Drexel University MEM 415 Chapter2-solutions

Chapter 2

Calculation/Short Answer Problems:

2.1 Define the units of the following in terms of the most basic SI units
a) Volt
b) Ampere
c) Ohm
d) Faraday’s constant
e) n (as in iA/nF)

Soln:
V= J/C (where J = Nm = kgm2/s2)
A=C/s (note…A is actually an SI unit)
Ohm= Js/C2 (where J = Nm = kgm2/s2)
F=C/eq
N= mol/s


Instructors note: Do not assign parts b and c of question 2.2, the electrochemistry is too
complex and the question is not posed well.

2.2 Determine the theoretical open circuit voltage of the following fuel cells, and
determine which reactant would be the oxidizer and which would be the
fuel for a galvanic reaction.
a. Oxygen and hydrogen gas
b. Lithium and oxygen gas
c. Magnesium and oxygen gas


Soln:

Part a: Fuel is hydrogen, oxidizer is oxygen
H 2 → 2 H + + 2e − = 0.0 V (SHE)
O2 + 4e − + 4 H + → 2 H 2Ol = 1.23 V(SHE)
Net Reaction:
1
H 2 + O2 → H 2Ol Ecell = Eanode + Ecathode = 0.0 + 1.23 = 1.23V
2




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,2.3 Determine the minimum theoretical open circuit that would be required to generate
hydrogen peroxide H2O2, with hydrogen gas.

Soln:

H 2 → 2 H + + 2e − = 0.0 V (SHE) fuel
O2 + 2e − + 2 H + → H 2O2 = 0.68 V(SHE) oxidizer
Net Reaction:
1
H 2 + O2 → H 2O2 Ecell = Eanode + Ecathode = 0.0 + 0.68 = 0.68V
2

The reaction potential for the production of water is larger, so that production of peroxide
is a minor species resulting from intermediate steps beyond the scope of simple
thermodynamics.




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, 2.4

Besides the desired hydrogen oxidation and oxygen reduction reactions, there are several
other potential reactions listed in Table 2-1 that can occur in a hydrogen/air fuel cell stack
(e.g. they only involve atomic hydrogen, oxygen, and nitrogen species). List the potential
reactions, then determine the theoretical voltage for these reactions and if they could
occur in a fuel cell or not. Could any of these reactions occur normally? Note, the
species besides H2, O2, N2, and H2O must be generated and balanced by the overall
reaction, so you will have to combine some reactions to achieve this. Using your results,
explain why the hydrogen oxidation and oxygen reduction reaction is the reaction that
occurs, rather than other reactions along the same potential series.

Soln:

We could have the following reactions is a H2/O2 fuel cell, taken from Table 2.1

Half Reaction Voltage Eo (V)
2 H (+aq ) + 2e − → H 2,( g ) 0.000
2 H 2O( l ) + 2e − → H 2,( g ) + 2OH (−aq ) -0.830
− − −
aq ) + H 2O( l ) + 2e → 3OH ( aq )
HO2,( +0.880
H 2O2,( aq ) + 2 H (+aq ) + 2e − → 2 H 2O( l ) +1.776
N 2,( g ) + 4 H 2O(l ) + 4e − → 4OH (−aq ) + N 2 H 4,( aq ) -1.16
N 2,( g ) + 5 H (+aq ) + 4e − → N 2 H 5,(
+
aq )
-0.23
− + −
aq ) + 4 H ( aq ) + 3e → NO( g ) + 2 H 2 O( l )
NO3,( +0.96
O2,( g ) + 4 H (+aq ) + 4e − → 2 H 2O( l ) +1.23
O2,( g ) + 2 H (+aq ) + 2e − → H 2O2,( aq ) +0.68
O2,( g ) + 2 H 2O(l ) + 4e − → 4OH (−aq ) +0.40
O3,( g ) + 2 H (+aq ) + 2e − → O2,( g ) + H 2O( l ) +2.07

Note that Table 2.1 is obviously non-exhaustive, and does not contain carbon-reactions
that occur. The point here is to think about thermodynamics and kinetics a little and to
realize that there can be side reactions that occur to some extent as well (e.g. to realize
that it is not all the global reaction, and that the rate of reaction is as important as the
potential of reaction).
There are many reaction combinations you can come up with here using 2, 3, or more
reactions. Some make sense, others not. In general, the conventional reaction has a
higher potential, or if not, the kinetics must be very slow so it does not occur to a great
extent. Some manipulations can show the potential for a peroxide reaction, which does
indeed occur as an intermediate and minor species, as well as OH radical production.




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