Pd doping-weakened intermediate adsorption to promote electrocatalytic nitrate reduction on TiO2 nanoarrays for ammonia production and energy supply with zinc–nitrate batteries

Abstract

The (photo)electrochemical nitrogen reduction reaction for ammonia (NH₃) production is an appealing alternative to the traditional high-energy Haber–Bosch reaction. However, the future of this approach is bleak because of the ultralow N₂ solubility and the nonpolar NN bond causing the NH₃ yield and selectivity to be unsatisfactory. Nitrate electroreduction (NORR) into NH₃ brings promise for the future landscape of NH₃ electrosynthesis due to the low NO bond energy and high nitrate solubility. Here, we report a highly efficient Pd-doped TiO₂ nanoarray electrode for NH₃ production from the NORR. With weakened adsorption abilities to the intermediates induced by Pd introduction, the catalyst delivers a record-high NH₃ yield of 1.12 mg cm⁻² h⁻¹ (or 0.066 mmol cm⁻² h⁻¹), an impressive NH₃ faradaic efficiency (FE) of 92.1%, and an exceptional nitrate conversion of 99.6%. Considering an eight-electron nitrate-to-ammonia reaction and the excellent electrocatalytic activity of Pd/TiO₂, we, for the first time, propose and develop a Zn–nitrate battery system, which delivers striking bifunctionality for harnessing the electrons related to the NORR to generate electricity and directly produce NH₃, specified by a power density of 0.87 mW cm⁻² and a high NH₃ FE of 81.3%. Our work not only verifies the positive effect of Pd doping on facilitating the NORR, but also demonstrates a galvanic nitrate-based cell providing a promising strategy for NH₃ production and broadening the field of Zn-based batteries.