(399e) Structure of the Electric Double Layer during Nitrate Reduction

Nitrate pollution of wastewater from agricultural runoff and industrial waste streams is a worldwide challenge. Nitrate negatively impacts the environment through harmful algal blooms and can be dangerous for human consumption. Rather than treat nitrate as a waste, we can electrochemically reduce nitrate to ammonia. Ammonia is an important chemical precursor with potential applications in sustainable energy as a fuel or hydrogen carrier or can be used as a fertilizer.

A nitrate reduction process must be selective to avoid the formation of undesired products and be efficient to minimize operating costs from electricity. To address these challenges, we must understand the molecular mechanisms in the electric double layer (EDL) that forms at the electrode–electrolyte interface, where heterogeneous electrochemical reactions occur. The EDL structure comprises ordered molecular layers extending up to 10 angstroms from the surface with different composition, molecular orientation, and density from the bulk. This EDL structure—which is dependent on the bulk electrolyte concentration, applied potential, and electrode composition and structure—can impact electrochemical reactions in many ways, including blocking catalytic sites and stabilizing reactants.

We studied the EDL with in situ X-ray reflectivity (XRR), a synchrotron technique which provides atomic-level resolution of the near-surface electron density profile. XRR can reveal the structure of the EDL including the number of ordered layers, the cation degree of hydration, and the molecular density of each layer. We probed the EDL structure at a graphene electrode under an applied potential at the Stanford Synchrotron Radiation Light Source (SSRL). We compared the EDL structures of perchlorate electrolytes with different alkali metal cations (Na+, K+, Rb+, Cs+) with and without nitrate to determine if the nitrate ion adsorption and concentration in the EDL was influenced by the cation identity. This understanding could lead to the development of electrolyte engineering strategies to optimize ammonia production in electrochemical nitrate reduction.