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

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.