Types of fuel cells

There are a number of different fuel cells being investigated, which are classified based on the electrolyte used. Each type of fuel cell has intended applications based on power output limitations, operating temperature, and size of the power system. There are presently five major fuel cell types at varying stages of development and commercialization: (1) alkaline fuel cell (AFC), (2) phosphoric acid fuel cell (PAFC), (3) solid oxide fuel cells (SOFC), (4) molten carbonate fuel cells (MCFC), and (5) polymer electrolyte membrane fuel cells (PEMFC). Figure 2-1 shows the operational characteristics of each fuel cell family. A brief

description of different electrolyte cells is as follows.

[1] Alkaline fuel cell (AFC)

The AFCs typically utilize KOH as the electrolyte with the concentration of 35-50 wt%

and have the highest electrical efficiency of all fuel cells but suffer economically from the necessity for ultra pure gases for its fuel. The electrolyte is retained in a matrix (usually asbestos), and a wide spectrum of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinals, and noble metals). The operating temperature for AFCs is below 100^○C, but higher temperatures are desirable for improved hydrogen oxidation kinetics. Furthermore, the AFCs are among the first fuel cells to have been studied and taken into development for practical applications, and they are the first type of fuel cells to have reached successful routine applications, mainly in space programs such as space shuttle missions. However, almost all of the AFC development activities have come to an end now.

Figure 2-1 Simplistic representation of a various types of fuel cells.

The major challenge is that alkaline electrolytes, like potassium or sodium hydroxide, do not reject carbon dioxide, even the 300-350 ppm of carbon dioxide in atmospheric air is not tolerated, whereas terrestrial applications almost invariably require the use of atmospheric air as oxidant due to technical and economic considerations. The expected power output of an AFC is in the range of tens of MW [27].

[2] Phosphoric acid fuel cell (PAFC)

The PAFCs are commercially the most advanced system due to simple construction and the thermal and chemical stability of the phosphoric acid electrolyte at an operating temperature in the range of 150-200^○C. Phosphoric acid concentrated up to 100% is used as the electrolyte in PAFC. The matrix universally used to retain the acid is silicon carbide, and the electrocatalyst in both the anode and cathode is the platinum catalyst. The stability of concentrated phosphoric acid is relatively high compared to other common acids;

consequently, PAFC is capable of operating at the high end of the temperature range (100 to 220^○C) among the acid-type fuel cells. It is mainly used for stationary power ranging from dispersed power to on-site generation plants. Power outputs of 0.2-20 MW are able to supply shopping malls and hospitals with electricity, heat and hot water and are commonly used as primary or backup power for these sites [27].There are several disadvantages associated with the PAFC design. These include the need to use the expensive noble metal, platinum, as electrodes. Furthermore the electrodes are susceptible to CO poisoning and the electrolyte in the fuel cells is a corrosive liquid which, is consumed during operation [28].

[3] Solid Oxide Fuel Cell (SOFC)

The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y²O³- stabilized ZrO². The fuel cell operates at 800-1000^○C where ionic conduction by oxygen ions takes place. The high-temperature operation results in fast electrochemical kinetics and no need for

noble-metal catalysts. Typically, the anode is Co-ZrO² or Ni-ZrO² cermet, and the cathode is Sr-doped LaMnO³. The fuel may be gaseous hydrogen, H²/CO mixture, or hydrocarbons because internal in-situ reforming of hydrocarbons with water vapor can occur at high temperatures.

The SOFC has been investigated for applications ranging from industrial and home generators, telecommunication systems, and in hybrid electric vehicles. The SOFC provides high-quality waste heat that can be utilized for cogeneration applications or combined cycle operation for additional electric power generation. The operating condition of SOFC is also compatible with the coal gasification process, which makes the systems highly efficient when using coal as the primary fuel. It has been estimated that the chemical to electrical energy conversion efficiency is 50–60%, even though some estimates go as high as 70%–80%. Also, nitrogen oxides are not produced, and the amount of carbon dioxide released per kWh is around 50% less than for power sources based on combustion because of the high efficiency.

There are several advantages to using SOFC systems for practical power generation as compared with the other types of fuel cell. SOFCs have a solid electrolyte, which eliminates the corrosion and liquid management problems of the PAFCs [27].

[4] Molten carbonate fuel cell (MCFC)

The MCFCs uses liquid lithium potassium or lithium sodium carbonate stabilized in a matrix as the electrolyte for the system, which is supported on Al²O³ fibers for mechanical strength. The fuel cell operates at 600 to 800^○C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) can promote reaction;

therefore, noble metals are not required.

