Effect of Water Quality on PEM Electrolyzer Performance

Among renewable energy sources, hydrogen production by electrolysis is very promising due to its potential as an energy storage medium. Proton exchange membrane (PEM) is one of the mainstream technologies for hydrogen production by electrolysis due to its advantages such as high efficiency, large current density, low temperature range, and fast response speed. Most of the research on hydrogen production by PEM electrolysis focuses on the demonstration of hydrogen production by PEM electrolysis, the development of new catalysts, and the development of new proton exchange membrane electrolytes. However, the optimization of the system and feed water remains a challenge. Therefore, this study investigates the effect of water quality on the energy consumption of PEM electrolyzers, focusing on total dissolved solids (TDS), water pH, and conductivity (of course, these three factors often affect each other).

The efficiency and energy consumption of PEM electrolyzers depend on the quality of the influent water. This study validated three parameters that affect water properties: pH (3, 7, 9), total dissolved solids (TDS) (300ppm, 600ppm, 900ppm), and conductivity ( conductivity : 30mS/cm, 70mS/cm, 100mS/cm) to understand and optimize the process of producing hydrogen using PEM electrolyzers. The results showed that the amount of hydrogen produced was significantly affected by pH, total dissolved solids, and conductivity, and the optimal level of each variable was determined through extensive testing.

The working principle of PEM electrolyzers is to electrochemically separate water into oxygen and hydrogen at their respective electrodes. Since water is the medium for producing hydrogen, its quality may affect the results of the electrolysis process. Water qualities that may affect the efficiency of PEM electrolyzers include pH, total dissolved solids (TDS), and conductivity. For example, the pH value of the electrolyte affects the production of hydrogen and the energy consumption of the PEM electrolyzer; a lower pH value can reduce the overall oxygen reduction reaction (oxygen evolution reaction: OER) potential, thereby reducing energy consumption, but there is a problem of membrane degradation; another important factor is conductivity, low conductivity will also reduce the overall potential, thereby reducing the required energy, and high conductivity will also damage the membrane; the active overpotential between hydrogen and oxygen reduction reactions also has an asymmetric and dependent distribution on pH. Therefore, it is necessary to optimize the pH value, TDS value and conductivity to ensure the improvement of the performance of the PEM electrolyzer. The American Society for Testing and Materials (ASTM) recommends that commercial PEM electrolyzers use Type I deionized water, that is, water with a total organic carbon content of less than 50ppb, a resistivity of more than 1 MΩ.cm, and a sodium and chloride content of less than 5µg/L. However, almost all water resources are impure, which means that water purification for PEM electrolyzers requires additional costs. A study on the effect of TDS on photovoltaic cell efficiency showed that the higher the TDS level of water (0-2000ppm), the better the yield, while when the TDS level dropped to zero, there was no yield. Similarly, the results of a study using artificial river water (soft water) as the electrolyte of a PEM electrolyzer showed that the performance of the electrolyzer decreased due to the increase in calcium and magnesium ion concentrations. The cell performance and mechanical life of the PEM electrolyzer were reduced.

1. The effect of pH value on gas production and electrolyzer energy consumption

1.1. The effect of pH value on gas production

As the pH value of the electrolyte changes, the amount of hydrogen and oxygen produced also changes. The function relationship between the amount of hydrogen and oxygen produced and the time and pH value, the pH value changes from 3 to 11 at regular intervals. Interestingly, the initial results show that the amount of hydrogen and oxygen produced decreases as the pH value rises from 3 to 7, which indicates that the electrolysis process may be slow at neutral pH. Surprisingly, the results showed that the production of hydrogen and oxygen increased significantly at a pH of 11, indicating that the alkalinity of the water sample may contribute to the production of hydrogen and oxygen.

1.2 Effect of pH on energy consumption

The pH of the electrolyte affects the energy consumption of the system. The pH affects the conductivity of the electrolyte, which in turn affects the efficiency of the electrolysis process. Typically, the optimal pH range for PEM electrolyzers is between 7 and 9. The higher the pH, the more conductive the electrolyte is, which can improve the efficiency of the electrolysis process. However, if the pH is too high, the membrane in the electrolyzer may be damaged, resulting in decreased performance and increased energy consumption. On the other hand, if the pH is too low, the conductivity of the electrolyte may decrease, resulting in decreased efficiency and increased energy consumption. In addition, too low a pH can cause the membrane to dry out, which can also reduce performance and increase energy consumption. The energy consumption of the PEM electrolyzer increases at lower pH values. At a pH of 8, the energy consumption is the lowest, at 45kwh/m3 H2. As the pH value increases or decreases, the energy consumption begins to increase.

