Direct use of sodium borohydride by electrochemical oxidation in fuel cells

Boron is a metal with wide industrial applications. The fact that Turkey holds 72% of total boron reserves put her a very advantageous position. In addition to being a critical material in some industrial applications boron has a very good potential as a fuel in fuel cell as a metal hydride (borohydride). The depletion oil and coal reserves and rapidly increasing environmental pollution and energy demand increase the importance of borohydride as an alternative and clean energy source. Apart from that the difficulties in the storage and transportation of hydrogen, the fuel of future, make the development of alternative hydrogen storage method increasingly important. Borohydride received great interest as hydrogen storage hydride in hydrogen energy studies. It has also been used as direct fuel in fuel cells in recent years.Fuel cells are the systems which convert the chemical energy into electrical energy. The major types of commercial fuel cells are polymer electrolyte membrane (PEM) fuel cells which use hydrogen fuel and direct methanol fuel cells (DMFC). In addition to these there are fuel cells working with other alcohols and natural gas. There are extensive researches on fuel cells concerning the fuels involved, anode and cathode materials and cell design. This study is related to an important class of fuel cells namely direct borohydride fuel cells (DBFC).DBFCs are the fuel cells which have high theoretical cell potential and energy yield. Also the fuel is easily and safely transportable as a aqueous basic solution. That is why they are very suitable for mobile applications. The fact that the raw material of its fuel boron is abundantly available in our country makes these cells additionally attractive for the utilization of this mineral. Taking the fact that these types of fuel cells have additional advantages over other types of fuel cells and the position of the country as regards to boron reserves, it was decided to investigate the direct borohydride/peroxide fuel cells in this study.This study is mainly concerned with the direct use of NaBH4 as a fuel in fuel cells. The critical point in the use of DBFCs is the selection of suitable anode catalyst material. As a result of electrochemical investigations there was test cell designed with an Ag anode and Pt cathode. The fuel and oxidant in this test cell were basic NaBH4 and acidic H2O2 solutions. The use of H2O2 as an oxidant in place of air is important for airtight applications such as space studies or under water devices.The experimental studies revealed that Ag2O layer formed on Ag surface has a considerable catalytic effect on cell performance. The test cell gave a maximum power density of 7 mW/cm2 at a current density of 11 mA/cm2 and a cell potential of 0.6 V. These values are comparable with those obtained in literature.A mathematical model has been developed for the proto type cell including the ohmic overpotential, anode overpotential and cathode overpotential of the cell. The mathematical model was found to be in good accordance with the experimental results. The biggest polarization loss was caused by the ohmic resistance. The activation overvoltage of NaBH4 makes quite a bit contribution to the loss of polarization.The cost analysis of the proto type cell showed that carbon plates and membrane cause the major shares of the cost. But 69% of the total cost comes from NaBH4.Science Code: 912.1.038Key Words: Borohydride, fuel cells, modelingPage Number: 219Adviser : Prof.Dr.Bekir Zühtü UYSAL