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.
It can be expressed as follows:. This part of the resistance can be reduced by reducing the length of the wire, increasing the cross-section area and adopting more conductive wire material. The presence of bubbles in the electrolyte solution and on the electrode surfaces causes additional resistances to the ionic transfer and surface electrochemical reactions. One of the accepted theoretical equations to study the bubble effect in the electrolyte is given as follows [ 41 ]:.
Convective mass transfer plays an important role in the ionic transfer, heat dissipation and distribution, and gas bubble behavior in the electrolyte. The viscosity and flow field of the electrolyte determines the mass ionic transfer, temperature distribution and bubble sizes, bubble detachment and rising velocity, and in turn influence the current and potential distributions in the electrolysis cell.
As the water electrolysis progresses the concentration of the electrolyte increases, resulting in an increase in the viscosity. Water is usually continuously added to the system to maintain a constant electrolyte concentration and thus the viscosity.
The conductivity of the solution is enhanced by the use of strong electrolytes that deliver ions with high mobility [ 43 ], such as sodium, potassium for positive ions, and hydroxide or chlorides as negative ions. During electrolysis, the water molecules move to the cathode by diffusion as they are consumed, and the hydroxide ions move to the anode by migration because they have an opposite charge and diffusion because they are consumed.
A diaphragm separates the two anode and cathode compartments and the gases formed are thus collected: hydrogen at the cathode and oxygen at the anode as shown in Figure 3.
Principle of an alkaline water electrolysis. Concentrated solutions of potassium hydroxide are generally used as the electrolytic solution because they have very high conductivities and fewer corrosion problems compared with other alkaline electrolytes.
The electrode materials often used are based on nickel because of its low cost, high activity [ 44 ]. Electrolysis cells can be of two types of configurations: monopolar and bipolar [ 14 ]. Figure 4 a gives a schematic of the monopolar configuration. The electrodes are altered in the electrolyzer and are all directly connected to the terminals of the DC power supply: the anodes at the positive terminal and the cathodes at the negative terminal.
Figure 4 b depicts conflation in bipolar mode. Only the two end electrodes are connected directly to the DC power source. The other inner electrodes have a dual role: one side acts as the cathode for a unit cell and the other side acts as the anode for the adjacent unit cell. These cells are electrically linked thanks to their electrodes which are bipolar and ionically via the electrolytic solution.
The electrical energy consumed is the same in the two configurations. Schematics of cell configurations of monopolar a and bipolar b electrolyzers [ 14 ]. The wide range of flammability limits of the mixture of hydrogen and oxygen requires a careful design of the electrolyzer system.
The separator diaphragm or membrane must avoid the mixing of the two gases inside the cell. Furthermore, the corrosive nature of the electrolyte does not allow leaks that are often likely to take place at the connections and seals of the electrolyzer. The bipolar configuration is more risky in mixing oxygen and hydrogen because of their simultaneous productions on the same bipolar electrode on each side and also electrolyte leakage as the monopolar design.
Obviously, the life of the system is an important criterion. It is extremely linked to the quality of the materials used. Indeed, these materials must be resistant to high concentrations of the alkaline electrolyte and operating conditions of the electrolyzer pressure and temperature.
In particular, connections and seals are subject to corrosion, which is why it is recommended to use sealing materials that are also stable in this environment [ 14 ].
PEM electrolyzers are characterized by their very simple construction and their compactness. The operating principle of electrolysis of water with an electrolyte protons exchange membrane PEM is simple. When operating in electrolysis, the water decomposes at the anode into protons and molecular oxygen. The oxygen is evacuated by the water circulation, and the protons migrate to the cathode under the effect of the electric field. There, they are reduced to molecular hydrogen. Each proton carries with it a procession of several molecules of solvation water: it is the electro-osmotic flow.
