Differential electrochemical mass spectrometry as well as online mass spectrometry in combination with singe cell testing has been used to study carbon corrosion of typical carbon materials discussed as support for electro-catalyst in fuel cells, e.g. carbon blacks or carbon nanotubes. Beside standard tests used to study the stability under automotive LT-PEMFC conditions, additional tests were performed to try to test the stability under the operating conditions of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). It was shown that under LT-PEMFC conditions a strong catalytic effect of platinum on the carbon corrosion rate is observable. High temperatures in HT-PEMFC do accelerate the corrosion rate. It was further found that corrosion does not only occur at high potentials but also to a minor amount at lower potentials. This low potential corrosion is in particular observed after a prior potential excursion of the electrode indicating that such excursions do not only lead to the direct corrosion of the support but also to the formation of unstable surface groups, which can be removed subsequently. Throughout all tests, CNT exhibited a higher stability than the tested carbon blacks. In single cell testing, the contribution of other carbon materials in particular the MPL was evaluated. It was found that it is present but small compared to the catalyst support itself. In order to reduce the expenditure of platinum group metals in polymer electrolyte membrane fuel cells the dispersion of the catalyst on a conductive support is the current state-of the art. Only few examples do not follow this approach. Most prominent among the exemptions is the Nano-Structured Thin Film (NSTF) type of catalyst developed by 3M.1 For most applications, carbon has become, however, a sort of standard support material. To accomplish the goal to reduce costs for the fuel cells, inexpensive carbon black materials like Vulcan XC72R from Cabot are used most often. However, as carbon in presence of water is thermodynamically unstable for potentials higher than 0.207 V 2 carbon corrosion can become an issue if the oxidation of the carbon is not strongly kinetically hindered e.g. via high degrees of graphitization. Consequently, carbon materials exhibiting such high degrees of graphitization like carbon nanotubes or graphene have attracted attention as alternative supports.The focus of the research on carbon corrosion is on LT-PEMFC for automotive application. With respect to carbon corrosion the reversed current decay mechanism proposed by Reiser et al.3 is an accepted cause for this type of degradation processes. This is confirmed e.g. in the review of PEMFC degradation processes published by Yu et al. 4 The process which is induced by the formation of a hydrogen oxygen front on the anode side upon hydrogen admission after longer standstill can cause potential excursions of the cathode side to potentials between 1.4 and 1.75 V.5 Accordingly, the common tests suites published by US Department of Energy (DoE) and the Fuel Ce...

The oxidation of ethanol and ethylene glycol at platinum as model catalyst was investigated in potassium hydroxide solution. Experiments comprised the investigation of formed adsorbates by stripping voltammetry of pre-adsorbed alcohols and the investigation of the bulk oxidation as function of the alcohol bulk concentration by cyclic voltammetry and chronoamperometry. For all measurements online mass spectrometry was used to detect the volatile reaction products. For both alcohols it was found that during stripping experiments only the C 1 products methane and CO 2 are formed. For the bulk oxidation of ethanol the CO 2 current efficiency rapidly decreases with increasing ethanol concentration showing hardly any CO 2 formation at technical relevant concentrations. Although CO 2 current efficiencies also dropped for the ethylene glycol oxidation with increasing concentration,the effect was much less pronounced. CO 2 formation was thus also observed at technical relevant ethylene glycol concentrations even after longer times under potentiostatic conditions indicating true bulk oxidation of ethylene glycol to CO 2 .

In this document, a newly designed electrochemical plug flow reactor system is presented. For a first preliminary test, ethanol in sulfuric acid was oxidized at a platinized titanium mesh electrode as working electrode of the reactor. Qualitative concentration behaviors were measured with mass spectrometric methods. A first result is that the reactor, in this configuration, seems to work properly.

The electro-oxidation of ethanol was investigated in alkaline solution (0.5 M ethanol and 0.1 M NaOH) on a polycrystalline platinum electrode. The electrode was positioned in a flow channel (channel electrode) which ensures controllable and defined convection conditions in the electrode surrounding. Moreover, a flow through UV-Vis absorbance cell was used for intermediate (acetaldehyde) detection and its quantification. It was experimentally observed that the intermediate current efficiency is a function of flow rate. Furthermore, the intermediate current efficiency increases with increasing surface poisoning during ethanol oxidation. From the experimental results it can be estimated that the intrinsic current efficiency for the pathway to acetaldehyde is at least 73(3)%. Hence, the current efficiency for pathways leading directly via surface reactions to other reaction products is at most 27(3)% (both are integral values over 15 min of oxidation at 0.6 V vs. RHE).

Polymer electrolyte membrane water electrolysers today mainly still use classic extrusion cast PFSA membranes of significant thickness such as Nafion® N117 or N115. In PEM fuel cells, much thinner solution cast membranes are used today in order to reduce ohmic resistance. In spite of the fact that reduced ohmic resistance would also benefit the goal to increase current density of operation for PEMEL so far it is not done for the sake of limiting hydrogen and oxygen cross-over. The necessity of this arises from the fact that hydrogen in oxygen concentrations exceeding certain limits can cause severe safety risk for the operation of PEMEL systems. An alternative approach to mitigate the issue is the introduction of recombination layers1, 2 which scavenge hydrogen and oxygen within the membrane and combine them back into water. In order to appropriately design such layer an online measurement of the hydrogen cross-over is required.In this contribution we will report on the use of online mass spectrometry to measure and quantify the hydrogen cross-over. The used set-up consists of a balticFuelCells cell holder with 4 cm2 cell with titanium flow fields as well as two pumps to supply water to anode and cathode compartment. Downstream of the cell on the anode side a water separator removes the remaining water from the product gas line. The capillary of a mass spectrometer with differential pump system is inserted into the exhaust gas line. Hydrogen concentration was measured by monitoring the m/z = 2 signal. For the calibration of the cell only a PTL was mounted into the cell holder. During the calibration measurement water was flown through the anode compartment while the anode side was fed with a 5% H2 in N2 mixture. The observed m/z = 2 signal was set for 5 vol% of hydrogen.For the tests, home-made CCMs were used with a Pt/C (JMFC) cathode layer and an IrO2 (Alfa Aesar) anode layer. A Freudenberg GDL was used as cathode PTL and a sintered Ti element as anode PTL. Tests were performed for CCM using Nafion® N115 or Nafion® XL electrolyte membrane. For some of the experiments a Nafion® overlayer was applied to the Nafion® XL to further determine the effect of Nafion® thickness. Also, a first test with a Pt black layer as recombination layer insert between the Nafion® XL membrane and the Nafion® overlayer was performed.Results reveal that the hydrogen cross-over for the Nafion® XL membrane is significantly higher than for the Nafion® N115 membrane as has to be expected. Already a rather thin Nafion® over cast to the Nafion® XL membrane can reduce the cross over to the Nafion® XL level without the need to achieve the full thickness of the Nafion® N115 membrane. The addition of the Pt layer is very effective to reduce H2 cross-over and will be further investigated. Of further importance is the strong spike of H2 cross over during current increase slopes which can be observed with this technique. These spikes bear the risk of the transient existence of flammable mixture. The observation is only possible due to the high ti...