Go with the flow: A novel spectroelectrochemical flow cell with well‐defined mass transport allows time‐resolved electrochemical and in situ FTIR spectroscopy measurements under continuous ...electrolyte flow (e.g. during electrolyte exchange). Its potential for mechanistic and kinetic studies was demonstrated in studies on the electrooxidation of formic acid.
► Glycerol oxidation kinetics/mechanism at Pt electrode under controlled electrolyte flow. ► Simultaneous detection of Faradaic current, volatile products (MS), adsorbed side products (IR). ► ...Reaction kinetics via spectro-electrochemical adsorption/reaction transients. ► Glyceraldehyde is an intermediate to COad formation and its further oxidation to CO2. ► Glyceric acid is a dead end in the electrochemical oxidation of glycerol.
Using a combined spectro-electrochemical DEMS/ATR-FTIRS set-up, we have investigated the adsorption/oxidation of glycerol on a Pt thin film electrode by potentiodynamic, potentiostatic, and adsorbate-stripping experiments. The potential and time dependent development of the glycerol adlayer was followed by highly surface sensitive in situ ATR-FTIR spectroscopy, while the volatile products formed during the glycerol adsorption/oxidation process were simultaneously monitored by online differential electrochemical mass spectrometry (DEMS). For the assignment of the detected glycerol adsorbates, comparative experiments were carried out, investigating the adsorption/oxidation of glyceraldehyde and glyceric acid. The data show that (i) during the adsorption/oxidation of glycerol, COad is formed in the potential range between 0.1 and 0.7 VRHE, which can be oxidized further to CO2 at potentials above 0.5V, that (ii) beside COad, adsorbed glycerol (at 0.3 and 0.4V) and glycerate (between 0.6 and 0.9V) are formed, that (iii) glyceric acid is a dead end in the electrochemical oxidation of glycerol, and that (iv) glyceraldehyde is an intermediate to COad formation.
The potential of in-situ Fourier transform infrared (FTIR) spectroscopy measurements in an attenuated total reflection configuration (ATR-FTIRS) for the evaluation of reaction pathways, elementary ...reaction steps, and their kinetics is demonstrated for formic acid electrooxidation on a Pt film electrode. Quantitative kinetic information on two elementary steps, formic acid dehydration and COad oxidation, and on the contributions of the related pathways in the dual path reaction mechanism are derived from IR spectroscopic signals in simultaneous electrochemical and ATR-FTIRS measurements over a wide temperature range (25−80 °C). Linearly and multiply bonded COad and bridge-bonded formate are the only formic acid related stable reaction intermediates detected. With increasing temperature, the steady-state IR signal of COad increases, while that of formate decreases. Reaction rates for COad formation via formic acid dehydration and for COad oxidation as well as the activation energies of these processes were determined at different temperatures, potentials, and surface conditions (with and without preadsorbed CO from formic acid dehydration) from the temporal evolution of the IR intensities of COad during adsorption/reaction transients, using an IR intensity−COad coverage calibration. At potentials up to 0.75 V and temperatures from 25 to 80 °C, the “indirect” CO pathway contributes less than 5% (at potentials ≤0.6 V significantly below 1%) to the total Faradaic reaction current, making the “direct” pathway by far the dominant one under the present reaction conditions. Much higher activation energies for COad formation and COad oxidation compared with the effective activation energy of the total reaction, derived from the Faradaic currents, support this conclusion.
Fuel cell electric vehicles (FCEVs) are a possible sustainable and long-term solution to satisfy the demand for mobility. Cost targets are achieved by lowering platinum (Pt) loadings on the ...electrodes and using thinner membranes in the fuel cell system (FCS), maximizing the current density. Due to these measures, the FCS becomes more sensitive to contaminants, including those originating from crossover through the membrane. One of these contaminants - carbon dioxide (CO2) - causes effects that can be reversed, however, this process negatively influences the FCS lifetime. For this reason, operating conditions should be chosen such that minimal temporary losses are caused without causing further negative effects, such as degradation, in the FCS. This paper shows the effects of CO2 and O2 permeation occurring during operation of an FCS and proposes the possibility of mitigating the crossover via the right choice of membrane.