MCFCs are aimed at stationary application such as the distributed power plants for industrial and commercial applications. The MCFCs system can attain efficiencies of up to

50 %, or up to 70 % with the combination with other power generators. MCFCs can operate on a wide range of different fuels and are not prone to CO or CO² contamination, as is the case for low temperature cells.

[5] Polymer electrolyte membrane fuel cell (PEMFC)

The PEMFCs uses a proton-conducting polymer membrane as the electrolyte at an operating temperature of 80-105^○C. The most commonly used reactants for this system are hydrogen and methanol. Due to its low operating temperature, hydrogen and methanol fuel cells are popular for use in automotive and portable electronic applications. The standard membrane is a perfluorinated sulfonic acid membrane developed by DuPont and trademarked as Nafion^®. The platinum catalyst is widely employed as the catalyst in both the anode and cathode electrode.

Moreover, the PEMFC has fast-start capability and yields the highest output power density among all types of the fuel cells. Because of the solid membrane as the electrolyte, there is no corrosive fluid spillage hazard, and there is lower sensitivity to orientation. It has no volatile electrolyte and has minimal corrosion concerns. It has truly zero pollutant emissions with potable liquid product water when hydrogen is used as fuel. As a result, the PEMFC is particularly suited for vehicular power applications, although it is also being considered for stationary power applications to a lesser degree [29]. The fuel – gas hydrogen can be produced by steam reforming or partial oxidation of hydrocarbons; however, reforming these fuels produces impurities such as carbon monoxide, to which the Pt-based electro-catalyst have very low tolerance. Even a trace amount of CO drastically reduces the performance levels, although CO poisoning effect is reversible and does not cause permanent damages to the PEMFC system [30]. Furthermore, the performance reduction due to CO poisoning takes a long time (on the order of two hours) to reach steady state. This transient effect may have profound implication for transportation applications. Therefore, the PEMFC

requires the use of a fuel virtually free of CO (must be less than a few ppm). Purifying the fuel stream through a water gas shift reaction and preferential oxidation adds both extra equipments and costs to a fuel cell system. Therefore, a fuel cell researcher’s dream is to directly electrooxidize an organic fuel, rather than process it to hydrogen. Methanol is one alternative because it is a common, widely used and inexpensive substance that can be easily achieved from natural gas. Methanol offers other advantages over gas hydrogen, including liquid state at room temperature, ease to fuel-feeding, distribute and store. Hence, direct methanol fuel cell (DMFC) can be regarded as a subcategory of PEMFC, in which liquid methanol is directly converted into the electrical energy without the further use of reformer. In this thesis we shall focus our attention on the DMFCs and described in more detail below.

2.2 Direct methanol fuel cell (DMFCs)

2.2.1 Introduction

DMFCs using polymer electrolyte membranes are promising candidates for transportation applications and portable power sources such as replacing batteries. By eliminating reformer, DMFC offer simple system design and potentially higher overall efficiency than the reformate-fed fuel cells. Significant advances in H2/air polymer electrolyte fuel cells have been reported including the low electrocatalyst consumption and high power density.

However, fuel processor-fuel cell stack system on board the vehicle presents problems of packaging, complexity, and an overall system efficiency significantly lower than that of the fuel cell itself. Moreover, methanol is the liquid fuel that has substantial electroactivity and can be directly oxidized to CO2 and water on catalytically active anodes in DMFC. In addition to high efficiency and environmental compatibility, liquid methanol is inexpensive, widely available and can be handled and distributed to consumers to such an extent that the present supply networks of gasoline can be used for methanol without difficulty. There are several

conditions, which fuel cells must meet in order to become a real alternative to the internal combustion engines. One of the most important requirements is system size, because of the need to generate sufficient power within limited space on car board. This requirement is not met by liquid electrolyte fuel cells as they have considerably lower power density than solid electrolyte fuel cells. Highly efficient molten carbonate and solid oxide electrolytes, operating at temperatures in the range 700 to 1000^○C require extended, power consuming periods to reach working temperature. Therefore, they cannot start rapidly and respond quickly to the change in power demand of the particular vehicle. Using a gaseous fuel (commonly hydrogen) is not suitable for small light duty electric cars due to difficulties with fuel distribution and safe handling, and on-vehicle space and weight constraints. Currently methanol and also other organic fuels are steam reformed to hydrogen rich gas before entry into the anode area of a cell. This fuel feed usually contains traces of carbon monoxide, which acts as catalyst poison and needs to be purified. Purification of the fuel feed with water gas shift reaction will reduce the overall system efficiency and increase the weight, volume and start-up time of the device.

