Results

Synthesis nitrogen-doped carbon nanostructures

Three “free template" methods for polyaniline synthesis were developed to obtain nitrogen doped carbon materials with tubules-like morphology and two “free template” methods for obtaining polypyrrole with different morphologies ( granular shape and tubules-like). For the "hard-template" method, two methods were used: a) suspension of the alumina membrane in the polymerization solution of pyrrole monomer and b) wetting of the polymeric membrane with a solution of a polymer containing is nitrogen (polyacrylonitrile).

The results obtained from the corresponding syntheses and characterizations of these materials have shown that the “free template” method is a feasible and inexpensive method for making polymeric precursors with different morphologies which can then be converted into nitrogen-doped carbon materials preserving precursor morphology.

The characterization of the obtained carbon nanomaterials, especially XPS, showed that the synthesis method influences the type of functional groups of N on the surface of the material. The type of surface functional groups is important in the behavior of these materials in oxygen reduction reactions and in the improvement of MEC performance (power density, the adhesion of biofilm to the substrate, wastewater treatment and heavy metal reduction).

Consequently, two methods have been proposed for obtaining nitrogen-containing polymeric materials, precursors for nitrogen-doped carbon materials:

A. Synthesis of polyaniline nanotubes

The synthesis of polyaniline nanotubes was done using “free template” method, in an appropriate mixed solution of ethanol and acetic acid and ammonium peroxydisulfate as an oxidant.

SEM image of polyaniline nanotubes

B. Synthesis of Polypyrrole Nanotubes

Polypyrrole tubules-like morphology were synthesized by chemical oxidative polymerization (using FeCl3 as oxidant) and an anionic azo dye MO (methyl orange) as structuring agent.

SEM image of polypyrrole nanotubes

Electocatlytic activity of nitrogen-doped carbon materials in oxygen reduction reaction (ORR)

The electrocatalytic activity of nitrogen doped carbon materials in the oxygen reduction reaction was investigated using rotating ring disk electrode (RDDE). The RDDE results showed that the best behavior was obtained for the nitrogen-doped carbon materials obtained form the carbonization of polyaniline nanotubes, for which the apparent number of electrons transferred to the ORR is between 3.8 and 4. The better activity of the sample in the ORR reaction can be attributed, according with XPS analyzes, to the quaternary nitrogen, which is the dominant species of surface nitrogen.

Design of bicameral MEC reactor

The proposed design for bicameral MEC reactor is shown in the figure below, and is feasible for testing the various materials used to modify the MEC anode:

Design of MEC reactor

Evaluation of MEC performance in term of power density

The best MEC results in terms of maximum power density assessment were obtained for nitrogen-doped carbon form polyaniline nanotubes precursors - modified anodes. Thus, we can suggest that the determining factors in choosing the material for the anode modification are: tubular-like morphology of the structures; the total nitrogen content of the material as well as the nature and individual content of the nitrogen atoms functionality on the surface.

Performance of MEC in the wastewater treatment

In order to assess the efficiency of waste water treatment through the MEC system, the effluent in the anode chamber was examined for chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total organic carbon (TC) in accordance with standard wastewater treatment methods. Results of water analyzes of municipalities used before and after MEC experiments show that MEC reactors are able to significantly reduce TC (between 68-84%), COD (75-86%) and BOD (74-90%). The results show that MEC cells besides producing energy can simultaneously and treat municipal waters.

Performance of MEC in cooper removal

For testing MEC systems for removing / recovering heavy metals, Cu has been used since it is one of the most toxic metal ion pollutants that is released into the aquatic environment due to various industrial and natural processes. Determination of Cu from cathode solutions after MEC experiments was performed using atomic absorption spectroscopy. The results of copper content in the analyzed water sample after MEC experiments show that modified electrodes with nitrogen-doped carbon materials form polyaniline nanotube precursors lead to Cu removal efficiency from the solution at about ~80%.

Based on the analysis of the electrochemical activity results (the apparent number of electrons transferred in the ORR reaction is between 3.8 and almost 4), the power density (362.7 mW / m2 - without Cu in catholite and 115.9 mW / m2 with Cu in catholite), wastewater treatment (reduction of TC ~ 84%, COD ~ 86% and BOD ~ 90%) and Cu removal efficiency (solution for removal with ~ 85.9%), we propose an experimental model of bicameral MEC reactor, with both electrodes modified with nitrogen-doped carbon materials obtained from the polyaniline nanotubes precursors.

Other activities

Improving the material infrastructure:

Throughout the project, specific equipments were purchased:

- OrigaFlex potentiostat with accessories

- Impedance module - with accessories

- 8-channel ADC-24 data logger

- Heating plate dedicated to MEC experiments

Human resource development:

The activities involved in the project have enabled the development of professional team members and especially young researchers in an interdisciplinary field, the acquisition of new scientific knowledge in the field of electrochemical microbial cells, the accumulation of new knowledge and expertise on synthesis and characterization of advanced materials and electrochemical behavior of the materials, in order to improve the performance of MEC reactors.