Development and application of proton exchange membrane water electrolysis hydrogen production technology under wind and solar power fluctuations II

Development and application of proton exchange membrane water electrolysis hydrogen production technology under wind and solar power fluctuations II

II. Basic characteristics of hydrogen production by PEM water electrolysis under wind and solar fluctuating power supply
Under the fluctuating power supply of wind and solar power, the working parameters of the electrolyzer undergo transient changes, which can cause irreversible damage to  the main components. Exploring the performance characteristics of PEM water electrolysis for hydrogen production under the fluctuating power supply of wind and solar power, the attenuation mechanism and evaluation methods of PEM electrolyzer components are of great value to the research and development of key technologies for PEM electrolyzer components.

1. Wind and solar power fluctuations have a significant impact on electrolytic cells
Usually, the input voltage of the electrolytic cell is controlled within a certain range; when the input power of the electrolytic cell fluctuates, the voltage of the electrolytic cell  changes slightly, while the current fluctuates significantly. When voltage stabilization control is adopted in practical applications, once the input power of the electrolytic cell changes, the current will fluctuate sharply, which will cause a sharp change in the electrode reaction rate, causing the electrolytic cell to deviate from the stable operating condition. Due to the existence of the electrode reaction overpotential, the voltage input is significantly higher than the theoretical voltage; although the electrolysis reaction of water is an endothermic reaction, the Joule heat generated by ohmic loss causes the temperature of the electrolytic cell to gradually increase over time even under stable power supply conditions. From the working characteristics of the electrolytic cell under simulated wind power conditions, it can be seen that the temperature changes with the fluctuation of the power generation under transient operating conditions. After the temperature of the electrolytic cell drops, the electrode reaction rate slows down and the efficiency decreases. Increasing the power leads to an increase in temperature, and the increase in the oxygen and hydrogen yields on the electrode surface leads to the attachment of bubbles to the electrode surface, thereby increasing the ion transfer resistance of the catalyst layer and reducing the effective reaction area, thereby generating a higher reaction overpotential, resulting in an increase in the voltage of the electrolytic cell. The attachment and flow of bubbles also lead to uneven supply of electrolyte on the electrode surface, causing uneven reaction and local hot spots on the electrode surface.
In recent years, the topic of the impact of wind and solar fluctuating power supply on the performance attenuation or aging of electrolytic cells has received much attention from scholars at home and abroad, but some conclusions are different. Through the 500-h durability test of the PEM electrolytic cell, the performance characteristics of the electrolytic cell under different operating modes were clarified, and it was found that in the fast cycle operation mode (simulating photovoltaic power generation), as the ohmic resistance decreased, the performance of the electrolytic cell was improved. After the 1000-h durability test of the PEM electrolytic cell, it was found that the performance attenuation rate of the electrolytic cell was 194 μV/h, and 78% of the attenuation came from the increase in the ohmic resistance of the anode-porous layer; the performance attenuation of the electrolytic cell was significantly alleviated under the conditions of wind and solar fluctuating power supply, because the wind and solar fluctuating power supply partially restored the reversible degradation and weakened the electrode degradation problem. The long-term stability of the electrolytic cell performance under different input characteristics and its attenuation mechanism still need further study.

2. Wind and solar power fluctuations accelerate the degradation of electrolytic cell components
1). Catalytic layer
The catalytic layer of the electrolytic cell is generally composed of a catalyst (such as precious metals such as Pt, RuO2, Ir, IrO2) and a binder (such as perfluorosulfonic  acid). In order to enhance durability, the catalytic layer is usually loaded with some conductive carrier materials, such as TiO2, SnO2, Ta2O5, Nb2O5, Sb2O5, TaC, TiC. The above catalysts can meet the high performance requirements of PEM electrolytic cells, but the durability under harsh operating conditions is difficult to be satisfactory. The performance of the anode is more seriously degraded under low catalyst loading conditions, and the corresponding attenuation mechanisms mainly include dissolution, agglomeration, and carrier passivation. After a 5500 h durability test on the PEM electrolytic cell, it was found that the corrosion of the catalytic layer and the degradation of the Pt catalyst were the main factors leading to performance degradation.
2). Exchange membrane
In traditional PEM electrolyzers, the exchange membrane is used to separate gaseous reaction products, transport protons, and support the cathode and anode catalyst layers. It needs to have excellent chemical stability, mechanical strength, thermal stability, proton conductivity and other characteristics. The performance degradation of the  exchange membrane is mainly due to membrane contamination or chemical degradation. From the perspective of safety and reliability, the durability of the membrane is crucial to the electrolyzer. Membrane damage may cause the generated hydrogen and oxygen to mix directly. The degradation mechanism of the exchange membrane is mainly divided into three types: mechanical degradation, thermal degradation, and chemical/electrochemical degradation.
3). Bipolar plate
The bipolar plate is a multifunctional component of the electrolytic cell. It effectively conducts electrons, provides channels for reactant/product transport, maintains the mechanical stability and integrity of the equipment, and serves as a component of thermal management. As the main component of the electrolytic cell, the cost accounts for  about 48% of the PEM electrolytic cell. Its design and manufacturing should meet the requirements of high conductivity, corrosion resistance, low cost, and high mechanical strength. However, the voltage/current changes under the fluctuating power supply of wind and solar power lead to uneven or drastic changes in the temperature of the electrolytic cell, resulting in uneven stress distribution or repeated stress changes, resulting in increased contact resistance and mechanical performance strain, which ultimately affects the durability of the electrolytic cell.

3. Wind and solar fluctuating power supply simulation method
Developing accelerated decay test, life assessment and durability research schemes for electrolytic cells and their components will help evaluate the decay behavior of materials and better understand the decay mechanism of materials. The durability of PEM electrolytic cells is mainly evaluated by constant current under specific temperature and pressure conditions. The life test time of electrolytic cells is relatively long (>4×104 h), and the corresponding durability assessment cost is relatively high. At present, there is no standardized and generally accepted durability assessment method for PEM electrolytic cell components. The academic and industrial circles in Europe have long been committed to characterizing, testing and evaluating the performance, efficiency and durability of electrolytic cells, and have accumulated rich experience. Representative works include: using accelerated stress test methods to evaluate the chemical stability of membranes in PEM electrolytic cells; studying the effects of different wind and solar fluctuating power input waveforms on the degradation of PEM electrolytic cells, and believing that square wave and sawtooth wave power supplies significantly accelerate electrode degradation; proposing to simulate the start-up and shutdown operation mode of electrolytic cells by constant current and open circuit voltage, and finding that open circuit conditions can accelerate the performance decay of electrolytic cells. It is generally believed that accelerated attenuation is usually related to current density, pressure, and temperature, but there is still a lack of accelerated attenuation test methods for electrolytic cells under wind and solar fluctuating power sources and related standardized implementation plans. Testing methods under single factor conditions are difficult to comprehensively evaluate the attenuation characteristics of electrolytic cells under wind and solar fluctuating power sources.