Fuel cells, basic requirements

Two emerging trends endorse the concept of heat-integrated processes first, the production of basic chemicals is moved close to oil and gas wells where crude oil or natural gas is processed in large stand-alone units [1]. Second, fuel cell systems require on-site and on-demand hydrogen production from primary fuels (i.e., natural gas, liquid hydrocarbons or alcohols) [2]. Net heat generation in these processes is equivalent to raw material and energy loss, and is therefore undesirable. 

The performance of a fuel ceU is determined by the total surface area of the catalyst particles that participate in the reactions. Ideally, aU the surface of the Pt particles is used and the Pt achieves 100% dispersion (e.g., Pt exists as individual atoms). In reality, the ideal situation does not exist and only a small fraction of the Pt atoms can participate in the fuel cell reaction. The reasons are that the Pt can not achieve 100% dispersion and the fuel cell reactions require the so-called three-phase boundaries. It can be seen from Eqs. 1 and 2 that both the anode and the cathode reactions involve protons, electrons, and reactants. So, only the Pt surface that is accessible to protons, electrons, and the reactant is active, and such regions are often called catalyst-electrolyte-reactant three phase boundaries as illustrated in Fig. 2. All the other Pt surface area is basically wasted. For an electrode composed of Pt (or... 

Ballard s original PEM design has been the prototype for most automobile development. This has been the basic design that has been used to demonstrate fuel cell power in automobiles. But, it may not be the best architecture and geometry for commercial automobiles. The present geometry may be keeping the price up. Commercial applications require a design that will allow economies of scale to push the price down. 

Although a fuel cell produces electricity, a fuel cell power system requires the integration of many components beyond the fuel cell stack itself, for the fuel cell will produce only dc power and utilize only processed fuel. Various system components are incorporated into a power system to allow operation with conventional fuels, to tie into the ac power grid, and often, to utilize rejected heat to achieve high efficiency. In a rudimentary form, fuel cell power systems consist of a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or bottoming cycle to utilize the rejected heat. A simple schematic of these basic systems and their interconnections is presented in Figure 9-1. 

The requirements for plate materials in a fuel cell stack for different markets or applications can be quite different due to fuel cell working conditions and specific needs for the power, lifetime, weight, volume, size, and acceptable cost range. For example, in addition to basic requirements of all plate materials for their common functions, the plate material used in transportation fuel cells, such as that used in automotive applications, would be significantly different from requirements in stationary stacks in terms of working temperature range, density, durability, and lifetime. 

In this chapter, we will pay attention to the basic or common materials requirements of the plate according to its functions in fuel cells. The emphasis will be put on plate materials used in transportation fuel cells because these applications, more directly for automotive, have potentially the largest market for fuel cells and the related material requirements are most challenging [1]. The various plate materials, fabrication process, and major challenges will be introduced and analyzed. The underlying mechanism and development trends will also be discussed. 

Although EIS offers many advantages for diagnosing fuel cell properties, clear difficulties exist for applying impedance methods and fitting the data to the model to extract the relevant electrochemical parameters. The limitations of the EIS technique derive from the several requirements required to obtain a valid impedance spectrum, because the accuracy of EIS measurement depends not only on the technical precision of the instrumentation but also on the operating procedures. Theoretically, there are three basic requirements for AC impedance measurements linearity, stability, and causality. 

As mentioned above, an anode serves as the HOR site in a H2/O2 fuel cell. As such, it must fulfill the following basic functional requirements (1) transport H2 to the catalyst sites, (2) catalyze the HOR process, (3) carry protons away from the reaction sites to the membrane electrolyte, (4) remove electrons from the anode, and (5) transfer heat in or out of the reaction zone. Water management is also an important consideration, as it is for all PEM fuel cell components. 

The simultaneous utilization of FCS and battery within a fuel cell propulsion system can be accomplished by two basic ways (i) the battery pack can be minimized (but not eliminated as in full power) assigning the role of generating most energy required by the load to the FCS (soft hybrid configuration), (ii) the FCS can be sized to provide the base load, i.e. a power value close to the average power of the expectable road mission (hard hybrid configuration), while larger battery pack are necessary to satisfy the dynamic requirements. 

The standard for a modem vehicle requires it to start within 2 seconds at worst. A fuel cell starts well within 1 second. However, fuel cells, including hydrogen fuel cells, do not operate well at subfreezing temperatures. This is because fuel cells are basically a liquid interface device and need liquid-phase water to operate. Running the system under the conditions of a highway environment is possible, but the current cost is too great for commercialization. 

---