Elektrolyse van waterdamp met aard-abundante elektrokatalysatoren

Since the Industrial Revolution the use of energy and resources has substantially increased. This is related to the consumption of fossil fuels, that provided mankind with immense amounts of cheap energy. The energy content in one barrel of crude oil, which is available at a price of less than € 100, represents about 10 years of human labor. Fossil fuels are finite resources and their use is increasingly contributing to global warming and air pollution. To meet energy demand and stay within the planetary boundaries, renewable energy should be accessible at a price that is comparable to energy obtained from fossil fuels. Electricity production from solar and wind energy has fallen dramatically in price in the last decades, and is nowadays competitive with electricity production from fossil fuels in many applications. As a result, the global share of electricity consumption from solar and wind energy increased from 1 to 8% between 2007 and 2017. Global electricity consumption currently contributes to only 20% of total energy consumption. Fuels are responsible for the other 80% of energy consumption. Therefore, there is still need for renewable fuels at a competitive price, which are currently not yet available. Hydrogen production from water and solar energy is an appealing strategy for production of renewable fuels, since solar energy and water are abundantly available. Water splitting into hydrogen and oxygen gas is also a very efficient and selective reaction. For hydrogen production from solar energy, direct connection between solar electricity production and water splitting modules has shown to be most energy efficient. Water splitting is usually performed with pure liquid water or liquid electrolyte. However, starting from the area required for solar energy capture, sufficient water in outside air is available to produce hydrogen gas. This has enabled the concept of a solar hydrogen panel that produces hydrogen gas solely from sunlight and outside air. This type of hydrogen panel would be as convenient as solar panels; modular, self-sufficient and can be placed at almost any location. Compared to liquid water electrolysis, electrolysis of water vapor requires higher electrolysis unit areas due to increased presence of water diffusion limitations. Therefore, it is imperative to confine the catalyst material choice to non-noble metals to keep hydrogen production costs low. The major goal of this doctoral research was to investigate whether non-noble metal catalysts can be used to produce hydrogen with water vapor electrolysis efficiently. A water vapor electrolysis device requires catalysts to perform the water splitting reactions and a membrane to provide ion conduction and to separate product gases. Previous studies on water vapor electrolysis used noble metal catalysts in combination with a (acidic) proton exchange membrane. Most non-noble metal catalysts were found to be unstable in an acidic liquid environment. In the first part of this work, the possibility is assessed of using non-noble metal nickel electrodes in combination with a Nafion 117 proton exchange membrane for producing hydrogen gas from water vapor. Another approach to guarantee the stability of earth-abundant catalysts is providing an alkaline environment. Therefore, in the second part of this work different anion exchange materials are used for water vapor electrolysis. Firstly, layered double hydroxides are used that are composed of positively charged brucite-type layers and interlayer anions. The effect of changing the interlayer anion is examined on both ion conductivity and water vapor electrolysis performance. Secondly, different types of homogeneous anion exchange membranes are screened on their performance towards water vapor electrolysis. These membranes contain an organic backbone with covalently attached cationic head groups. The membranes, that are tested in this work, differ in backbone, head groups, reinforcement or thickness. In addition, the effect of varying electrode thickness and relative humidity is investigated. Lastly, an ion-solvating membrane is used that contains potassium hydroxide as anion conductor imbibed in a polymer matrix. This membrane is coupled with highly active non-noble metal catalysts to perform water vapor electrolysis. The activity of this device is compared to liquid electrolyte water electrolysis at different applied potentials. In the last part of this work, the water vapor electrolysis device, based on the potassium hydroxide doped membrane and non-noble metal catalysts, is coupled with silicon solar cells to produce hydrogen gas directly from water vapor and light, without any applied bias. Long-term tests on the water vapor electrolysis device were also performed to obtain insights in the stability behavior. These tests were done at various relative humidity levels and with isotope labeling to gain more understanding in the effect of the water source. This work demonstrates that different types of ion exchange membranes can be used to produce hydrogen from water vapor electrolysis with non-noble metal catalysts. It also provides insights in the bottlenecks of each system so that with further research practical devices can be made that produce hydrogen efficiently from outside air and sunlight.

Publication year:2020

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