A Spectroscopic Study on the Electrochemical Nitrogen and Nitrate Reduction Reactions on Rhodium and Ruthenium Surfaces
Authors: Yao Yao , Wang Haijiang , Li Hui , Shao Minhua 
Published: February 2020

Ammonia is one of the most promising carbon-free alternative energy storage media for several reasons, including its high weight fraction of hydrogen (17.65%), ease of decomposition to produce high-purity hydrogen, ease of liquefaction at room temperature, and low transportation cost.1 The N2 reduction reaction (NRR) is an attractive approach to synthesizing ammonia with renewable electrical energy by using water and nitrogen as the reactants.2-3 Rh and Ru are promising N2 reduction reaction electrocatalysts, given their suitable nitrogen adsorption energy and low overpotential. However, the N2 reduction reaction pathway on these surfaces is still unclear, as there is a lack of information about the reaction intermediates. In this study, we employed surface-enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS) to study the reaction mechanisms of nitrogen reduction on Rh and Ru.

A Rh nanofilm deposited on the Si prism by a chemical deposition method was used as the working electrode. Figure 1a shows the IR spectra collected in the first CV segment in N2-saturated KOH solution with the spectrum at 0.4 V as the reference. The bands at 3250 and 1612 cm-1 attribute to the O-H stretching and H-O-H bending of water molecules. Those bands intensity increase significantly as a result of the change in the adsorption configuration of water molecules upon decreasing the potential. A weak band at 1865 cm−1 emerged below −0.2 V, and the band intensity increased as the potential became more negative, which was assigned to the adsorbed H atoms. An obvious band at ~2020 cm−1 was attributed to the N=N stretching of end-on adsorbed N2HX (1≤X≤2) on the Rh surface. These bands were clearly revealed in a single spectrum at 0 and −0.4 V, shown in Figure 1b. The potential dependence of the N=N stretching band absorption and frequency are given in Figure 1c, as well as the IR absorption potential dependence of hydrogen on the Rh surface. When the potential fell from 0.2 V to −0.05 V, the band intensity of N2HX was significantly enhanced, peaking at −0.05 V, which suggested that the coverage of the adsorbed N2HX species on the Rh surface was boosted in this potential range. Simultaneously, the frequency of the N=N stretching band was blue-shifted from 1997 cm−1 at 0.2 V to 2034 cm−1 at −0.05 V, due to the weakened adsorption strength of the N2HX species on the Rh surface. The highest N2HX coverage was achieved between 0 and −0.2 V. When the potential further decreased from −0.2 to −0.4 V, the N=N stretching band intensity decreased, indicating less coverage of the adsorbed N2HX species. Figure 1d presents the DEMS data collected during the NRR in an alkaline electrolyte with Rh/C as the electrocatalyst. As occurred during nitrate reduction, the pulse DEMS signal of H2 (m/e = 2) was detected at potentials below 0 V as the HER occurred on the Rh surface. The pulse DEMS signal of N2H2 (m/e = 29) was also detected with H2, but its intensity was much lower than that of H2 (m/e=2), and the signal of N2H2 (m/e = 30) was undetectable. Similar spectroscopic studies on Ru surface will be also discussed.

Figure 1. (a) FTIR spectra during the first segment from 0.4 to −0.4 V on Rh film electrode in N2-saturated 0.1 M KOH solution. The reference spectrum was taken at 0.4 V. (b) Single spectrum at 0 V (black line) and −0.4 V (red line) in the range of 1250−3000 cm−1. (c) Potential dependence of the N=N stretching band IR absorption (black dot) and frequency (red triangle), derived from (a). The potential dependence of the hydrogen IR absorption magnified by a factor of two (pink dot) is also given. (d) DEMS of H2 (m/e = 2) and N2H2 (m/e = 29, 30) on Rh/C during a scan from 0.4 to −0.8 V in N2-saturated 1 M KOH solution. Potential scan rate: 5 mV s−1.

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Figure 1