How Conventional Solid Oxide Fuel Cells Work

All fuel cells combine oxygen and fuel to chemically generate electricity without combustion. They provide the best pathway to clean, efficient power. Solid Oxide Fuel Cells (SOFC’s) use ceramic materials which are more robust and typically operate more efficiently, on a wider range of fuels than other fuel cells. The electrolyte in these systems is typically Yttria-stabilized Zirconium Oxide (YSZ) - hence the name "solid oxide". When operated at high temperatures, the solid-state YSZ electrolyte becomes an excellent conductor of oxygen ions. This electrolyte is typically coated on one side by an electrically conductive ceramic cathode material and on the other side by a nickel/YSZ cermet anode. Unlike PEM fuel cells, which are poisoned by carbon monoxide, the SOFC system has the advantage that it can use both hydrogen and carbon monoxide as fuel.

Fig. 1 Conventional Fuel Cell

LTA-SOFC Technology
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The Liquid Tin Anode Solid Oxide Fuel Cell (LTA-SOFC) is a game-changing fuel cell technology. It combines the efficiency and reliability of conventional SOFC while simplifying the system and dramatically expanding the range of fuels which can be used. Our technology has several independent capabilities, each of which is unique and substantial in its own right.


1. LTA-SOFC stacks produce electricity directly from many useful fuels. We have operated stacks directly on JP-8, diesel, coal, natural gas and plastic as well as specialty fuels such as hydrogen. No other fuel cell stack has demonstrated this capability. As a result, systems using our technology are simpler:

• No reformer or pre-reformer
• No shift reactor or purifier
• Increased system efficiency from direct fuel utilization
• Ability to use coal, plastic and bio-mass without a gasifier

2. Sulfur, which is present in JP-8, gasoline, diesel and natural gas is not a poison to our cells. In fact, sulfur is utilized as a fuel by our cell.

3. Our anode stores chemical energy, allowing our fuel cell to operate as if it has an integral rechargeable battery. This means that systems using our technology can reduce or eliminate batteries that are typically used for peaking power. Also, system control complexity is greatly reduced because precise control of air/fuel ratio is no longer required to maintain system stability.
CellTech has built and tested cells, stacks and systems to validate our unique capabilities. To our knowledge, none of our direct competitors have tested their technology in a system environment as we have.

How LTA-SOFC Works
Our technology builds upon conventional solid oxide fuel cell materials and components. Oxygen ions are extracted at the cathode and pass through the electrolyte to the anode (fuel) side where they combine with hydrogen or carbon monoxide from the fuel to form water or carbon dioxide. Meanwhile, electrons are released at the anode and travel through the load producing useful electrical energy on their way to the cathode. The cathode in our cell can be any one of a number of conventional cathode materials such as lanthanum manganite doped strontium (LSM) while our electrolyte is also conventional ytria stabilized zirconia (YSZ). In our current designs, we use the cathode as a support structure and a tubular configuration although electrolyte supports and a planar geometry are possible.
The anode of our cell is where our differentiation begins. Our anode is a p-orbital-electron metal such as tin which is molten at operating temperatures. The tin is held in place by a ceramic matrix. Electricity is produced in our fuel cell by converting tin to tin oxide at the anode.

Fig 2 CellTech Power LTA-SOFC

 

  In a separate reaction, the tin is “recharged” by the fuel (carbon, plastic, JP-8). A key aspect of our technology is that we have integrated the reforming reaction (the reduction of tin oxide to tin) into the fuel cell, but uncoupled that reaction from the electrical production reaction ( the oxidation of tin to tin oxide), allowing each to proceed at a pace dictated by fuel quality/flow and electrical demand.
  No Reformer required
Conventional SOFC and PEM fuel cells cannot operate on hydrocarbon fuels without pre-treatment because soot will form, clogging passages and destroying the catalyst structure. In the case of PEM fuel cells, only hydrogen can be used as a fuel so a fuel processor with multiple catalyst beds is required to convert hydrocarbon fuels to hydrogen. In the case of SOFC, some methane can be part of the fuel mix, but when other hydrocarbons such as JP-8 are used, the nickel catalyst promotes the formation of soot. Soot quickly clogs the anode and it also irreversibly destroys the nickel microstructure.
 
 

Battery Function

As with a battery, the CellTech cell can operate for significant amounts of time with no fuel being added to the system. Current will continue to flow until the liquid metal anode is completely converted to oxide. "Recharging" the system requires the second step of the two-step process where fuel is added to the anode to reduce the metal oxide back to the base metal of the anode. Unlike a fuel cell, the CellTech cell can “load follow” extremely quickly. Because energy is stored in the cell itself, the power output can vary far more quickly than the fuel flow rate.

 

The graph shows the power output versus time of one of CellTech’s kW-class alpha systems. As can be seen, the power varies from zero to 2.3 kW in only 60 microseconds. Single cells have shown rise times as fast as 2 microseconds. This stored energy also allows the cell to deliver “peak” power that is 25% to 75% higher than the rated continuous power. It can deliver this peak power for up to a minute. This peaking capability allows the LTA-SOFC to meet the varying power profile typical of many applications without the need for a separate battery which is typically used with other fuel cells.