Anode Surface Engineering for Tunable Carbon Nanotube Growth via Integrated Carbon Capture and Conversion in Low‐Cost Molten Carbonates

The electrochemical conversion of CO 2 into carbon nanotubes (CNTs) using molten salt electrolysis is a promising route for carbon capture and utilization, yet the role of anode surface chemistry in governing CNT growth remains poorly understood. Herein, we demonstrate that controlled acid pretreatment of Inconel 718 anodes provides an effective surface engineering strategy to regulate metal dissolution behavior and control CNT formation. By systematically varying the HCl concentration for acid pretreatment, we show that partial removal of the passivation layer and the generation of localized surface pits enrich Ni and Fe metals at the near‐surface layer. Combined XPS, SEM‐EDS, and electrochemical analyses demonstrate that moderate acid treatment (6–8 M HCl) induces a regulated release of catalytically active Fe and Ni species, resulting in the formation of multi‐walled CNTs with tunable outer diameters down to 18.7 ± 4.1 nm and a maximum Faradaic efficiency (FE) of 93.6% in a low‐cost eutectic molten salt composed of Li 2 CO 3 –Na 2 CO 3 –K 2 CO 3 –KF (30.5:22.1:17.5:30.0 mol%) at 500°C. This work highlights the critical role of anode surface engineering in the conversion of captured carbonate within molten carbonate electrolytes and provides a simple and effective strategy to enable rational control of CNT growth behavior.