A simple fabrication route of porous palladium/palladium oxide/carbon nanostructures using one-step combustion waves for high-performance pH sensors

Reduction-oxidation controls and synthesis of organic-inorganic hybrid structures are key features associated with their electrochemical properties. However, they involve bulky setup and long-time processes that incur high-cost. Herein, we report one-step combustion waves (CWs) for the fabrication of hybrid-porous palladium/palladium oxides/carbon (Pd/Pd_xO_y@C) nanostructures with controllable atomic composition and their application as a high-performance pH sensor. Hybrid composites of Pd/formaldehyde (FA)/nitrocellulose (NC) were prepared as precursors. Self-propagating CWs through the NC networks carried out the transformation from Pd to PdO₂ in the higher oxidation state and the formation of a carbon layer, while the fast release of chemicals in CWs developed the porous nanostructures with large surface areas. The flexible pH sensor using the synthesized Pd/Pd_xO_y@C powders exhibited the outstanding specific voltage potential (2.08 V/mg at pH 2.83) and sensitivity (167 mV/mg pH). It showed no hysteresis during cycling of pH values and high stability over 4 h (voltage change <0.89%). The high oxidation states and conductive carbon layers of Pd/Pd_xO_y@C with large surface areas improved the specific potential and the stability in minute changes of pH values and forward-reverse proton exchanges. The physicochemical synthesis using CWs would contribute to developing scalable processing for electrochemical applications, including chemical sensors.