Basic model for membrane electrode assembly design for direct methanol fuel cells

This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density.     

How platinum oxide affects the degradation analysis of PEM fuel cell cathodes

In this work, proton exchange membrane fuel cell cathodes are degraded with accelerated-stress-tests.These PtCo containing cathodes are analyzed at begin-of-life and end-of-test with a dedicated diagnostic procedure. For every individual load point, the oxygen transport resistance and voltage losses due to the formation of platinum oxides were obtained in addition to commonly measured electrochemical surface area, high frequency resistance, as well as cathode ionomer resistance. These data were used to break down the voltage losses into six different contributors. With this break down, performance gains and performance losses were determined at end-of-test. At low current densities, it was found that voltage losses due to degradation are dominated by the loss of specific activity and catalyst surface area - in line with the state-of-the-art knowledge. But by quantifying the losses from platinum oxide formation explicitly, we show that end-of-test an unassigned voltage loss is not only present at highest current densities, but already at low current density. More precisely, the unassigned voltage loss shows a linear increase with decreasing half cell voltage and is independent from the chosen accelerated stress test. As this unassigned loss depends on half cell voltage, it might arise from ionomer adsorption.     

Low and high temperature storage characteristics of membrane electrode assemblies for direct methanol fuel cells

This paper investigates changes in the performance of membrane electrode assemblies (MEAs) of Direct Methanol Fuel Cells (DMFC) that are caused by undergoing storage at −10 °C and 60 °C under different experimental conditions. Storage at 60 °C exhibited negative effects on an MEA’s performance only when storing the MEA at a 4 M CH₃OH solution. Here, application of a reverse current for 10 s was found to reinstall the original performance. The effect of storage at −10 °C on an MEA’s performance strongly depends upon the MEA’s properties. MEAs are grouped into three different categories with regard to their suitability for low temperature storage: not affected, temporarily affected, irreversibly affected. The temporarily affected MEAs could be instantly and completely reactivated by a reverse current. Changes in the MEA properties that had been caused by being stored at −10 °C were investigated for two MEAs using electrochemical methods, scanning electron microscopy and porosity measurements. The following MEA materials and manufacturing methods had been found to be principally suitable to build MEAs tolerant to storage at −10 °C: the manufacturing methods CCM (catalyst coated on the membrane) and CCS (catalyst coated on the substrate), several hydrocarbon membranes, high Pt and Pt-Ru catalyst loadings. By carefully selecting the proper MEA material, MEAs with tolerance towards low and high storage temperatures can be designed.     

Asymmetric Synthesis and Biological Evaluation of Glycosidic Prodrugs for a Selective Cancer Therapy

A severe limitation in cancer therapy is the often insufficient differentiation between malign and benign tissue using known chemotherapeutics. One approach to decrease side effects is antibody‐directed enzyme prodrug therapy (ADEPT). We have developed new glycosidic prodrugs such as (?)‐(1S)‐ 26 b based on the antibiotic (+)‐duocarmycin SA ((+)‐ 1 ) with a QIC₅₀ value of 3500 (QIC₅₀=IC₅₀ of prodrug/IC₅₀ of prodrug+enzyme) and an IC₅₀ value for the corresponding drug (prodrug+enzyme) of 16 pM . The asymmetric synthesis of the precursor (?)‐(1S)‐ 19 was performed by arylation of the enantiomerically pure epoxide (+)‐(S)‐ 29 (≥98 % ee).     

