Quantitative SNIFTIRS studies of (bi)sulfate adsorption at the Pt(111) electrode surface

Subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was applied to study (bi)sulfate adsorption on a Pt(111) surface in solutions of variable pH while maintaining a constant total bisulfate/sulfate ((bi)sulfate) concentration without the addition of an inert supporting electrolyte. The spectra were recorded for both the p- and s-polarizations of the IR radiation in order to differentiate between the IR bands of the (bi)sulfate species adsorbed on the electrode surface from those species located in the thin layer of electrolyte. The spectra recorded with p-polarized light consist of the IR bands from both the species adsorbed at the electrode surface and those present in the thin layer of electrolyte between the electrode surface and ZnSe window whereas the s-polarized spectra contain only the IR bands from the species located in the thin layer of electrolyte. A new procedure was developed to calculate the angle of incidence and thickness of the electrolyte between the Pt(111) electrode surface and the ZnSe IR transparent window. By combining these values with the knowledge of the optical constants for Pt, H(2)O and ZnSe, the mean square electric field strength (MSEFS) at the Pt(111) electrode surface and for thin layer of solution were accurately calculated. The spectra recorded using s-polarization were multiplied by the ratio of the average MSEFS for p- and s-polarizations and subtracted from the spectra recorded using p-polarization in order to remove the IR bands that arise from the species present within the thin layer cavity. In this manner, the resulting IR spectra contain only the IR bands for the anions adsorbed on the Pt(111) electrode surface. The spectra of adsorbed anions show little change with respect to the pH ranging from 1 to 5.6. This behavior indicates that the same species is predominantly adsorbed on the metal surface for this broad range of pH values and the results suggest that sulfate is the most likely candidate for this adsorbate.     

Heavy phosphate adsorption on amorphous ITO film electrodes: nano-barrier effect for highly selective exclusion of anionic species

We prepared an amorphous indium tin oxide (ITO) film and studied it with respect to its surface characterization and the effect of phosphate adsorption on its electrochemical properties. The film was deposited using RF sputtering under ambient low-oxygen conditions at room temperature. The XPS results revealed that the amount of phosphate adsorbed on the amorphous ITO film was more than 4.6 times greater than that adsorbed on commercially available polycrystalline ITO film in spite of the smaller microscopic surface area of the former. Electrochemical responses for anionic species such as L-ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) on the phosphate-adsorbed ITO film electrodes were more effectively suppressed at the amorphous ITO film electrode than at the polycrystalline ITO film electrode when a phosphate-containing electrolyte was used. Such suppression could be attributed to the electrostatic repulsion between the anionic species and more heavily adsorbed phosphate on our amorphous ITO film electrode surface. This effect is made more pronounced by increasing the phosphate concentration to 1 mM. With 1 mM phosphate, the amorphous ITO film electrode showed the highest selectivity for dopamine (DA) against the anionic species, namely, 880 for DA/AA and 330 for DA/DOPAC, respectively. In contrast, the selectivity was 120 for DA/AA and 20 for DA/DOPAC with the polycrystalline ITO film electrode.     

Electorchemical Studies of Cytochrome c on Electodes Modified by Single—Wall Carbon Naotubes

Single-wall carbon nanotubes(SWNTs) modified gold electrodes were prepared by using two different methods.The electrochemical behavior of cytochrome c on the modified gold electrodes was investigated.The first kind of SWNT-modified electrode (noted as SWNT/Au electrode)was prepared by the adsorption of carboxylterminated SWNTs from DMF dispersion on the gold electrode.The oxidatively processed SWNT tips were covalently modified by coupling with amines (AET) to form amide linkage.Via Au-S chemical bonding,the self-assembled monolayer of thiol-unctionalized nanotubes on gold surface was fabricated so as to prepare the others SWNT-modified electrode (noted as SWNT/AET/Au electrode).It was shown from cyclic voltammetry cxperiments that cytochrome c exhibited direct electrochemical responses on the both electrodes, but only the current of controlled diffusion existed on the SWNT/Au electrode while both the currents of controlled diffusion and adsorption of cytochrome c occurred on the SWNT/AET/Au electrode.Photoelastic Modulation Infared Reflection Absorpthion Spectroscopy (PEM-IRRAS) and Quartz Crystal Microbalance (QCM) were employed to verify the adsorption of SWNTs on the gold electrodes.The results proved that SWNTs could enhance the direct electron transfer proecss between the electrodes and redox proteins.     

