Low and Medium Temperature Fuel Cells

A whole family of fuel cells now exists that can be characterized by the electrolyte used — and by a related acronym as listed in Table 1.1. All of these fuel cells function in the same basic way. At the anode, a fuel (usually hydrogen) is oxidized into electrons and protons, and at the cathode, oxygen is reduced to oxide species. Depending on the electrolyte, either protons or oxide ions are transported through the ion-conducting but electronically insulating electrolyte to combine with oxide or protons to generate water and electric power. A more detailed analysis of the power generation process is presented in Chapters 3 and 4.

Table 1.1 lists the fuel cells that are currently undergoing active development. Phosphoric acid fuel cells (PAFCs) operate at temperatures of 200°C, using molten H3PO4 as an electrolyte. The PAFC has been developed mainly for the medium-scale power generation market, and 200 kW demonstration units have now clocked up many thousands of hours of operation. However, in comparison with the two low-temperature fuel cells, alkaline and proton exchange membrane fuel cells (AFCs, PEMFCs), PAFCs achieve only moderate current densities.

The alkaline fuel cell, AFC, has one of the longest histories of all fuel cell types, as it was first developed as a working system by fuel cell pioneer F.T. Bacon since the 1930s (compare Chapter 2). This technology was further developed for the Apollo space program and was key in getting people to the moon. The AFC suffers from one major problem in that the strongly alkaline electrolytes used (NaOH, KOH) adsorb CO2, which eventually reduces electrolyte conductivity. This means that impure H2 containing CO2 (reformate) cannot be used as a fuel, and air has to "scrubbed" free of CO2 prior to use as an oxidant in an AFC. Therefore, the AFC has so far only conquered niche markets, for example space applications (the electric power on board the space shuttle still comes from AFCs).

Some commercial attempts has been made to change this. Most notably, ZETEK/ZEVCO started in the mid-1990s to reexamine the AFC technology developed by ELENCO, a Belgian fuel cell developer that had previously gone into bankruptcy. A number of ZETEK's activities attracted extensive publicity. In the late 1990s, ZETEK presented a so-called fuel-cell-powered London taxi. Little is known about the technology of the engine in this vehicle. However, the AFC employed had a power range of only 5 kW, which means it cannot be the main source of power and merely served as a range extender to some onboard battery. Other recent activities based on AFC technology include the construction of trucks (by ZEVCO) and boats (etaing GmbH). A big advantage of the AFC is that it can be produced rather cheaply. This may help this technology penetrate the highly specialized market for indoor propulsion systems, such as airport carrier vehicles, and possibly a number of segments in the portable sector.

The proton exchange membrane fuel cell, PEMFC, takes its name from the special plastic membrane1 that it uses as its electrolyte. Robust cation exchange membranes were originally developed

1Therefore, it is also known as a solid polymer fuel cell (SPFC).

TABLE 1.1 Currently Developed Types of Fuel Cells and Their Characteristics and Applications

Electric

Charge

Operating

Efficiency

Power Range/

Fuel Cell Type

Electrolyte

Carrier

Temperature

Fuel

(System)

Application

Alkaline FC

KOH

OH-

60-120°C

Pure H2

35-55%

<5 kW, niche

(AFC)

markets (military, space)

Proton

Solid polymer

H+

50-100°C

Pure H2

35-45%

Automotive,

exchange

(such as

(tolerates

CHP (5-250

membrane FC

Nafion)

CO2)

kW),

(PEMFC)a

portable

Phosphoric acid

Phosphoric

H+

~220°C

Pure H2

40%

CHP (200 kW)

FC (PAFC)

acid

(tolerates CO2, approx. 1% CO)

Molten

Lithium and

CO32-

~650°C

H2, CO, CH4,

>50%

200 kW-MW

carbonate FC

potassium

other

range, CHP

(MCFC)

carbonate

CO2)

and standalone

Solid oxide FC

Solid oxide

O2-

~1000°C

H2, CO, CH4,

>50%

2 kW-MW

(yttria, zirconia)

CO2)

range, CHP and standalone

a Also known as a solid polymer fuel cell (SPFC).

a Also known as a solid polymer fuel cell (SPFC).

for the chlor-alkali industry by DuPont and have proved instrumental in combining all the key parts of a fuel cell, anode and cathode electrodes and the electrolyte, in a very compact unit. This membrane electrode assembly (MEA), not thicker than a few hundred microns, is the heart of a PEMFC and, when supplied with fuel and air, generates electric power at cell voltages up to 1 V and power densities of up to about 1 Wcm-2.

The membrane relies on the presence of liquid water to be able to conduct protons effectively, and this limits the temperature up to which a PEMFC can be operated. Even when operated under pressure, operating temperatures are limited to below 100°C. Therefore, to achieve good performance, effective electrocatalyst technology (Chapter 6) is required. The catalysts form thin (several microns to several tens of microns) gas-porous electrode layers on either side of the membrane. Ionic contact with the membrane is often enhanced by coating the electrode layers using a liquid form of the membrane ionomer.

The MEA is typically located between a pair of current collector plates with machined flow fields for distributing fuel and oxidant to anode and cathode, respectively (compare Fig. 4.2 in Chapter 4). A water jacket for cooling may be inserted at the back of each reactant flow field followed by a metallic current collector plate. The cell can also contain a humidification section for the reactant gases, which helps to keep the membrane electrolyte in a hydrated, proton-conduction form. The technology is given a more thorough discussion in Chapter 4 (compare Section 10.2.3).

Having served as electric power supply in the Gemini space program, this type of fuel cell was brought back to life by the work of Ballard Power Systems. In the early 1990s, Ballard developed the Mark 5 fuel cell stack [Fig. 10.4(a)] generating 5 kW total power at a power density of 0.2 kW per liter of stack volume. With the Mark 900 stack [Fig. 10.4(b)] jointly developed by Ballard and DaimlerChrysler in late 1990s, the power density had increased more than fivefold to over 1 kW/l. At a total power output of 75 kW, this stack meets the performance targets for transportation (compare Section 10.2.3).

PEMFCs are also being developed for stationary applications. In the 250-kW range, Ballard Generation Systems is currently the only PEMFC-based developer. More recently, the micro-CHP range has been claimed by a wide range of developers. Here, high power density is not the most crucial issue. In a

(domestic) micro-CHP system, high electric efficiency and reliability count. The overall goal is the most economic use of the fuel employed, usually natural gas, in order to generate electric power and heat.

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