## 4 State-of-the-Art Review
### Water Electrolysis  
[PERSON] approaches the subject of water electrolysis, especially PEMEL, as a viable alternative to produce hydrogen from renewable energy sources; in his study [68], several possible applications are discussed, including grid-independent and grid-assisted hydrogen generation, the use of support-electrolyzers, and integrated systems where electrolytically generated hydrogen is stored and then--via fuel cell--converted back to electricity when needed. It goes even deeper, by addressing specific issues regarding the use of PEMEL electrolyzers in renewable energy systems, such as sizing of equipment, the issue of intermittent operation, output pressure, oxygen generation, water consumption, and, naturally, efficiency. His findings are indeed interesting: PEMEL is a viable alternative for hydrogen generation in conjunction with renewable energy sources, particularly solar photovoltaics. PEMEL electrolyzers are simpler than conventional alkaline electrolyzers, being able to generate hydrogen (and optionally even oxygen) at pressures up to 200 bar with very little additional power consumption (which may be attractive for applications where it needs to be stored). This method is also capable of producing hydrogen with very high purity, at very high efficiencies--between 70% and 90% depending on the generation rate. [PERSON] and [PERSON] decided to approach this thematic from another perspective, developing an electrical equivalent model for a PEMEL electrolyser [126]. Using experimental results, the authors managed to model the input current-voltage characteristic for a single PEM electrolyser cell under steady-state conditions; useful power conversion and losses were taken into account, following Faraday's Law. They found that the developed model matches very closely the experimental results in the active operating electrolysis region, and, using the developed model and a simplified equivalent circuit, the hydrogen production rate and electrolysis efficiency can be estimated. It was observed that the hydrogen production rate is proportional to the input current, and efficiency decreases with input voltage, being up to 68% in this study. [PERSON] agrees on hydrogen as a future energy carrier having a number of advantages [35]. One of them is that it can be produced from a variety of primary resources, through water electrolysis; another important advantage is that its only major oxidation product is water vapor--so its use produces no CO\({}_{2}\), if generated from renewable energy sources and nuclear energy. The author asserts that hydrogen also has good properties as a fuel for internal combustion engines in automobiles, being able to be used as a fuel directly (not much different from engines using gasoline nowadays). The main problem here is that while hydrogen supplies three times the energy per kilogram of gasoline, it has only one tenth the density (when in a liquid form--very much less when it is stored as a compressed gas). [PERSON] and his co-authors presented an extensive literature review on increasing the efficiency of hydrogen production, stating that \"although hydrogen presents an excellent option as an energy carrier, much of hydrogen's current uses are based on its ability to chemically react with other molecules\" [76]. Some examples its uses as a reactant include petroleum processing, the production of petrochemicals, and the process for recycling plastics [127]. Besides, over 96% of the presently produced hydrogen is still generated using fossil fuels, only 4% coming from commercial electrolysis (yet with low efficiency and high production costs). [PERSON], [PERSON], and [PERSON] have studied the impact of nanomaterials in photoelectrochemical water splitting, a technique that could effectively couple solar energy with hydrogen production [128]. This promising recent technology has the potential to become an easy, cheap, and sustainable method of generating hydrogen, simply by adjusting the bandwidth of the photocatalyst material.