Co‐adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single‐Crystal Electrode

The successful deployment of advanced energy‐conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl *OH) at electrified metal–aqueous electrolyte interfaces. The effect of alkali metal cations (Li⁺, Na⁺, K⁺, Cs⁺) on the non‐Nernstian pH shift of the step‐related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. The co‐adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step‐related peak on Pt(553). Density functional calculations explain the observations on the identity and concentration of alkali cations on the non‐Nernstian pH shift, and demonstrate that cation–hydroxyl co‐adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.     

铂电极上苯和苯磺酸吸附定向的现场红外反射吸收光谱电化学方法研究

本文用现场红外反射吸收光谱电化学方法和循环伏安法研究铂电析上苯和苯磺酸的吸附定向。对于苯/铂势系, 电势在-0.6至0.0V(相对饱和甘汞电极)内, 苯主要以垂直方式吸附; 在0.0至0.8V内则主要以平躺方式吸附。对于苯磺酸/铂体系,电势在-0.4V至0.0V内, 苯磺酸分子中的苯环主要呈垂直吸附且SO~3H基团远离电极表面; 在0.0至1.0V内则主要以倾斜平躺方式吸附, SO~3H基团通过其中的两个氧原子吸附于电极表面上。     

Anion-induced adsorption of ferrocenated nanoparticles

Au nanoparticles fully coated with omega-ferrocenyl hexanethiolate ligands, with average composition Au225(omega-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing submonolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and there exhibit stable ferrocene voltammetry over long periods of time. The kinetics of forming the robustly adsorbed layer are slow; monolayer and submonolayer deposition can be described by a rate law that is first order in nanoparticle concentration and in available electrode surface. The adsorption mechanism is proposed to involve entropically enhanced (multiple) ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by scanning to positive potentials (through the ferrocene wave) and by high concentrations of Bu4N+ X- electrolyte (X- = ClO4(-), PF6(-)) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is not appreciably diminished by the adsorbed ferrocenated nanoparticles, which are gradually desorbed by scanning to potentials more negative than the electrode's potential of zero charge. At very slow scan rates, voltammetric current peaks are symmetrical and nearly reversible, but exhibit E(fwhm) considerably narrower (typically 35 mV) than ideally expected (90.6 mV, at 298 K) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is qualitatively explicable by a surface-activity effect invoking large, attractive lateral interactions between nanoparticles and, or alternatively, by a model in which ferrocene sites react serially at formal potentials that become successively altered as ion-pair bridges are formed. At faster scan rates, both deltaE(peak) and E(fwhm) increase in a manner consistent with a combination of uncompensated ohmic resistance of the electrolyte solution and of the adsorbed film, as distinct from behavior produced by slow electron transfer.     

Pd/Pt二元合金电极的制备及氧还原性能

本文利用欠电位沉积亚单层的Cu及Pt置换取代Cu的方法, 制备了具有不同表面元素组成的Pd/Pt二元合金电极(用Pd/Ptx表示, x指欠电位沉积Cu-Pt置换取代Cu过程的次数),并对其表面元素组成、氧还原性能进行了表征. 在控制欠电位沉积Cu的下限电位恒定(0.34 V)的前提下, 表面Pt/Pd的元素组成比通过重复欠电位沉积Cu及Pt置换取代Cu的次数(1~5次)来可控地调变. 光电子能谱(XPS) 以及红外光谱实验表明,Pd/Ptx电极表层区的Pt：Pd元素组成比随着Pt沉积次数增加而增加, 对Pd/Pt4电极, 在电极表层区约2~3 nm内的Pt/Pd的原子比大约是1:4,而最表层裸露Pd原子的比例仍在20%以上。循环伏安结果显示, 随着Pt沉积次数的增加(1-5次), Pd/Ptx电极表面越不易被氧化。氧还原测试结果显示随着Pt沉积次数的增加(1~4次), Pd/Ptx二元金属电极的氧还原活性依次增加, 经过第3次沉积后其氧还原活性已优于纯Pt,而经4次以上沉积,其氧还原活性基本不变。在其它反应条件相同条件的前提下, Pd/Pt4电极上氧还原的半波电位与纯Pt相比右移约25 mV。结合本文与文献的实验结果,我们初步认为Pd/Ptx二元金属体系氧还原性能改善主要源自表层Pd原子导致其邻近的Pt原子上含氧物种吸附能的降低.     

