## 5 Conclusions  
The development of renewable hydrogen production technologies is a vital step moving forward into a truly sustainable human existence; the use of renewable resources for energy generation is pivotal. Although renewable hydrogen production technologies have made some very important advances lately--increasing its feasibility as a broad-scale energy generation method--there remains the need to develop methods with greater \begin{table}
\begin{tabular}{p{113.6 pt} p{113.6 pt} p{113.6 pt} p{113.6 pt} p{113.6 pt} p{113.6 pt}} \hline \hline
**Production Method** & **Advantages** & **Disadvantages** & **Efficiency (\%)** & **Cost (EUR/kg)** & **Ref.** \\ \hline \multicolumn{4}{p{113.6 pt}}{**Hydrocarbon Reforming**} \\ Steam methane reforming & Developed technology; existing infrastructure & Unstable supply; production of greenhouse gases & 74–85 & 1.90 & [105; 106; 107; 108; 109] \\ Partial oxidation & Established technology & Production of heavy oils and petroleum coke & 60–75 & 1.24 & [111] \\ Auto-thermal reforming & Well established technology; existing infrastructure & Use of fossil fuels; production of greenhouse gases & 60–75 & 1.24 & [112; 113] \\ \multicolumn{4}{p{113.6 pt}}{**Hydrocarbon Gasification**} \\ Coal gasification & Abundant, cheap feedstock; less GHG emissions & Fluctuating H\({}_{2}\) yields because of feedstock impurities, seasonal availability and formation of tar & 35–55 & 1.60 & [114; 115] \\ \multicolumn{4}{p{113.6 pt}}{\(\uparrow\) Non-Renewable \(\uparrow\)} \\ \hline \multicolumn{4}{p{113.6 pt}}{**Therrochemical Biomass Processes**} \\ Pyrolysis & Abundant; cheap feedstock; CO\({}_{2}\)-neutral & Tar formation; fluctuating & Tar formation; fluctuating & \(\rm H_{2}\) amount because of feedstock impurities; seasonal availability & 35–50 & 1.38 & [119] \\ \multicolumn{4}{p{113.6 pt}}{**Biological Biomass Processes**} \\ Bio-photolysis & Consumption of CO\({}_{2}\) production of O\({}_{2}\) & Low yields of H\({}_{2}\); sunlight required; large reactor; high cost & 10–11 & 1.79 & [119] \\ Dark fermentation & Simple method; no light required; CO\({}_{2}\)-neutral; waste recycling & Fatty acids elimination; large reactor required & 60–80 & 2.15 & [121] \\ Photo fermentation & Waste water recycling; CO\({}_{2}\)-neutral & Low efficiency; low production rates, large reactor and sunlight required & 10 & 2.37 & [122] \\ \multicolumn{4}{p{113.6 pt}}{**Water Splitting**} \\ Thermolysis & Clean and sustainable; production of O\({}_{2}\); abundant feedstock & High capital costs; element’s toxicity; corrosion problems & 20–45 & 6.96 & [122] \\ Photolysis & Production of O\({}_{2}\); abundant feedstock; no emissions & Low efficiency, non-effective photocatalytic material; sunlight required & 6 & 7.54 & [121] \\ Electrolysis & Established technology; zero-emissions; existing infrastructure; production of O\({}_{2}\) & Storage and transportation problem & 60–80 & 8.63 & [68; 126] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Hydrogen production technologies summary. efficiency for them to be economically competitive with current hydrogen production methods sourced on fossil fuels. In this review, a short introduction was made about the various strategic configurations for the hydrogen value chain and the best storage systems currently available. Some background concepts were discussed regarding hydrogen production technologies, with particular attention given to water electrolysis. In addition, research related to alkaline water, solid oxide, and proton-exchange membrane electrolysis was deepened, giving focus to their respective characteristics, operational and economical parameters, nominal features, system details, advantages/disadvantages, and market maturity.