Jürgen Mergel

Recent developments of the measurement of the methanol permeation in a direct methanol fuel cell

The performance and the efficiency of direct methanol fuel cells (DMFCs) are affected by the methanol permeation from the anode to the cathode. A widely used method to measure the methanol permeation in a DMFC is the analysis of the carbon dioxide content of the cathode exhaust. During the operation of a DMFC large amounts of carbon dioxide are produced in the anodic catalyst layer which can diffuse partially to the cathode. As a consequence the carbon dioxide in the cathodic exhaust gas stream is expected to consist of two fractions: the carbon dioxide resulting from the oxidation of the permeating methanol and the carbon dioxide diffusing from the anode to the cathode. In this work we describe a way to separate the distribution of the two fractions under real DMFC operating conditions. As a results we found that with low methanol concentrations (<1 M) and high current densities the amount of carbon dioxide passing from the anode to the cathode can even be higher than the amount of carbon dioxide formed at the cathode by methanol oxidation.

Interaction between the diffusion layer and the flow field of polymer electrolyte fuel cells—experiments and simulation studies

The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers.

Heat and power management of a direct-methanol-fuel-cell (DMFC) system

In this paper, we describe the heat and the power management of a direct methanol fuel cell system. The system consists mainly of a direct methanol fuel cell stack, an anode feed loop with a heat exchanger and on the cathode side, a compressor/expander unit. The model calculations are carried out by analytical solutions for both mass and energy flows. The study is based on measurements on laboratory scale single cells to obtain data concerning mass and voltage efficiencies and temperature dependence of the cell power. In particular, we investigated the influence of water vaporization in the cathode on the heat management of a direct-methanol-fuel-cell (DMFC) system. Input parameters were the stack temperature, the cathode pressure and the air flow rate. It is shown that especially at operating temperatures above 90 °C, the combinations of pressure and air flow rate are limited because of heat losses due to vaporization of water in the cathode.

Improved components for advanced alkaline water electrolysis

The industrial realization of an advanced electrolysis cell for alkaline water electrolysis is gradually achieved and semitechnical cell units were constructed. The cells work with ceramic diaphragms and galvanically-deposited Raney nickel electrodes. The working temperature is 100–120°C and total pressure between 1 and 5 bar. The average energy consumption at 0.4 A cm−2 and 100°C is 3.8 kWh m−3 hydrogen. The influence of cell components on total cell performance data is discussed. The variation of manufacturing parameters on the quality of single components (electrodes and diaphragm) is considered. Corrosion problems at operating conditions and their elimination by using proper construction techniques were investigated.

Advanced water electrolysis and catalyst stability under discontinuous operation

Abstract Raney nickel produced from an Ni/Zn alloy was tested as a catalyst for discontinuously operating alkaline water electrolysers. The cells were loaded with a current density of 400 mA cm−2 at 100°C for 10 h every day. They were then left in the depolarized state without current application for 14 h daily maintaining the temperature of 100°C. Under these very severe conditions the residual zinc partially dissolved from the electrodes. Bipolar cells are thus more strained than monopolar cells. It is therefore appropriate in the bipolar case to either maintain a low protective current during standstill or reduce the temperature. Calculations of the shunt currents in a bipolar cell stack have shown that the behaviour of the central cells in the block must be taken as a measure for the protective effect, the boundary cells being less strained.

The current voltage plot of PEM fuel cell with long feed channels

Abstract The formula for voltage loss on the cathode side of the polymer electrolyte membrane fuel cell (PEMFC) is derived, taking into account oxygen consumption in the feed channel. Limiting current density due to the oxygen exhaustion along the channel is obtained. Theory predictions are in line with experiments that were performed to test the theory. The results reveal new reserves for the optimization of the cell performance. They also show new in situ electroanalytical options: the study of electrochemical reaction in the catalyst layer via the cell voltage–current plots and via detecting of the feed gas consumption or local current distribution along channel.

