A. K. Sinha

9.1 Introduction

Global primary energy consumption (i. e., energy used for space heating, transportation, generating electricity, etc.) is expected to triple from about 400 exajoules (EJ = 1018 joules) per year in 2000 to about 1200 EJ/yr in 2050 at the present rate of increase in consumption. However, due to increased energy efficiency of the devices, the actual increase is expected to be about 800-1000 EJ.

More than 80% of the present primary energy requirements are met by fossil fuels. The consequences of burning hydrocarbons at such a large scale for our energy needs are already evident in the form of global warming and its disastrous environmental effects. In order to permit sta­bilization of anthropogenic greenhouse gases, fossil fuel consumption will have to be limited to about 300 EJ/yr by 2050. Hopefully, the con­cern about global warming, limit on fossil fuel supplies, and rise in their prices will force us to gradually decrease the use of fossil fuels in the future. Reducing hydrocarbon consumption to 300 EJ requires carbon — free energy sources to supply the difference ~700 EJ/yr. This shortfall is a problem that requires immediate attention and proactive action for sustainable development.

The need for an efficient, nonpolluting energy source for transportation, large-scale generation, and portable devices has spurred the develop­ment of alternative energy sources. Fuel cells are a promising alternative energy source that fits the above requirements [1-6]. A fuel cell is an electrochemical device that converts the chemical energy of a fuel (hydro­gen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity, with water and heat as by-products. Since no combustion

251

is involved in the hydrogen fuel cell process, no NOx are generated. Since sulfur is a poison to fuel cells, it has to be removed from fuel before feed­ing it to a fuel cell; therefore, no SO2 is generated in the fuel cell.

The trend toward portability and miniaturization of computing and communication devices has created a requirement for very small and lightweight power sources that can operate for long periods of time with­out any refill or replacement. Also, advances in the medical sciences are leading to an increasing number of electrically operated implantable devices like pacemakers, which need power supplies to operate for an extremely long duration (years) without maintenance, as any mainte­nance would necessitate surgery. Ideally, implanted devices would be able to take advantage of the natural fuel substances found in the body [7-8]. The idea of a biofuel cell that can generate electricity based on var­ious metabolic processes occurring in our own cells is very appealing. A biofuel cell converts chemical energy to electrical energy by the catalytic reaction of microorganisms. Most microbial cells are electrochemically inactive, and electron transfer from microbial cells to the electrode requires mediators such as thionine, methyl viologen, methylene blue, humic acid, and neutral red. In recent years, mediatorless microbial fuel cells have also been developed; these cells use electrochemically active bacteria (Shewanella putrefaciens, Aeromonas hydrophila, etc.) to transfer elec­trons to the electrode. A major advantage of the biofuel cell over the hydro­gen fuel cell is the replacement of expensive and precious platinum (Pt) as a catalyst by much cheaper hydrogenase enzymes. A brief description of the development and state of the art of hydrogen and biofuel cells is presented in this chapter.