During the twentieth century, several major innovations have significantly increased the energy and faradic efficiencies of electrolyzers. The concept of zero-gap cells has been developed in order to overcome the disadvantages of the electro-osmotic flow. Frischknecht, Todd M. Macromolecules , 52 3 , Antonov , Vadim S. Efimchenko and Marek Tkacz. The Journal of Physical Chemistry B , 3 , Bartels, , Timothy W.
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Applied Surface Science , , Modeling phase equilibrium of hydrogen and natural gas in brines: Application to storage in salt caverns. International Journal of Hydrogen Energy , 46 5 , International Journal of Hydrogen Energy , 45 56 , Ratnakar , Birol Dindoruk , Albert Harvey. Thermodynamic modeling of hydrogen-water system for high-pressure storage and mobility applications. Journal of Natural Gas Science and Engineering , 81 , Chemistry — A European Journal , 25 32 , Schmidt , Felix N.
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Place the cut cup, open-side down, over the battery. The cup bottom and the battery should fit together, creating one level surface on which you can place the other cup.
Put the cup with the graphite pins on the upside-down, cut cup, on top of the battery. It should sit there firmly and each of the graphite pins should contact one of the battery poles. Seal one end of both jumbo drinking straws with Play-Doh or clay. Procedure Take the cup with the graphite pins and pour approximately milliliters of distilled water into the cup, away from the battery.
Make sure that it is not leaking. If it is, you might need to add a bit more glue to make a tight seal. Note: You want to avoid touching the water or the electrodes once the cup is placed on top of the battery as you might feel the electricity tingling your fingers. As you did previously, place it on the upside-down, cut cup, on top of the battery so that each of the graphite pins contacts one of the battery poles. You might need to press it down a little to make a good connection. Observe the two graphite electrodes.
What do you see? Is anything happening at the electrodes? Remove the cup filled with distilled water from the battery. Fill one teaspoon with baking soda and stir it into the distilled water until everything is dissolved.
What do you think the baking soda will change? What function does it have? Now put the cup back on top of the battery and connect the graphite electrodes with the battery poles. What do you observe now?
Does anything happen at the graphite pins? What do you think the reaction's products are? Compare the reactions that happen at each of the graphite electrodes. Can you see a difference between both sides? Is there one graphite electrode at which the reaction is more pronounced? Which pole of the battery is this graphite pin connected to, the positive or negative?
Put your nose inside the cup and smell the reaction products. Is there any smell? If so, how does it smell? Remove the cup from the battery again. With the medicine dropper, fill both plugged-up jumbo straws with the baking soda solution from inside the cup that has the graphite pins. Once they are full, close each with one of your fingers and turn them upside down.
Submerge them into the cup with the baking soda solution and carefully place them on top of the graphite pins one straw on each so that the straws stay completely filled with baking soda solution.
If the straws do not stay upright, you can lean them against the side of the cup. What do you think will happen with the straws? Once the straws are placed on top of the graphite pins, put the cup back on top of the battery.
Leave it there for 10 minutes and press the cup down a little to make sure that the electrodes stay connected and the electrode reactions are happening continuously throughout that time. Observe the jumbo straws that you put on top of the graphite pins. What is happening to the water that you put in there? Do you notice a difference between the two water levels in both straws? Which one is higher, which is lower; to what battery poles are each of them connected?
After the 10 minutes are over, mark the water level in each of the straws with the permanent marker. How much more water was displaced by the reaction products on the negative pole compared with the positive pole?
Is it the same, double or triple? Extra: If you have any pH strips that can measure the acidity or basicity of solutions, use them to measure the pH in each of the jumbo straws once the water level decreases by about 50 percent. Carefully remove the jumbo straws from the electrodes and immediately seal each one with a finger once you lift it off the electrodes. Making sure you do not lose the water that is inside, dip a pH test strip inside.
What color does the test strip show and what pH does this represent? Is there a difference between the solutions in the two straws? How do they differ, and why do you think this is the case?
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