RHS Chemistry department
Name ……………………………………………………………………………………………………
Cells
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Date ………………….
Fuel Cells
Why do we need fuel cells?
We live in a power-hungry society. This power comes at an environmental price in terms of increasing
CO2 emissions (CO2 is a greenhouse gas that contributes to global warming). In recognition of this
problem, and with the prospect of dwindling fuel reserves, Government plans state that the UK should
move to a more energy-efficient economy and cut CO2 emissions to 60 per cent of the 1990 level by 2050.
As energy demands are increasing, this is not a trivial requirement.
How can this be achieved?
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nuclear energy: zero CO2 emissions, but environmental concerns
renewable energy: solar, wind, tidal, biofuel crops (concerns about displacing food crops as they
can be more profitable)
more efficient energy conversion: fuel cells.
Fuel cells versus conventional power generation
Conventionally, electrical energy is generated by burning fossil fuels:
Chemical energy → mechanical power to drive turbine → electricity
As a power station is only able to convert about 30 per cent of the available energy from fuels into
electricity, this is an inefficient process by any standards (100 per cent efficiency is theoretically
impossible). It also results in unacceptably high pollution levels.
A fuel cell is an electrochemical device which operates like a battery, converting chemical energy directly
to electricity in a chemical reaction between the fuel and oxygen, which is effectively a combustion
reaction. As long as fuel is supplied to the cell it will produce energy.
Chemical energy from fuel cell → electricity
Consider the combustion reactions of hydrogen and methanol. The reactants, hydrogen and oxygen, are
stable with respect to the product, water. Methanol and oxygen are also stable with respect to the
products carbon dioxide and water. In both reactions, the reactants will mix together and not react
unless a high activation energy barrier [the activation energy is the minimum amount of energy needed
for a reaction to occur] is overcome (typically, by application of a spark). An exothermic chemical reaction
then occurs:
hydrogen + oxygen → water + energy (heat)
methanol + oxygen → carbon dioxide + water + energy (heat)
We are accustomed to hydrogen and methanol being used to produce heat energy. Methanol is used by
campers for cooking, and the ability of hydrogen to generate heat may be demonstrated by exploding a
hydrogen-filled balloon.
A fuel cell converts chemical energy directly to electricity, not via heat. The energy
released is in the form of electricity and a small amount of heat.
Figure 1 — A hydrogen explosion in controlled laboratory conditions
How the fuel cell works
Typically, a fuel cell consists of two electrodes, between which is an electrolyte. Oxygen in the air reacts
at one electrode and the fuel, hydrogen, at the other. The product is water. The products from hydrogen
fuel cells are so pure that astronauts on the space shuttle use the water to drink. The catalyst is the most
important part of the fuel cell. It allows a chemical reaction to proceed at reasonable temperatures and
pressures by following an alternative reaction route of lower activation energy [the activation is the
minimum amount of energy required for a reaction to occur] than the combustion reaction.
In a hydrogen fuel cell, the half-equations that occur at the electrodes are:
2H2(g)  4H+(aq) + 4e- (oxidation half-equation)
O2(g) + H2O(l) + 4e-  4OH-(aq) (reduction half-equation)
The hydrogen ions and oxygen ions are free to move within the electrolyte and combine to form water,
H2O(l):
H+(aq) + OH-(aq) H2O(l)
Comparing the fuels used in fuel cells and petrol engines
Hydrogen
Hydrogen is the cleanest fuel to use — the product of the reaction is only non-polluting water, with no
greenhouse gases formed, except for the carbon dioxide produced in its manufacture from methane or, if
made by the electrolysis of water, the carbon dioxide released when the electricity is generated.
H2(g) + ½O2(g)  H2O(l)
ΔH* = -286 kJ mol-1
* The more negative the value of
H, the more energy is released.
It requires a heavy container to store gaseous hydrogen under pressure, or a refrigerated container to
store hydrogen as a liquid. Hydrogen can only exist as a liquid at a temperature below 33 K or – 240 oC.
