# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 3 Hydrogen Production Methods
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**Hydrogen Production Technology** & **Estimated Proces Efficiency [\%]** & **Emission of CO2[kgO2/kg H2]** & **Hydrogen Colour** & **Advantages** & **Disadvantages** & **Estimated Hydrogen Production Cost [EUR/kg H2]** \\ \hline Hard coal gasification process & 30–40 & 15–20 & Bosch & & high-emission technology, low availability technology, low efficiency, low efficiency, low efficiency & high-emission technology, low  
Power-to-gas (PtG) technology envisages the making of hydrogen on an industrial scale through water electrolysis using electricity. Water electrolysis is a process with unfavourable thermodynamics that requires external energy. The four most commonly used methods of water electrolysis for hydrogen production are proton exchange membrane water electrolysis (PEM), solid oxide water electrolysis (SOE), alkaline anion exchange membrane water electrolysis (AEM), and alkaline water electrolysis (AEL) [19; 20; 21; 22; 23; 24; 25; 26]. It involves reducing protons at the cathode (negative electrode) to hydrogen gas and simultaneously oxidising water at the anode (positive electrode) to oxygen gas. \[\text{Cathode: 4H}^{+}+4\text{e}\to 2\text{H}_{2}\quad\text{E}^{\circ}=0\text{ V} \tag{6}\]  
\[\text{Anode: 2H}_{2}\text{O}\rightarrow\text{O}_{2}+4\text{H}^{+}+4\text{e} \quad\text{E}^{\circ}=+1.23\text{ V} \tag{7}\]  
The efficiency of water electrolysis is approximately 70-80% [22; 23], and the goal is to achieve an efficiency of up to 82-86% by 2030 using proton exchange membrane (PEM) electrolysers [24]. These electrolysers are characterised by high efficiency but also a high equipment cost. Therefore, they should be used for small-scale green hydrogen production. Hydrogen produced by means of water electrolysis is much more costly (3.6-12.1 USD/kg H\({}_{2}\), which is dependent on the price of, among others, electricity and electrolysers) [25] than hydrogen made by the classic methane reforming method (1.2-2.7 USD/kg H\({}_{2}\)) because of the high energy input needed for the water electrolysis process [25]. However, hydrogen obtained in the synthesis process by processing gas or coke cannot be considered a clean energy carrier. In order to ensure the production process of hydrogen is climate neutral, it is essential to use water as a source of hydrogen in addition to electricity from renewable energy sources (RES)--we assign such hydrogen the colour green. To avoid transmission losses, it is best to locate PtG electrolysers near renewable energy sources. If we use nuclear energy as a source of electricity to produce hydrogen, then as a result of the electrolysis process, we will obtain hydrogen assigned a purple colour. It is also possible to obtain hydrogen using the photocatalytic and photoelectrocatalytic decomposition of water. To this end, semiconductor materials and catalysts are used, capable of absorbing solar radiation, whose energy is utilised to reduce water [14; 15; 16; 17; 22]. Nevertheless, systems of this type are not yet used on an industrial scale. The decision whether hydrogen is cleaner than other alternative energy sources should be assessed based on its entire life cycle and not just on selective stages, e.g., compared to zero emissions during electrochemical oxidation in a PEM fuel cell. It should be remembered that hydrogen fuel is not emission-free. As shown by the authors of [27], the hydrogen technologies with the lowest carbon footprint are the steam reforming of waste alcohol (586.7-768.1 kg CO\({}_{2}\) equivalent/t H\({}_{2}\)) and water electrolysis using renewable energy sources (2329.8 kg CO\({}_{2}\) equivalent/t H\({}_{2}\)). In addition, the use of green hydrogen in the most efficient fuel cells results in obtaining electrical power two times lower than the energy used for electrolysis [27].