Alcohol oxidation DAFCs

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

Operating Principle of DAFCs 1.5.1 Thermodynamics of Alcohol Oxidation... 

Table 1.1 Number of electrons, standard potential, theoretical specific energy and density energy, pure compound capacity, and theoretical energy conversion efficiency for alcohol oxidation in DAFC...

The faster kinetics of alcohol oxidation and oxygen reduction reactions in alkaline direct alcohol fuel cells opens up the possibility of using less expensive Pt-free catalysts, as nickel, gold, palladium and their alloys [30]. Thus, the cost of ADAFC could be potentially lower compared to the acid DAFC technology if non-precious metal alloys are used for the alcohol electrooxidation, being the nanoparticulated Ni-Fe-Co alloys developed by Acta (Italy) with the trade name of HYPERMEC a good example. 

If DAFCs fuelled with ethanol and higher alcohols have a commercial future, this seems to be indissolubly linked to Pd-based electrocatalysts and anion-exchange membranes. As shown in this Chapter, the known catalytic architectures for alcohol oxidation are extremely valid, yet they suffer the scarce ability to cleave C-C bond in a selective way as well as poisoning by COads. Therefore increasing research efforts are required to design new catalysts with better performance and higher electrochemical stability. 

In direct alcohol fuels cells (DAFCs), some simple organic molecules such as methanol, ethanol, formic acid, and ethylene glycol are used as alternative fuels. Besides the slow kinetics of ORR in the cathode, the slow alcohol oxidation reaction on Pt is another major contribution to low DAFC performance. 

In recent decades, direct alcohol fuel cells (DAFCs) have been extensively studied and considered as possible power sources for portable electronic devices and vehicles in the near future. The application of methanol is limited due to its high volatility and toxicity, although it is relatively easily oxidized to CO2 and protons. So other short chain organic chemicals especially ethanol, ethylene glycol, propanol, and dimethyl... 

C. Lamy, E. M. Belgsir, and J.-M. Leger, Electrocatalytic oxidation of aliphatic alcohols Application to the direct alcohol fuel cell (DAFC), J. Appl. Electrochem. 31, 799-809 (2001). 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

In addition to hydrogen as a fuel, methanol or ethanol can be directly converted into electricity in a DAFC, the great progress of which resulted from the use of a proton exchange membrane acting both as an electrolyte (instead of the aqueous electrolytes previously used) and as a separator preventing the mixing of fuel and oxidant. A DAFC can work at moderate temperatures (30-50 °C) for portable applications, but now the tendency is to look for new membranes that are less permeable to alcohol and... 

Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. The alcohol is fed directly into the fuel cell without any previous chemical modification and is oxidized at the anode while oxygen is reduced at the cathode. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. Among different electrocatalysts tested in the methanol oxidation, PtRu-based electrocatalysts were the most active [1-3]. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electrooxidation [3],... 

Ethanol is considered as the ideal fuel for the so-called direct alcohol fuel cells (DAFCs). This is because ethanol has a number of advantages over methanol it can be produced in a sustainable manner, easily stored and transported, and is less toxic or corrosive than methanol. The theoretical mass energy of ethanol is 8.0 kWh kg compared to 6.1 kWh kg" for methanol. The complete oxidation of ethanol releases 12 electrons per molecule its standard electromotive force E° q =1145V, is similar to that of methanol. 

The general hemi-reactions for a CnH2n+iOH mono-alcohol that oxidizes completely to CO2 in a DAFC employing an acid electrolyte are ... 

There are different kinds of DAFC operation conditions depending of the way the fuel and the oxidant (oxygen/air) are fed into the cell. In complete active fuel cells the liquid fuel (neat alcohol or aqueous solution) is pumped and gas is compressed, using auxiliary pumps and blowers, in order to improve mass transport and reduce concentration polarization losses in the system. On the other hand, in complete passive DAFC the alcohol reaches the anode catalyst layer by natural convection and the cathode breathes oxygen directly from the air. A number of intermediate options have been also studied and tested. 

However, CO2 is produced in the anode of a DAFC as final product of the alcohol electro-oxidation and, if the CO2 bubbles cannot be efficiently removed from the surface of the DL, they block the access of alcohol to the catalyst layer decreasing the cell efficiency. On the cathode, the formation of water on the catalyst layer and the electro-osmotic drag of water through the membrane can lead to GDL flooding if the excess water cannot be eliminated, reducing the transport of oxygen/air to the catalyst. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

Independently of the selectivity parameter chosen for comparing the performance of membranes for DAFC, it is clear that a high proton conductance (higher than 0.08 S cm ) and low impermeability to alcohol and oxygen, along with good thermal and oxidation stability and low cost, are desirable properties. 

Figure 8.1 summarizes the operation principle and the main mechanisms occurring at multiple scales in a DAFC alcohol and water transport in the anode (air and water in the cathode) at the macroscale (within the distributor), mesoscale (within the secondary pores formed by C in the electrodes) and microscale (within the primary pores of the C), nanoscale electrochemical double layer formation around the catalyst nanoparticles, alcohol electrochemical oxidation in the anode and ORR in the cathode. 