From the point of view of system simplicity and convenience in operation the direct methanol fuel cell where methanol fuel is supplied directly to the anode is a most attractive technological solution for automotive application. The specific advantages in comparison to other fuel cell types such as high energy efficiency (weight and volume) stationary electrolyte, hence no corrosive liquids, self starting at ambient temperature, long-term experience, up to several 10000 h, stability, etc. make the PEM-DMFC the most promising transportation power source.

2.2.2 Principle of DMFC operation

A schematic drawing of a DMFC is given in Fig. 2-2, which demonstrates the principle of operation of a DMFC. The DMFC works by oxidizing the liquid methanol to CO2 and water.

This eliminates the need for an external hydrogen supply. A proton conducting solid membrane, used both as electrolyte and separator between anode and cathode, is sandwiched between porous structures (i.e. carbon). The latter serve as current collectors and at the same time as a support for catalyst particles. Before catalyst deposition the current collectors are impregnated with polymer electrolyte to provide the intimate contact of the metal particles both with electron and proton conductors. At the anode a methanol molecule reacts with a water molecule liberating CO2, six protons which are free to migrate through the electrolyte towards cathode, and six electrons which can pass through the external load. The CO2

produced in the reaction is rejected by the acid electrolyte.

Figure 2-2 Schematic diagram of the DMFCs.

The protons, migrating through electrolyte and electrons, moving via external loaded circuit, have to reach a particle of catalyst on the cathode, where oxygen is electro-catalytically reduced producing water. The water produced is removed by the oxygen flowing through the cathode compartment. An electric potential appears between the electrodes because of the excess of electrons at the anode (where they are generated) compared with the

cathode (where they are consumed). It is this potential difference that drives current through the external load, making fuel cell a real source of power. The maximum voltage attainable from the overall reaction in the methanol-air fuel cell in theory is ~1.21 V with a theoretical efficiency of 96.5%, but in practice it is not achieved due to the poor electrode kinetics and ohmic losses the electrolyte.

The relevant electrochemical reactions at the electrode are:

Anode reaction: CH3OH + H2O → CO2 + 6H⁺ + 6e^- E⁰ = 0.02 V (2.5) Cathode reaction: 3/2 O2 + 6H⁺ + 6e^- → 3H2O E⁰ = 1.23 V (2.6) Overall reaction: CH3OH + 3/2O2 → CO2 + 2H2O E⁰ = 1.21 V (2.7)

2.2.3 DMFC Anode

The electrooxidation of methanol requires the presence of Pt-based catalysts. Pt is involved in the two key steps occurring during oxidation route. One is the dehydrogenation step and the second is the chemisorption of CO. The methanol electrooxidation reaction is a slow process and it involves the transfer of six electrons to the electrode for complete oxidation to carbon dioxide. Various reaction intermediates may be formed during methanol oxidation. Some of CO-like species are irreversibly adsorbed on the surface of the electrocatalyst and severely poison Pt for the occurance of the overall reaction, which has the effect of significantly reducing the fuel consumption efficiency and the power density of fuel cell. Thus it is very important to develop new electrocatalysts to inhibit the poisoning and significantly increase the rate of electro-oxidation by at least a factor of two to three times.

Until now Pt is proved to be the only effective anode catalyst for DMFC. The research in methanol electrooxidation using Pt anode reached important breakthroughs during the last 15 years. The most significant issue in the development of useful low-cost high efficiency methanol fuel cells for generating electric current is the poisoning of the platinum anode by

carbon monoxide that is generated during the oxidation. Carbon monoxide molecules formed from the early steps of methanol oxidation adsorb on and block polycrystalline platinum electrode surfaces and are not oxidized away by the reaction with water to make carbon dioxide unless the anode potential is increased to about 0.6 V (SHE). The net result of doing this is an unacceptable loss of cell voltage and efficiency. It has been found that carbon monoxide can be oxidized at a lower potential by adding oxygen to the system, but the gain in the cell voltage is not large. Furthermore, there is a loss of power because no current is generated when carbon monoxide is oxidized by oxygen on the anode surface whereas, on the other hand, oxidation of carbon monoxide by water yields two electrons and two protons and the COads poison that forms is oxidized by water

COads + H2O → CO2 + 2H⁺ + 2e^- (2.8) It was proposed that reaction (2-8) proceeds by direct attack by a H2O molecule on the adsorbed CO [31]. A molecular orbital study on Pt(111) suggested that for this to occur the H2O molecule would not need to be adsorbed on the surface [32]. Results of the kinetic isotope study were consistent with the formation of an activated complex of H2O and COads

for which deprotonation was not rate limiting [33]. The involvement of OHads, formed from H2O decomposition on electrode surfaces in oxidizing organic fuels was proposed four decades ago [34]. Its formation was advanced as the rate-determining step for the electrooxidation process.

