Bifunctional Catalyst Design Integrating Copper Nickel and Tungsten Trioxide on Defective Titanium Dioxide Enables Reaction Pathway Steering in Nitrate Electroreduction

Metal oxide subnanometric size clusters can be “small” but “powerful” in suppressing side-reactions such as the hydrogen evolution reaction (HER), thereby improving ammonia (NH ₃) product during the nitrate reduction reaction (NO ₃-RR). This study presents the synthesis of a carbon-vulcanized (C)-defective TiO ₂ nanosheet (TNS) composite, modified with subnanometric WO ₃ clusters. It is found that among various loadings, the electrocatalyst with 3 wt % WO ₃ (C-3%WO ₃-TNS) suppresses HER. NH ₃ production higher than 97% is achieved by incorporating CuNi (40:60 wt %) onto C-3%WO ₃-TNS (Cu ₄₀Ni ₆₀/C-3%WO ₃-TNS), as confirmed by in situ differential electrochemical mass spectrometry (DEMS). Chemical characterizations reveal that WO ₃ clusters influence the Ti ³⁺/Ti ⁴⁺ ratio, thereby potentially suppressing HER. It has also been found that NH ₃ formation is further facilitated by Cu ₄₀Ni ₆₀, which promotes faster NO ₃ ^– reduction via a multistep reaction on the C-WO ₃-TNS supports. The synergy between Cu ₄₀Ni ₆₀, C, WO ₃, and defective TNS modulates the production of H ₂ and NH ₃. This synergy can be attributed to the morphological and structural characteristics of the electrocatalyst, which indicate that Ni is positioned at specific edge sites over the C and TNS, while WO ₃ and Cu are well-distributed over the TNS. A mechanistic approach is proposed to explain the observed products by DEMS. This work highlights the dual potential of Cu ₄₀Ni ₆₀/C-3%WO ₃-TNS to suppress HER and promote NH ₃ synthesis, offering a promising strategy for tuning reaction pathways during NO ₃-RR.