Novel Cathode Structures for the Development of a Fuel Cell/Battery System

Energy storage is crucial in numerous applications. Various technologies, such as super capacitors, batteries and fuel cells are being developed to satisfy the demands in terms of cost, life cycle, safety, power and energy density, etc. Generally speaking a tradeoff has to be made depending on technology; fuel cells, for example, show satisfying energy density but suffer from low power density. Batteries, on the other hand, are more cost effective and offer higher power densities, but cannot achieve as high energy densities. Research groups in different fields try to overcome the drawbacks through the incorporation of novel materials, compositions, designs, etc.

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.

Anode

Electrochemical charge/discharge:             M + H₂O + e^-  ↔ MH + OH^-                       (1)        

Charge with H₂:                                       M + 1/2 H₂  → MH                                    (2)

Cathode

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.