Green Hydrogen

Many of you would have heard about green hydrogen, but do you know what it actually is. Hydrogen is one of the most abundant elements on our planet, and it can be used as a fuel in energy generation, but it is rarely found in isolation as it is usually part of other compounds. Hydrogen can either be completely combusted with oxygen in the reaction: 2H2 + O2 —> 2H2O, or it can be used in hydrogen fuel cells, both of which cause energy and electricity to be generated. Therefore, hydrogen is a highly demanded fuel, however most hydrogen, approximately 95% of all hydrogen, is produced from the steam reformation of natural gas, which of course releases greenhouse gases. So, green hydrogen effectively aims to produce this hydrogen fuel and then to use it to generate energy, all without producing greenhouse gases. 

Firstly, the process of Green Hydrogen produces the hydrogen gas through the electrolysis of water (H2O) into hydrogen gas and oxygen gas, which are separated from one another. In electrolysis, a circuit is connected to two electrodes: the anode and the cathode. The positive terminal of the circuit is connected to the anode, which is given a positive charge and so, it attracts anions (negatively charged ions), or the OH- ions in water. While the negative terminal of the circuit is connected to the cathode, which is given a negative charge and so, it attracts cations (positively charged ions), or the H+ ions in water, thus hydrogen gas is formed at the cathode. Now these two electrodes are firstly made of graphite, in order to conduct electricity, and is placed into the chosen solution, which is called the electrolyte.

However, this process of electrolysis requires electricity to run the circuit, so in order to maintain green hydrogen’s stance of having net zero emissions, this electricity is supplied by renewable energy resources. Now as I said before, hydrogen fuel can be used in two ways to generate energy: Combustion or hydrogen fuel cells, but let’s start with combustion. Hydrogen is combusted with oxygen (in the air) in the reaction:  2H2 + O2 —> 2H2O, forming water, whilst also releasing a significant amount of energy. This process occurs in Hydrogen combustion turbines. Firstly, hydrogen gas is introduced into the combustion chamber, where it is ignited, producing high temperature gases. The high temperature gases mainly consist of water vapour, as the hydrogen reacts with oxygen in the air, but there is also nitrogen oxides (NO and NO2) as nitrogen in the air also reacts with oxygen under high temperatures. Finally, there can also be some unreacted hydrogen gas. Then, these hot, expanding gases flow through the turbine blades, causing them to spin and since the turbine is connected to the generator, the mechanical energy of the gases is converted to electrical energy as electricity is generated. This process is more environmentally friendly than burning fossil fuels as the byproduct of this process is primarily water vapour. However, sometimes nitrogen oxides are produced which are pollutants that also cause health issues. However, these are present whenever there is combustion at high temperatures because it is hard to restrict their production.

On the other hand, fuel cells work in a way that seems to be the opposite of the process of electrolysis to produce electrical energy from the chemical energy store of the hydrogen fuel. Like in electrolysis, fuel cells have the two electrodes:  cathode and an anode, as well as an electrolyte. The hydrogen fuel is fed to the anode (positively charged), while oxygen is fed to the cathode (negatively charged). At the anode, a catalyst (usually platinum) reacts with and oxidises the hydrogen, splitting it into positively charged protons and negatively charged electrons, which both take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity, which is electrical energy. The protons however, move through the electrolyte to the cathode as they are oppositely charged and so, the protons experience an attractive force. Then at the cathode, the electrons and the hydrogen protons combine with the oxygen to produce heat and water.   

However, the process of obtaining hydrogen gas from electrolysis actually requires more energy than what is generated when hydrogen is used as a fuel. This poses the question: Why is green hydrogen even considered and used an energy source if there is a net energy loss during the conversion process. This is due to a number of reasons. 

Hydrogen as a fuel can be stored and transported more easily than electricity can, especially over long distances, where the transmission of electricity through power lines results in energy losses. So, the hydrogen can be produced when and where renewable energy is abundant and then, be stored for use either when renewable energy generation is insufficient or when energy demands are high. 

Hydrogen is also used as a fuel in various applications from power generation for the masses to being used as a fuel for vehicles. Hydrogen gas as a fuel, has a high energy, meaning that relatively more energy can be released from hydrogen for a given volume of it, as opposed to other fuels. This is why it is particularly favourable in aviation, where there is limited amounts of space.