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A new concept that enables fully passive CO 2 gas bubble removal in micro direct methanol fuel cells (µDMFCs) is presented. The original concept behind the presented degassing structure (flowfield) is based on microchannels with a T-shaped cross section. These channels have defined tapering angles over their cross section (α) and along their axis (β). The tapered channel design creates an intrinsic transport mechanism that removes the gas bubbles from the electrodes by capillary forces only. Computational fluid dynamic (CFD) simulations have been used to determine applicable opening angles of α = 5 • and β = 1.5 •. The experimental verification was done by using a transparent flowfield to show the passive bubble removal as well as with a fully operational µDMFC. During the operation, the fuel cell delivered an output of up to 8 mW cm −2 without the need for external pumping in short-term measurements. During the long-term measurements, discontinuous pumping showed the highest fuel cell efficiency compared to the continuously pumped fuel supply.

Reference cells are used to distinguish between polarization losses of the anode electrode and the cathode electrode in an electrochemical system. In the case of a direct methanol fuel cell ͑DMFC͒, a reference electrode can either be attached using a salt bridge or be printed directly on the same ionomer as the working electrode. Several effects influence the reference readings in this setup by disturbing the symmetrical potential distribution of the working electrode inside the membrane. Precise alignment of the electrodes, for example, is required to prevent boundary effects. Thus, a new preparation technique for improved alignment by laser ablation of the catalyst layer is introduced. Results of this segmentation method are validated microstructurally by scanning electron microscopy and energy-dispersive X-ray analysis. It is shown that the relative error in distinguishing anode polarization losses using the reference cell compared to the true value for such a postprocessed membrane electrode assembly is well below 1%. Various influencing factors that cause deviations in the symmetric potential distribution are studied systematically. A simple 2D DMFC model is developed to support the experimental conclusion and to study additional effects. Finally, losses of anode potential derived from half-cell measurements are compared to losses of anode potential derived from reference readings and the results are discussed.Distinguishing between cathode and anode polarization losses is of great interest to gain enhanced knowledge of the electrode processes of a direct methanol fuel cell ͑DMFC͒ and consequently to improve a DMFC system. Effects like flooding of the cathode, methanol depletion at the anode, or temperature-dependent methanol oxidation have a severe influence on the overall performance of a DMFC. Therefore, half-cell measurements can provide data such as voltage losses due to kinetics, mass transfer, and ohmic resistance. This data can be obtained either by half-cell operation of the fuel cell or by using a reference-cell test setup.Half-cell operation can be achieved by filling the cathode compartment with hydrogen, 1 nitrogen, 2 or inert gases like argon. 3 The electrochemical reaction is forced by a connected external power supply. Under these conditions it is assumed that the cathodic hydrogen evolution reaction experiences only a small overvoltage compared to the large anode overvoltage of the methanol oxidation reaction ͑MOR͒. Thus, the polarization losses due to hydrogen evolution are usually neglected. The polarization losses of the anode electrode can be determined by correcting the cell voltage for the ohmic drop of the membrane.Several concepts of reference cells, including a dynamic hydrogen electrode ͑DHE͒ 4-6 and a normal hydrogen electrode ͑NHE͒, 7,8 have been reported on both proton exchange membrane fuel cells ͑PEMFCs͒ and solid oxide fuel cells ͑SOFC͒. In a DHE hydrogen is usually produced by placing a Pt working electrode on both sides of the membrane and applying a small cathodic current....

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