopy (SEIRAS) in a flow cell through potential modulation and CO displacement. We found that while Ru had a relatively constant OH coverage at potentials between 0.1 and 0.8 V, Pt had a maximum OH coverage at 0.6 V in 0.1 M HClO4 and 0.7 V in 0.1 M KOH. CO oxidation kinetics on Ru were sluggish, although adsorbed OH appeared on Ru at very low potentials. Binary Pt/Ru electrodes promote CO oxidation through a synergistic effect in which Ru promotes OH adsorption and Pt catalyzes the reaction between the CO and OH adspecies. In addition, water coadsorbed with CO at Ru sites of Pt/Ru also plays an important role. These new spectroscopic results about OH adspecies could advance the understanding of the mechanism of fuel cell related electrocatalysis.

Hyphenated DEMS and ATR-SEIRAS techniques for in situ multidimensional analysis of lithium-ion batteries and beyond

A wide spectrum of state-of-the-art characterization techniques have been devised to monitor the electrode-electrolyte interface that dictates the performance of electrochemical devices. However, coupling multiple characterization techniques to realize in situ multidimensional analysis of electrochemical interfaces remains a challenge. Herein, we presented a hyphenated differential electrochemical mass spectrometry and attenuated total reflection surface enhanced infrared absorption spectroscopy analytical method via a specially designed electrochemical cell that enables a simultaneous detection of deposited and volatile interface species under electrochemical reaction conditions, especially suitable for non-aqueous, electrolyte-based energy devices. As a proof of concept, we demonstrated the capability of the homemade setup and obtained the valuable reaction mechanisms, by taking the tantalizing reactions in non-aqueous lithium-ion batteries (i.e., oxidation and reduction processes of carbonate-based electrolytes on Li1+xNi0.8Mn0.1Co0.1O2 and graphite surfaces) and lithium-oxygen batteries (i.e., reversibility of the oxygen reaction) as model reactions. Overall, we believe that the coupled and complementary techniques reported here will provide important insights into the interfacial electrochemistry of energy storage materials (i.e., in situ, multi-dimensional information in one single experiment) and generate much interest in the electrochemistry community and beyond.

Oxygen-Bridged Indium-Nickel Atomic Pair as Dual-Metal Active Sites Enabling Synergistic Electrocatalytic CO2 Reduction

Single-atom catalysts offer a promising pathway for electrochemical CO₂ conversion. However, it is still a challenge to optimize the electrochemical performance of dual-atom catalysts. Here, an atomic indium-nickel dual-sites catalyst bridged by an axial oxygen atom (O-In-N₆ -Ni moiety) was anchored on nitrogenated carbon (InNi DS/NC). InNi DS/NC exhibits superior CO selectivity with Faradaic efficiency higher than 90 % over a wide potential range from -0.5 to -0.8 V versus reversible hydrogen electrode (vs. RHE). Moreover, an industrial CO partial current density up to 317.2 mA cm^-2 is achieved at -1.0 V vs. RHE in a flow cell. In situ ATR-SEIRAS combined with theory calculations reveal that the synergistic effect of In-Ni dual-sites and O atom bridge not only reduces the reaction barrier for the formation of *COOH, but also retards the undesired hydrogen evolution reaction. This work provides a feasible strategy to construct dual-site catalysts towards energy conversion.

Plasmon-Enhanced C-C Bond Cleavage toward Efficient Ethanol Electrooxidation

Ethanol, as a sustainable biomass fuel, is endowed with the merits of theoretically high energy density and environmental friendliness yet suffers from sluggish kinetics and low selectivity toward the desired complete electrooxidation (C1 pathway). Here, the localized surface plasmon resonance (LSPR) effect is explored as a manipulating knob to boost electrocatalytic ethanol oxidation reaction in alkaline media under ambient conditions by appropriate visible light. Under illumination, Au@Pt nanoparticles with plasmonic core and active shell exhibit concurrently higher activity (from 2.30 to 4.05 A mg_Pt^-1 at 0.8 V vs RHE) and C1 selectivity (from 9 to 38% at 0.8 V). In situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) provides a molecular level insight into the LSPR promoted C-C bond cleavage and the subsequent CO oxidation. This work not only extends the methodology hyphenating plasmonic electrocatalysis and in situ surface IR spectroscopy but also presents a promising approach for tuning complex reaction pathways.

