Fuels' Paradise - How can Chemistry find a way out of the energy crisis?

Introduction and Contents[1-8]:

The title of this literature research project is a challenging one: How can chemistry find a way out of the energy crisis?

That an energy crisis is at hand is a commonly accepted fact, also that the burning of fossil fuels is very inefficient and creates heavy pollution.

So what can chemistry do and is the future going to be a fuels’ paradise? In this paper I want to show, that chemistry (in combination with physics, engineering etc.) offers some means to get out of the energy crisis, namely the technology. What is also needed is political and social acceptance and the backing and understanding that we must put an end to our reliance on fossil fuels.

What will the future look like then? There are many paths to get there but most people share the opinion, that we are going towards a hydrogen future. Fuel cells are going to be the major energy sources in the upcoming decades and centuries. The fuel cell itself was invented in 1839 but it is only in the last 40 years that we got really interested in them. We are now on the step of seeing them applied in everyday life. In many cities buses are already running on fuel cell power and in California, since this year, 10% of all new vehicles must be zero emission vehicles.

Many researchers predict a shift towards hydrogen in the coming decade and many papers show how this can be achieved. The most pressing question is however, how the hydrogen is going to be supplied, i.e. what the infrastructure is going to look like.

As I will outline in chapter 6, hydrogen can be produced from many different sources and it looks like using fossil fuels for hydrogen production would be the easiest option, as it can use the existing infrastructure. But, as R. Buckmister Fullerene has put it, the fossil fuels are our “savings account”. Surely it is easier to live from the savings, but much more realistically and especially far more responsible to our children, is to live from the “current account”, i.e. tapping renewable energy sources. This refers to the cover picture of this paper: renewable sources of energy (solar, hydro, biomass, geothermal) can be used to produce hydrogen, effectively leaving us with one of the cleanest energy systems imaginable.

The most optimistic thinkers outline a future with decentralized power systems. Solar panels on roofs and big wind farms produce the electricity to electrolyse water into hydrogen which will a) fuel people’s cars and b) provide fuel for stationary fuel cells. These provide energy and heat for a whole village or a skyscraper. An electric grid would not be necessary anymore (at least not in the current size), reducing losses from resistance and prohibiting power outages such as the one in the north-east United States a couple of months ago. The electricity providers would become service providers who offer the service of supplying PVs, wind farms and fuel cells. This would mean that they have an interest in long life-time and would design the systems so that waste is avoided and they could be recycled.

This may sound like Utopia, but I believe that we have not got much choice and have to dare to take the step forward.

In this paper I want to outline the general working principle of Fuel cells in Chapter 1; briefly discuss the physics behind Fuel Cells in Chapter 2; introduce different FCs in Chapter 3; and have a look at the latest research on fuel cell catalysts in Chapter 4. Finally I want to comment briefly on Safety in Chapter 5 and finish with Chapter 6 on Hydrogen Production and Storage.

1. Principle [9-11,13-17] :

The basic working principle of a fuel cell is very simple and the reaction well known: A hydrogen molecule H2 and an oxygen molecule O2 combine to yield a water molecule via the following reaction scheme:

Abbildung in dieser Leseprobe nicht enthalten[1]

This reaction is probably so well known because it is quite exothermic and the acoustic and visual effect quite impressing, when hydrogen is “burnt” in air.

In a fuel cell however, instead of heat, electrical energy is produced.

In order for a fuel cell to work properly, i.e. to draw a useful current from it, a large contact area between the gas, the electrolyte and the electrode is needed, as well as a short distance between the two electrodes as the electrolyte resists the flow of the electric current.

These parameters govern the shape of fuel cells in such a way that they are usually flat, with a rather thin layer of electrolyte being dispersed between the anode and the cathode. The figure below outlines the basic construction of a fuel cell combined with the anodic and cathodic reactions:

Abbildung in dieser Leseprobe nicht enthalten

Figure 1

These reactions differ in the various fuel cells, however as the Proton Exchange Fuel Cell -based on an acidic electrolyte- is the most common and probably simplest cell it is taken as an example.

