The process of splitting water into hydrogen and oxygen with the help of electricity has been used since the beginning of 19 th century. One important precondition for hydrogen electrolysis is that both water and electricity sources do not conflict with sustainability standards see our EESG Framework for Sustainable Power-to-X. Sustainable water supply must also be ensured: The water input should not compete with crucial processes such as local drinking water supply or agricultural irrigation.
The basic principle of electrolysis is to split water into oxygen and hydrogen with the help of electricity. In practice, electrolysers consist of several interconnected electrolysis cells, also called stacks. When voltage is applied, hydrogen is produced at the cathode and oxygen at the anode.
Between the two partial reactions, charge equalisation takes place in the form of ion conduction via an electrolyte which is an electrically conductive substance.
In addition, a membrane is needed to spatially separate the two reactions and prevent the product gases from mixing. Both the ion charge and the type of electrolyte differ in the various electrolysis technologies. AEL works with a liquid electrolyte in the form of potassium hydroxide.
The electrodes are made of metal. Between the two electrodes is a diaphragm that is non-permeable to hydrogen and oxygen. They are more sensitive to impurities in the product gas as the gases dissolved in the electrolyte remain in the cycle. In addition, the AEL has a long cold start time of 50 minutes. However, it is also already available on an industrial scale and nominal outputs of over 10MW can be achieved. PEM electrolysis works in an acidic environment.
Precious metals such as iridium or platinum must therefore be used to protect the electrodes from corrosion. Polymer Electrolyte Membrane Electrolyzers In a polymer electrolyte membrane PEM electrolyzer, the electrolyte is a solid specialty plastic material. Water reacts at the anode to form oxygen and positively charged hydrogen ions protons. The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.
At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas. Solid Oxide Electrolyzers Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions O 2- at elevated temperatures, generate hydrogen in a slightly different way.
Steam at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit. It is important to note Today's grid electricity is not the ideal source of electricity for electrolysis because most of the electricity is generated using technologies that result in greenhouse gas emissions and are energy intensive.
Electricity generation using renewable or nuclear energy technologies, either separate from the grid, or as a growing portion of the grid mix, is a possible option to overcome these limitations for hydrogen production via electrolysis. The U. Department of Energy and others continue efforts to bring down the cost of renewable-based electricity production and develop more efficient fossil-fuel-based electricity production with carbon capture, utilization, and storage.
Its molecules contain two hydrogen atoms. If this gas does not exist in its natural state, it is found in many molecules: water, sugar, proteins, hydrocarbons, and so on. Hydrogen is a very light gas, colorless, odorless, and extremely flammable and reacts very easily in the presence of other chemicals.
The properties of this gas are summarized in Table 1 [ 3 ]. Characteristics of hydrogen [ 3 ]. The advantages of using hydrogen as a fuel in fuel cells are: its high electrochemical reactivity,.
Its low density under normal conditions, the difficulty of storage, and the risk of explosion can summarize the major drawbacks of the use of pure hydrogen in fuel cells. Notwithstanding the increasing interest in hydrogen as an energy carrier, its main uses continue to be in petroleum refining, ammonia production, metal refining, and electronics fabrication, with an average worldwide consumption of about 40 million tons [ 4 , 5 , 6 , 7 , 8 ].
This large-scale hydrogen consumption consequently requires large-scale hydrogen production. Presently, the technologies that dominate hydrogen production include reforming of natural gas [ 9 ], gasification of coal and petroleum coke [ 10 , 11 , 12 ], as well as gasification and reforming of heavy oil [ 13 , 14 ].
Water electrolysis is the process whereby water is split into hydrogen and oxygen through the application of electrical energy, as in Eq.
Typically, a water electrolysis unit consists of an anode, a cathode separated with an electrolyte, and a power supply. The electrolyte can be made of an aqueous solution containing ions, a proton exchange membrane PEM or an oxygen ion exchange ceramic membrane. A direct current DC is applied from the negative terminal of the DC source to the cathode seat of the reduction reaction , where the hydrogen is produced.
At the anode, the electrons produced by the electrochemical reaction return to the positive terminal of the DC source. For the case of water electrolysis in an acid aqueous electrolyte, the processes that occur at the anode and the cathode are described, respectively, by Eqs.
Electrolysis of water is not a spontaneous phenomenon because the standard global reaction potential is negative. Therefore, it needs an external intervention power source and the global reaction can be written as:. The equation of the German chemist Walther Nernst can be obtained from thermodynamics. Some species involved in the electrode reaction are solids or pure liquids. For these pure substances, the activity is constant and its value is considered unitary.
