Electrochromic Rutile with Dynamically Tailored Surfaces in Formaldehyde-Mediated Hydroxylamine Electrosynthesis.

Electrocatalytic nitrate reduction is an attractive route for sustainable hydroxylamine synthesis, but its selectivity is limited by over-reduction and competing hydrogen evolution, highlighting the need for in-depth mechanistic understanding to guide catalyst design. Here, we systematically investigate the electrochemical synthesis of hydroxylamine via a formaldehyde-mediated method on titanium oxides. An electrochromic rutile array prepared via a wet-chemical route achieved a Faradaic efficiency (FE) of 92.6% (for formaldehyde oxime) and a corresponding yield rate of up to 2085 μmol cm-2 h-1 under ambient conditions. Mechanistic studies reveal that the electrochromism is a macroscopic manifestation of the protonation of Ob (bridging oxygen) sites and the formation of Ov (oxygen vacancies) and Ti3+, which act as proton "sponges" and electron reservoirs. Formaldehyde not only serves as the capturing agent but also helps to stabilize *NH2OH through molecular tuning, thereby achieving high selectivity. Through formaldehyde-nitrate electro-reforming, hydrogen, formic acid, and hydroxylamine can be coproduced at 200 mA cm-2 under an ultralow cell voltage of 0.78 V. This work links the catalytic performance of hydroxylamine electrosynthesis to the dynamic surface of titanium oxides, offering insights into selectivity control in nitrate electroreduction and providing a green, cost-effective alternative to conventional hydroxylamine synthesis.