# An Overview of Hydrogen's Application for Energy Purposes in Lithuania
## 3 Hydrogen Value Chain
Alkaline water electrolysis is the most popular and widely-used process. It is an electrochemical water splitting technique in the presence of electricity. This process consists of two individual half-cell reactions: hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. According to [24], the main advantages  
Figure 1: Example of hydrogen value chain (developed by the authors). of such technique would be well established technology; electrocatalysts are made from non-noble metals, etc. However, the disadvantage would be a highly-concentrated alkaline solution (5M potassium hydroxide (KOH)) used as a liquid electrolyte, which causes the corrosion. The efficiency of this type of water electrolysis in 2020 was 50-78% [22]. AEM water electrolysis is a process that utilises a semipermeable membrane (anion exchange membrane) that conducts hydroxide ions with help of electricity. This electrochemical reaction consists of the same two half-cell reactions (HER and OER) as in alkaline water electrolysis; it has the advantage of lower concentration alkaline solution needed (1M KOH), but it has limited stability and it is not yet commercialised [24]. According to [22], in 2020 the efficiency of AEM water electrolysis was 57-69%. PEM water electrolysis is a process where water is electrochemically split into hydrogen and oxygen. During this electrolysis initially water molecule is decomposed to generate oxygen (O\({}_{2}\)) and protons (H\({}^{+}\)), and electrons (e\({}^{-}\)) at the anode side. The main advantages are the high purity of hydrogen gases being produced and compact system design; however, the major issues are its dependence on iridium, one of the rarest and most expensive elements, and the acidity of the electrolyte [24,25]. The efficiency for this type of electrolysis was between 50% and 83% in 2020 [22]. Solid oxide water electrolysis typically operates at higher temperatures, consumes water in the form of steam and splits it to green hydrogen and oxygen. As well as AEM water electrolysers, they are still under development, but their overall efficiency is the highest compared to the other types of water electrolysers and could reach around 89% at a lab scale [24]. Intensive research is performed at laboratories in an attempt to increase the efficiency of hydrogen production process, some of these have demonstrated promising results. A team of researchers implementing a GrInHy2.0 hydrogen project have achieved a technological breakthrough by demonstrating the high production efficiency of green hydrogen: the solid oxide electrolyser produced 200 Nm\({}^{3}\) of green hydrogen per hour and proven electrical efficiency is 84% [26]. Even more promising results come from [PERSON] et al. [27], they have published the results of their team's experiments with an alkaline capillary-fed electrolysis (CFE) cell demonstrating water electrolysis performance exceeding commercial electrolysis cells, with a cell voltage at 0.5 A cm\({}^{-2}\) and 85 \({}^{\circ}\)C of only 1.51 V, equating to 98% energy efficiency. These examples clearly show that higher electrolysis efficiency is possible to achieve and combined with the potential of balance-of-plant simplification, brings cost-competitive green hydrogen closer to reality. Attention should also be given to green hydrogen production through biomass fermentation such as dark or photo-fermutation, pyrolysis, direct or indirect biophotolysis and gasification, during which a small amount of CO\({}_{2}\) is produced [28,29]. This CO\({}_{2}\) is part of the natural carbon cycle as a form of biomass; these processes are therefore environmentally friendly ways of producing hydrogen. Dark fermentation is considered as most productive and technically available and it is also one of the technologies with the lowest production cost [30]. According to [PERSON] and [PERSON] [31], hydrogen production from biomass has shown a net HHV (higher heating value) energy efficiency of 56-64%.