Water electrolysis is the chemical reaction that occurs in water when it is passed through by an electric current, resulting in its separation into molecules ofH2 and oxygen (O2) according to Equation2.1.
2H2O→2H2+O2 (2.1)
This is done using an electrolytic cell, whose generic structure is shown in Figure2.2. The electric current flows between two separated electrodes, called anode and cathode, which are im-mersed in an electrolyte to increase conductivity. The electrodes must have good electric con-ductivity, be resistant to corrosion and show appropriate structural integrity. Also, the electrolyte cannot undertake any changes during the process, so it must not react with the electrodes. A sep-arator (diaphragm) is essential to avoid the recombination of the generated oxygen and hydrogen, and it should have high ionic conductivity and stability [25].
The term electrolysis was first used by Michael Faraday when formulating the basic physical laws behind this chemical process in 1834, also known as Faraday’s Laws of Electrolysis. This word results from the combination of the Greek words "elektron" (amber, a material associated with electrical phenomena for centuries) and "lysis" (breakdown/decomposition) [27].
It is a process known for over two centuries, but it is not very clear who discovered it first since different statements can be found in the literature. However, most sources agree that, in the year 1789, Adriaan Paets van Troostwijk and Johan Rudolph Deiman were the first to produce hydrogen using electricity, with an electrostatic generator being used to break water into "combustible air"
(H2) and "life-giving air" (O2). Their experiments were successfully repeated by George Pearson, who reported his results to the Royal Society of London in 1797, noting that "...hydrogen and
Figure 2.2: Generic structure of an electrolytic cell [26].
oxygen gases were produced by passing electric discharges through water”. At a subsequent time, William Nicholson republished Pearson’s paper in his own journal, making a contribution with several comments [28].
With the introduction of the Volta pile in 1800 by Alessandro Volta, an easily reproducible electricity source became available. With the ability to provide an almost constant voltage and to deliver energy over a longer period of time, this apparatus allowed for wider use of electricity, marking the beginning of electrochemistry as a science. Using this new device and their knowledge of water electrolysis, Nicholson and Anthony Carlisle repeated the experiments of van Troostwijk and Deiman, being able to analyse the anodic and cathodic products separately. The accessibility of this new battery increased the interest of many other researchers to perform more studies, but, according to Faraday’s first law, it takes high currents to mass produce chemicals, something that Volta’s pile could not achieve [28].
In the years that followed, new and more sophisticated electrochemical cell models were stud-ied, leading to the creation of several patents, the publication of numerous papers and books, and the development of new devices. Among these can be found the electrochemical reactor for hy-drogen production, designed by Charles Renard around 1890 and used to generateH2for French military airships [28].
Around 1900 over 400 industrial electrolysers were already operating. Hydrogen was needed to produce ammonia for fertilizers and explosives, and so the electrolyser industry felt significant growth during the 1920s and the 1930s. Manufacturers like Oerlikon, Norsk Hydro and Com-inco started to supply plants in the multi-megawatt range. Many of these were installed close to Hydropower plants (HPP), so as to make use of an inexpensive electricity source [29].
Today these devices are the basic units of hydrogen production from water, converting elec-tricity into chemical energy in a P2G process [28].
Depending on the type of electrolyte used in the cells, it is possible to distinguish between several types of electrolysers, from which the most important ones are [28]:
• Alkaline Water Electrolysers (AWE);
• Proton Exchange Membrane Electrolysers (PEME);
• Solid Oxide Electrolysers (SOE).
2.3.1 Available technologies
AWE are the most mature water electrolysis technology and, nowadays, they are still the stan-dard systems for large-scale electrolysis applications [25].
Each alkaline cell is made of a pair of electrodes, which are immersed in an aqueous elec-trolyte with a microporous membrane (diaphragm) separating them. The cathode is usually made of nickel with a catalytic coating, such as platinum. The anode can be made of nickel or copper metals coated with metal oxides, such as manganese, ruthenium or tungsten. Due to its high con-ductivity, Potassium Hydroxide (KOH) is typically the chosen electrolyte, but Sodium Hydroxide (NaOH) is also an option. During the reaction, the electrolyte is not consumed but has to be re-plenished over time because of diverse losses. The diaphragm keeps the produced gases apart and has to be permeable both to water and hydroxide ions. In the past, asbestos was used as separator material, but due to high corrosion and adverse health effects, many alternatives were developed, with current membranes being based on sulfonated polymers, polyphenylene sulphides, polyben-zimides and composite materials [27] [25].
The main advantages of AWE are a high Technology Readiness Level (TRL), lower costs and higher lifetime when compared to other technologies, and a good tolerance to impurities.
Meanwhile, the main disadvantages are a lower flexibility and slower start-up, a lowerH2 purity and a large stack size [30].
Regarding PEME, the usage of exchange polymers for electrochemical applications dates back to the middle of the 20th century. In 1966, General Electric created the first electrolyser based on the proton conducting concept using a polymer membrane as the electrolyte, which they started to commercialize in 1978 [25].
Nowadays, PEME are perceived as the second most important electrolysis technology. Owing to the growing interest in the production of greenH2and the opportunity to overcome some issues related to AWE, PEME have been receiving more attention [27].
The materials used for the electrode catalysts are usually platinum black, iridium, ruthenium, or rhodium and the most commonly used membrane is Nafion, which separates the electrodes and acts as a gas separator [31]. As an alternative to the liquid electrolytes employed in alkaline cells, in a Proton Exchange Membrane (PEM) cell, thin proton conducting membranes are used as electrolytes [27].
The main advantages of PEME are higher efficiency, higher flexibility, lower minimum load, lower power-up and power-down times, higherH2purity and a more compact design. The main disadvantage is the higher cost [30].
