Fuel cell technology is electrochemical in nature and involves extracting hydrogen from fuel, combining it with oxygen from air and producing electricity. Advances in materials science and engineering, ceramics and computational chemistry techniques are refining fuel cell technology into an efficient means of generating electrical power. Fuel cells offer the potential of a much higher specific energy capacity than the Pb-acid batteries (50 Wh/kg) that are currently the reliable mainstay of Navy ship batteries and the silver-zinc battery technology (130 Wh/kg) typically used for long-range missions. Continuing advances in fuel cell designs offer an increasing number of energy source applications for mission critical power requirements for the Navy.

Types of fuel cells
There are several types of fuel cells, and each has a different charge carrier (Table 2-1).

What are fuel cells made of?
Standard fuel cell components include an anode, an electrolyte, and a cathode. When combined, these form the basic structure where hydrogen and oxygen can react together to form electricity and water as a byproduct. Typical polymeric electrolyte membrane fuel cells (PEMFCs) utilize an anode consisting of a reactive platinum coated surface. At this surface, hydrogen gas splits into protons and electrons. The proton charge migrates through the PEM and the electrons flow as usable current through an outside circuit to the cathode. At the cathode, the protons, electrons and oxygen form water by catalysis.

The EMPF interviewed John Hardy, a research scientist at Pacific Northwest National Labs (PNNL), in order to determine fuel cell technical requirements. He is developing reactive air brazing methods for joining ceramic to ceramic cells, particularly for Solid Oxide Fuel Cells (SOFCs). His technical requirements for SOFCs are listed below.

• Hermeticity
• Durability at 650-800oC for 5,000 hours (in air)
• CTE (Coefficient of Thermal Expansion) matching
• Thermal cycling capability
• Thermal shock resistance
• High mechanical strength

Fuel Cells and the Navy
Fuel cells have primarily been researched for special warfare or undersea applications. A Ship Service Fuel Cell (SSFC) system for future ships is undergoing feasibility studies to prove that commercial fuel cell systems can use NATO F-76 diesel fuel. The quarter build system is currently under evaluation and provides non-propulsion service aboard a Navy vessel. Currently, gas turbines operate at less than 20% efficiency, as Navy ships do not typically cruise at peak power. A full SSFC system is expected to achieve 40-50% efficiencies. The system will also be capable of being housed at appropriate locations throughout the ship, in addition to the typical location next to the propeller shaft.

The components that make up SOFCs are listed in Table 2-2. A cross section of a SOFC (Figure 5-1) shows a dense YSZ electrolyte and a porous anode and cathode.

The chemical storage infrastructure is equally important to fuel cell technology. An infrastructure already exists consisting of housing and transporting diesel fuel. The Navy is currently examining small man-portable fuel cells systems that could replace BA-5590 batteries (12 or 24 volt DC, 170 Watt-hr). In addition, prototype hydrogen and methanol storage and distribution systems are being investigated as a packaged fuel.

Forecast
Fuel cell technology is being driven by market factors such as a need for a distributed and reliable energy supply, energy security and environmental concerns. As the technology matures, the Navy has identified areas of benefit and has begun the integration of fuel cell systems into its own architecture.