## 2 Hot Topics
### Feeding Solutions  
In AEMWE, deionized water, ultrapure water, 1 wt.% K\({}_{2}\)CO\({}_{3}\), and 1 M KOH have all been used as reactants [3]. Hydroxide solutions are used as an alternative to pure water to feed AEMWE to increase the ionic conductivity of the AEM because resistance losses decrease almost linearly (in a limited range of concentrations) when increasing electrolyte concentration [21]. However, higher hydroxide concentration conducts in elevated corrosion rates, reducing the lifetime of AEMWEs [7]. Better performances have been reported using 1 M KOH [2]. The predicted hydrogen demand in the distant renewable future is 2.3 Gt per year [22]. Considering the reaction stoichiometry, for every kg of hydrogen produced, 9 kg of water must be consumed. Therefore, 2.3 Gt of hydrogen requires 20.5 Gt, or 20.5 billion m\({}^{3}\) of water [23]. Although in other sectors, i.e., agriculture, more water is used than necessary for the most ambitious scenario for hydrogen, the scarcity of freshwater limits its use. As mentioned before, the feeding condition significantly impacts on the electrocatalytic activity and, consequently, AEMWE performance. The use of demineralized water is the most preferred option to make AEM electrolysis a competitive technology. Nevertheless, using pure water carries critical new challenges. Ni is currently the standard material for electrode construction in AEM electrolysis. However, under anodic polarization, it is stable only at a pH higher than 9, conditioning the use of a pure water feed without performance degradation. Another problem with using deionized water, also occurring using hydroxide solutions, is the negative influence of CO\({}_{2}\) on performance. [PERSON] et al. [24] described that this phenomenon was due to dissolved carbonate and bicarbonate anions that diminish the ionic conductivity of the membrane and catalytic layer binders. The use of impure water in AEMWE is also an important research topic. Even if most pollutants in wastewater are successfully removed by the current wastewater treatment processes, small quantities of impurities (residual organics and ions) remain in the treated water. No studies show how the impurities affect water electrolyzer performance and durability, despite studies reporting how impurities affect either the membrane or electrode. There is a need to understand more fully the impact of water impurities present in wastewater on the performance and durability of AEMWE. For example, despite the fact that most cationic impurities have undesirable effects during electrolysis, cationic Fe impurities seem to be essential to stabilize OER activity in alkaline conditions with Earth-abundant catalysts [6]. [PERSON] et al. [25] reported that Fe cations intensely improve OER activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, acting as possible OER active sites. However, pure Fe (oxy)hydroxide is not a good catalyst for OER, and its effect still needs to be better understood. Cation poisoning can be avoided by adjusting the water flow rate and current density to minimize co-ion transport or through engineering approaches. It is also important to understand the relationships between the morphology, composition, and phase of the catalyst material and its durability in impure water AEM electrolysis. Analysis of Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), and several metals (i.e., zinc, copper, iron) present in treated wastewater will be necessary to identify its composition. If CO\({}_{2}\) (previously identified as a detrimental compound) is present, it must be removed before using the wastewater. It is also possible that supplementary water treatments will be needed to eliminate specific impurities affecting AEMWE performance and durability. For example, precipitation or adsorption treatments could be able to remove undesirable metals. Another possible solution is the use of seawater to feed AEMWEs. However, this latter approach leads to a side reaction, competitive with OER: the chloride oxidation reaction (CIER).