Electrochemically active site-rich nanocomposites of two-dimensional materials as anode catalysts for direct oxidation fuel cells: new age beyond graphene

Abstract

Direct oxidation fuel cell (DOFC) has been opted as a green alternative to fossil fuels and intermittent energy resources as it is economically viable, possesses good conversion efficiency, as well as exhibits high power density and superfast charging. The anode catalyst is a vital component of DOFC, which improves the oxidation of fuels; however, the development of an efficient anode catalyst is still a challenge. In this regard, 2D materials have attracted attention as DOFC anode catalysts due to their fascinating electrochemical properties such as excellent mechanical properties, large surface area, superior electron transfer, presence of active sites, and tunable electronic states. This timely review encapsulates in detail different types of fuel cells, their mechanisms, and contemporary challenges; focuses on the anode catalyst/support based on new generation 2D materials, namely, 2D transition metal carbide/nitride or carbonitride (MXene), graphitic carbon nitride, transition metal dichalcogenides, and transition metal oxides; as well as their properties and role in DOFC along with the mechanisms involved.

Fig. 1. Schematic of the mechanism of DAFC in acidic medium and electrochemical fuel oxidation on the 2D surface.

Fig. 2. Schematic of the mechanism of DAFC in basic medium.

Fig. 3. Schematic of the formation of 3D Pt-doped BN-graphene aerogel. Reproduced with permission. Copyright 2018, ACS.

Fig. 4. Structure of different MAX phases depending upon the number of n. Reproduced with permission. Copyright 2014, Wiley.

Fig. 5. Schematic diagram of the synthetic method of g-C₃N₄ by the condensation of different precursors-cyanamide, dicyandiamide, melamine, thiourea, and urea; here, different colors represent different atoms: black-C atom; red-N atom; blue-H atom; purple-S atom; white-O atom. Reproduced with permission. Copyright 2015, Wiley.

Fig. 6. Predicted structures of heptazine-based g-C₃N₄, (a) phase 1, (b) phase 2, (c) phase 3. Middle layer atoms are represented by yellow crosses. Reproduced with permission. Copyright 2017, ACS.

Fig. 7. Metal coordination and stacking sequence. The octahedral coordination gives tetragonal symmetry (1T), whereas distinct stacking sequences of trigonal prismatic layers yield different symmetries: 2H and 3R. Reproduced with permission under the terms of Creative Commons Attribution 3.0 Unported License (CC BY 3.0, facilitated open access). Copyright 2017 RSC.

Fig. 8. (a) CVs of Pd with MXene and C as the support in 1 M methanol + 1 M potassium hydroxide, (b) current densities of Pd/C and Pd/MXene, (c) Tafel plots, (d) TOF plots, (e) current densities in the forward scan vs. the cycle number, (f) CVs of Pd/MXene and MXene in 1 M methanol + 1 M potassium hydroxide solution, (g) second derivative spectra of the catalysts after methanol adsorption, (h) CO₂-TPD spectra. Reproduced with permission. Copyright 2020, ACS.

Fig. 9. Fabrication route of Pt NW/PDDA-Ti₃C₂T_x. Reproduced with permission. Copyright 2020, ACS.

Fig. 10. SEM images of (a) MXene and (b) PtRu/MXene; response surface between the factors with response, (c) 2D contour and (d) 3D surface plot. Reproduced with permission. Copyright 2020, Elsevier.

Fig. 11. TEM of Pt supported on (a) Vulcan, (b) gCNM, (c) PTILi⁺Cl⁻, (d) B-gCNM; (e) XRD pattern of the Pt catalyst supported on (i) Vulcan, (ii) gCNM, (iii) PTI/Li⁺Cl⁻, (iv) B-gCNM; (f) MOR of the Pt electrocatalysts supported all the three graphitic carbon nitrides in 1 M methanol + 0.1 M perchloric acid at a scan rate of 2 mV s⁻¹. Reproduced with permission. Copyright 2014, ACS.

Fig. 12. (a) CV of Pd–MoS₂-modified ITO electrodes in 0.5 M potassium hydroxide + 1.0 M methanol solution before and after annealing, (b) EIS measurement of Pd–MoS₂-modified ITO electrodes before and after annealing. Reproduced with permission. Copyright 2014, RSC.

Fig. 13. FESEM images of Co₃O₄ grown on NF using different anions, (a–c) microflower, (d–f) microsphere, and (g–i) nanograss-like Co₃O₄. Reproduced with permission. Copyright 2018, Elsevier.

Kashmiri Baruah

Pritam Deb