Producing ‘Green’ Hydrogen Gas (VCE SSCE Chemistry): Revision Notes

Producing 'Green' Hydrogen Gas

Why hydrogen as a fuel?

As society works to reduce carbon dioxide emissions from traditional fuels, hydrogen gas has become an important alternative fuel option. Unlike fuels such as methane, petrol and biodiesel, the combustion of hydrogen produces only water as a product.

The thermochemical equation for hydrogen combustion is:

$2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$ $\Delta H = -564 \text{ kJ}$

Hydrogen releases significantly more energy per gram than other fuels. When 1 g of hydrogen gas burns completely, it releases 141 kJ of energy, compared to just 55.6 kJ from burning 1 g of methane.

Advantages and disadvantages of hydrogen gas as a fuel

Understanding both the benefits and challenges of hydrogen fuel is essential for evaluating its potential as an alternative energy source.

Advantages of hydrogen gas as a fuel	Disadvantages of hydrogen gas as a fuel
High energy density: $282 \text{ kJ mol}^{-1}$	Not found naturally as an element, so energy is needed to produce it
Abundant on Earth: present in water and most carbon compounds	Very low boiling point of $-253°\text{C}$, requiring significant energy to liquefy it. High pressures needed for efficient gas storage
Sole combustion product is water: $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$	Explosive, requiring careful handling and storage
	Difficult to transport safely as either gas or liquid

Hydrogen colour classification system

While hydrogen itself burns cleanly, its production methods vary in environmental impact. Hydrogen is classified by colour according to how it is produced:

Brown hydrogen is derived from fossil fuels. Historically, most hydrogen has been produced by steam reforming of methane:

$\text{CH}_4(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}(g) + 3\text{H}_2(g)$ $\Delta H = +206 \text{ kJ}$

The carbon monoxide produced is then converted to carbon dioxide. This process releases $\text{CO}_2$ as a by-product, which is harmful to the environment.

Grey hydrogen is derived from industrial processes.

Blue hydrogen is derived from fossil fuels but with carbon capture technology, reducing emissions.

Green hydrogen is produced using renewable energy sources and represents the cleanest production method.

Electrolysis of water

In the laboratory, producing hydrogen by electrolysis of dilute sulfuric acid ($\text{H}_2\text{SO}_4$) solution is straightforward. Two electrodes are placed in the acid solution and connected to a power supply.

The overall equation for water electrolysis is:

$2\text{H}_2\text{O}(l) \rightarrow 2\text{H}_2(g) + \text{O}_2(g)$

This is an acidic electrolytic cell. The half-equations occurring at each electrode are:

The process produces two useful products: hydrogen gas and oxygen gas. According to the overall equation, the volume of oxygen gas produced should be half the volume of hydrogen gas.

Industrial production of green hydrogen

Industry cannot use simple electrolysis of acidic or alkaline solutions for large-scale hydrogen production because:

• The process cannot easily accommodate the intermittent nature of renewable energy sources like solar arrays and wind turbines
• The solutions used are corrosive to equipment
• The hydrogen gas produced is not compressed, requiring additional processing

Polymer electrolyte membrane (PEM) electrolyser

Modern industrial hydrogen production uses advanced electrolysers. An electrolyser is a system that uses electricity to decompose water into hydrogen and oxygen through electrolysis.

A polymer electrolyte membrane (PEM) electrolyser uses a solid electrolyte instead of a liquid solution. The initials PEM can also stand for proton exchange membrane, as the membrane allows protons to flow through to complete the electrical circuit.

The half-equations for PEM electrolysis are:

Anode: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + \text{O}_2 + 4e^-$

Cathode: $2\text{H}^+ + 2e^- \rightarrow \text{H}_2$

Overall equation: $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$

PEM electrolysers are high-technology devices. The electrodes are made from expensive metals such as ruthenium and iridium. These metals must be porous enough to allow gas passage while preventing liquid flow. They also act as catalysts to improve the efficiency of gas production. The membrane contains advanced polymers called polysulfones, which conduct protons but not electrons.

Electrolyser stacks and hydrogen hubs

A single electrolyser does not produce enough hydrogen to be economically viable. Modern electrolysers are designed to be assembled in clusters called electrolyser stacks. Multiple units can be combined into a large hydrogen plant, making them more cost-effective.

A hydrogen hub is a facility where renewable electrical energy powers an electrolyser stack to produce compressed hydrogen for storage. The concept behind hubs is to locate as many connected industries as possible in one area, so hydrogen can be produced and used without requiring transport or liquefaction.

Australian hydrogen projects

Several Australian states are developing hydrogen production facilities:

• Hydrogen Park South Australia: Australian Gas Networks (AGN) is investing $12 million in a demonstration project with a 1.25 MW electrolyser. Plans exist for a 50 MW plant linked to South Australia's wind energy and battery storage network.
• Victoria and Western Australia: The Australian Renewable Energy Agency (ARENA) is investing over $100 million to build 10 MW electrolysers.
• Port of Gladstone: This location is emerging as a potential world leader in hydrogen production, with multiple companies planning manufacturing and export facilities.

How hydrogen hubs operate

Hydrogen hubs follow a systematic production and distribution process:

1. Water is purified
2. Purified water is split into hydrogen and oxygen using an electrolyser powered by electrical current
3. The electrolyser is powered by green energy sources such as solar panels, wind turbines or tidal energy
4. Hydrogen is used locally as fuel, or converted into ammonia or synthetic natural gas (SNG) for easier transport
5. Ammonia or SNG can be shipped overseas, then converted back to hydrogen and used as fuel

Hydrogen cars

When a hydrogen fuel cell is installed in an electric car, the hydrogen gas generates the electrical energy needed to power the vehicle. The only emission from the car is water vapour.

For hydrogen vehicles to become practical, two key requirements must be met:

• A hydrogen gas distribution system (refuelling stations) must be established
• Hydrogen storage tanks must be safely installed in vehicles to prevent explosions in crashes

Artificial photosynthesis

While producing hydrogen from renewable energy is a major advancement in reducing emissions, scientists are developing an even more efficient approach. Current solar panel systems require energy storage in batteries before the electricity can be used in an electrolyser. Artificial photosynthesis eliminates this intermediate step by using solar energy directly to produce fuel.

Photoelectrochemical cells

A photoelectrochemical cell operates like a solar panel immersed in solution:

• Sunlight strikes the anode
• Solar energy excites the metal atoms in the electrode
• This energy oxidises water to produce oxygen gas and hydrogen ions: $2\text{H}_2\text{O}(l) \rightarrow \text{O}_2(g) + 4\text{H}^+(aq) + 4e^-$
• Hydrogen ions migrate to the cathode
• At the cathode, hydrogen ions are reduced to hydrogen gas: $2\text{H}^+(aq) + 2e^- \rightarrow \text{H}_2(g)$

The overall result is that solar radiation directly produces hydrogen gas. This process mimics natural photosynthesis. In plants, sunlight converts carbon dioxide and water into glucose. In artificial photosynthesis, the end-product is hydrogen gas instead of glucose.

Challenges and future potential

Like fuel cells and lithium-ion batteries, photoelectrochemical cells use complex combinations of metals in the electrodes, catalysts and conducting membrane polymers. Energy efficiencies of up to 30% have been achieved for converting solar energy to the chemical energy of hydrogen.

Alternative method: microbial production of hydrogen

An alternative research approach for commercial hydrogen production involves using bacteria to produce hydrogen gas from biomass such as forest waste. Microorganisms break down cellulose in the waste into glucose at the anode of an electrolysis cell, where it is then converted to ethanoic acid. The hydrogen ions produced migrate to the cathode, where they are reduced to hydrogen gas.

This process is called dark fermentation because no light is required for the bacteria to function. A typical equation for this process is:

$\text{C}_6\text{H}_{12}\text{O}_6(aq) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{CH}_3\text{COOH}(aq) + 2\text{CO}_2(g) + 4\text{H}_2(g)$

Currently, the low reaction rate makes this process commercially non-viable, but researchers are working to improve its efficiency. This method offers potential energy savings because the voltage required is much lower than that needed to electrolyse water.

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