Finally, the SOE is the least mature technology among these three. Based on steam electrolysis at high temperatures, between 600 ºC and 900 ºC, the SOE provides an opportunity to significantly
reduce the consumption of electrical energy for electrolysis processes when compared to both AWE and PEME [27]. This is achieved by replacing part of the required electrical energy with thermal energy [31].
This concept is not new, as it started to be developed in the USA by General Electric and the Brookhaven National Laboratory during the late 1960s. In Germany, Dornier followed with the HOT ELLY project, which lasted until the mid-1980s. Significant technical progress was made, but the commercialization of these devices was still far from being a reality [28].
In solid oxide cells, the electrolyte is made of a thin, dense, and solid oxide layer, that turns conductive for ions at high temperatures [31].
The main advantages of SOE are high efficiency and low minimum load. The main disadvan-tage is the low TRL, higher costs, high operating temperatures and a large size [30].
2.3.2 Efficiency
The efficiency of an electrolyser is an important aspect to compare the performance ofH2PP and to calculate the cost of electrolytic hydrogen. In general, it is given by the ratio between the energy contained in the producedH2 and the energy used by the electrolyser to produce it [25].
However, there are two slightly different definitions available for this indicator [32].
According to the first definition, the energy contained in the producedH2is given by its HHV, and as such, the efficiency of an electrolyser is:
ηELEC(%) =HHVH2
CE
·100 (2.2)
whereηELEC is the electrolyser’s efficiency,HHVH2 is the HHV ofH2 andCE is the energy con-sumed by the device.
The second definition considers that the energy contained in the H2 is given by its LHV, resulting in:
ηELEC(%) =LHVH2
CE ·100 (2.3)
whereLHVH2 is the LHV ofH2.
As there is no definitive agreement about which method should be used, instead of a percentage many electrolyser manufacturers and suppliers choose to provide the energy consumed by the device, in kWh, to produce a certain amount ofH2[33].
In this dissertation, it is the HHV ofH2 that will be used to calculate the efficiency of an electrolyser.
This decision is supported by the majority of the works consulted that also use the HHV.
Besides, according to [27], due to the fact that electrolysers often use liquid water as feedstock, the energy required for evaporation of H2Ohas to be taken into account, meaning that it is the HHV ofH2that should be used for this calculation.
Additionally, the heat released by the electrolyser can be used for other purposes, such as district heating, effectively increasing the efficiency of the electrolytic process and potentially reducingH2production costs [34].
2.3.3 Water consumption
As shown, electrolysis relies on two fundamental inputs: electricity as the energy source and water as the main feedstock. Based on the stoichiometry of this reaction, for every kg of H2 produced, at least 9 L (≈9 kg) of water is needed but, in reality, and due to process inefficiencies and losses, up to 20 L of water are required to obtain 1 kg ofH2[35,36].
Since one of the goals of the creation of a H2 economy is to decarbonize energy systems and fight environmental change, it is not irrelevant to analyse possible negative side effects of H2 production for the environment. One of those is the consumption of a significant amount of freshwater and, as such, some authors have already examined this [35].
The water demand of electrolysis is not necessarily larger than that of other hydrogen produc-tion methods. Generating H2 from NG with CCS takes between 13 and 18 L ofH2Oper kg of H2, while coal gasification uses between 40 and 86 L of water per kg ofH2. Under these circum-stances, water is not a bottleneck for scaling up electrolysis, even in territories with a higher level of water stress where seawater desalination is an option, since reverse osmosis for desalination takes 3-4 kWh of electrical energy perm3 of water and should only have a minor impact on the final cost of hydrogen [35].
That last premise might be particularly important for countries like Portugal and Spain, which are investing in and planning the conception of multiple hydrogen projects while facing a high risk of water stress caused or aggravated by rising temperatures [37], more frequent and intense heatwaves [38] and longer droughts periods [39].
Additionally, according to other authors, a future replacement of fossil fuels byH2for energy-related applications has the potential to result in the conservation of hydric resources, because nowadays, the exploitation of these hydrocarbons consumes a great amount of water in mining, hydraulic fracturing, cooling and refining. Furthermore, the water consumed for producing elec-trolyticH2is expected to be particularly marginal when compared to other sectors, such as irri-gated agriculture. Concerns about freshwater scarcity call for a reduction in water extractions at all possible angles, and, therefore, pursuing solutions which allow theH2economy to make use of Earth’s saltwater resources can further reduce its water footprint [36].
2.3.4 Costs
Regarding the cost of these devices, as seen in Table2.2, for an AWE, the Capital Expendi-ture (CAPEX) was around 600 C/kW in 2020, but it is expected to fall to 480 C/kW by 2024.
Conversely, the capital cost of a PEME was around 900 C/kW in 2020 and is expected to drop to 700 C/kW by 2024 [40,41]. According to these data, an electrolyser with a lower rated power has a higher CAPEX, in terms ofe/kW, when compared to a more powerful device.
Table 2.2: Cost of different electrolysers, according to type and rated power.
Source Year Type Rated Power CAPEX (e/kW)
Demo4Grid [42] 2017 AWE 2.5 MW 680
Haeolus [43] 2018 PEME 2.5 MW 1328
Demo4Grid [42] 2017 AWE 5.0 MW 550
Demo4Grid [42] 2017 AWE 10 MW 530
Refhyne [44] 2018 PEME 10 MW 1000
Demo4Grid [42] 2017 AWE 20 MW 515
FCH JU [40] 2020 AWE ——— 600
FCH JU [40] 2020 PEME ——— 900
(future prospects[41]) 2024 AWE ——— 480
(future prospects[41]) 2024 PEME ——— 700