Overcoming Nitrogen Reduction to Ammonia Detection Challenges: The Case for Leapfrogging to Gas Diffusion Electrode Platforms

The nitrogen reduction reaction (NRR) is a promising pathway toward the decarbonization of ammonia (NH3) production. However, unless practical challenges related to the detection of NH3 are removed, confidence in published data and experimental throughput will remain low for experiments in aqueous electrolyte. In this perspective, we analyze these challenges from a system and instrumentation perspective. Through our analysis we show that detection challenges can be strongly reduced by switching from an H-cell to a gas diffusion electrode (GDE) cell design as a catalyst testing platform. Specifically, a GDE cell design is anticipated to allow for a reduction in the cost of crucial 15N2 control experiments from €100–2000 to less than €10. A major driver is the possibility to reduce the 15N2 flow rate to less than 1 mL/min, which is prohibited by an inevitable drop in mass-transport at low flow rates in H-cells. Higher active surface areas and improved mass transport can further circumvent losses of NRR selectivity to competing reactions. Additionally, obstacles often encountered when trying to transfer activity and selectivity data recorded at low current density in H-cells to commercial device level can be avoided by testing catalysts under conditions close to those in commercial devices from the start.


Miniaturized alkaline impurity trap
The removal of NH3 and NOx contamination from the gas feed of the cell is crucial to avoid false positives while testing catalysts for NRR. Purifiers to achieve this typically consist of an inner and an outer tube. The inner tube is immersed into the outer tube which is filled with an oxidizing solution that traps the NH3/NOx. During operation, gas is bubbled into the oxidizing solution through the inner tube. The gas exits the outer tube through its headspace. Typically, both outer tube and inner tube are made of glass, because glass is very inert and easy to reshape. However, the smallest commercially available impurity traps of this design have several mL of headspace volume which would make them very expensive to flush during a 15 N2 experiment. Therefore, we propose to use an impurity trap made from inert polymers instead, as shown in Figure S1. The working principle of the design is identical to that of glass impurity traps but the inner and outer tube are made of inert polymer tubing. The headspace of the 1/32" outer diameter (OD) inner tubing and the tee is negligible so that the total headspace of the purifier can be estimated from the headspace of the ¼" outer diameter (OD) outer tube. In our experience approximately 1 cm of headspace in the outer tube is sufficient to prevent liquid from entering the gas channel. With an inner diameter (ID) of 5.6 mm the headspace of the outer tube is approximately 250 µL. This low headspace makes it ideal for cheap 15 N2 experiments in GDE cell. Additionally, it is comprised of standard connectors for easy, leak-tight, contamination-free connections. Unlike with glass impurity traps, it is possible to easily adjust the length of the outer tube depending on the required removal efficiency. The trap only consists of readily available, off-the-shelf parts which should improve standardization of this critical component.

Calculation: nitrogen reduction reaction mass-transport limiting current in H-cell
We estimate the mass transport limiting current of the nitrogen reduction reaction jlim,NRR from the mass-transport limiting current of the CO2 reduction reaction to CO jlim,CO2RR according to: Calculation: accumulated NH3 in the electrolyte The concentration of NH3 in the electrolyte was calculated according to: , where cNH3 is the concentration of NH3 after electrolysis, iNH3 is the partial current density of NRR, t is the duration of the experiment, z is the number of electrons transferred per molecule of NH3 produced, F is the Faraday Constant and V is the half-cell volume of the electrolyte, respectively.
Mathematical modelling of influence of ECSA on ammonia production and faradaic efficiency The specific activity (defined as the ECSA normalized current density) jECSA was calculated by assuming Butler-Volmer kinetics according to: , where j0 is the exchange current density, α is the symmetry factor, f is the Faraday Constant F divided by the ideal gas constant R and the temperature T and η is the overpotential. 6 The current density normalized by geometric surface area jgeometric was calculated by multiplying jECSA with the roughness factor of the electrode.
The faradaic efficiency was modelled by assuming that a potential window exists where NRR is favorable over HER and that the faradaic efficiency (FE) of NRR within this potential window can be described by a quadratic function: , where a,b,c are constant parameters.
The partial current density of NRR jNRR was calculated by multiplying the faradaic efficiency with jgeometric. The partial current density of NRR including mass transport effects jmt,NRR was calculated by replacing jNRR with the mass transport limiting current jlim,NRR wherever jNRR would otherwise have been lower than jlim,NRR: Literature summary of reported levels of NH 3 /NO x contamination