Novel Cathode Structures for the Development of a Fuel Cell/Battery System
While lithium based batteries and proton exchange fuel cells are chosen by a vast number of research groups as the basic technology in each field, we have chosen a quite uncommon approach by using a nickel metal-hydride (NiMH) battery as a basis for meeting the demands in power and energy density. By modifying both its structure and materials, we work to achieve a hybrid Fuel Cell/Battery (FCB) system satisfying our goals.
Metal hydride, used as the anode in NiMH batteries, is well known to absorb hydrogen efficiently, wherefore we regard it as the material of choice in the FCB system. As the electrolyte, we also use what is commonly chosen in NiMH batteries, i.e. a potassium hydroxide (KOH) solution. On the cathode side, however, bigger changes have to be made, as nickel hydroxide cannot be oxidized with oxygen once discharged. Thus, another material had to be found.
Manganese Dioxide (MnO2), used for example in primary alkaline batteries, is a very interesting material due to its low-cost, environmental friendliness and catalytic capabilities. In alkaline batteries, discharging MnO2 leads to Mn2O3, which is electrochemically stable. To avoid this state and instead obtain rechargeable MnOOH, it was found in previous research that limiting the discharge potential to -0.5 V vs. Ag/AgCl inhibits the creation of Mn2O3. Thus, if oxidization of MnOOH with oxygen can be obtained, a FCB which runs according to following reaction is achieved.
Electrochemical charge/discharge: M + H2O + e- ↔ MH + OH- (1)
Charge with H2: M + 1/2 H2 → MH (2)
Electrochemical charge/discharge: MnOOH + OH- ↔ MnO2 + H2O + e- (3)
Charge with O2: MnOOH + 1/4 O2 → MnO2 + 1/2 H2O (4)
According to equation (1) and (3), the FCB can work as a rechargeable alkaline battery, thus providing high power density. Additionally, by charging the FCB with hydrogen and oxygen according to equations (2) and (3), it can run as a fuel cell, leading to high energy densities.
In this work, we have focused on the cathode side – both improving electrochemical rechargeability and oxidization under O2 exposure. In order to achieve satisfying results, we first had to analyze different crystalloid and particle structures of MnO2. The crystalloid structure of MnO2 consists of MnO6 building blocks which are arranged to create different tunnel structures. In our work, we investigated the capabilities of α-, β-, γ- and δ-MnO2 structure on their performance as cathode for both secondary battery (i.e. rechargeability) and alkaline fuel cell (i.e. oxidization with O2). Furthermore, we studied the effects of the particle size by synthesizing all four MnO2 structures through a sol-gel method and via hydrothermal treatment. While former method leads to particles with comparatively large diameters, latter synthesis leads to nano-sized rods with diameters of few dozen nanometers and thus high surface areas for improved FCB capabilities.