The Direct Electrochemical Detection of Amino Acids at a Platinum Electrode in an Alkaline Chromatographic Effluent

Abstract

It is the general experience that most organic compounds including amino acids do not produce reversible or even quasi-reversible anodic waves at a Pt electrode under conditions of conventional cyclic voltammetry. Furthermore, amperometric detection of these compounds at a constant electrode potential is not successful because of the accumulation of adsorbed reaction products and/or an oxide film at the electrode surface. However, it is observed that a Pt electrode surface is cleaned quite effectively of adsorbed organic molecles and radicals simultanaeously with the anodic formation of the oxide layer. This oxidation of adsorbed organic species is concluded to be electrocatalyzed by PtOH formed as the first step in the production of the oxide layer (PtO). A pulsed-potential waveform applied at a frequency of ca. 1 Hz is demonstrated to provide direct amperometric detection of adsorbed amino acids at a Pt electrode. Satisfactory analytical precision (i.e., < 3% rel. std. dev.) results because the waveform reproducibly generates the catalytically active surface state at the Pt electrode. Both primary and secondary amino acids are determined with satisfactory detection limits: e.g., ca. 13 ng for glycine, 7 ng for phenylamine and 23 ng for hydroxyproline in 50-μL samples. Analytical response is concluded to depend on the adsorption isotherm of the amino acid being detected. Hence, the calibration plot of I/I_peak vs. 1/C^D is linear for low surface coverages. Results are shown for amperometric detection of a synthetic mixture of amino acids by anion-exchange chromatography using NaOH as the eluent and supporting electrolyte.     

铂电极上锑吸附原子对1,2-丙二醇电催化氧化性能的影响

运用电化学循环伏安(CV)和电化学原位石英晶体微天平(EQCM)研究了Pt电极表面不可逆吸附Sb原子的电化学特性以及Pt电极上Sb吸附原子对0.1mol·L-1H2SO4溶液中1,2 丙二醇电催化氧化性能的影响.研究发现,当扫描电位的上限Eu≤0.50V(SCE)时,Sb可以稳定地吸附在Pt电极表面,饱和覆盖度为0.34;通过控制电位扫描上限和扫描圈数剥离部份Sb可方便地得到Sbad的不同覆盖度;Pt电极表面Sb吸附原子能在较低的电位下吸附氧,可显著提高1,2 丙二醇电催化氧化活性.与Pt电极相比较,Sb饱和吸附原子修饰的Pt电极使1,2 丙二醇氧化的峰电流增加了近2倍.作者还从表面质量变化提供了吸附原子电催化作用的新数据.     

Sum-frequency generation of acetate adsorption on Au and Pt surfaces: molecular structure effects

The reversible adsorption of acetate on polycrystalline Au and Pt surfaces was investigated with broadband sum-frequency generation (SFG) and cyclic voltammetry. Specifically adsorbed acetate as well as coadsorbed sulfuric acid anions are observed for the first time with SFG and give rise to dramatically different SFG intensities on Au and Pt surfaces. While similar coverages of acetate adlayers on Au and Pt surfaces are well established by previous studies, an identification of the interfacial molecular structure has been elusive. However, we have applied the high sensitivity of SFG for interfacial polar ordering to identify different acetate structures at Au and Pt surfaces in contact with HClO(4) and H(2)SO(4) electrolytes. Acetate competes with the formation of surface oxides and shifts the oxidation threshold of both Au and Pt electrodes anodically. Effects of the supporting electrolyte on the formation of acetate adlayers are revealed by comparing SFG spectra in HClO(4) and H(2)SO(4) solutions: Sulfuric acid anions modify the potential-dependent acetate adsorption, compete with adsorbed acetate on Au and coadsorb with acetate on Pt surfaces.     

Interfacial Structure of Water as a New Descriptor of the Hydrogen Evolution Reaction

Driven by the persisting poor understanding of the sluggish kinetics of the hydrogen evolution reaction (HER) on Pt in alkaline media, a direct correlation of the interfacial water structure and activity is still yet to be established. Herein, using Pt and Pt–Ni nanoparticles we first demonstrate a strong dependence of the proton donor structure on the HER activity and pH. The structure of the first layer changes from the proton acceptors to the donors with increasing pH. In the base, the reactivity of the interfacial water varied its structure, and the activation energies of water dissociation increased in the sequence: the dangling O−H bonds < the trihedrally coordinated water < the tetrahedrally coordinated water. Moreover, optimizing the adsorption of H and OH intermediates can re-orientate the interfacial water molecules with their H atoms pointing towards the electrode surface, thereby enhancing the kinetics of HER. Our results clarified the dynamic role of the water structure at the electrode–electrolyte interface during HER and the design of highly efficient HER catalysts.     

