# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 1 Introduction  
Hydrogen is the most abundant element in the universe (it makes up about 74% of the matter of the universe). It is only ninth on Earth in terms of its prevalence (0.9% of the mass of the Earth's crust and hydrosphere) and occurs mainly in the form of water and numerous organic substances (natural gas, crude oil, biomass). Hydrogen fuel is considered emission-free because during its combustion with pure oxygen, as well as during electrochemical oxidation in fuel cells, the only by-product is water or water vapour. Since the use of the above-mentioned energy carrier is technically complicated and expensive, for a long time, virtually the only area where hydrogen fuel was used was rocket propulsion.  
The current need to reduce greenhouse gas emissions makes it necessary to develop fuels other than hydrocarbons to minimise the effects of global warming. Widespread awareness of the described problem, both among politicians and societies, favours the announcement of further programmes for the development of emission-free energy sources. In July 2020, the European Commission published a document entitled \"A hydrogenstrategy for a climate neutral Europe\" [1]. It indicates that \"hydrogen is a key element in achieving the European Green Deal and a clean energy transformation in Europe\", and also indicates the directions in which the use of hydrogen in the economy and industry should be developed. The priority in the strategy adopted by the European Commission is the production of emission-free hydrogen based on energy from renewable sources. Ambitious plans for the development of the hydrogen economy were also adopted by Poland in the \"Polish Hydrogen Strategy until 2030 with a perspective until 2040\" (PHS), published in December 2021, developed by the Ministry of Climate and Environment [2]. It formulates six goals relating to the priority areas of hydrogen use, which are industry, transport, and energy, as well as its production and distribution.  
To date, Poland is a leading hydrogen producer (approximately 1.3 million tons of H\({}_{2}\) per year) in the EU [3]. Hydrogen produced conventionally (mainly from natural gas), which is accompanied by greenhouse gas emissions, is used by industries. Therefore, in the light of PHS and other applicable regulations, it is crucial to implement new hydrogen technologies producing clean hydrogen that could be utilised in other sectors of the economy. Taking into account the current political situation and the need to diversify energy supply sources, as well as reduce dependence on the import of fossil fuels, the development of the hydrogen economy in Poland is of particular importance.  
This work discusses the physicochemical properties of hydrogen fuel compared to other energy carriers, its production technologies, including costs in addition to its environmental impact, and storage methods. The following part of the work presents the current state of the hydrogen economy in Poland in addition to examples of practical implementations of Polish hydrogen technologies important for its development. The motivation for writing this comprehensive review was to provide researchers and decision-makers with up-to-date knowledge about the possibilities and limitations regarding the transition to hydrogen energy with the example of Poland and to indicate a direction for further actions. The strengths and weaknesses of obtaining, storing, transporting, and using hydrogen were analysed. The goals of Poland's current energy policy in the context of implementing hydrogen energy were also presented.