## 1 Introduction  
Nowadays, an ever-expanding human population coupled with a growth in anthropogenic activities and general better standards of living have led to a significant surge in overall energy consumption [1, 2]. Presently, most of the energy generation comes from fossil fuel sources; Figures 1 and 2 show how the widespread use of coal, oil, and natural gas since the beginning of the 19 th century has led to the continued emission of greenhouse gases--such as carbon dioxide, methane, and nitrous oxide--causing a gradual increase in the concentration of these gases in the Earth's atmosphere and contributing to environmental degradation and climate change (it should be noted that the most recent measurement has already peaked at 419 ppm, in May of this year) [3, 4]. As is well known, the presence of these gases in the atmosphere traps heat radiating from the Earth toward space, effectively warming it. Figure 3 shows how there is mounting evidence that this global warming is man-made, namely, by observing the rise of world temperatures, the warming of the oceans, shrinking ice sheets, glacial retreats, decreased snow cover, the declining of the Arctic Sea ice, a broad sea level rise, widespread ocean acidification, and more extreme weather events in general [5, 6, 7]. Figure 1: CO\({}_{2}\) atmospheric concentration: long-term overview. Based on data retrieved from [8]. Figure 3: Relationship between CO\({}_{2}\) concentration in the atmosphere and global temperature anomaly. Based on data retrieved from [9]. Figure 2: Relationship between fossil fuel consumption and CO\({}_{2}\) concentration in the atmosphere. Based on data retrieved from [8]. Furthermore, fossil fuels are naturally a finite resource; so, using them is inherently limiting the use of such energy sources by future generations. While these factors are enough to motivate a total replacement to alternative sources of energy, it turns out that we are actually increasing the use of these conventional fuels, whose impacts are already rapidly approaching tipping-points that will bring disastrous consequences for humanity [10, 11, 12]. Fighting climate change might be the greatest challenge of this generation; it all boils down to halting the temperature rise, which in turn means decreasing the atmospheric concentration of greenhouse gases, which, again, in turn means finding solutions to replace fossil fuels as our primary energy source; to reverse the on-going global warming, we urgently need to decarbonize the world economy--hence, the development of renewable energy sources has become essential. While such renewable sources like solar and wind can provide environmentally friendly alternatives to fossil fuels, their intermittent nature brings the need of an energy storage medium that allows for the continual provision of energy; as there is no one-size-fits-all solution, we need a multi-faceted approach to accomplish that. For instance, instead of using common batteries, these sources could grant the energy needed to produce hydrogen from water, which can then be stored as a means to generate electrical and mechanical energy, as well as heat--thus ensuring the continuous production of emissions-free energy, which is necessary to fulfill modern society's consumption requirements [13, 14]. The push for environmentally friendly energy solutions has renewed the interest to accelerate the development of hydrogen production methods. Currently, around 96% of global hydrogen production comes from non-renewable fossil fuels [15, 16]. However, besides releasing harmful greenhouse gases to the atmosphere, these methods can only produce low-purity hydrogen [17, 18, 19]. This article focused on studying green hydrogen production methods, namely, through the analysis of different types of water electrolysis technologies currently being developed and used in modern industry--including their characteristics and modes of operation, their advantages and disadvantages, and their similarities and differences.