Electrochemical Investigation of Hydrogen Storage in Metal Hydrides

Abstract

A new technique has been developed to study and control hydrogen storage in solid metal hydrides. The formation of metal hydrides and their thermodynamic and kinetic properties have been investigated by electrochemical methods employing a low temperature molten salt galvanic cell of the typeThe choice of the electrolyte is of great importance in this method. Besides having to be thermodynamically stable when in contact with the electrodes, it must conduct charge by transport of hydrogen containing species. For this purpose, the low temperature organometallic salt (melting point 128°C) has been synthesized. It is saturated with , the hydride ions of which act as hydrogen transmitters. This electrolyte has the additional advantage that its oxygen activity is extremely low, so that no oxide layers form which could block hydrogen reaction with the alloy and thus reduce its apparent capacity. It even cleans the surface of oxide layers which may have been present initially. This technique allows easier investigation of metal hydrides than the conventional pressure‐temperature experiments and, in addition, avoids the main problems involved in using aqueous solution electrolytes, i. e., the corrosion of the metal alloys and, especially, the formation of oxide or hydroxide layers. With the aid of this cell, charge/discharge experiments have been carried out and hydrogen has been added to and deleted from the hydride forming metal alloy systems Mg‐Ni, Mg‐Cu, and Mg‐Al at 142° and 170°C. Equilibrium open‐circuit voltage measurements provide information about the thermodynamic properties as a function of the overall composition of the electrode material. Thermodynamic data for the different metal hydride systems have been determined as a function of hydrogen content and interpreted in terms of the corresponding ternary phase diagrams (Mg‐Ni‐H, Mg‐Cu‐H, and Mg‐Al‐H, respectively). The Gibbs free energies corresponding to the various reactions of hydrogen with the binary metal alloys measured in this way have been found to be in good agreement with data determined by conventional pressure‐temperature experiments. For the Gibbs free energy of formation of , for instance, we find at 142°C, whereas the data reported in literature range from −4.2 to −5.0 kcal/mol. The results are also in accordance with a model in which these systems are treated as ternaries rather than as pseudo‐binary reactions. Using this new approach, the relevant three‐phase triangles and two‐phase tie lines of the ternary phase diagrams can be calculated from the Gibbs formation energies of the phases present in these systems. The hydrogen capacities in these several metal alloy systems are also readily interpreted in terms of the phase boundaries of the corresponding three‐phase equilibria.