Electrochemical energy storage in formate ions

Renewable energy sources are increasingly important in meeting society’s energy demand. In particular, penetration of wind and photovoltaic power is growing rapidly. This development is essential for the wellbeing of future generations, but is accompanied by many challenges. One of the most important technical challenges is security of supply of renewable energy. After all, there are windless, dark periods. This thesis focusses on a potential novel energy storage system to bridge those periods with stored energy from light, windy periods. The storage occurs through conversion of dissolved potassium bicarbonate into potassium formate, creating a safe and scalable system. Safe, as only non-flammable, non-toxic chemicals are used and scalable due to the liquid storage medium that can be pumped into tanks. Before the system can be realised, many questions need to be answered. Some of which, in this thesis.
In Chapter 2, the question is answered under which circumstances electrochemical conversion of bicarbonate to formate can occur. Electrodes designed for the reduction of carbon dioxide can be utilized in the bicarbonate-formate storage system. Chapter 3 considers lead (Pb) electrodes, which are initially highly active, but show reduced activity within minutes. Brief anodic polarization of the electrode, restores the initial activity and selectivity. In chapter 4, electrodes with a palladium catalyst are studied. Palladium has a low overpotential, but also produces relatively large amounts of undesired hydrogen. An increase in the partial pressure of hydrogen results in reduced net hydrogen production and only 4 bars can be sufficient to results in nearly exclusive formate production. In chapter 5, an engineering method is applied to determine a suitable reaction mechanism for the palladium based electrodes. A mechanism based on both an α- and β-palladium hydride phase can describe the experimental results and correctly predicts the outcome of isotopically labelled experiments.