Studies involving partial substitution of Pt with other transition metals like W, Pd, Ni, Ti, Rh, Mo have not yielded fruitful results [35]. Accordingly, most work has addressed to the modification of the Pt environment by alloying it with other elements or through the synthesis of multifunctional electrocatalyst. Until now the most successful results have been obtained through the alloying route. Thus the electrocatalyst activity of the new material for anodic oxidation of methanol is ambiguous. The mechanism of oxidation of methanol is discussed in section 2.3.

2.2.4 DMFC Cathode

In the direct methanol fuel cell the oxygen is reduced at the cathode, and so the electrode configuration is the same as that of H2/O2 fuel cell because of the same cathodic reaction. So, most of the cathodes developed originally for H2/O2 fuel cells are used in DMFC, which is Pt supported on carbon. In direct methanol fuel cell using polymer electrolyte membrane, there is a serious problem of methanol crossover from the anode region to cathode region, which causes a decrease in cathode performance leading to a loss in overall fuel cell efficiency. Not much is known about the chemical and electrochemical processes which methanol is undergoing at the cathode of an operating DMFC. Wang et al. [36] found that methanol is oxidized to carbon dioxide by oxygen in the presence of platinum, which inhibits the oxygen reduction reaction and results in lower cathode potential (depolarization). Keeping in mind that the highest cathode potential is 1V in practice, this kind of loss is certainly a serious problem that needs to be resolved. Some work is being done to develop methanol tolerant catalyst (cathode), however, even if the cathode depolarization is resolved, the methanol crossover is still an issue of considerable significance, which results in a decrease in fuel cell efficiency. From a practical point of view it should be important to stop this methanol crossover through the membrane. Beside the methanol crossover, a general problem related with Pt cathode still exists. The equilibrium potential for oxygen gas reduction to water according to equation 2 is 1.23 V (RHE) at 25^○C. In practice the cathode potential on Pt/C electrode does not exceed 1 V. The main reason for this voltage loss is the formation of an oxide film (OHads) and the presence of strong water dipole, which interacts with positively charged metal surface. It is concluded that the neutral oxygen molecules are unable to displace the water dipoles from the surface when the potential is above 1 V. Consequently, the oxygen molecules are unable to exert their maximum thermodynamic potential in Pt electrode

system.

2.3 Mechanism of methanol oxidation

The basic mechanism for methanol oxidation was reviewed in 1988 [37], and can be summarized in terms of two basic functionalities [38]:

(a) Electrosorption of methanol onto the substrate.

(b) Addition of oxygen to adsorbed carbon-containing intermediates to generate CO2.

Very few electrode materials are capable of adsorption of methanol; in acid solution only platinum and platinum-based catalysts have been found both to show sensible activity and stability, and almost all mechanistic studies have concentrated on these materials. On platinum itself, adsorption of methanol is now believed to take place through a sequence of steps as:

Additional reactions that have also been suggested include:

Pt-CH2OH → Pt(s) + HCHO + H⁺ + e^- (2.17)

Pt2CHOH+Pt-OH → 3Pt(s) + HCOOH + H⁺ + e^- (2.18) Or

Pt2CHOH + H2O → 2Pt(s) + HCOOH + 2H⁺ + e^- (2.19) Pt3C-OH + Pt - OH→ 3Pt(s) + Pt-COOH +H⁺ + e^- (2.20) Or

Pt3C-OH + H2O → 2Pt(s) + Pt-COOH +2H⁺ + 2e^- (2.21) Reactions (2.9, 2.12) are electrosorption processes, whereas subsequent reactions involve oxygen transfer or oxidation of surface bonded intermediates. Considerable controversy surrounds the relative rates of the above processes and the identity of the dominant species on the surface.

2.4 Features

DMFCs possess a wide spectrum of advantages as compared with PEMFCs that use hydrogen as the fuel. The theoretical energy conversion efficiency of all DMFCs exceeds

DMFCs possess a wide spectrum of advantages as compared with PEMFCs that use hydrogen as the fuel. The theoretical energy conversion efficiency of all DMFCs exceeds