Advanced energy materials for fuel cell systems and flow batteries

Hydrogen production and energy storage are two major research fields in the renewable energy area. Hydrogen is a promising clean energy carrier with zero-emission. However, safe, efficient and cheap on-board storage of hydrogen as a transportation fuel is the most important obstacle for the widespread usage of hydrogen. Conventional hydrogen storage techniques such as cryogenic and high-pressure vessels do not supply the needs of required demand. On the other hand, chemical hydrides are promising solid hydrogen storage candidates for hydrogen energy having higher hydrogen density. Hydrolysis and thermolysis are two major methods to produce hydrogen from chemical hydrides. However, thermolysis requires thermal energy and hydrolysis has slower kinetics. Hence, catalytic hydrolysis is the best way in order to achieve higher hydrogen generation rates. Typically, precious and non-precious metals have been used as catalysts. Due to the lower durability of non-precious metals, precious metals are in great importance for this subject. Furthermore, price of precious metals is the biggest problem for their commercialization. Hence, decreasing the amount of use of these metals by increasing their catalytic activity is important in terms of cost-effectiveness. For this purpose, by alloying-dealloying approach, catalysts with higher surface area were aimed within the scope of this thesis. Platinum amount in thin films are only about 44.0 µg cm-2 which is one of the smallest value in the literature. Moreover, very high hydrogen generation rates (up to 130 L min-1 gcatalyst-1) from these thin films was observed. These thin films could also be applied on flexible substrates and they retained their performances for more than 25 hours. It has been also showed that it is possible to produce porous powder catalysts with this approach. Besides this method, with the aid of controllable proton transfer, stable hydrogen generation with fast kinetics from chemical hydrides has been introduced by the use of proton conductive membranes especially used in fuel cells and flow batteries. A fuel cell was powered along 300 hours with this catalyst free hydrogen generation system. Hydrogen is a renewable energy carrier that can be used instantaneously and stored as chemical hydride with higher density. On the other hand, wind and solar powers are auxiliary energy generation systems used in order to decrease the dependency to grid energy. Moreover, when the energy demand is lower than the energy produced by these renewable energy resources especially at nights, they are in need of storage. Flow batteries have become prominent as one of the best alternative for storage of wind and solar powers. Especially, vanadium redox flow batteries (VRFB) have great advantage since same material is used as anolyte and catholyte during the VRFB's operation. The most important problem for the commercialization of these batteries is the use of commercially available NafionTM membranes and vanadium electrolytes which are the most expensive parts of the cell. Also, NafionTM membranes suffer from higher vanadium permeability which leads to a dramatic decay in capacity of the batteries. In order to increase cost-effectiveness and the battery performance, directly sulfonated poly(arylene ethersulfone) membranes which have been used in fuel cells are tested in vanadium redox flow batteries. Problems arising from post-sulfonation were prevented by direct sulfonation. Batteries assembled with these membranes showed higher coulombic, voltage and energy efficiencies than batteries assembled with NafionTM membranes.