Water electrolysis works well at a small scale, and the process is even more sustainable if electricity used is derived from renewable sources e. In a conceptual energy system involving production, conversion, storage, and use in remote communities, water electrolysis may play an important role.
When abundant renewable energy is available, extra energy may be stored in the form of hydrogen using water electrolysis. The stored hydrogen can then be used in FCs to generate electricity or used as a fuel gas for heating applications. Therefore, electricity generated by the renewable energy is either directly merged into the grid or used to produce hydrogen. Figure 1 illustrates such an energy system. Small-scale water electrolyzers may avoid the need for a large number of cryogenic, liquid hydrogen tanks or a huge hydrogen pipeline system.
The existing electrical power grid can be used as the backbone of the hydrogen infrastructure system, contributing to the load leveling by changing operational current density in accordance with the change in electricity demand. Although hydrogen possesses the advantages of availability, flexibility, and high purity, for its widespread applications hydrogen production using water electrolysis needs improvements in energy efficiency, safety, durability, operability, and portability, and, above all, reduction in installation and operation costs.
These features open up many opportunities for research and development leading to technological advancements in water electrolysis. This paper presents the electrochemical fundamentals of water electrolysis and a theoretical basis for scientific analysis of the available electrolysis systems.
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The various water electrolysis techniques and a broad range of applications are analyzed. The main research needs are identified and future trends are discussed. From the discovery of the phenomenon of electrolytic splitting of water into hydrogen and oxygen to the development of modern electrolyzers, water electrolysis technology has seen continuous progresses over the past years.
Hydrogen production by alkaline water electrolysis
Deiman and A. With the development of electrochemistry, the proportional relationship between electrical energy consumption and the amount of gases produced became established through Faraday's law of electrolysis. Finally, the concept of water electrolysis was defined scientifically and acknowledged. A technique for industrial synthesis of hydrogen and oxygen through water electrolysis was developed later in by Dmitry Lachinov.
By , more than industrial water electrolyzers were already in operation. The period between the s and the s was the "golden age" for the development of water electrolysis technology, 28 when most of the traditional designs were created. Driven by the industrial need for hydrogen and oxygen, the knowledge established in the first stage was applied to the industrialization of water electrolysis technologies.
The first membranes to be commercialized were made of asbestos. However, asbestos is not very resistant to corrosion caused by a strongly alkaline environment at high temperatures. Moreover, due to its seriously adverse health effects, asbestos was gradually replaced by other materials. Configuration of the water electrolysis cell also underwent several improvements through time. Typical conventional tank cells, with a unipolar configuration, are simple, reliable, and flexible.
On the other hand, filter press cells, with a bipolar configuration, have lower Ohmic losses and are more compact. High-pressure water electrolyzers, which use the bipolar configuration, would be difficult to accomplish with unipolar cells. The electrode material selected should have good corrosion resistance, high conductivity, high catalytic effect, and low price. Noble metals were found to be too expensive to be used as bulk electrode materials. Ni was then recognized as an electroactive cathode material with good corrosion resistance in an alkaline solution when compared to other transition metals and rapidly became popular during the development of water electrolyzers.
Ni-based alloys have then started to be the object of extensive research efforts. These progresses motivated commercialization of water electrolyzers. The first records of commercial water electrolysis date back to , when the technique was still in its early life. Two decades later, large-size electrolysis plants, rated at MW, were developed in Canada, primarily to feed the ammonia fertilizer industries. The Brown Boveri electrolyzer is another highly modularized unit, which is able to produce hydrogen at a rate of about m 3 h Stuart Cell Canada is a well-known unipolar tank-type cell manufacturer.
In the first half of the 20 th century, there was a huge demand for hydrogen in the production of ammonia fertilizers. This need for hydrogen stimulated the development of water electrolysis technology, which was helped by the low cost of hydroelectricity at the time. However, then hydrocarbon energy started to be applied massively in industry. Hydrogen could be produced in large scale through coal gasification and natural gas reforming, and at much lower costs, gradually fading the economic advantage of water electrolysis. At that point, progresses on water electrolysis for hydrogen production simply ceased.
The oil crisis of the s renewed the worldwide interest in water electrolysis. Improving the efficiency of water electrolysis became a major goal. Novel breakthroughs were achieved at the cell level, with the emergence of PEM and pressurized water electrolyzers. Compact, high-pressure water electrolyzers were used to produce oxygen on board the nuclear-powered submarines, as part of the life-supporting system. The compact design eliminates the gaskets between cells, requiring high-precision machining of the cell frames.
However, high operating pressures of these water electrolyzers up to 3. In , General Electric for the first time used a Nafion membrane to supply energy for space projects. However, the short durability of the membrane makes PEM electrolyzers too expensive for general applications. Currently, many efforts are under way to integrate renewable energy technologies as energy sources in water electrolysis for hydrogen production, as a means for distributed energy production, storage, and use, particularly in remote communities.
