Single-step synthesis of functionally graded carbon electrodes via off-axis MPECVD: Decoupling structural and electrochemical heterogeneity

Functionally Graded Materials (FGMs) are essential for next-generation electrochemical interfaces, yet their fabrication often requires tedious, multi-step layering processes. This study demonstrates a process intensification strategy using off-axis Microwave Plasma-Enhanced Chemical Vapor Deposition (MPECVD) to synthesize spatially graded boron-doped carbon nanowalls (BCNWs) in a single operational step. By exploiting the inherent gradients in plasma species flux and substrate temperature that arise from a 45° off-axis geometry, we achieved a continuous structural transition from amorphous carbon films to highly crystalline, porous graphitic mazes on a single electrode. To map the resulting electrochemical heterogeneity, a dual-probe voltammetric strategy was employed. The outer-sphere redox probe hexaammineruthenium(III) highlighted an increase in capacitive behavior and electroactive surface area in the nanowall-like regions. Conversely, the surface-sensitive inner-sphere probe ferrocyanide/ferricyanide indicated high kinetic resistance in the same regions, implying that high physical surface area does not strictly correlate with catalytic activity due to local variations in sp 2 /sp 3 hybridization and defect density. These findings validate off-axis MPECVD as a robust, intensified route for the rapid prototyping of multifunctional electrodes, eliminating the need for complex masking, post-growth thermal treatments, or multi-stage deposition protocols.