What is hydrogen and what are the application areas?

What is hydrogen?

Hydrogen (H₂) is the first element on the periodic table and it consists of one electron and one proton. Under the standard conditions, hydrogen is in the gas phase and it is the most abundant gas in the universe. Hydrogen is colorless, odorless, tasteless, non-toxic and non-poisonous [³]. Hydrogen does not naturally occur as an energy source like oil or coal. It is obtained by consuming energy through methods such as SMR or electrolysis and can then be converted back into energy with a fuel cell. In this respect, it is an energy carrier, not an energy source.

Characteristics of Hydrogen

Hydrogen’s energy content is 143 MJ/kg, which is up to three times higher than fossil fuels by weight [³].

Hydrogen has a wide range of flammability (4-75%) therefore, the energy required to initiate the combustion of hydrogen is much lower than other common fuels [³]. Flash point indicates the lowest temperature that vapor formed on a liquid can be ignited at atmospheric pressure. Since, hydrogen has the lowest flash point among common fuels, so it is more prone to flammability [⁴].

The Role of Hydrogen in Decarbonization

Hydrogen does not contain any carbon. Depending on the production process, it does not contribute to carbon emissions [²]. Hydrogen is classified by color depending on the production source.

White hydrogen: Naturally occurring hydrogen [⁵]
Yellow hydrogen: Hydrogen produced by electrolysis. The energy provided by a power grid.[⁵]
Pink hydrogen: Hydrogen produced by electrolysis. The energy provided by nuclear power.[⁵]
Green hydrogen: Hydrogen produced by electrolysis. The energy provided by renewable energy sources.[⁵]
Black hydrogen: Hydrogen produced via gasification of bituminous coal [⁵]
Brown hydrogen: Hydrogen produced via gasification of lignite coal [⁵]
Gray hydrogen: Hydrogen produced via hydrocarbon reaction process from fossil fuels [⁵]
Turquoise hydrogen: Hydrogen produced via methane pyrolysis [⁵]
Blue hydrogen: Hydrogen produces via fossil fuels along with carbon capture, utilization and storage methods. [⁵]

An ideal situation is known as “Net Zero,” in which the quantity of greenhouse gases absorbed from the atmosphere is equal to the amount emitted. Governments and unions publish their strategy and regulations to reach the net zero goal or for transformation to net zero. For instance, Global Net Zero Target, 2oC Target and Global Hydrogen Mobility Initiatives, European Green Deal, European Union 2030 Climate Target Plan, EU 2050 Long Term Strategy, Paris Climate Agreement, Turkey 2053 Net Zero Emission Target, Turkey Hydrogen Technologies Strategy and Road Map. As the trend to reduce carbon emissions increases, the use of hydrogen and its applications are also diversifying.

Area of applications

• Direct fuel in the scope of hydrogen concept covers the use of hydrogen as a fuel directly. This technology is still progressing and depends on mixing hydrogen with natural gas [⁶].
• Power-to-gas technology is the idea of using existing excess renewable electricity and producing hydrogen gas via electrolysis. The produced hydrogen can be stored for later use, using produced hydrogen gas directly via injecting the existing natural gas grid, and converting it to methane to store it in a natural gas network [⁶].
• Fuel upgrading is a process that aims to transform raw or low-quality fuels into products with desirable properties for specific applications. Hydrogen plays a pivotal role in several of these processes, significantly enhancing fuel quality and environmental performance [⁶].
• Syngas is a mixture of gases primarily consisting of hydrogen (H₂) and carbon monoxide (CO). Syngas fermentation is a biological process that converts syngas into ethanol, higher alcohols, and other valuable bioproducts [⁶]. In this process hydrogen acts as an energy source and reduces the equivalents for microbial metabolism and the subsequent synthesis of biofuels and chemicals.
• Fuel cells transform chemical energy stored in a fuel (hydrogen, depending on the type) directly into electrical energy. Fuel cells have several advantages over batteries and combustion engines. As long as the fuel is supplied, a fuel cell will continue to produce electrical energy.There are 5 major types of fuel cells which are phosphoric acid fuel cell (PAFC), polymer electrolyte membrane fuel cell (PEMFC), alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), solid-oxide fuel cell (SOFC). Depending on the fuel cell type, it produces no emissions, or only trace amounts, of pollutants such as NOx, SOx, and particulate matter. This makes them a more environmentally friendly option than combustion engines, which are a major source of air pollution [⁷].

• Aerospace applications: The demand for air travel requires sustainable alternatives to traditional fuels (kerosene-guzzling jets). Liquid hydrogen is promising due to low emissions and high energy density, but faces challenges like complex combustion, hydrogen risks, storage difficulties, and high costs. Advances in hydrogen production, engine optimization, and hybrid fuel systems could revolutionize aviation [⁶].
• Maritime applications: The growth in maritime trade raises concerns over CO2 emissions and fuel costs. Hydrogen and ammonia are potential alternative fuels. Hydrogen is noted for its safety and compatibility with existing ships, while ammonia, though carbon-free, faces challenges with NOx emissions. Both require further research for a cleaner maritime future [⁶].
• Ammonia production: Ammonia, essential for fertilizers and chemicals, currently has production issues like high energy use, greenhouse gas emissions, and fossil fuel reliance. Greener production methods, such as electrochemical hydrogen production and photocatalytic nitrogen fixation, are being explored [⁶].
• Pharmaceutical applications: Hydrogen is crucial in the pharmaceutical industry, used in various precursors and sustainable oxidation reactions. It’s also being investigated as a therapeutic gas for conditions like inflammation and cancer. Its isotope, deuterium, has unique applications in drug metabolism, medical imaging, and protein analysis, highlighting hydrogen’s versatility in pharmaceuticals [⁶].
• Metallurgical applications: Hydrogen is gaining prominence in metallurgy as a versatile and efficient reducing agent for metal production. It offers cleaner alternatives to traditional methods by reducing metals from their salts in solution. Its diverse chemical interactions and ability to trap metals enhance its utility. Hydrogen is particularly effective in processes like nickel production, providing high yields and improved efficiency. It also benefits pig iron production by enhancing reaction kinetics and furnace heat balance, highlighting its potential to transform metal production [⁶].

[1] Hydrogen as an energy vector (sciencedirectassets.com)
Hydrogen Production | Deparment of Energy
[2] The prospects for hydrogen as an energy carrier: an overview of hydrogen energy and hydrogen energy systems | Energy, Ecology and Environment (springer.com)
[3] Hydrogen as an energy carrier: Prospects and challenges (sciencedirectassets.com)
[4] 22 – Specifications (sciencedirectassets.com)
[5] JMSE | Free Full-Text | Color-Coded Hydrogen: Production and Storage in Maritime Sector (mdpi.com)
[6] Futuristic applications of hydrogen in energy, biorefining, aerospace, pharmaceuticals and metallurgy – ScienceDirect
[7] Fuel Cell Fundamentals, 3rd ed. Ryan O’hayre, Suk-won Cha, Whitney Colella, Fritz B. Prinz, 2016