Synthesis, properties and electrocatalytic application of g-C₃N₄ for oxygen electrodes of fuel cells

Figures & data

Figure 1. Mock-up of the fuel half-cell. 1 - cell housing: 2 - pressure clutch; 3 – metal current lead and tube for oxygen supply; 4 - metal grid of the oxygen electrode; 5 - hydrophobic layer of oxygen electrode; 6 - the active layer of the oxygen electrode from the g-C₃N₄ particles; 7 - polytetrafluoroethylene gasket; 8 - reference electrode; 9 - zinc anode.

Figure 2. X-ray diffraction patterns of the prepared carbon nitride samples: I (curve 1); III (curve 2); II (curve 3); IV (curve 4).

Figure 3. The TEM image of a rather large g-C₃N₄ particle (of about 1 µm) demonstrating the complicated shape of the prepared particles of II and their high transparency to electron beam. The SAED pattern in the inset shows two very weakly expressed wide (similar to amorphous) diffraction rings, with two pairs of symmetric reflections and a separate reflection. These bright reflections are located in the middle of weak diffraction rings indicating that the large crystalline particles are of the same g-C₃N₄ origin as the small ones.

Figure 4. IR spectra of the prepared C₃N₄ samples I-IV in the regions of: (a) 3800–2600 and (b) 1800–700 cm⁻¹. For comparison, IR spectrum of bare KBr disk is shown (blue curve).

Figure 5. Optical absorption spectra of g-C₃N₄ samples: I – curve 1; II –curve 2; IV –curve 3; III – curve 4. As the nitrogen content in the obtained materials decreases in order of I < II < III = IV, the band gap increases in the same order due to a decreased number of the defects states. The upper location of melamine precursor over the urea powder during the synthesis of III results in the production of nitrogen-doped С₃N₄, showing the largest band gap of 2.4 eV for the 1:1 components weight ratio.

Figure 6. Cyclic voltammograms with oxygen flow for the g-C₃N₄ samples: III –curve 1; IV – curve 2; II – curve 3; I –curve 4.

Figure 7. Current-voltage characteristics of the oxygen electrodes with an active layer made of different electrode materials: C₃N₄ (IV) - curve 1; C₃N₄ (III) – curve 2; C₃N₄ (I) – curve 3; C₃N₄ (II) – curve 4; and MWCNTs with 10 wt.% Pt - curve 5.

Figure 8. Results of the long-term testing of a two-layer oxygen electrode with an active layer consisting of 0.014 g cm⁻² of C₃N₄ (II) coupled with a zinc anode under constant operating current density at the oxygen electrode of 200 mA cm⁻².