Fuel Cell Comparisons

by: Dr. Karl Kordesch

All fuel cells convert chemical energy into electric energy. In principle, any exothermic chemical reaction (where heat energy is released) can be used to generate electricity, if suitable electrodes and an electrolyte supporting the reaction can be found. 

Fuels like natural gas, propane, methanol, gasoline, diesel fuel, or hydrogen are either oxidized (burned) in a furnace or in an internal combustion engine. The heat energy may be converted to mechanical energy in a piston engine or a turbine and then a dynamo or generator produces electricity. This complicated and inefficient process (only 20 to 30 percent efficiency) is avoided by a fuel cell, which is a galvanic cell, like a battery. A basic law of nature (called Carnot´s law) says, that the efficiency of a process involving heat energy conversion is far less efficient at high temperatures than at room temperatures. Therefore, fuel cells are often called “Low temperature combustion engines”.  Hydrogen-oxygen fuel cells, e.g. operating with liquid alkaline electrolytes at 80 deg. C can have efficiencies in the 60 to 70 percent ranges.

The hydrogen–oxygen (air) electrochemical reaction is responsible for the production of electricity and water at the electrodes of many fuel cells.  However, hydrogen is not a fuel, which is generally available, therefore efforts are made to use commercial hydrocarbon fuels and convert them to pure hydrogen and carbon dioxide. For alkaline cells the CO₂ must be removed.  Fuel conversion is usually done with separate units using special catalysts operating at high temperatures. It can also be done by running the fuel cells at such high temperatures that their own catalysts can convert the fuel to hydrogen at the electrodes. Except for space fuel cells, commercial fuel cells use air as oxidant.

The conditioning of the fuel, to make it digestible for a particular fuel cell does not change the overall chemistry. As chemical species are conserved, the number of hydrogen and carbon atoms supplied with the fuel is exactly the same as the number of hydrogen and carbon atoms discharged from the fuel cell system in the form of water vapor and carbon dioxide. 

        The overall efficiency of different fuel cells and fuel conditioning options is not the same. Fuel cells with low efficiency need more fuel than efficient ones to generate the same amount of electricity. Also, elaborate fuel conditioners cost money and add complexity to the system.  And, if the fuel is a complex hydrocarbon there are likely to be pollutants emitted.  Therefore, the cost effectiveness of a fuel cell technology is highly dependent on the complexity of the overall fuel processing requirements. 

Fuel cells can be classified by looking at the electrolytes. See Table 1. The electrolyte can be acidic (PEM, PAFC DMFC) or alkaline (MCFC, AFC). A circulating liquid electrolyte can be used as a heat management, concentration adjusting and water-balancing feature. Immobilization can be achieved by a micro porous matrix (Asbestos) like in Space AFCs, or by crystallizing/gelling the electrolyte like in a PAFC. Polymer Electrolyte Membranes (PEMs) serve as fixed acidic electrolyte. A high temperature solid-state electrolyte is the ceramic material used in the SOFCs.

  Table 1:    Technical Fuel Cell Systems

In acidic PEMFC and PAFC positively charged hydrogen ions or protons migrate from the anode to the cathode, water is produced at the oxygen (air) cathode. In alkaline fuel cells the charge transport is made by negatively charged ions and the water is produced at the hydrogen electrode (anode).
  
The high costs of a PEM-fuel cell are due to the use of high catalyst loadings, expensive membranes, and peripheral unit operations like fuel reformers in the process. Also, to extend the life of the membrane, a humidification system is required. In some models an air compressor is used to drive accumulated liquid water out of the cathodic airflow field.

In AFCs and MCFCs, the charge transfer in the electrolyte is managed by (OH)- and (CO3)2-, ions respectively. To operate MCFCs efficiently, carbon dioxide has to be recycled from the anodic exhaust to the cathodic (air) stream.

