Fuel Cells in Energy Technology. Tutorial 5 / SS solutions. Prof. W. Schindler, Jassen Brumbarov / Celine Rüdiger

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1 Fuel Cells in Energy Technology Tutorial 5 / SS solutions Prof. W. Schindler, Jassen Brumbarov / Celine Rüdiger Homework 3: What hydrogen flow rate (g/hour) is required to generate 1 ampere of current in a fuel cell? 1A = 1C/s = 3600C/h (1h = 3600s) The Faraday constant F = 96485C/mol tells us, how much charge 1 mol electrons have. considering that H 2 2H + + 2e - ; the oxidation of 1 mol H 2 leads to the transfer of 2 mol electrons. Thus we get 1A 3600/96485 mol e - /h = mol e - /h = mol H 2 /h, Finally the weight of H 2 is looked up in the periodic system and the value M hydrogen = 2 * g/mol = g/mol gives us the desired flow rate: 1A = g H 2 /h How is the faradaic efficiency defined? Calculate the actual needed flow rate of hydrogen for a faradaic efficiency of 80%! The faradaic efficiency is defined as: faradaic I nf v If all the Hydrogen was oxidized and all the electrons were transferred to the electrode, the flow rate of hydrogen to produce 1 ampere would be mol H 2 / 3600s = μmol H 2 /s. With an efficiency of 0.8, the actual needed flow rate to produce 1 ampere is mol H 2 / 3600s = μmol H 2 /s or g H 2 /h. You would like the cell to have an operating lifetime of 100h. If this H2 fuel is stored as a compressed gas at 500 bars, what volume would it occupy (assume ideal gas, room temperature, pv = nrt)? If it is stored as a metal hydride at 5 wt % hydrogen, what volume would it occupy? (Assume the metal hydride has a density of 10 g/cm 3.) fuel ideal gas: pv=nrt p = 500*10 5 Pa n H2 = 100h*0.0234mol H 2 /h = 2.34 mol H 2 R = J/(mol K) T = 300 K compressed gas: V = nrt/p = 2.34 mol * J/(mol*K)*300K/(500*10 5 N/m 2 ) = 1.17*10-4 m 3 = 117 cm 3 = 117 ml For the metal hydride with 5 wt% hydrogen, 2.34 mol hydrogen would be stored in 4.72g/0.05 = 94g metal hydride. The volume of the metal is 94g/(10g/cm 3 )=9.4 cm 3. 1.) Overview of fuel cells a) In the lecture you were introduced to different types of H 2 /O 2 - fuel cells (AFCs, PEMFCs, PAFCs, MCFCs, SOFCs). What are the typical temperature ranges for operating the different types of fuel cells?

2 Sketch these different types of fuel cells with all relevant components and give the electrochemical reactions at anode- and cathode side assuming air (O 2 /N 2 ) and hydrogen as reactants. Name some applications for each type of fuel cell. b) What is a direct alcohol fuel cell? What is the temperature range of a DMFC? Make a sketch of this cell type and give the electrochemical reactions at anode- and cathode (air) side. What would be the advantages of using ethanol instead of methanol? Sketches see lecture. Disadvantages and advantages see lecture. Overview of different fuel cells: Fuel H 2 H 2 CH 3OH H 2 H 2 + CO H 2 + CO Electrolyte KOH Nafion Nafion H 3PO 4 Li 2CO 3, K 2CO 3 YSZ Comparison of the five major fuel cell types (based on hydrogen). In a direct alcohol fuel cell, alcohols like methanol or ethanol are the fuel. The protons are produced directly from the oxidation of alcohol. No previous reforming to produce hydrogen is needed. The temperature range of the DMFC is given in the table above. Sketch see lecture. The problem with methanol is that it is toxic. Ethanol is not toxic and has a higher energy density (ethanol: C 2 H 5 OH, methanol: CH 3 OH the second carbon atom in ethanol increases energy density). Problem: no good catalyst identified by now! 2.) Electrocatalysis

3 a) What determines the electrocatalytic activity of a fuel cell electrode? The whole cell determines the electron transfer. To obtain high electron transfer rates, we need: Efficient mass transport High electronic conductivity of the electrode High proton conductivity of the membrane The electrode (catalyst) properties determine the reaction rate (described by the rate constant k) of the electron transfer reaction. It depends on: Composition of the catalysts Particle size, number of defects (low coordinated atoms) of catalyst Chemistry and physics of substrate (interaction catalyst - support) b) Discuss for the catalyst - substrate systems depicted below which parameters influence the catalytic activity! The electrocatalytic activity is influenced by the following parameters Electronic and geometric effects Influence of alloy formation / clustering Influence of support material In the case of a monolayer catalyst, depending on the relative atom sizes of the catalyst material and the substrate material, strain or tension may occur. The interaction between the substrate and the catalyst may influence the electronic properties of the catalyst and therewith affect its catalytic activity. If the attractive interaction among the catalyst particles is stronger than the attraction to the substrate, clusters may form. In this case, geometric effects play a crucial role. Experiments have shown that the electron transfer reactions mainly occur at low coordinated catalyst atoms. Atoms at step sites and defects of the catalyst layers or clusters and atoms at the border between catalyst and substrate are low coordinated. Also here electronic effects play a crucial role since the electronic state of the catalyst material at low coordinated sites is different from the electronic state in the bulk or on top of a layer/cluster. The electronic state of the catalyst determines its properties concerning adsorption and desorption of reactants in an electrocatalytic reaction. In the case of catalytic nanoparticles the density, distances and sizes of the particles influence the catalytic activity of the system. c) What is a volcano curve in electrocatalysis? Which information can you gain from it? Consider the volcano curve for hydrogen for your discussion!

4 One essential step of electrocatalytic reactions in fuel cells is the adsorption of reactants to the surface of a catalytic material. The Sabatier principle says that there is an optimum of the rate of a catalytic reaction as a function of the heat of adsorption ΔG H. If the adsorption is too weak, the catalyst has little effect (too positive ΔG H ). If it is too strong, the adsorbates won t desorb from the surface (too negative values of ΔG H ) and the reaction process does not proceed. The relation between the exchange current density j 0 and the heat of adsorption ΔG H of a reactant material on different catalyst materials results in a volcano shaped curve. From the maximum of the curve one obtains the optimum catalyst material for a chosen catalytic reaction. For the hydrogen oxidation or evolution reaction, for example platinum, palladium, rubidium, iridium or alloys of these materials show a high reactivity (given by the current per surface area). 3.) Electrocatalytic reactions a) Describe the elementary steps in electrocatalytic reactions. Diffusion of reactants to electrode surface In some cases followed by adsorption (physical and/or chemical) Surface reaction (Two mechanisms predicted: Langmuir-Hinshelwood and Eley-Rideal mechanism); electrochemical reactions are always related to an electron transfer Desorption and diffusion of the product species b) What are the elementary steps of the hydrogen evolution reaction? Write down the reactions and discuss the mechanisms.

5 In the case of the hydrogen evolution reaction, the two surface reaction mechanisms were approved experimentally. The Volmer and Tafel found a Langmuir-Hinshelwood mechanism, Volmer and Heyrovsky an Eley-Rideal mechanism. The reaction mechanisms of other electrochemical reactions, like e.g. the methanol oxidation are not that well understood so far. There are many possible reaction steps to oxidize methanol. The experimental determination of the single reaction steps is difficult, especially if the overall reaction is too fast to be time resolved (like the hydrogen evolution).