**1. Introduction**

138 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Zhang, C.; Hu, J.; Nagatsu, M.; Meng, Y.; Shen, W.; Toyoda, H. & Shu, X. (2011). High-

Zhao, X.; Tian, H.; Zhu, M;, Tian, K.; Wang, J.J.; Kang, F. & Outlaw, R.A. (2009). Carbon

Zhu, F.; Hu, J.; Matulionis, I.; Deutsch, T.; Gaillard, N.; Kunrath, A.; Miller, E. & Madan, A.

Directly from Water Using Sunlight. *Philos. Mag.*, Vol. 89, pp. 2723-2739 Zhu, L.; Susac, D.; Teo, M.; Wong, K.C.; Wong, P.C.; Parsons, R.R.; Bizzotto, D. & Mitchell,

Oxygen Reduction Reaction. *J. Catal.*, Vol. 258, pp. 235-242

1024-1032

pp. 1208-1212

Performance Plasma-Polymerized Alkaline Anion-Exchange Membranes for Potential Application in Direct Alcohol Fuel Cells. *Plasma Process. Polym.*, Vol. 8, pp.

Nanosheets as the Electrode Material in Supercapacitors. *J. Power Sources*, Vol. 194,

(2009). Amorphous Silicon Carbide Photoelectrode for Hydrogen Production

K.A.R. (2008). Investigation of CoS2-Based Thin Films as Model Catalysts for the

Environmental concerns related to energy, already widely held today, will substantially increase in coming years. The energy is one of the main factors to consider in environmentalists discussions, since there is an intimate connection between energy, environment and sustainable development (Stambouli & Traversa, 2002a). In response to a critical need for cleaner energy technology, the solutions have evolved, including energy conservation by improving the efficiency of global energy, a reduction in the use of fossil fuels and an increase in the supply of renewable energies (hydro (Mendez et al., 2006), solar (Akorede et al., 2010), wind (Mendez et al., 2006), biomass (Kirubakaran et al., 2009), geothermal (Fridleifsson, 2001), hydrogen (Louie & Strunz, 2007), etc…). In the context of renewable energy, a good alternative can be found in the development and popularization of fuel cells (Akorede et al., 2010).

A fuel cell is an energy conversion device that generates electricity and heat by combining, electrochemically, fuel gas (hydrogen, for example) and an oxidant gas (oxygen in air). During this process, water is obtained as a reaction product. The fuel cell does not work with charging system such as a battery, it only produces power while the fuel is supplied. The main characteristic of a fuel cell is its ability to convert chemicals directly into electrical energy, with a conversion efficiency much higher than any conventional thermo-mechanical system, thus extracting more electricity from the same amount of fuel to operate without combustion. It is virtually free of pollution and has a quieter operation because there are no moving mechanical parts (Stambouli & Traversa, 2002a).

The initial concept of fuel cells is attributed to the German physical-chemist Friedrich Wilhelm Ostwald in 1894. His idea was to modify the internal combustion engines, eliminating the intermediate stage of combustion and convert chemical energy into electrical energy in a single step. His devices project provided the direct oxidation of natural fuel and oxygen from the air, using the electrochemical mechanism. The device that would perform this direct conversion was called a fuel cell (Wand, 2006).

Ostwald's concepts marked the beginning of a great deal of research in the fuel cells field. Ostwald examined only the theoretical aspect of energy conversion in fuel cells, but completely ignored other practical aspects: the question of whether the electrochemical

Fuel Cell: A Review and a New Approach

**1.1.3 Phosphoric acid fuel cell (PAFC)** 

of platinum as a catalyst (O'Sullivan, 1999).

(MCFC) and solid oxide fuel cell (SOFC).

**1.2.1 Molten carbonate fuel cell (MCFC)** 

**1.2.2 Solid oxide fuel cell (SOFC)** 

its launching is slow, these are its main disadvantages.

**1.2 High temperature fuel cells** 

**1.1.2 Alkaline fuel cell (AFC)** 

About YSZ Solid Oxide Electrolyte Deposition Direct on LSM Porous Substrate by Spray Pyrolysis 141

Initially, it was called Bacon cell, by virtue of its inventor Francis Thomas Bacon. It operates at low temperature around 100 °C and it is able to achieve 60-70% efficiency. It uses an aqueous solution of potassium hydroxide (KOH) as electrolyte solution. This fuel cell has quick startup speed, one of its great advantages. The main disadvantage is that it is very sensitive to CO2 (Farooque & Maru, 2001). It needs an external system to remove CO2 from the air. Furthermore, the use of a liquid electrolyte is also a disadvantage because it reduces

The phosphoric acid fuel cell (PAFC) operates at around 175-200 °C. This range of operating temperature is almost twice as high as the PEM's. It uses phosphoric acid as electrolyte. Unlike PEMFC and AFC, the PAFC is very tolerant to impurities in reforming hydrocarbons. The chemical reaction involved in this type of fuel cell is the same as PEMFC, where hydrogen is used as fuel input, however PAFC is more tolerant to CO2 (Farooque & Maru, 2001). Cogeneration is also possible due to its relatively high operating temperature. The disadvantage of the PAFC is the same as the PEM's, its cost also increases due to the use

The second major group of fuel cells, the high temperature ones (650 - 1000 °C) has as its main feature the high efficiency, since the high operating temperature facilitates the reactions. One application of this type of fuel cell is stationary generation, such as primary or secondary source of energy. Two cell types in this group are molten carbonate fuel cell

The molten carbonate fuel cell (MCFC) operates at high temperature, which is about 600- 700 °C. It consists of two porous conductive electrodes in contact with an electrolyte of molten carbonate. This type of cell allows the internal reform. The main advantage of the MCFC is its high efficiency (50-60%) without external reformer and metal catalyst, due to the high operating temperature (Farooque & Maru, 2001). This cell is intolerant to sulfur and

The solid oxide fuel cell (SOFC) is the cell that operates at the highest temperatures (800- 1000 °C). It uses a solid electrolyte, which consists of a dense ceramic material with high ionic conductivity. In this cell, oxygen ion is transported through the electrolyte and, in the interface with the anode, it combines with hydrogen to create water and energy. The main advantages of the SOFC are that it produces electricity with high efficiency of 50-60% and does not require an external reformer to extract hydrogen from fuel due to its ability to internal reform. The waste heat can be recycled to produce additional electricity in the operation of cogeneration. The high temperature, which provides satisfactory characteristics

the cell lifetime and makes the assembly handling and transport more difficult.

reactions that involve natural fuels are feasible or not and how they can be efficient. The first experimental studies, conducted after the publication of Ostwald's document, indicated that it was very difficult to build devices for the direct electrochemical oxidation of natural fuels (Wand, 2006).

Ceramic fuel cells came up much later, initially with the discovery of the Nernst solid oxide electrolytes in 1899 [9]. Nearly forty years later, the first ceramic fuel cells began operating at 1000 °C, developed by Baur and Preis in 1937 (Farooque & Maru, 2001). Since 1945, three research groups (USA, Germany and the USSR) made studies on a few main types of generators by improving their technologies for industrial development. This work yielded the current concepts on fuel cells (Wand, 2006).

Nowadays, the development of fuel cells is being mainly driven by environmental reasons mentioned above. Over the last decades, advances in research made possible for a considerable improvement to happen, regarding the characteristics of the cells, in particular their stability and efficiency. Currently, it is possible to classify the fuel cells into two major groups, which differ by basic operational characteristics: the low-temperature and high temperature fuel cells.