New water electrolysis concepts, such as photovoltaic PV electrolysis and steam electrolysis, are now emerging. Figure 3 presents the simplest water electrolysis unit, consisting of an anode and a cathode connected through an external power supply and immersed in a conducting electrolyte. A direct current DC is applied to the unit; electrons flow from the negative terminal of the DC power source to the cathode, where they are consumed by hydrogen ions protons to form hydrogen atoms. In the general process of water electrolysis, hydrogen ions move toward the cathode, whereas hydroxide ions move toward the anode.
A diaphragm is used to separate the two compartments.
Gas receivers are used to collect hydrogen and oxygen gases, which are formed at the cathode and anode, respectively Figure 3. For the case of water electrolysis in an acid or neutral aqueous electrolyte, the processes that occur at the electrodes' surface are described by Eqs.
The sum of these two equations leads to the overall reaction of water electrolysis, as given in Eq. However, for the specific case of alkaline water electrolysis, where a strong base is used as the electrolyte, the hydroxide anions are transferred through the electrolyte to the anode surface, where they lose electrons that then return to the positive terminal of the DC power source. Nickel Ni is a popular choice due to its low cost, good activity, and easy availability. Of course the sum of Eqs. For these reactions to proceed, a number of barriers have to be overcome, which depend on the electrolysis cell components: the boundary layers at an electrode surface, electrode phase, electrolyte phase, separator, and electrical resistances of the circuit.
Clearly, the performance of the electrolytic system must involve the understanding of the cell components.
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Therefore, it is appropriate to discuss them, at least in a relatively simple way. Boundary layers at an electrode surface. The electrolytic reactions just described are heterogeneous reactions, i. The discontinuous "interphase" region experiences differences in electrolyte velocity, concentration of electroactive species, and electrical potential with distance from the electrode.
Each of these gradients gives rise to a different boundary layer near the electrode surface and to different physical consequences. It is most important to appreciate the nature of the phenomenon resulting from the gradients and the typical extent of each boundary layer Table 1. The hydrodynamic and concentration boundary layers lead to mass transfer of reactants and products within the solution and will be discussed later; here the focus is on a simplified treatment of the electrical double layer resulting from the potential gradient. It arises from the charge separation between an electrode and the surrounding electrolyte.
Figure 4a shows a greatly simplified model of the double layer for the case of a negatively charged electrode surface, i.
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The inner, or "compact," layer consists of cations that are electrostatically held at the electrode surface and adsorbed solvent molecules. Outside of this highly structured layer lies the "diffuse" layer where the ions retain a structure that is higher in degree than that of the bulk electrolyte. The potential field resulting from this model is shown in Figure 4b. The potential decays linearly over the compact layer and then exponentially over the diffuse layer. The system behaves as two capacitances in series, one for the compact and another for the diffuse layers.
In case of large electrodes used in industrial reactors, power supplies must be designed and controlled to handle start-up, shut-down, and other conditions where the electrode potential changes abruptly. In the section devoted to the rate of electrode processes, kinetic expressions will be written in terms of the electrode potential, E, which is the difference between the potentials of a working electrode and a reference electrode located close to its surface in the solution.
Any change in electrode potential, DE, can be determined from a change in potential difference across the interface, i. Both electrodes in a cell must have adequate mechanical strength and be resistant to erosion and other types of physical attacks by the electrolyte, reactants, and products. The physical form of the electrodes is often very important, as it must be readily accommodated within a selected reactor design to achieve sound electrical connections and can also be removed for inspection and maintenance.
The shape and condition of the surface of an electrode should be designed taking into account the need for product separation such as disengagement of gases or solids. In many cases, there is a need to maintain a high surface area, a structure having turbulence-promoting properties, or a porous matrix having a reasonably low pressure drop.
The active electrode material may also be a thin coating rather than the surface of a bulk material. The electrode surface has to achieve and maintain the desired reaction and, in some cases, must possess specific electrocatalytic properties that are essential to promote a high reaction rate for the product of interest at a low overpotential, while inhibiting all competing chemical changes. The electrical conductivity must be reasonably high throughout the electrode system, including the current feeder, electrical contacts, and the entire electrode surface, in order to avoid voltage penalties, generation of unwanted heat, as well as uneven current distribution.
Finally, the electrode performance must be obtained at a reasonable capital cost and maintained over an acceptable lifetime, possibly for several years.
Some of the most common electrode materials used as cathodes for the hydrogen evolution reaction HER are steel, stainless steel, high-area Ni on steel, and Ni and those used as anodes for the oxygen evolution reaction OER are Ni oxides, and Ni- and Co-based spinels, e. Electrical resistivity is a key property of electrode materials and electrode structures.