Hydrogen fuel cells

How much energy is needed to make each kilogram of gasoline?  This may sound like an unusual question, because we tend to think of gasoline as a source of energy.  When one kilogram of gasoline is burned it releases about 45 megajoules (MJ) of energy in the form of heat, that can then be converted into other forms of useful energy.  However, 9 MJ are needed to produce a kilogram of fuel.  Though this number is non-zero, the important point is that less energy goes into the fuel than is recovered from it.  This feature makes petroleum good as a fuel source, among other properties.

In a previous post, I wrote about the energy requirements of producing hydrogen.  Every kilogram of hydrogen contains roughly three times as much energy as a kilogram of petroleum—about 140 MJ.  Steam methane reformation requires 135 MJ per kilogram of hydrogen; this represents a net gain in energy, but a very small net gain.  Furthermore, it relies on the same non-renewable resources that it is meant to replace.  However, even if we rely on electrolysis to manufacture hydrogen, the fuel cells extract energy by performing the electrolysis reaction in the opposite direction.  That is to say, the energy recovered by fuel cells is the same as the energy used in manufacturing the fuel.  At best, this only allows electricity to be converted to a more portable form.

There are difficulties in making hydrogen portable, as well.  Because gasoline is relatively dense, it can carry a large amount of energy in a compact space.  Hydrogen pressurized to 5,000 psi, would occupy over eight times as much space as the equivalent amount of gasoline, at room temperature.  Instead, hydrogen is typically cooled to about −400 degrees Fahrenheit.  At this temperature, the same hydrogen would occupy a similar amount of space as gasoline, but this temperature would need to be maintained.  Gasoline can sit for years before being used, but hydrogen would likely need to be used shortly after it is brought to temperature.

Electrolysis of water

In a previous post, I stated that steam methane reforming requires only half the energy of the electrolysis of water.  The so-called hydrogen economy will be a topic that I will come back to from time to time, and while doing more calculations, I may have encountered an error in the numbers.  It is important to always present accurate information—my goal is, of course, to promote education and critical thinking.

Steam methane reforming is a process in which natural gas and water are placed under high temperature—up to 1000 degrees Celsius—and pressure.  This ensures that the reactants are fully converted into hydrogen and carbon dioxide.
$$ \text{CH}_4 + 2 \text H_2 \text O + 165 \text{ kJ} \rightarrow \text{CO}_2 + 4 \text H_2 $$This source claims that the process requires approximately 135 megajoules (MJ) of energy.  However, my best efforts cannot reconcile this with naïve thermo-chemistry.  Using the enthalpy of formation, I can only account for 21 MJ per kilogram of hydrogen.  Even including all the heat required to create the high temperatures needed does not reconcile the difference.

Electrolysis, as the name implies, involves the use of electricity to disassociate the oxygen and hydrogen in water.  Though this process can be demonstrated without high temperatures in table-top experiments, industrial applications often require them.$$2 \text{H}_2\text{O} + 484 \text{ kJ} \rightarrow \text{O}_2 + 2 \text{H}_2$$Because of the stability of the bonds in water, the energy requirement is considerably larger than steam reformation.  Naïve thermo-chemistry yields about 120 MJ per kilogram of hydrogen.  I am unable to find sources that give the energy requirements for the industrial state of the art.  From a purely theoretical stand point, though, steam reformation has only one fifth the energy requirement of electrolysis.  Furthermore, if electrolysis could more easily manufacture hydrogen, we would already use this method more widely; though, it remains unclear exactly how different the energy requirements are.

Nitrogen

"It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty... The fixation of atmospheric nitrogen is one of the great discoveries, awaiting the genius of chemists." ― William Crookes, Chemical News

I've tried to avoid equations, but this one is too relevant to today's discussion.  Ammonia is necessary to making fertilizer, and it can be easily made with hydrogen and nitrogen, in the Haber-Bosch process: $$ \text{N}_2 + 3 \text{H}_2 \rightarrow 2 \text{NH}_3 $$

Schematic of the Haber-Bosch process.

Nitrogen is essential to life.  Though only 3% of the human body is nitrogen, by weight—9%, if water is ignored—it gives proteins and DNA much of their chemical identity.  We don't simply absorb nitrogen from the atmosphere, like oxygen.  It is supplied through the food we eat, after it has been collected by other plants and animals.  However, modern agriculture has become dependent on artificial fertilizers.  In fact, 80% of the nitrogen in your body comes from the Haber-Bosch process.

This process relies on hydrogen as a reactant, which is abundant on Earth, in the form of water.  Most of our hydrogen does not come from water, though.  95% of it comes from hydrocarbons, with only 5% coming from the electrolysis of water.  The most common process used to manufacture hydrogen is steam reformation of methane.  This process is able to efficiently produce hydrogen at lower cost than electrolysis, though it does produce a substantial amount of carbon dioxide as a by-product.  It also requires about half the energy as electrolysis per kilogram of hydrogen.

In the United States, most hydrogen in used to refine other hydrocarbons—for example, it is used to remove sulfur from petroleum.  Only 1.6 million tonnes of hydrogen could supply our need for ammonia, which would require three times as much hydrogen to be produced by electrolysis. However, other countries use nitrogen more for the production of ammonia, and meeting the world need for cheap fertilizers could be more difficult.