Increasing Electrocatalytic Nitrate Reduction Performance By Tuning Adsorption through PtRu Alloying
Authors: Wang Zixuan , Young Samuel D. , Goldsmith Bryan R. , Singh Nirala 
Published: December 2020

Nitrate (NO3
−) is one of the world’s most common water pollutants because of massive global fertilizer overusage, which imbalances the global nitrogen cycle. High nitrate levels in water are not only toxic to humans but also promote harmful algal blooms and dead zone formations.1 Electrocatalytic nitrate reduction (NO3RR) is a promising approach to convert nitrate into value-added (NH3) or benign (N2) products by using renewable electricity sources without an external H2 stream.2 Devices using NO3RR could ultimately reduce anthropogenic effects on the nitrogen cycle. However, NO3RR requires improved electrocatalysts that are sufficiently active, selective, stable, and inexpensive to be industrially and economically viable.

Using first-principles microkinetic modeling, we predicted that a Pt3Ru alloy could be an active and selective NO3RR catalyst.3 In this talk, we confirm our computational predictions by experimentally synthesizing different compositions of Pt
x
Ru
y
/C (x = 48–100%) and characterizing their intrinsic activity and selectivity for nitrate reduction. By increasing Ru content, we show optimal nitrate and hydrogen binding energies that maximizes the electrocatalytic activity. Additionally, we demonstrate that these catalysts are selective towards ammonium (NH4
+) at 0.1 V vs. RHE.

To accomplish this work, a series of Pt
x
Ru
y
/C alloys were synthesized using a modified NaBH4 reduction method with varying Ru compositions ranging from 0 to 52 at% of the total metal loading. Bulk structural characterization was performed by using X-ray diffraction, extended X-ray absorption fine structure, X-ray absorption near edge structure, and transmission electron microscopy. To ensure that intrinsic catalyst activities were normalized properly, we used hydrogen and copper underpotential deposition to measure the electrochemical active sites and compared these values to surface compositions determined by X-ray photoelectron spectroscopy. The catalysts were deposited onto a glassy carbon electrode for intrinsic steady-state current measurements at different operating potentials. Steady-state intrinsic activity measurements show that PtxRuy/C alloys are more active than Pt/C, with Pt78Ru22/C being greater than 10 times more active (−55 μA/cm2) than Pt at 0.05 V vs RHE. Product quantification of the catalysts gives faradaic efficiencies between 70–98% towards ammonium at 0.1 V vs RHE at pH = 1. Density functional theory calculations predict increasing Pt
x
Ru
y
activity as Ru content increases to 25 at%, followed by a decrease in activity at higher Ru content, which is qualitatively consistent with experimental measurements. Ultimately, this study clarifies how nitrate reduction electrocatalyst performance can be tuned by changing the adsorption strength of nitrate, hydrogen, and intermediate species through alloying.

References

Yao, F., Yang, Q., Zhong, Y., Shu, X., Chen, F., Sun, J. Ma, Y., Fu, Z., Wang, D., Li, X., Water Res, 157, 191-200 (2019).

Singh, N. and Goldsmith, B. R. ACS Catal. 2020, 10, 5, 3365-3371 (2020).

Liu, J.-X., Richards, D., Singh, N., Goldsmith, B. R., ACS Catal. 9, 8, 7052-7064 (2019).