Improvements of water electrolysis in alkaline media at intermediate temperatures

Abstract Studies were carried out on the electrochemical splitting of hydrogen from water in an aqueous KOH solution at 110–140°C and in molten hydroxides at 300–400°C with the aim of increasing the energy efficiency of hydrogen production. The investigations on the electrolysis in an aqueous KOH solution were concentrated on developing metal oxide diaphragms and improved activated Ni electrodes. At 140°C and a pressure of 8 bar, a cell voltage of 1.55 V was obtained at a current density of 500 mA cm−2. In molten NaOH at 350°C, a cell voltage of 1.3 V was achieved at a current density of 500 mA cm−2. However, current yields were only ca 90%, due to side reactions producing peroxides. The formation of peroxides is significantly reduced in a LiOH/NaOH melt. Current yields of 100% have now been obtained at a cell voltage of 1.45 V and current density 500 mA cm−2. The hydrogen and oxygen formed are separated by a Ni-diaphragm which is cathodically protected to eliminate corrosion.

Preparation and properties of raney nickel electrodes on Ni-Zn base for H2 and O2 evolution from alkaline solutions Part I: electrodeposition of Ni-Zn alloys from chloride solutions

Experimental results for the electrodeposition of Ni-Zn alloys from chloride solutions, with addition of H3BO3 and without other additives, in a laboratory cell with a perforated nickel sheet cathode and with recirculating electrolyte are presented. The dependence of the zinc content in the alloy on the following operating conditions was investigated: cathodic current density, 1.0–20.0 A dm−2; temperature, 35–65°C; pH 1.5–5.5; total molarity,Mtot=MNi2++MZn2+=1.1–2.8 M; and, molar ratio,P=MNi2+/MZn2+=1.0–15. Depending on the operating conditions the Zn content in the alloy varied over the range 22–88 mol%. In separate experiments galvanostatic polarization curves were measured in the direction of increasing and then decreasing cathodic current density in the range 0.1–20.0 A dm−2 with all other operating conditions as used for electroplating experiments. In all cases significant hysteresis effects were observed. It was found that the current efficiency for the electrodeposition of Ni-Zn alloys from chloride solutions as a function of the zinc content in the alloy showed a sharp minimum of about 55% atXZn=55 mol % irrespective of other operating conditions.

Preparation and properties of Raney nickel electrodes on Ni-Zn base for H2 and O2 evolution from alkaline solutions Part II: Leaching (activation) of the Ni-Zn electrodeposits in concentrated KOH solutions and H2 and O2 overvoltage on activated Ni-Zn Raney electrodes

The partial dissolution of zinc from electrodeposited Ni-Zn alloys (withXZn0=22–87.3 mol %) was studied, in cold and nearly boiling 10m KOH. It was found that alloys withXZn0≤22 mol % are not dissolved at all. The dissolved zinc fraction,A, increased rapidly with further increase in zinc content and after having passed a maximum withA=82–90% atXZn0=55–58 mol % and a sharp minimum withA=52–65% atXZn0=65–69 mol %, it asymptotically approached toA → 100% atXZn0 → 100 mol %. The discontinuous dependence ofA againstXZn0 may be explained by differences in the crystallographic composition of the alloy deposits. Alloys withXZn0<50–60 mol % can be allocated to solid solutions of zinc in the Ni matrix (α-phase); the range of 50–60<XZn0<70–80 mol % corresponds to the coexistence of α+γ phases. The pure γ-phase exists within a narrow range atXZn0=75–80 mol %. No zinc dissolution from Ni-Zn alloys withXZn0≤22 mol % was explained by extremely low zinc activities in dilute solid solutions of the α-phases shifting the Gibbs energy of the dissolution reaction to very low negative, or even to positive values. The dependence of the hydrogen and oxygen overvoltage atj=0.4 A cm−2 in 10m, KOH at 100°C on the original zinc contentXZn0 showed, in both cases, a clear minimum atXZn0=75–78 mol %. This points to a practically pure γ-phase in the original Ni-Zn alloy with an approximate composition NiZn3.

Improved construction of an electrolytic cell for advanced alkaline water electrolysis

Abstract The basic set-up and structural elements of an advanced electrolytic cell for alkaline water electrolysis are described. This involves in detail the choice of catalysts and diaphragm, and computation of the potential distribution of a bipolar circuit. The experimental data obtained are shown.