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.

Electrolyte-Layer-Tunable ATR-SEIRAS for Simultaneous Detection of Adsorbed and Dissolved Species in Electrochemistry

A balanced detection of both adsorbates and dissolved species is very important for the clarification of the electrochemical reaction mechanism yet remains a major challenge for different modes of electrochemical infrared (IR) spectroscopy. Among others, conventional attenuated total reflection-surface-enhanced IR absorption spectroscopy (ATR-SEIRAS) is far less sensitive to low-concentration solution species than to surface species. We report herein an electrochemical wide-frequency ATR-SEIRAS with a novel thin-layer flow cell design, fulfilling the simultaneous detection of the variations of surface and solution species. This setup consists of a silicon wafer (with one side micromachined and the other side metallized), a thin-layer electrolyte structure with tunable thickness and flow rate, and a tilt-correction system based on laser collimation, enabling a well-controlled mass transport within the electrolyte layer and the spectral differentiation of solution species from adsorbates. Using acidic methanol oxidation on a Pt film electrode as a model system, besides SEIRA bands for adsorbed CO and formate intermediates, IR spectral signals for dissolved products CO₂, formic acid, and methyl formate can be readily identified for a quiescent electrolyte layer of ∼20 μm, which are otherwise undetected with conventional ATR-SEIRAS, as indicated by the trend of spectral features with increasing thickness or flow rate.

Bridge Sites of Au Surfaces Are Active for Electrocatalytic CO2 Reduction

Prior in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) studies of electrochemical CO2 reduction catalyzed by Au, one of the most selective and active electrocatalysts to produce CO from CO2, suggest that the reaction proceeds solely on the top sites of the Au surface. This finding is worth updating with an improved spectroelectrochemical system where in situ IR measurements can be performed under real reaction conditions that yield high CO selectivity. Herein, we report the preparation of an Au-coated Si ATR crystal electrode with both high catalytic activity for CO2 reduction and strong surface enhancement of IR signals validated in the same spectroelectrochemical cell, which allows us to probe the adsorption and desorption behavior of bridge-bonded *CO species (*COB). We find that the Au surface restructures irreversibly to give an increased number of bridge sites for CO adsorption within the initial tens of seconds of CO2 reduction. By studying the potential-dependent desorption kinetics of *COB and quantifying the steady-state surface concentration of *COB under reaction conditions, we further show that *COB are active reaction intermediates for CO2 reduction to CO on this Au electrode. At medium overpotential, as high as 38% of the reaction occurs on the bridge sites.

Understanding the complementarities of surface-enhanced infrared and Raman spectroscopies in CO adsorption and electrochemical reduction

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

Infrared and Raman spectroscopies are often assumed to provide similar insights into heterogeneous reaction mechanisms. This study shows that these techniques provide similar data when CO is strongly bound to a surface, yet distinct subpopulations of CO are probed when binding is weaker.

Electrolyte pH-dependent hydrogen binding energies and coverages on platinum, iridium, rhodium, and ruthenium surfaces

The weakened Hatop binding strength and increased Hatop coverage are universal phenomena on Pt, Ir, Rh, and Ru surfaces from acidic to alkaline media, which are important factors in the pH-dependent hydrogen reaction kinetics.

Accessing Organonitrogen Compounds via C-N Coupling in Electrocatalytic CO2 Reduction

Given the limited product variety of electrocatalytic CO₂ reduction reactions solely from CO₂ and H₂O as the reactants, it is desirable to expand the product scope by introducing additional reactants that provide elemental diversity. The integration of inorganic heteroatom-containing reactants into electrocatalytic CO₂ reduction could, in principle, enable the sustainable synthesis of valuable products, such as organonitrogen compounds, which have widespread applications but typically rely on NH₃ derived from the energy-intensive and fossil-fuel-dependent Haber-Bosch process for their industrial-scale production. In this Perspective, research progress toward building C-N bonds in N-integrated electrocatalytic CO₂ reduction is highlighted, and the electrosyntheses of urea, acetamides, and amines are examined from the standpoints of reactivity, catalyst structure, and, most fundamentally, mechanism. Mechanistic discussions of C-N coupling in these advances are emphasized and critically evaluated, with the aim of directing future investigations on improving the product yield and broadening the product scope of N-integrated electrocatalytic CO₂ reduction.

Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics

The production of molecular hydrogen by catalyzing water splitting is central to achieving the decarbonization of sustainable fuels and chemical transformations. In this work, a series of structure-making/breaking cations in the electrolyte were investigated as spectator cations in hydrogen evolution and oxidation reactions (HER/HOR) in the pH range of 1 to 14, whose kinetics was found to be altered by up to 2 orders of magnitude by these cations. The exchange current density of HER/HOR was shown to increase with greater structure-making tendency of cations in the order of Cs⁺ < Rb⁺ < K⁺ < Na⁺ < Li⁺, which was accompanied by decreasing reorganization energy from the Marcus-Hush-Chidsey formalism and increasing reaction entropy. Invoking the Born model of reorganization energy and reaction entropy, the static dielectric constant of the electrolyte at the electrified interface was found to be significantly lower than that of bulk, decreasing with the structure-making tendency of cations at the negatively charged Pt surface. The physical origin of cation-dependent HER/HOR kinetics can be rationalized by an increase in concentration of cations on the negatively charged Pt surface, altering the interfacial water structure and the H-bonding network, which is supported by classical molecular dynamics simulation and surface-enhanced infrared absorption spectroscopy. This work highlights immense opportunities to control the reaction rates by tuning interfacial structures of cation and solvents.

Hydrazine Detection during Ammonia Electro-oxidation Using an Aggregation-Induced Emission Dye

Ammonia electro-oxidation is an extremely significant reaction with regards to the nitrogen cycle, hydrogen economy, and wastewater remediation. The design of efficient electrocatalysts for use in the ammonia electro-oxidation reaction (AOR) requires comprehensive understanding of the mechanism and intermediates involved. In this study, aggregation-induced emission (AIE), a robust fluorescence sensing platform, is employed for the sensitive and qualitative detection of hydrazine (N₂H₄), one of the important intermediates during the AOR. Here, we successfully identified N₂H₄ as a main intermediate during the AOR on the model Pt/C electrocatalyst using 4-(1,2,2-triphenylvinyl)benzaldehyde (TPE-CHO), an aggregation-induced emission luminogen (AIEgen). We propose the AOR mechanism for Pt with N₂H₄ being formed during the dimerization process (NH₂ coupling) within the framework of the Gerischer and Mauerer mechanism. The unique chemodosimeter approach demonstrated in this study opens a novel pathway for understanding electrochemical reactions in depth.

In situ monitoring of the selective adsorption mechanism of small environmental pollutant molecules on aptasensor interface by attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS)

In situ monitoring of the interactions and properties of pollutant molecules at the aptasensor interface is being a very hot and interesting topic in environmental analysis since its charming molecule level understanding of the mechanism of environmental biosensors. Attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) provides a unique and convenient technique for the in situ analysis, but is not easy for small molecules. Herein, an ATR-SEIRAS platform has been successfully developed to in situ monitor the selective adsorption mechanism of small pollutant molecule atrazine (ATZ) on the aptasensor interface by characteristic N‒H peak of ATZ for the first time. Based on the constructed ATR-SEIRAS platform, a thermodynamics model is established for the selective adsorption of ATZ on the aptasensor interface, described with Langmuir adsorption with a dissociation constant of 1.1 nM. The adsorption kinetics parameters are further obtained with a binding rate constant of 8.08×10⁵ M^-1 s^-1. A promising and feasible platform has therefore successfully provided for the study of the selective sensing mechanism of small pollutant molecules on biosensors interfaces, further broadening the application of ATR-SEIRAS technology in the field of small pollutant molecules.

Determining intrinsic stark tuning rates of adsorbed CO on copper surfaces

This work reports a general and effective strategy of determining the intrinsic Stark tuning rate by removing the impact of the dynamical coupling of adsorbed CO on the Cu surface with surface enhanced infrared absorption spectroscopy (SEIRAS).

Stability of the ketyl radical as a descriptor in the electrochemical coupling of benzaldehyde

Electroreductive coupling is an emerging pathway for the renewable upgrading of biomass derived oxygenates. This work investigates electrochemical benzaldehyde reduction on Au, Cu, Pt and Pd using reactivity testing and in situ spectroscopy.

Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liquid side of the electrocatalytic interface. The electrocatalytic reduction of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liquid reaction environment poses a formidable experimental challenge. As a result, the molecular origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examined the chemical and electrostatic interactions of surface-adsorbed CO with its liquid reaction environment. Using a series of quaternary alkyl ammonium cations ([Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]), we systematically tuned the properties of this environment. With differential electrochemical mass spectrometry (DEMS), we show that ethylene is produced in the presence of [Formula: see text] and [Formula: see text] cations, whereas this product is not synthesized in [Formula: see text]- and [Formula: see text]-containing electrolytes. Surface-enhanced infrared absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. However, SEIRAS shows that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a critical molecular-level insight into how interactions of surface species with the liquid reaction environment control the selectivity of this complex electrocatalytic process.

Electro-Oxidation of CO Saturated in 0.1 M HClO4 on Basal and Stepped Pt Single-Crystal Electrodes at Room Temperature Accompanied by Surface Reconstruction

The electro-oxidation of CO on Pt surface is not only fundamentally important in electrochemistry, but also practically important in residential fuel cells for avoiding the poisoning of Pt catalysts by CO. We carried out cyclic voltammetry on Pt(111), (110), (100), (10 10 9), (10 9 8), (10 2 1), (432), and (431) single-crystal surfaces using a three compartment cell to understand the activity and durability towards the electro-oxidation of CO saturated in 0.1 M HClO4. During the potential cycles between 0.07 and 0.95 V vs. the reversible hydrogen electrode, the current for the electro-oxidation of CO at potentials lower than 0.5 V disappeared, accompanied by surface reconstruction. Among the electrodes, the Pt(100) electrode showed the lowest onset potential of 0.29 V, but the activity abruptly disappeared after one potential cycle; the active sites were extremely unstable. In order to investigate the processes of the deactivation, potential-step measurements were also conducted on Pt(111) in a CO-saturated solution. Repeated cycles of the formations of Pt oxides at a high potential and Pt carbonyl species at a low potential on the surface were proposed as the deactivation process.

Enhancement of Electrocatalytic Oxygen Reduction Activity and Durability of Pt-Ni Rhombic Dodecahedral Nanoframes by Anchoring to Nitrogen-Doped Carbon Support

Pt-based nanostructured electrocatalysts supported on carbon black have been widely studied for the oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells. Because sluggish ORR kinetics are known to govern the cell performance, there is a need to develop highly active and durable electrocatalysts. The ORR activity of Pt-based electrocatalysts can be improved by controlling their morphology and alloying Pt with transition metals such as Ni. Improving the catalyst durability remains challenging and there is a lack of catalyst design concepts and synthetic strategies. We report the enhancement of the ORR activity and durability of a nanostructured Pt-Ni electrocatalyst by strong metal/support interactions with a nitrogen-doped carbon (NC) support. Pt-Ni rhombic dodecahedral nanoframes (NFs) were immobilized on the NC support and showed higher ORR electrocatalytic activity and durability in acidic media than that supported on a nondoped carbon black. Durability tests demonstrated that NF/NC showed almost no activity loss even after 50 000 potential cycles under catalytic conditions, and the Ni dissolution from the NFs was suppressed at the NC support, as confirmed by energy dispersive X-ray spectroscopy analysis. Physicochemical measurements including surface-enhanced infrared absorption spectroscopy of surface-adsorbed CO revealed that the strong metal/support interactions of the NF with the NC support caused the downshift of the d-band center position of the surface Pt. Our findings demonstrate that tuning the electronic structure of nanostructured Pt alloy electrocatalysts via the strong metal/support interactions with heteroatom-doped carbon supports will allow the development of highly active and robust electrocatalysts.

Surface enhanced spectroscopic investigations of adsorption of cations on electrochemical interfaces

The adsorption of alkali and tetraalkylammonium cations on Pt is investigated using surface enhanced infrared absorption spectroscopy and carbon monoxide as a probe molecule. Alkali cations exhibit a stronger adsorption than organic cations, with potassium showing the strongest effect, followed by sodium and lithium.