1.1 What is Chemistry?

23.1. Electrochemical Cells http://www.ck12.org

In a nickel-metal hydride battery, the negative electrode is composed of nickel (II) hydroxide and some other alkaline
electrolyte, such as KOH. The nickel serves as the source of electrons via the following half-reaction:

Ni(OH) 2 + OH−→NiO(OH) + H 2 O + e−

Looking only at the element that changes oxidation state (the nickel), we could also write the following:

Ni^2 +→Ni^3 ++ e−

The positive electrode is a complex mixture of neutral metals, some of which are generally rare earth metals
(e.g., lanthanum, cerium, and neodymium) and others that are more plentiful (e.g., nickel, cobalt, manganese, and
aluminum). The half-reaction at this electrode has the following general form:

H 2 O + M + e−→OH−+ MH

where M is a neutral metal (oxidation number = 0), and MH is a metal hydride. Note that in metal hydrides, hydrogen
has an oxidation number of -1, so the metal exists as a cation. Both half-reactions are readily reversible, allowing
the battery to be recharged.

Fuel Cells

There are a wide variety of differentfuel cellsavailable. Many are large devices that would be most suitable for
stationary generators. The type we will consider in this section is portable and can be used to power a car or similar
vehicle. This cell is referred to as a polymer exchange membrane fuel cell (PEMFC).

FIGURE 23.6

Typical hydrogen fuel cell.

The basic operation of this type of fuel cell involves an initial reaction of hydrogen gas with the platinum catalyst
(purple area), which facilitates the following half-reaction:

H 2 →2 H++ 2 e−