Despite this, hydrogen is a highly desirable fuel, as the energy density is relatively large, at – 142.9 kJ g-1.
Petrol has an energy density of approximately – 50 kJg-1.
Methanol
When methanol is used in a fuel cell the products of the reaction are water and carbon dioxide.
CH3OH(l) + 1½O2(g)  CO2(g) + 2H2O(l) ΔH = -726 kJ mol-1
Methanol, a liquid, is used as a fuel because it is easy to store and transport in a conventional,
unpressurised fuel tank, and to transfer to a fuel cell. These benefits help offset that the energy density
of methanol is -22.7 kJ g-1, far less than that of hydrogen, and that methanol produces the greenhouse
gas carbon dioxide as a product, as well as the CO2 produced during its manufacture.
Fossil fuels
Compare the reactions of hydrogen and methanol with that of, say, octane used in a petrol engine. For
complete combustion to occur with only carbon dioxide and water vapour as products, the combustion
reaction of octane is:
C8H18(l) + 12½O2(g)  8CO2(g) + 9H2O(l) ΔH = -5512 kJ mol-1
Petrol is easy to transport and transfer to combustion engines and the energy density of octane,
-48.4 kJ g-1, is higher than hydrogen [or methanol]. However, such fuels are non-renewable and highly
polluting. Methanol and hydrogen are both currently made from non-renewable fossil fuels, as well as
using fossil fuels as a source of energy during the manufacturing process. The complete combustion
reaction above actually occurs in many steps, as part of a chain reaction. With alkanes such as C8 found in
petrol, so many moles of oxygen per mole of fuel are required that it is difficult to achieve the necessary
ratios of gases in an internal combustion engine. For example, in a car engine where the oxygen is taken
from air that contains 20 per cent oxygen, 1500 dm3 of air would be required to completely burn one
mole of octane (given that one mole of gas occupies 24 dm3). Incomplete combustion therefore occurs
with both carbon dioxide and toxic carbon monoxide as products.
Environmental impacts of fuel cells
Figure 6 — Daimler Chrysler NECar (New Electric Car) — one of the earliest fuel cell powered cars developed
1. Fossil fuels, formed by the decomposition of marine plants and animals millions of years ago, are
non-renewable. There are great concerns over diminishing fossil fuel reserves and therefore a
need for more efficient energy conversion devices such as fuel cells. Because they convert
chemical energy directly to electricity and do not burn fuel to drive mechanical systems which
then produce electricity, fuel cells are fundamentally more efficient than combustion systems.
Where a power station is only able to convert around 30 per cent of chemical energy to
electricity, fuel cells are able to operate to efficiencies of between 50 and 80 per cent. It is
estimated that if just 20 per cent of its cars used fuel cells, America could cut oil imports by 1.5
million barrels every day. No other energy-generating technology currently available holds the
combination of benefits that fuel cells offer.
2. Fuel cells can be used in conjunction with other renewable energy sources, such as solar or wind
power, offering the promise of a totally emission-free energy system. For example, NASA is
investing in ‘regenerative fuel cells’ as a closed-loop form of power generation. Water is
separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen
are then fed into the fuel cell, which generates electricity and water. The water is then recirculated back to the solar-powered electrolyser and the process begins again.
3. Fuel cells can help to reduce air pollution, which continues to be a primary health concern.
Scientists are now directly linking air pollution to heart disease, asthma and cancer. Recent health
studies suggest polluted urban air is a comparable health threat to passive smoking. Fuel cell
vehicles, operating on hydrogen stored on-board, produce no pollution at point of use, as no CO,
CO2 or NOx are emitted. Their only by-product is water.
Adapted from Context study (Fuel Cells) – Edexcel AS/A GCE in Chemistry (8CH01/9CH01) – Issue 1 October 2007
© Edexcel Limited 2007