3D‑printed boron‑nitrogen doped carbon electrodes for sustainable wastewater treatment via MPECVD

As global concerns over emerging contaminants (such as pharmaceuticals) in wastewater grow, traditional treatment methods like ozone oxidation and activated carbon adsorption face limitations—from high energy consumption to reliance on critical raw materials. Now, a collaborative team of researchers from Gdansk University of Technology (Poland), Università Politecnica delle Marche (Italy), and Lund University (Sweden) has developed a game-changing solution: 3D-printed boron-nitrogen (B,N)-doped carbon electrodes fabricated via a synergistic combination of 3D printing, phase inversion, and microwave plasma-enhanced chemical vapor deposition (MPECVD). Published in Nano-Micro Letters, this technology delivers unprecedented performance in electrochemical oxidation (EO) of persistent pollutants, offering a scalable, metal-free path to sustainable water treatment.

Why These 3D-Printed Electrodes Stand Out

The core innovation lies in the integration of topology optimization, precision fabrication, and catalyst-free nanostructure growth—addressing key pain points in wastewater treatment while boosting efficiency:

• Hierarchical Porosity & Enhanced Mass Transport: By combining 3D-printed triply periodic minimal surface (TPMS) geometries (e.g., diamond, gyroid) with phase inversion, the electrodes achieve a 180% higher surface area-to-volume ratio than non-optimized counterparts. Computational fluid dynamics (CFD) simulations further refine the design, reducing pressure drop by 40% and improving reactant mixing—critical for fast pollutant degradation.
• Catalyst-Free B,N-Doped Nanostructures: MPECVD enables direct growth of vertically aligned carbon nanowalls (CNWs) on polymer-derived scaffolds without metal catalysts. Boron doping introduces p-type carriers, increasing electrical conductivity and active defect sites, while nitrogen incorporation enhances redox reactivity. This leads to a 20-fold increase in electrochemically active surface area (EASA) and a 6.2-fold higher charge transfer rate constant.
• Superior Pollutant Degradation: The optimized electrodes achieve dramatic improvements in EO of β-blockers (common pharmaceuticals found in wastewater):
  1. 4.7-fold faster degradation of atenolol
  2. 4-fold faster degradation of metoprolol
  3. 6.5-fold faster degradation of propranolol
     Removal efficiencies reach 75–99.9%, outperforming conventional carbon-based electrodes.

Key Design, Fabrication, and Performance Details

1. Topology Optimization via CFD Simulation

To maximize efficiency, the team tested 15 modular geometries (6 TPMS and 2 fractal structures) using CFD, focusing on three critical metrics: mixing efficiency, pressure drop, and surface area-to-volume ratio.

• Optimal Geometries Selected: Structures like fks− (Fischer–Koch S solid) and gyr+ (gyroid sheet) stood out for their low friction factor (reduced pressure drop) and high mixing uniformity (coefficient of variation, CoV < 0.1).
• TPMS Advantages: Sheet-like TPMS geometries (e.g., dia+) minimized flow resistance, while solid networks (e.g., fks−) maximized surface area—striking a balance between mass transport and reaction sites.

2. Precision Fabrication: From 3D Printing to MPECVD

The fabrication process combines three scalable techniques to create hierarchical porosity at micro- and nanoscales:

1. 3D-Printed Molds: Water-soluble filaments (polyvinyl alcohol, PVA; butenediol vinyl alcohol copolymer, BVOH) were 3D-printed into TPMS molds using fused deposition modeling (FDM).
2. Phase Inversion: A polyacrylonitrile (PAN)/dimethylformamide (DMF) solution was cast into the molds. Immersion in water dissolved the mold and induced phase separation, forming porous PAN scaffolds. BVOH molds produced larger, more uniform pores than PVA, while adding 20% acetone further refined micron-scale porosity.
3. MPECVD Nanostructure Growth: Simultaneous pyrolysis of PAN and MPECVD growth (using B2​H6​ as a boron precursor) yielded B,N-doped CNWs. Taguchi optimization identified ideal conditions (550 °C, 40 Torr, 700 W microwave power) for uniform, vertically aligned nanowalls—eliminating the need for metal catalysts and reducing contamination risks.

3. Electrochemical Performance & Pollutant Degradation

Critical Material Properties

• Crystallinity & Conductivity: Raman spectroscopy confirmed graphitic CNWs (D/G ratio = 1.2), while X-ray photoelectron spectroscopy (XPS) verified B-N bonding (B 1s peak at 187 eV) and nitrogen incorporation (pyridinic/pyrrolic N at 398–400 eV).
• Electrochemical Activity: The best-performing electrode (cPAN3) exhibited a high EASA of 0.57 cm² mg^-1 and a low charge transfer resistance (1.7 Ω)—enabling fast electron transfer during EO.

β-Blocker Degradation Results

In flow-through reactors, the electrodes targeted three common β-blockers (atenolol, metoprolol, propranolol), with key findings:

• Kinetics: Propranolol degraded fastest (first-order rate constant = 0.021 min^-1), followed by metoprolol (0.018 min^-1) and atenolol (0.015 min^-1)—all outpacing commercial carbon electrodes.
• Degradation Pathways: UHPLC-MS/MS analysis revealed hydroxyl radical (•OH)-driven breakdown: metoprolol and propranolol underwent C-O bond cleavage, while atenolol followed a unique amide bond scission pathway—highlighting the electrodes’ selectivity.
• Stability: After 120 minutes of continuous operation, the electrodes retained 95% of their initial activity, with no detectable leaching of B or N.

Future Outlook & Sustainability Impact

This technology addresses two critical goals for wastewater treatment: performance and sustainability. By eliminating metal catalysts, using low-cost PAN as a precursor, and leveraging scalable 3D printing, the electrodes reduce reliance on critical raw materials (e.g., rare metals in conventional catalysts) and cut fabrication costs by 30% compared to boron-doped diamond electrodes.

Looking ahead, the team aims to:

1. Optimize thin-film formulations for large-area roll-to-roll production;
2. Test the electrodes on complex wastewater matrices (e.g., hospital effluents) containing multiple contaminants;
3. Explore other dopant combinations (e.g., B,P) to enhance reactivity for recalcitrant pollutants.

With the EU’s new Urban Wastewater Directive mandating removal of organic micropollutants, these 3D-printed electrodes offer a timely, scalable solution—bridging advanced materials science and practical environmental engineering.

Stay tuned for further innovations from this team as they advance toward commercializing this sustainable water treatment technology!