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 (NH3) product during the nitrate reduction reaction (NO3-RR). This study presents the synthesis of a carbon-vulcanized (C)-defective TiO2 nanosheet (TNS) composite, modified with subnanometric WO3 clusters. It is found that among various loadings, the electrocatalyst with 3 wt % WO3 (C-3%WO3-TNS) suppresses HER. NH3 production higher than 97% is achieved by incorporating CuNi (40:60 wt %) onto C-3%WO3-TNS (Cu40Ni60/C-3%WO3-TNS), as confirmed by in situ differential electrochemical mass spectrometry (DEMS). Chemical characterizations reveal that WO3 clusters influence the Ti3+/Ti4+ ratio, thereby potentially suppressing HER. It has also been found that NH3 formation is further facilitated by Cu40Ni60, which promotes faster NO3– reduction via a multistep reaction on the C-WO3-TNS supports. The synergy between Cu40Ni60, C, WO3, and defective TNS modulates the production of H2 and NH3. 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 WO3 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 Cu40Ni60/C-3%WO3-TNS to suppress HER and promote NH3 synthesis, offering a promising strategy for tuning reaction pathways during NO3-RR.