Single-walled Carbon Nanotubes as Electrode Materials for Supercapacitors

Large-scale synthesized single-walled carbon nanotubes （SWNT） prepared by electric arc discharge method and a mixture of NiO and Y2O3 as catalyst have been used as electrode materials for supercapacitors. N2 adsorption/desorption measurement shows that the SWNT is a microporous and mesoporous material with specific surface area 435 m^2·g^-1. The specific capacitance of the nitric acid treated SWNT in aqueous electrolyte reaches as high as 105 F/g, which is a combination of electric double layer capacitance and pseudocapacitance. The SWNT-based capacitors also have good charge/discharge reversibility and cycling perdurability.     

Electrochemical detection of amino acids at carbon nanotube and nickel-carbon nanotube modified electrodes

The oxidation and enhanced detection of traditionally 'non-electroactive' amino acids at a single-wall carbon nanotube (SWNT) surface and at a nickel hydroxide film electrochemically deposited and generated upon the SWNT layer is reported. Different CNT are compared, with Nafion-dispersed SWNT offering the most favorable layer for constant-potential amperometric detection. Factors affecting the oxidation process, including the pH or applied potential, are assessed. The response of the SWNT-Nafion coated electrode compares favorably with that of copper and nickel disk electrodes under flow injection analysis (FIA) conditions. The electrodeposition of nickel onto the SWNT-Nafion layer (Ni-CNT) led to a dramatic enhancement of the analytical response (vs. that observed at the SWNT or nickel electrodes alone). The oxidative process at the Ni(OH)(2) layer has been studied and the increase in sensitivity rationalized. In the presence of amino acid the Ni-CNT layer undergoes an electrocatalytic process in which the amino acid reduces the newly formed NiO(OH) species. Furthermore, the attractive response of both the CNT and Ni-CNT layers has allowed these electrodes to be used for constant-potential FIA of various amino acids and indicates great promise for monitoring chromatographic effluents. Once again an improved signal was observed at the Ni-CNT electrode compared to nickel deposited upon a bare glassy carbon electrode (Ni-GC).     

Voltammetric and radioactive labeling studies of single crystal and polycrystalline rhodium electrodes in sulfate-containing electrolytes

Adsorption of sulfate/bisulfate anions on single crystal Rh(111) (ordered/disordered) and polycrystalline rhodium electrodes in perchloric acid solution was studied by the use of cyclic voltammetry and the radioactive labeling method. The ordered Rh(111) surface was characterized by LEED and Auger spectroscopy to verify its well-defined character. Details of the surface chemistry of the anions interacting with the three rhodium substrates are different. This is considered as evidence that a long-range order of the metal electrodes alters the structure of chemisorbed hydrogen and oxygen significantly and that this structure is affected by the reversibly interacting species. Likewise, the adsorbed layer of sulfate anions on Rh(111) is more stable than that on other rhodium surfaces due to a favorable spatial configuration of the anions and surface water molecule network, which are proposed to be cross linked by hydrogen bonding. Overall, the adsorption reversibility of sulfate was confirmed with respect to the bulk concentration of the adsorbate and the electrode potential (provided that the surface oxidation is avoided). Radiochemical data reveal that, in the studied bulk concentration range (up to about 10⁻³ M), the uptake of anions by Rh(111) is limited to ca. 40% of the theoretical maximum coverage. The interaction of sulfate with Pt(111), and corresponding voltammetry in perchloric acid electrolyte are also brought into focus. It is concluded that only a part of the Pt(111) “butterfly” can be accounted for by adsorption of high energy hydrogen. No high energy hydrogen has been found to exist on Rh(111), as inferred earlier in our laboratory.     

纳米碳管作为氧扩散电极催化层第二相碳载体的电极特性

A novel gas diffusion electrode using binary carbon supports (carbon nanotubes and active carbon) as the catalyst layer was prepared. The electrochemical properties for oxygen reduction reaction (ORR) in alkaline electrolyte were investigated by polarization curves and electrochemical impedance spectroscopy. The results show that the binary-support electrode exhibits higher electrocatalytic activity than the single-support electrode, and the best performance is obtained when the mass ratio of carbon nanotubes and activated carbon is 50 ∶50. The results from their electrode kinetic parameters indicate that the introduction of carbon nanotubes as a secondary support provides high accessible surface area, good electronic conductivity and fast ORR kinetics. The electrocatalytic activity of binary-support electrodes is obviously improved by the deposition of Pt nanoparticles on carbon nanotubes, even at very low Pt loading (45.7 μg/cm2). In addition, the EIS analysis results show that the process of ORR may be controlled by diffusion of oxygen in the thin film for binary-support electrodes with or without Pt catalyst.     