Despite the promising characteristics of DAFCs, several factors limit their performance, including the slow kinetics of the electrochemical reactions in the electrodes (oxidation of the alcohol at the anode and ORR in the cathode) and the... 

Electrocatalysis in DAFC anodes is complex because the reaction mechanism involves adsorption of alcohol and several elementary reaction steps including the CO oxidation. Figure 8.2 shows a possible network of reaction pathways by which the electrochemical oxidation of methanol occurs. Since more than 50 years detailed catalysis studies have attempted to analyze possible reaction pathways to find the main pathway of methanol oxidation [11, 12] (see next Section). Most studies conclude that the reaction can proceed according to multiple mechanisms and that the most significant reactions are the adsorption of the alcohol and the oxidation of CO. 

The development and application of carbonate as well as anion-exchange PEM electrolytes have significantly renewed interest in the development of alkaline-based DAFC [134]. Many of the reaction intermediates, products and paths discussed above for the catalytic oxidation of alcohols in alkaline media have also been identified or speculated to take part in the electrocatalytic oxidatiOTi of these same alcohols. 

The main problems regarding the replacement of batteries by direct alcohol fuel cells are related to the largest volume required by the fuel cells, as compared to the batteries which have become highly compact (because DAFCs have not reached yet high efficiencies), elimination of residues of the methanol partial oxidation (generally mixtures of water with formic acid, methyl formate, and formaldehyde), and the high temperature which can reach the DAFC (up to around 85 °C for cells using Nafion membranes) [11, 12]. 

The dilution of Pd with non-noble metals in a smart catalytic architecture capable of rapidly and stably oxidizing alcohols on anode electrodes would knock down the main barriers to the commercialization of direct alcohol fuel cells (DAFC), especially those fed with primary alcohols, hi er than methanol, and polyalcohols. Indeed, apart from methanol for which there exist platinnm based catalysts capable of prodncing cnrrent densities of several tens of mW cm, the higher alcohols like ethanol and polyalcohols like glycerol are difficnlt to oxidize on platinum or platinum alloyed with either noble or non-noble metals. 

Palladium is more abundant in nature and sells at half the current market price of platinum. Unlike Pt, the Pd-based electrocatalysts are more active towards the oxidation of a plethora of substrates in alkaline media. The high activity of Pd in alkaline media is advantageous considering that non-noble metals are sufficiently stable in alkaline for electrochemical applications. Importantly, it is believed that the integration of Pd with non-noble metals (as bimetallic or ternary catalysts) can remarkably reduce the cost of the membrane electrode assemblies (MEAs) and boost the widespread application or commercialization of DAFCs [1]. Palladium has proved to be a better catalyst for alcohol electrooxidation in alkaline electrolytes than Pt [2]. Palladium activity towards the electrooxidation of low-molecular weight alcohols can be enhanced by the presence of a second or third metal, either alloyed or in the oxide form [3]. 

In a DAFC the total electro-oxidation to CO2 of an aliphatic mono-alcohol, CxHyO, involves the participation of water (H2O) or of its adsorbed residue (OHads) provided by the cathodic reaction (electro-reduction of dioxygen). 

Direct Alcohol Fuel Cells (DAFCs), Table 1 Thermodynamic data associated with the electrochemical oxidation of some alcohols (under standard conditions) ... 

Figure 1.20. Detailed reaction mechanism of methanol oxidation on a Pt electrode [94]. (Reprinted from Journal of Power Sources, 105(2), Lamy C, Lima A, LeRhun V, Delime F, Coutanceau C, Leger J-M, Recent advances in the development of direct alcohol fuel cells (DAFC), 283-96, 32002, with permission from Elsevier.)...

Membrane. Perfluorosulfonic acid (PFSA) is the most commonly used membrane material [4], PFSA membranes are relatively strong and stable in both oxidative and reductive environments, since the structure of PFSA is based on a PTFE backbone. The conductivity of a well-humidified PFSA membrane can be as high as 0.2s cm. As is well known, fuel cell operation at elevated temperatures can increase the rates of reaction, reduce problems related to catalyst poisoning, reduce the use of expensive catalysts, and minimize problems due to electrode flooding. Unfortunately, a PFSA membrane must be kept hydrated to retain its proton conductivity. Moreover, a PFSA membrane is alcohol permeable if it is used in DAFCs. Because of the disadvantages of PFSA membranes, many alternatives have been proposed [106]. Five categories of membranes are classified (1) perfluorinated, (2) partially fluorinated, (3) non-fluorinated, (4) non-fluorinated composite, and (5) others. 

The same problem is encountered during electrosynthesis by the oxidation of a chemical compound. In an electrochemical reactor, the oxidation reaction has to be counterbalanced with a reduction reaction in order to close the electrical circuit. Under these conditions, it is better for industrial applications to use oxygen from air, which is free, as the oxidative agent. Such a system then becomes very close to a fuel cell system, apart from the oxidation reaction that has to be controlled here, whereas the complete oxidation of alcohol into CO2 is sought in direct alcohol fuel cells (DAFCs). For this reason, fuel cell systems will be considered in this chapter to illustrate the important problem of oxygen activation for electrochemical processes. 

---