2. Effect of total dissolved solids (TDS) on gas production and energy consumption

2.1. Effect of TDS on gas production

When evaluating three different TDS concentrations, 300 ppm is a low concentration, 600 ppm is a medium concentration, and 900 ppm is a high concentration. The results are consistent with other studies. The results show that as the TDS concentration increases, the production of hydrogen and oxygen increases, which may be a catalyst for the formation of hydrogen. It can be concluded that the production of hydrogen from water is more favorable at higher TDS levels, while the production is limited at low concentrations, indicating that no hydrogen may be produced at zero TDS levels:

2.2 Effect of TDS on energy consumption

Total dissolved solids (TDS) have a significant impact on the energy consumption of proton exchange membrane (PEM) electrolyzers. TDS refers to the concentration of all inorganic and organic substances dissolved in water. When these substances are present in the water used in the electrolyzer, they affect the performance and efficiency of the electrolyzer. TDS in water increases the conductivity of the water, which leads to an increase in the electrolytic cell voltage required for electrolysis. The increase in cell voltage leads to an increase in the energy consumption of the electrolyzer. In addition, TDS can cause scaling of the electrodes and membranes, which reduces the efficiency of the electrolyzer and further increases energy consumption. In order to mitigate the impact of TDS on energy consumption, it is necessary to ensure that the water used in the PEM electrolyzer is of high purity and low TDS concentration. Water treatment technologies such as reverse osmosis and deionization can be used to remove TDS from water, thereby improving the efficiency of PEM electrolyzers and reducing their energy consumption.

3. Effect of Conductivity on Gas Production

Another key factor affecting the energy consumption of PEM electrolyzers is conductivity. Reducing the overpotential required for anode OER can reduce energy demand, which is reflected in the fact that higher conductivity values also mean higher ion concentrations in the electrolyte solution. However, high conductivity also increases the chance of membrane deterioration and increases the energy required for pumping. The production of hydrogen depends largely on conductivity, and several studies have shown that by using different solutions to increase conductivity, different conductivities can be achieved, thereby increasing hydrogen production.

4. Effect of different water qualities on PEM electrolyzer energy consumption

Seawater, well water, and deionized water are three different types of water that may affect the energy requirements of a proton exchange membrane (PEM) electrolyzer. Seawater contains a large amount of dissolved salts, minerals, and other contaminants. Because these contaminants increase the conductivity of the water, the resistance of the electrolyzer increases. Because more energy is required to overcome the resistance, the electrolysis process slows down. In order to provide the required current, a higher voltage is required, which also leads to an overall increase in energy usage. Well water is usually much lower in dissolved salts and contaminants than seawater. Minerals and other substances that may interfere with electrolysis may still exist. Exactly how the composition of well water affects energy usage is still uncertain to some extent. The energy required to treat well water is generally less than the energy required to treat seawater or deionized water. Deionized water is water that has had mineral ions removed through the deionization process. It is also called deionized water and distilled water. Deionized water has a much lower conductivity than seawater and well water. Therefore, it has a lower resistance during the electrolysis process and requires less energy to produce the same current. Using deionized water in PEM electrolyzers can improve energy efficiency. Deionized water has poor conductivity, which may help save energy, but it does not contain any ions required for the electrochemical reactions in the electrolyzer. Water quality requirements should be carefully considered based on the specific design and operation of the PEM electrolyzer system, because these ions are important to maintain the performance and life of the electrolyzer components.

In short, in PEM water electrolysis, we usually pay more attention to the electrolyzer itself and ignore the importance of BOP. Many people also think that the BOP of PEM is simpler than that of alkaline. In fact, although PEM does not require a large gas-liquid separation system like alkaline, it is also very important to manage the quality of pure water. Managing the quality of pure water not only ensures efficient operation but also helps to increase service life.