Systematic analysis of the direct methanol fuel cell

The dynamic operating behaviour of the direct methanol fuel cell (DMFC) is governed by several physico-chemical phenomena which occur simultaneously: double layer charging, electrode kinetics, mass transport inside the porous structures, reactant distributions in the anode and cathode flowbeds etc. Therefore it is essential to analyse the interactions of these phenomena in order to fully understand the DMFC. These phenomena were initially analysed independently by systematic experiments and model formulations. Electrode kinetics were determined by fitting models of varying complexity to electrochemical impedance spectroscopy (EIS) measurements. Reaction intermediates adsorbed on the catalyst seem to play a key role here. To describe mass transport across the DMFC a one-dimensional model was formulated applying the generalised Maxwell–Stefan equations for multi-component mass transport and a Flory–Huggins model for the activities of mobile species inside the membrane (PEM). Also swelling of the PEM as well as heat production and transport were considered. Finally, the anode flowbed was analysed by observing flow patterns in different flowbed designs and measuring residence time distributions (RTDs). Detailed CFD models as well as simpler CSTR network representations were used to compare to the experimental results. Even the simpler models showed good agreement with the experiments. After these investigations the results were combined: the electrode kinetics model was implemented in the mass transport model as well as in the CSTR network flowbed model. In both cases, good agreement, even to dynamic experiments, was obtained.     

Modelling,dynamics and control of a portable DMFC system

A dynamic model for a direct methanol fuel cell and its ancillary units is presented, in which all ancillary units perform only one operation each. The system’s losses and main dynamics (cathodic oxygen fraction, anodic methanol concentration, stack temperature, system water holdup) are analysed for stability and time constants. The system is found to be stable in all of its dynamics except for that of water holdup. The influence of external conditions, such as temperature and humidity, on system feasibility is analysed; the capability of system autonomous operation depends essentially on environmental conditions and on the chosen air excess ratio. Decoupled single-input, single-output controllers, some of which employing feedback, are applied to maintain the system at a certain set point. System simulations are performed, confirming the performance of the proposed controllers, their ability to stabilise the water holdup, and the absence of interaction-induced oscillations; the system can be started up in about ten minutes with the presented parameters.     

The Effect of Correlated Kinetic Parameters on (Bio)Chemical Reaction Networks

Exploiting the information provided by (bio)chemical reaction networks has proved beneficial for process analysis and design. To this end, parameter uncertainties have to be included in the analysis and design of (bio)chemical processes to ensure reliable model‐based results. The goal is to investigate the impact of parameter correlations on (bio)chemical reaction networks and parameter sensitivities. An efficient sensitivity analysis concept is demonstrated with two simulation studies, and the results indicate a significant impact of the parameter correlations on the derived parameter sensitivities and the model‐based results in general.     

The Development of Neuroendocrine Disturbances over Time: Longitudinal Findings in Patients after Traumatic Brain Injury and Subarachnoid Hemorrhage

Previous reports suggest that neuroendocrine disturbances in patients with traumatic brain injury (TBI) or aneurysmal subarachnoid hemorrhage (SAH) may still develop or resolve months or even years after the trauma. We investigated a cohort of n = 168 patients (81 patients after TBI and 87 patients after SAH) in whom hormone levels had been determined at various time points to assess the course and pattern of hormonal insufficiencies. Data were analyzed using three different criteria: (1) patients with lowered basal laboratory values; (2) patients with lowered basal laboratory values or the need for hormone replacement therapy; (3) diagnosis of the treating physician. The first hormonal assessment after a median time of three months after the injury showed lowered hormone laboratory test results in 35% of cases. Lowered testosterone (23.1% of male patients), lowered estradiol (14.3% of female patients) and lowered insulin-like growth factor I (IGF-I) values (12.1%) were most common. Using Criterion 2, a higher prevalence rate of 55.6% of cases was determined, which correlated well with the prevalence rate of 54% of cases using the physicians’ diagnosis as the criterion. Intraindividual changes (new onset insufficiency or recovery) were predominantly observed for the somatotropic axis (12.5%), the gonadotropic axis in women (11.1%) and the corticotropic axis (10.6%). Patients after TBI showed more often lowered IGF-I values at first testing, but normal values at follow-up (p < 0.0004). In general, most patients remained stable. Stable hormone results at follow-up were obtained in 78% (free thyroxine (fT4) values) to 94.6% (prolactin values).