In Solid Oxide Fuel Cells (SOFCs) the oxygen in transferred in its ionized form O2-. The oxygen ion conduction in an SOFC is a solid-state phenomenon. In the ceramic material oxygen ions jump from one lattice point to the next. High ion conductivities (high power densities) are obtained at the high temperature. In comparison, all other fuel cells have liquid electrolytes (for PEMFCs an acidic liquid phase is considered to be embedded in the polymer structure). Typically an SOFC has a much higher power potential than other fuel cells. Also, it does not require a complex periphery. At the cathode, oxygen is taken from the air and brought to the fuel on the anode side of the ceramic electrolyte. In principle, any combustible fuel can be directly converted into electricity. Most SOFC's operate at around 800 or 900 degrees C. According to Carnot´s law this means a lower efficiency of the heat conversion, but this can be remedied in the case of a power plant by co-generation with a turbine. The high temperature may be a disadvantage in the case of the application of an SOFC to mobile uses with frequent shutdown periods. Cooling down and heating up must be done carefully. The system of a solid oxide fuel cell can be reduced to a simple air blower and a regulating valve for the fuel gas flow. At lower operating temperatures carbon deposition within the anode chamber of an SOFC must be avoided by a preconditioning of the supplied fuel gas within the hot envelope of the fuel cell stack, preferably in the inlet manifolds. The presence of some catalyst material in this vicinity is sufficient for the control of the gas conditioning reactions. No steam reformers or shift reactors are needed for the process. In spite of all these advantages, the costs of SOFCs are calculated to be much higher than those of PEMFCs. Niche-applications are envisioned.

Direct Methanol Fuel Cells (DMFCs)

There are two types of DMFCs, an alkaline version and an acidic version, neither of which requires a reformer. Both operate in a temperature range between Room Temperature and about 70°C. Temperature for methanol-vapor-fed cells would go higher. The applications range from small units for computers to 150 W units to be used for the recharging of rechargeable batteries (Military use). Kilowatt sizes are envisioned, even for mobile uses. In the acidic DMFC the carbon dioxide is exhausted, resulting in CO2 emissions. In the alkaline version the CO2 is taken up by the caustic electrolytes (KOH or NaOH) forming nearly neutral carbonates. The alkaline electrolyte is in that case a part of the fuel supply in the tank.

In the acidic DMFC the tank must carry a corresponding methanol-water mix. The advantage of the acidic system is that the electrolyte stays essentially constant if a proper water exhaust management is done.

The advantage of the alkaline DMFC is the far better performance of the air electrode, which does not even need a noble metal catalyst (or at least a far smaller amount). Having no noble metal catalyst on the airside also removes the cross over energy loss problem. The cathodic wetting action by the methanol still exists, but that is where a low cost micro porous separator (not a PEM) in combination with a circulating electrolyte comes in as the problem solution. To a certain extent the alkaline DMFC is related to a Zinc-Air cell (where electrodes are replaced when discharged), but is far easier to empty and to refill with a pump.

The AFC and the alkaline DMFC, with circulating electrolyte, are being developed at the Technical University in Graz, Austria and the project is supported by the Electric Auto Corporation, Fort Lauderdale, Florida.

Limitations of carbon containing fossil fuel uses can arise as a result of a high complexity of the fuel molecule. Simple molecules (hydrogen, methanol, and ammonia) are more readily converted than complex hydrocarbon chains. When complex hydrocarbon chains are reformed for PEMs or converted in SOFC systems there are pollutants formed that in some cases can defeat the purpose of a fuel cell, i.e., to be an environmentally friendly alternative to the internal combustion engine. Also, the chemical compatibility between fuel and anode materials must be respected.

In Summary

 In solid oxide fuel cells (SOFCs) oxygen ions are transported through a solid ceramic electrolyte from the air at the cathode to the fuel at the anode. This oxygen transport is a highly efficient solid-state phenomenon. Most common hydrocarbon fuels can be directly converted into electricity at high rates. The conversion is accomplished with a simple but extremely expensive system.

In proton exchange fuel cells (PEMFCs) hydrogen ions are transported through the electrolyte from the anode to the cathode. Platinum catalysts are needed to combine hydrogen and oxygen at low temperature. Hydrocarbon fuels must typically be converted to pure hydrogen by reforming, shifting and preferential oxidation. The overall conversion requires a complex process technology and substantial investments in safety and controls.

In Direct Methanol Fuel Cells (DMFCs), with circulating electrolyte, there is the potential to use less catalyst, a lower cost membrane system and to react the fuel (methanol) at the electrode surface and therefore not require a reformer. These advantages will produce a more efficient and less costly fuel cell than traditional DMFC (without circulating electrolyte) or PEM systems. PEM systems today are at a cost that is at least 10 times higher than the cost of an equivalently sized internal combustion engine (ICE). The objectives are to get the cost of its fuel cell into a competitive range to the ICE.

Fuel Cell types and the Acronyms used here and in the literature:

The following links shows charts of comparison between the different types of Fuel Cells listed above.

The first link show comparison between the PEM and the AFC Fuel Cell - - Comparison 1

The second link show a chart of Fuel Cells being compared to the AFCs - - Comparison 2 -- PDF