The activity of the gases is usually taken to be the partial pressure of the gases expressed in the atmosphere, and the activity of the ions in dilute solution is generally considered to be their molar concentration. By substituting in Eq. In the specific case of an electrochemical cell, it is written:. In , Michael Faraday stated his two laws of electrolysis: The weights of substances formed at an electrode during electrolysis are directly proportional to the quantity of electricity that passes through the electrolyte.
The weights of different substances formed by the passage of the same quantity of electricity are proportional to the equivalent weight of each substance. In a cell in which a continuous current circulates, the majority of this current is connected to chemical reactions faradic current and a small part, often negligible can be used for other purposes non-Faradic current.
Thus, the amount of material that forms or disappears at the electrodes is proportional to the intensity of the current and the duration of the electrolysis t. Figure 1 shows the relationship between the electrolyzer cell potential and operating temperature [ 19 , 20 , 21 , 22 ].
The cell potential-temperature plane is divided into three zones by the so-called equilibrium voltage line and thermo-neutral voltage line. The equilibrium voltage is the theoretical minimum potential required to dissociate water by electrolysis, below which the electrolysis of water cannot proceed. The equilibrium voltage decreases with increasing temperature.
The thermo-neutral voltage is the actual minimum voltage that has to be applied to the electrolysis cell, below which the electrolysis is endothermic and above which, exothermic. The thermo-neutral voltage naturally includes the overpotentials of the electrodes, which are only weakly dependent on temperature. Thus, the thermo-neutral voltage only exhibits a slight increase with temperature. If water electrolysis takes place in the shaded area in Figure 4 , the reaction will be endothermic.
Cell potential for hydrogen production by water electrolysis as a function of temperature [ 19 ]. Under nonequilibrium potential conditions, the equation that best describes the current density versus potential is the Butler-Volmer expression:. The first step Eq. Hydrogen formation is intrinsically determined by the strength of the bond between hydrogen and the electrode surface. Electrode properties, type and concentration of the electrolyte, and temperature are parameters that also influence hydrogen formation.
If hydrogen adsorption is the rate-determining step, electrode materials with more edges and cavities in their surface structure will favor electron transfer and create more centers for hydrogen adsorption. If hydrogen desorption is the rate-determining step, physical properties such as surface roughness or perforation will prevent bubbles from growing and increase electron transfer by adding reaction area, consequently increasing the rate of electrolysis [ 26 ].
When the overpotential is low, electron transfer is not as fast as desorption and hydrogen adsorption will be the rate-determining step. In contrast, when the potential is high enough, hydrogen desorption will be the rate-determining step. The hydrogen adsorption energy is a good parameter to identify the most promising materials for the HER. Figure 1 also shows that the elements that interact strongly with H ads such as Ru and Ti are positioned on the descending slope of the volcano, supporting previous suggestions that the M — H ads binding energy can be used as a descriptor for the HER.
Not in passing, given that recent analysis has demonstrated that neither Ru nor Ti are bare metals in the HER region, it is suggested that, in fact, experimentally it is very difficult impossible to determine unambiguously solely based on the M — H ads energetics what would be the correct position of these two elements in the observed volcano relationship.
This is most likely also true for the HER in alkaline solutions, when the rates of the reaction are much slower than in acidic environments [ 22 ].
The HER exchange current of Pt in acid media is at least two orders of magnitude higher than that in alkaline electrolytes, including KOH. The long-term stability of Ni OH 2 in the strongly reducing environment occurring at the cathode is also not discussed. The most generally accepted mechanism for the OER is that described by Cappadonia et al. The mechanism is controlled by the charge transfer step 20 or 21 at low temperatures.
On the other hand, at high temperatures, the recombination step Eq. Generally, acid solutions or PEMs are used as electrolytes in water electrolyzers because acidic media show high ionic conductivity and are free from carbonate formation, as compared with alkaline electrolytes. Consequently, noble metals are used as electrocatalysts for OER in acidic media.
Ruthenium and iridium have shown strong activity for OER, but they were passivated at very high anode potentials [ 31 , 32 , 33 , 34 , 35 , 36 ]. Bifunctional electrocatalysts, which can work for both oxygen evolution and oxygen reduction, have also been proposed for water electrolysis.
A typical bifunctional electrocatalyst is composed of a noble metal oxide such as IrO2. At high current densities, are added to the polarization of the electrodes other resistances: ohmic losses in the electrolyte, resistances from bubbles, diaphragm, and ion transfer.
The electrical resistance in a water electrolysis system has three main components: 1 the resistance in the system circuits; 2 the mass transport phenomena including ions transfer in the electrolyte; 3 the gas bubbles covering the electrode surfaces and the diaphragm [ 15 ]. The nature and the dimensions of the materials used in the electrodes and the connections and the electric circuit, the methods of their preparations are responsible for the electrical resistance of the system.
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