Preparation of Indium Tin Oxide Nanoparticle‐modified 3‐Aminopropyltrimethoxysilane‐functionalized Indium Tin Oxide Electrode for Electrochemical Sulfide Detection

A sulfide sensor based on an indium tin oxide nanoparticle (ITONP)‐modified ITO electrode was developed. To prepare ITONP‐modified ITO, various methods were tested. Drop‐drying of ITONPs (aq.) on aminopropyltrimethoxysilane‐functionalized ITO (APTMS/ITO) was found to be the best method on the basis of voltammetric analysis of the sulfide ion. ITONP‐modified APTMS/ITO (ITONP/APTMS/ITO) yielded much better electrocatalytic properties toward sulfide electro‐οxidation than did bare or APTMS/ITO electrodes. The ITONPs and ITONP‐modified ITO were also characterized using transmission electron microscopy and field emission scanning electron microscopy, respectively. Optimization of the type of inert electrolyte and pH yielded an ITONP/APTMS/ITO detector whose amperometrically and chronocoulοmetrically determined limits of detection for sulfide were 3.0 μM and 0.90 μM, respectively. ITONP/APTMS/ITO electrodes displayed reproducible performance, were highly stable, and were not susceptible to interference by common contaminants. Thus, the developed electrode can be considered as a promising tool for sensing sulfide.     

Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.     

Observation of Ion Electrosorption in Metal–Organic Framework Micropores with In Operando Small‐Angle Neutron Scattering

A molecular‐level understanding of transport and adsorption mechanisms of electrolyte ions in nanoporous electrodes under applied potentials is essential to control the performance of double‐layer capacitors. Here, in operando small‐angle neutron scattering (SANS) is used to directly detect ion movements into the nanopores of a conductive metal–organic framework (MOF) electrode under operating conditions. Neutron‐scattering data reveals that most of the void space within the MOF is accessible to the solvent. Upon the addition of the electrolyte sodium triflate (NaOTf), the ions are adsorbed on the outer surface of the protrusions to form a 30 Å layer instead of entering the ionophobic pores in the absence of an applied charging potential. The changes in scattering intensity when potentials are applied suggests the ion rearrangement in the micropores following different mechanisms depending on the electrode polarization. These observations shed insights on ion electrosorption in electrode materials.     

SERS and FT-IR studies of CO adsorbed on underpotential deposited Ag/Pt electrodes

CO adsorbed on UPD and OPD (under- and overpotential deposited) Ag layers on a Pt electrode surface was studied by SERS and IRRAS in conjunction with cyclic voltammetry. Electrochemical activation of a uniform UPD Ag adlayer produced Ag clusters on the Pt electrode as well as bare Pt sites. The strong adsorption of CO on the UPD Ag/Pt electrode compared with a bulk Ag electrode is explained by the influence of the substrate Pt atoms. The degree of electron back-donation to CO increases the degree of lower frequency shifts of CO on the electrodes in the order Pt electrodes < monolayer Ag/Pt < multilayer Ag/Pt.     

Metals and alloys bonded on solid polymer electrolyte for electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte

Metals and alloys bonded on the solid polymer electrolyte (SPE) Nafion® 117 were studied as both the electrodes and electrolyte for the electrochemical reduction of pure benzaldehyde without liquid supporting electrolyte. The results indicated that SPE electrodes modified with metals such as Pt, Ni, Pb, Cu and Ag by the ion exchange chemical deposition method had a more stable structure and could provide a larger electrochemical active surface area than those prepared by other methods. The composition of reducing agents and the pH value have a significant effect on the characteristics of the prepared SPE electrodes. In this study a novel method was developed to prepare a Pt+Pb/Nafion® electrode which formed a protective Pt layer on the surface of Pb/Nafion®. The results of scanning electron microscopy further confirmed that Pt+Pb/Nafion® electrodes had obvious advantages for the electrochemical reduction of benzaldehyde. The results also revealed that the current efficiencies of benzylalcohol production at various SPE electrodes decreased in the order Pt+Pb/Nafion® > Pb/ Nafion® > Ni/Nafion® > Cu/Nafion® > Ag+Cu/Nafion® > Ag/Nafion® > Pt/Nafion®.