As can be seen from the figure above, di-hydrogen molecules which should come from hydrogen fuel are oxidised at the anode to protons via the following reaction:

Abbildung in dieser Leseprobe nicht enthalten

At the cathode molecular oxygen, which is very abundant in air is reduced to water by taking up two protons from the anodic reaction:

Abbildung in dieser Leseprobe nicht enthalten

In order for these two reactions to run continuously, the electrons released at the anode have to pass through an electrical circuit to the cathode, when at the same time protons have to pass through the electrolyte. This is straightforward in an acid electrolyte, as the liquid contains free H+ ions. In modern advanced fuel cells however, this electrolyte is a polymer through which the protons can diffuse and it is therefore called a proton exchange membrane. Therefore, these fuel cells are abbreviated PEMFC, or Proton Exchange Membrane Fuel Cells.

The most important feature of these proton exchange membrane is obviously, that it allows protons to pass through but must not let electrons pass it, as we would not be able to draw a useful current from the system.

What limits the current?

The energy released at the anode does not guarantee a reaction continuing at an unlimited rate. The energy level diagram looks as follows (note: “Energy released” is in physical terms the Gibbs free energy G):

Abbildung in dieser Leseprobe nicht enthalten

Figure 2

Although energy is released, a molecule must have enough energy to “overcome” the energy barrier of the activation energy, otherwise the reaction will proceed very slowly (unless the temperature is very high). There are three main possibilities to deal with slow rates of reaction:

1. The use of a catalyst
2. Raising the temperatures
3. Increasing the electrode area

Discussing the first point is one of the main goals of this paper and both the first and the second point are very common in chemical reactions in general. The third point however is very specific to fuel cells: As figure 1 lines out, protons, electrons and oxygen molecules have to “come together” in order to form water. As this reaction takes place on the surface of the electrode, this is often called three phase contact and is vitally important for the design of the fuel cells.

It shows, that the rate of the reaction is proportional to the surface area available on the electrode and it explains, why cell performance is quoted as current per area, or A cm-2.

It has been mentioned before, that the electrode has to be porous in order to let the gas pass through. This is also very useful when considering the surface area of the electrode, as it can not simply be calculated from the length and width of the electrode. In actual fact due to the microstructure of the electrode its surface area can be thousands of times larger than the “obvious” one. This micro-structural design is a vitally important issue for fuel cells, as the catalyst has to be added as well. Furthermore, the activity of the electrode must not be negatively affected by temperatures or corrosive material, the latter effect again being a main issue in this paper.

2. The Physics behind the fuel cell [9-17] :

On the next couple of pages I want to discuss fuel cell efficiency, efficiency limits, open circuit voltage, the effect of pressure and gas concentration and losses and voltage drops.

The second law of thermodynamics tells us the maximum work that we can get from an ideally reversible heat engine. It is dependent on the temperatures between heat that is transferred. Carnot came up with a term to calculate the maximum efficiency for heat engines.

In fuel cells, chemical energy is directly converted into electrical energy and the maximum energy available as work from a fuel cell is given by the Gibbs free energy of reactions (see Figure 2 in Chapter 1):

DG = DH - TDS [4]

where H is the enthalpy of reaction and S the entropy of reaction at the temperature T. In order to obtain energy from the system, DG has to be negative, i.e. the reaction has to be spontaneous. To be precise it should be noted, that it is the free energy of formation we are talking about, so

DGf = Gf (products) – Gf (reactants) [5]

In theory all Gibbs free energy could be converted into electrical energy, but in practice some is lost as heat (which however is used in combined systems, giving them a high efficiency).

Looking at reaction [2] in chapter 1, we can see, that 2 electrons pass through the circuit of the cell for every hydrogen molecule reacted. If N is Avogadro’s Number (6.02214x1023 mol-1), then 2N electrons pass the circuit for every mole of hydrogen used. So with –e being the charge of one electron, the charge that flows can be expressed as

-2Ne = -2F coulombs [6]

where F is Faraday’s constant (9.64853x104 C mol-1), also known as the charge on one mole of electrons. Knowing that work done can be expressed by multiplying charge by voltage (E) then

Work done = -2FE joules [7]

If, as mentioned above, our fuel cell system was ideal, this work done would be equal to the Gibbs free energy released from the fuel cell reaction:

DGf = -2 F E [8]

or

E = - D Gf / 2F [9]

This last equation is fundamental in fuel cell technology as it enables us to calculate the reversible open circuit voltage (OCV) also called electromotive force (EMF).