**1. Introduction**

### **1.1 Structure and operating principles of direct methanol fuel cell**

Today, population growth and economic growth lead to an increased energy demand. Current energy sources are mainly from fossil fuels (coal, oil and gas), which produce carbon dioxide (CO2) and other greenhouse gases that are responsible for global climate change. In order to minimize the bad impact of greenhouse effects (e.g. acid rain, ozone damage), the world needs an appropriate transition of the energy sources being used. Therefore, the development of clean energy is a common concern for balancing economic, social development, and environmental protection. Fuel cells are one of the most promising energy sources for use in transportation and communication applications. Compared with internal combustion engine, fuel cells are environmentally friendly, durable, reducing noise, and so on [1]. Currently, the main fuel cells include alkaline fuel cell (AFC), polymer electrolyte membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), phosphoric acid fuel cell (PAFC) and solid oxide fuel cell (SOFC).

Among the fuel cells, DMFC, apart from the advantage of being environmentally friendly, also has a high energy density [2, 3]. DMFC is one of the popular types of fuel cells using methanol directly as fuel. DMFC with liquid fuel can operate at ambient air temperature, has good energy density and is easy to store and transport. The membrane electrode assembly with acid or alkaline membranes is a main component of DMFC, in which both side of the polymer electrolyte membrane contact to anode and cathode catalyst layers. Conventionally, PtRu/C or PtRu catalyst is used in the anode, while Pt/C or Pt in the cathode [4]. The gas diffusion layers are closely aligned with the catalyst layers to aid reactant distribution, current collection and catalytic protection.

In DMFC, an electrochemical reaction will occur at the anode due to the interaction between methanol and water to produce protons and electrons. Specifically, at the anode, six protons and six electrons are formed by a methanol molecule reacting to a water molecule. These protons can move freely through the electrolyte toward the cathode, while electrons can travel through the external load (**Figure 1**). In addition, carbon dioxide will be also formed by oxidization of methanol. Meanwhile, at the cathode, water is formed by oxygen electrocatalytic reduction reaction. Therefore, the number of electrons at the anode is larger than the number of electrons at the cathode, resulting in a potential established between the two electrodes. Reaction formulas in DMFC are shown as follows [5]:

$$\text{Anode reaction}: \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{6H}^+ + \text{6e}^- \tag{1}$$

$$\text{Cathode reaction}: 3/2\,\text{O}\_2 + \text{6H}^\* + \text{6e}^- \rightarrow \text{3H}\_2\text{O} \tag{2}$$

$$\text{Overall reaction:}\,\text{CH}\_3\text{OH} + 3/2\,\text{O}\_2 \rightarrow \text{CO}\_2 + 2\,\text{H}\_2\text{O} \tag{3}$$

#### **1.2 Electrocatalytic materials in DMFC**

In the early 1950s, the anode and cathode electrocatalysts used for methanol fuel cell began to be investigated. Initially, methanol fuel cell used an alkaline electrolyte with an anode catalyst of nickel or platinum for methanol electro-oxidation reaction, and silver for the oxygen reduction process. At the same time, studies of acidic electrolyte replacement have shown that the kinetics of methanol electrooxidation are slower in this environment than in alkaline [6]. However, DMFC using liquid alkaline electrolyte has a main drawback of carbonate formation, meanwhile, DMFC using an acid electrolyte presents better perspectives. The Pt-Sn bimetallic catalyst has been systematically studied by Jansen and Molhuysen [7], which promoted the use of bimetallic catalysts for DMFC. Along with Pt-Sn, Pt-Ru was the most potential bimetallic catalyst for anode formulations, but it was still underestimated compared with Pt-Sn bimetallic catalyst.

During the 1960s, Pt-Ru system, particularly Pt combining Ru in solid solution, revealed great potential applications supported by the studies of Watanabe and Motoo [8]. In the 1960s and 1970s, the study of anode's processes was carried out by many different groups through the search or improvement of a suitable catalyst as a premise for the construction of the bifunction theory for bimetallic catalysts based on methanol oxidation. In twenty years later, the structural, surface and electronic properties of the most promising systems for DMFC, essentially Pt-Ru were investigated. Besides, the studies of electrode structure including diffusion and backing layers also attracted a lot of attention. Most of these studies aimed to enhance the catalytic activity, improve reaction rate, and minimize poisoning due to methanol

*Recent Advances in Pt-Based Binary and Ternary Alloy Electrocatalysts for Direct Methanol… DOI: http://dx.doi.org/10.5772/intechopen.95940*

**Figure 1.** *Structure and operating principle of DMFC.*

**Figure 2.**

*Poisoning CO on Pt surface to prevent methanol oxidation catalysis.*

residues by combining different metals with platinum. It has been found that the use of metal alloys can modify electronic surface structure, physical structure to prevent CO poisoning and absorb oxygen/hydroxyl species. The 1990s marked significant advances in DMFC technology with early applications for portable electronic devices. Briefly, fuel cells (including DMFC) were widely studied in the early 20th century.

DMFC is one of the most potential candidate of fuel cells, however, slow electrooxidation kinetics, methanol crossover, and gas management on the anode side in DMFC need to be improved. In methanol electro-oxidation, various surface

intermediates as CO, COHads, HCOads, HCOOads are formed and strongly adsorbed to the surfaces of catalysts. As a result, methanol molecules are prevented from the next actions, leading to slow down the oxidation reaction [5]. In addition, fuel efficiency is decreased because of small percentage of the intermediates desorbing before being oxidized to CO2. Therefore, research on developing suitable catalysts to prevent CO poisoning and improve efficiency for DMFC is one of the critical issues. One of the most popular intermediates, carbon monoxide was produced by adsorption and de-protonation on the anode catalyst, which limits the rate of methanol oxidation. Specifically, if CHO or COH are directly dehydrogenated, carbon monoxide will be formed, as shown in **Figure 2**. Consequently, the active sites of the catalyst will be decreased that limits the next reactions to be occurred.
