Novel catalyst systems and process optimization of electrochemical cells for ammonia synthesis

RWTH Publications (RWTH Aachen)

Abstract

The defossilization of energy production and the chemical industry requires new sustainable energy sources and new processes for the sustainable production of these and key basic chemicals using renewable energy. Ammonia plays a key role in this as both a chemical raw material and a potential energy source. While electrochemical nitrogen reduction is being intensively investigated as a direct alternative to the Haber-Bosch process, its technical implementation has so far been limited by low conversion rates, low selectivity, and methodological uncertainties. In contrast, the electrochemical reduction of nitrate represents a more robust and technically realistic route, but one that still needs to be implemented, particularly in the context of wastewater streams and the circular economy, as electrochemical processes have so far mostly been investigated on a laboratory scale and under idealized conditions.The starting point of the work is electrochemical nitrogen reduction, which was carried out using specially manufactured nitrogen-free catalysts and under meticulously controlled conditions to avoid contamination effects and false-positive results. Furthermore, nitrate reduction is established as a more promising synthesis route. Building on this, batch-based electrolysis systems are used to investigate the influence of electrolyte composition, pH value, and membrane type on selectivity and yield.A key contribution of this work is the transfer of nitrate reduction to continuously operated electrochemical flow cells and the associated avoidance of dynamic influences. The introduction of a feed-and-bleed operating mode enables stable steady-state operation at a constant nitrate concentration. The systematic variation of the hydrodynamic conditions allows a targeted increase in the ammonia formation current and the derivation of general relationships between mass transport and reaction performance. Through optimization, Faraday efficiencies for ammonia of almost 95 % are achieved at current densities of 600 mA/cm².In addition, process intensification is investigated in a zero-gap cell architecture with catalyst-coated membranes. This allows operation at high current densities without the addition of conductive electrolytes and leads to significantly increased ammonia concentrations. The work shows that electrochemical nitrate reduction can be developed into a technically relevant process through suitable reactor and process design and provides general concepts for electrochemical processes under industrially relevant conditions beyond the specific application.

Authors 1

1. Yannick Sebastian Mürtz corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen