Development of Suppressed Ion Chromatography for the Online Quantification of Cations in Electrochemical Ammonia Synthesis Research

Abstract

Practical research on the electrochemical nitrogen reduction reaction (eNRR) requires quantitative ammonia trace analysis because production rates are on the order of µg h−1 in aqueous electrolyte. This challenge is further aggravated by complex sample matrices. Ion chromatography is a powerful analytical technique for the quantitative determination of ammonium down to ppb-concentrations, but requires matrix elimination (ME) for these kinds of sample. We developed a suppressed cation chromatography method using automated matrix neutralization and ME to quantitatively determine ammonium in 0.2 M sulfuric acid electrolyte at µg L−1 concentrations for use in NRR research. Although direct conductivity detection of cations is less sensitive than unsuppressed indirect conductivity detection, baseline noise requires suppression at these concentrations. Nonlinearity of the calibration curve became noticeable below ≈ 1 ng ammonium. A method limit of detection of 2 µg L−1 (ppbmol) for ammonium was achieved at 100 µL injection volume. Direct coupling of the electrochemical cell and IC enabled online quantification. This online measurement of ammonium in 0.2 M sulfuric acid electrolyte revealed ammonium contamination rapidly liberated from the hitherto judged negligible Nafion ionomer of the gas diffusion electrode at open circuit voltage, showing prior production rates to be likely false positives.