•Carbon dioxide crosses over from cathode to anode depending on operating conditions.•The choice of membrane type and thickness is important to crossover.•The crossover is dependent on pressure ≫ relative humidity > temperature.•Hydrocarbon membranes have lower gas permeability properties.•Hydrocarbon membranes are preferable to minimize CO2 permeation.
A new membrane electrode assembly set up for catalytic processes containing carbon nanotubes has been developed. The process includes the nanotube synthesis, sputter deposition of platinum as ...catalyst and the membrane casting. Aligned nanotube carpets were grown from toluene/ferrocene solutions and sputtered with platinum. Subsequently the assembly was investigated using cyclic voltammetry to confirm a sufficient catalyst activity. A procedure was developed to embed the carbon nanotubes doped with catalyst into SPEEK membranes, while preserving the aligned structure and keeping some surface area of the catalyst-doped nanotubes free of membrane material to allow for easy access to reactants. So far the best results were obtained using an aligned but somewhat loose nanotube structure and a deposition of 0.034
mg/cm
2 Pt, forming a combination of small catalyst clusters and a thin film. The assemblies are optimized in respect to application in fuel cells and functional membranes.
The influence of ammonium ions on the activity and selectivity of the electrocatalytic oxygen reduction reaction (ORR) on polycrystalline Pt was investigated in model studies under continuous mass ...transport, both in sulfuric and perchloric acid solutions. Ammonium was found to increase the yield of hydrogen peroxide, particularly in sulfuric acid, but also in perchloric acid solutions, and also at higher potentials (0.80–0.90
V
RHE) typical for fuel cell cathode operation, which may severely impair the long-term stability of membranes and electrodes in fuel cells exposed to fuel gases and/or air containing ammonia. Adsorbed species, assigned to ammonia and nitric oxide, were identified on a Pt film electrode using
in situ FTIR spectroscopy. Adsorbed nitric oxide could only be observed in perchloric acid solutions. The higher coverage of adsorbed ammonia in sulfuric acid solution is attributed to a stabilization by coadsorbed (bi-)sulfate species; the higher total coverage in this electrolyte can explain the larger effect of ammonium ions on the ORR activity and selectivity in sulfuric compared to perchloric acid solution.
Quantum mechanical calculations of reaction probabilities for the ion−neutral molecule collisions H- + H2 ↔ H2 + H- are performed by means of time-dependent wave packet propagation. Results for two ...different potential energy surfaces (ab initio and diatomics-in-molecules) are compared. The calculated state-to-state reaction probabilities using product-Jacobi-coordinates are compared with energy resolved reaction probabilities calculated with the flux-operator using reactant-Jacobi-coordinates and with time-independent calculations. The shallow potential well of ca. 0.05 eV leads to some resonances in the reaction probabilities. In addition, we present results for integral cross sections using the J-shifting method.
During the past decade research into the German Final Paleolithic and Mesolithic has experienced an important revival. One clear sign of this renewed interest in the periods are the annual meetings ...of the "Arbeitsgruppe Mesolithikum" (Mesolithic Working Group) which have taken place every spring since 1992. At these meetings, which take place at changing venues, topical themes of Final Paleolithic and Mesolithic interest are presented by informal lectures and it is also possible to study regional collections (artifacts, raw materials) at first hand. Numerous contributions were subsequently published together in one volume (Conard and Kind (1998) Aktuelle Forschungen zum Mesolithikum/Current Mesolithic Research, Mo Vince, Tübingen). The present paper intends to complement that collection of papers with a synthesis of developments and perspectives and to present recent research highlights in the German Final Paleolithic and Mesolithic, together with a comprehensive bibliography, to a wider international audience.
PDF
