3.6.2　Fuel Cell Technologies

Text（课文）

It is possible to distinguish six major types of fuel cells，depending on the type of their electrolyte.They are proton exchange membrane（PEM）or polymer exchange membrane fuel cells（PEMFCs），alkaline fuel cells（AFCs），phosphoric acid fuel cells（PAFCs），molten carbonate fuel cells（MCFCs），solid oxide fuel cells（SOFCs），and direct methanol fuel cells（DMFCs）.Table 3.2 lists their normal operation temperature and the state of electrolyte.

Table 3.2　Operating Data of Various Fuel Cell Systems

Polymer Exchange Membrane Fuel Cells

PEMFCs use solid polymer membranes as the electrolyte.The polymer membrane is perfluorosulfonic acid，which is also referred to as Nafion（Dupont）.This polymer membrane is acidic；therefore the ions transported are hydrogen ions（H＋）or protons.The PEMFC is fueled with pure hydrogen and oxygen or air as oxidant.

The catalyst is a critical issue in PEMFCs.In early realizations，very high loadings of platinum were required for the fuel cell to operate properly.Tremendous improvements in catalyst technology have made it possible to reduce the loading from 28 mg/cm2 to 0.2 mg/cm2.Because of the low operating temperature of the fuel cell and the acidic nature of the electrolyte，noble metals are required for the catalyst layer.The cathode is the most critical electrode because the catalytic reduction of oxygen is more difficult than the catalytic oxidation of hydrogen.

Another critical issue in PEMFCs is water management.In order to operate properly，the polymer membrane needs to be kept humid.Indeed，the conduction of ions in polymer membranes requires humidity.If the membrane is too dry，there will not be enough acid ions to carry the protons.If it is too wet（flooded），the pores of the diffusion layer will be blocked and reactant gases will not be able to reach the catalyst.

The last major critical issue in PEMFCs is poisoning.The platinum catalyst is extremely active and thus provides great performance.The trade-off of this great activity is a greater affinity for carbon monoxide（CO）and sulfur products than oxygen.The poisons bind strongly to the catalyst and prevent hydrogen or oxygen from reaching it.The poisoning by carbon monoxide is reversible，but comes at a cost and requires the individual treatment of each cell.

New words and expressions（单词和短语）

1.New words（单词）

2.Expressions（短语）

Notes to the text（难点解析）

★1.The cathode is the most critical electrode because the catalytic reduction of oxygen is more difficult than the catalytic oxidation of hydrogen.因氧的催化还原作用比氢的催化氧化作用更为困难，所以阴极是最关键的电极。

★2.The last major critical issue in PEMFCs is poisoning.质子交换膜燃料电池中最后的关键是其毒化问题。

Exercises（练习）

◆1.Translate the following passages（expressions）into Chinese（英译汉）

（1）The catalyst is a critical issue in PEMFCs.

（2）Another critical issue in PEMFCs is water management.

（3）solid oxide

（4）proton exchange

◆2.Translate the following passages（expressions）into English（汉译英）

（1）固态聚合物

（2）氧化作用

Reading material（阅读材料）

The first PEMFCs were developed in the 1960s for the needs of the U.S.-manned space program.It is nowadays the most investigated fuel cell technology for automotive applications by manufacturers such as Ballard.It is operated at 60℃-100℃and can offer a power density of 0.35-0.6 W/cm2.The PEMFC has some definite advantages in its favor for EV and HEV applications.Firstly，its low-temperature operation and hence its fast start-up are desirable for an EV and HEV.Secondly，the power density is the highest among all the available types of fuel cells.The higher the power density，the smaller the size of the fuel cell that needs to be installed for the desired power demand.Thirdly，its solid electrolyte does not change，move，or vaporize from the cell.Finally，since the only liquid in the cell is water，the possibility of any corrosion is essentially delimited.However，it also has some disadvantages，such as the expensive noble metal needed，expensive membrane，and easily poisoned catalyst and membrane.