Edexcel Advanced Subsidiary GCE in Chemistry (8CH01)
Edexcel Advanced GCE in Chemistry (9CH01)
Fuel Cells October 2007
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Authorised by Roger Beard Prepared by Sarah Harrison
This document is designed to help teachers to understand the contemporary context of fuel cells. It should give teachers information on this context and on how to research it further if they wish. This document could also be given to students as introductory material.
2 Context study (Fuel Cells) – Edexcel AS/A GCE in Chemistry (8CH01/9CH01) – Issue 1 – October 2007
The chemistry of fuel cells can be related to many areas of the GCE Chemistry specification, including sections 2.13 — Green chemistry and 5.3.1j — discuss the use of hydrogen and alcohol fuel cells. References within this document relate to the GCE Chemistry specification. References are in the form ‘2.5d(ii) — water solubility of alcohols’ with the appropriate module highlighted and a short, paraphrased description of the reference. Hydrogen and methanol are discussed as possible fuels.
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?
• 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 kinetically stable with respect to the product, water. Methanol and oxygen are also kinetically 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
Context study (Fuel Cells) – Edexcel AS/A GCE in Chemistry (8CH01/9CH01) – Issue 1 – October 2007 3
is overcome (typically, by application of a spark). An exothermic chemical reaction1 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.
Figure 1 — A hydrogen explosion in controlled laboratory conditions
A fuel cell converts chemical energy directly to electricity, not via heat:
fuel + oxygen → water + carbon dioxide + energy The energy released is in the form of electricity and a small amount of heat.
The electrodes and the catalyst
Typically, a fuel cell consists of two electrodes containing a catalyst, between which is an electrolyte. Oxygen in the air reacts at one electrode and the fuel, hydrogen or methanol, at the other. The product is water, together with carbon dioxide if methanol is the fuel. 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 kinetically-stable chemical reaction to proceed at reasonable temperatures and pressures by following an alternative reaction route of lower activation energy2 than the combustion reaction. The fuel reacts at the anode (negative electrode) and oxygen (either pure, or in air) reacts at the cathode (positive electrode). In a hydrogen fuel cell, the half-equations that occur at the electrodes are:
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)
Figure 2 — A methanol fuel cell designed to operate at room temperature
The photographs show a methanol fuel cell designed for laboratory experiments and demonstrations. There are actually two sets of electrodes in this particular design, one at either side of the fuel reservoir. Methanol is easily introduced into the reservoir with a pipette, through a small hole at the top of the cell. A hydrogen-ion-conducting polymer membrane separates each set of electrodes. One side of the membrane is directly exposed to the air, the other to the central methanol reservoir. The chemical reaction occurs on the surface of the catalyst at the electrode.
Figure 3 — Fuel cell electrodes The polymer membrane is held between inert plastic spacers, which keep the anode and cathode from coming into contact with each other and short-circuiting the cell. For low temperature fuel cells, the catalysts are typically expensive transition metals such as platinum, although research into alternative, cheaper materials is underway3. The catalysts are sensitive to poisoning from impurities in the fuels which may block their active sites, thus reducing efficiency, lifespan and value. The optimisation of these catalysts is therefore an area of considerable worldwide research.
3 to be studied during A2 Chemistry — 5.3.2i — development of new catalysts
cathode containing catalyst
anode containing catalyst
‘Exploded’ schematic of electrodes
Photograph of fuel cell electrodes
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In high-temperature fuel cells, many of which operate in excess of 500°C, the hydrogen-ion-conducting electrolyte is of particular interest. One type of solid electrolyte used is a class of materials called ‘apatites’, an example of which is hydroxyapatite, Ca10(PO4)6(OH)2, a major constituent of teeth and bone. By using alternative elements but keeping the same crystal structure, similar versions of this material (eg lanthanum barium silicate, La9Ba(SiO4)6O2.5), can be used as electrolytes.
Figure 4 — Apatite-type lanthanum barium silicate, a hydrogen ion conductor
(Picture by Emma Kendrick) The structure of such materials need not be remembered.
The fuels used
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 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.
When methanol is used in a fuel cell the products of the reaction are water and carbon dioxide4,5.
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.
4 1.4f(ii) — Enthalpy of combustion measurements using alcohols 5 2.10.1c(i) — Combustion of alcohols
6 Context study (Fuel Cells) – Edexcel AS/A GCE in Chemistry (8CH01/9CH01) – Issue 1 – October 2007
In fuel cells of the type shown in the previous photographs, methanol can also be used in a dilute, aqueous solution6,7. A measurable current and potential difference (voltage) can be obtained across such methanol fuel cells at concentrations as low as 0.01 mol dm-3.
Figure 5 — Experiments to investigate the effect of methanol concentration on a) the voltage
and b) the current; this may also be used to study fuel cells connected both in parallel and series
Compare the reactions of hydrogen and methanol with that of, say, octane (or any of its C8 isomers8) that may occur 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 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)9. Incomplete combustion therefore occurs with both carbon dioxide and toxic carbon monoxide as products.
6 1.3d — Understand concentrations of solutions 7 2.5d(ii) — Water solubility of alcohols 8 1.7.2b — Structural isomers 9 1.3f — Moles of gases
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Fuel cells relate directly to section 2.13 — Green chemistry, and can be used to develop key skills such as communication if included in group discussions.
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 re-circulated 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.
10 2.13 — Green chemistry
8 Context study (Fuel Cells) – Edexcel AS/A GCE in Chemistry (8CH01/9CH01) – Issue 1 – October 2007
The following web-based resources can be used for background information and can help develop ICT key skills if used as background reference materials for projects. www.biologicalfuelcells.org.uk SuperGen BioFuelCells Consortium
www.bmweducation.co.uk BMW: looking at hydrogen cars
www.est.org.uk Energy Saving Trust: information about alternative fuel sources or cars
www.eyeforfuelcells.com Eye for Fuel Cells: The Business of Fuel Cells
www.fuelcells.org Fuel Cells 2000: The online fuel cell information resource
www.fuelcellsuk.org Fuel Cells UK: The UK organisation of Fuel Cells
www.fuelcelltoday.com Fuel Cell Today: The global internet portal for fuel cells with fuel cell news
www.gm.com General Motors: How GM are developing fuel cell cars
www.goodideacreative.com/fuel_cell.html Build Your Own Fuel Cell: All the information you need to make your own fuel cell
www.grovefuelcell.com Grove Fuel Cell: the Grove Fuel Cell website
www.matthey.com Johnson Matthey Fuel Cells: UK company based in Reading researching fuel cell developments
www.surrey.ac.uk/Chemistry/FuelCells Research at the University of Surrey
www.thecarbontrust.co.uk The Carbon Trust: gives press releases and information about low carbon technology
www.vcacarfueldata.org.uk Car Fuel Data: provides information about cars, emissions and other legal matters
www2.exxonmobil.com/corporate Exxon Mobil: the Esso website
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