Relating Near-Surface Ti Electrode Structure with Electrochemical Nitrate Reduction Performance Via Synchrotron X-Ray Characterization
Authors: Liu Matthew Junjie , Guo Jinyu , Hoffman Adam S. , Stenlid Joakim H. , Tang Michael T. , Corson Elizabeth R. , Stone Kevin H. , Ablid-Pedersen Frank , Bare Simon R. , Tarpeh William Abraham 
Published: September 2023

The global nitrogen cycle has been severely skewed since the widespread adoption of the Haber-Bosch process to produce ammonia (NH3). Currently, removal of reactive nitrogen (inorganic forms besides N2) from the environment lags behind its production and emission to the environment. Nitrate is one of the most prevalent waterborne nitrogen pollutants and, in its excess, threatens the health of ecosystems. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. Therefore, selective electrochemical nitrate reduction (NO3RR) to NH3 can couple water purification and NH3 production, helping offset the energy- and carbon-intensive Haber Bosch process. NH3 recovery from emissions could contribute over 20 million tons N per year by 2050 (~ 10% of projected N demand).

Noble and transition metal catalysts including single metals (e.g., Pt, Rh, Ru, Ir, Pd, Cu, Ag, Au)[1,2] and alloys (e.g., CuNi)[3] have been studied for NO3RR. Notably, under acidic conditions, polycrystalline titanium has been demonstrated to display robust and efficient NO3RR.[4] Ti is corrosion-resistant, a poor hydrogen evolution catalyst, and a readily available, abundant metal. However, under acidic and reducing conditions, Ti forms a water-stable hydride (TiHx, 0<x≤2).[5] It remains unclear how the degree of surface hydride formation – which alters the physical and electronic properties of the electrode surface – impacts NO3RR performance. Thus, rationally implementing Ti-catalyzed NO3RR requires improved understanding of how near-surface Ti-hydride forms and influences NO3RR activity and selectivity.

In this work, we show that near-surface Ti-hydride formation is a function of the duration and magnitude of applied NO3RR potential. A combination of ex situ grazing-incidence X-ray diffraction (GIXRD – probes long-range, crystalline order) and X-ray absorption spectroscopy (XAS – probes short-range, atom-specific probe into local coordination environments) enabled quantitative near-surface characterization of Ti-hydride electrodes. Through electrochemical testing and density functional theory calculations, we investigated the role near-surface Ti-hydride may play in electrochemical nitrate conversion. Our results preliminary suggest that near-surface hydride content plays a relatively minor role in steering NO3RR performance compared to applied potential and electrolyte effects. In addition, we are performing ongoing in situ GIXRD and XAS measurements to interrogate the transient nature and interplay of near-surface hydride formation and NO3RR. Put in context, our results help prioritize how Ti-catalyzed NO3RR processes can be optimized for electrified ammonia production and wastewater remediation.

[1] G. E. Dima, A. C. A. de Vooys, M. T. M. Koper, Journal of Electroanalytical Chemistry
2003, 554–555, 15–23.

[2] G. E. Dima, G. L. Beltramo, M. T. M. Koper, Electrochimica Acta
2005, 50, 4318–4326.

[3] Y. Wang, A. Xu, Z. Wang, L. Huang, J. Li, F. Li, J. Wicks, M. Luo, D.-H. Nam, C.-S. Tan, Y. Ding, J. Wu, Y. Lum, C.-T. Dinh, D. Sinton, G. Zheng, E. H. Sargent, J. Am. Chem. Soc.
2020, 142, 5702–5708.

[4] J. M. McEnaney, S. J. Blair, A. C. Nielander, J. A. Schwalbe, D. M. Koshy, M. Cargnello, T. F. Jaramillo, ACS Sustainable Chem. Eng.
2020, 8, 2672–2681.

[5] Y. Liu, Z. H. Ren, J. Liu, R. F. Schaller, E. Asselin, J. Electrochem. Soc.
2019, 166, C3096–C3105.