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. 2024 Apr 25;10(9):e29907.
doi: 10.1016/j.heliyon.2024.e29907. eCollection 2024 May 15.

Green and sustainable use of macadamia nuts as support material in Pt-based direct methanol fuel cells

N A Mojapelo  1 , N S Seroka  1   2 , L Khotseng  1
Affiliations

Green and sustainable use of macadamia nuts as support material in Pt-based direct methanol fuel cells

N A Mojapelo et al. Heliyon. .

Abstract

The successful commercialization of direct methanol fuel cells (DMFCs) is hindered by inadequate methanol oxidation activity and anode catalyst longevity. Efficient and cost-effective electrode materials are imperative in the widespread use of DMFCs. While Platinum (Pt) remains the primary component of anodic methanol oxidation reaction (MOR) electrocatalysts, its utilization alone in DMFC systems is limited due to carbon monoxide (CO) poisoning, instability, methanol crossover, and high cost. These limitations impede the economic feasibility of Pt as an electrocatalyst. Herein, we present the use of powdered activated carbon (PAC) and granular activated carbon (GAC), both sourced from macadamia nut shells (MNS), a type of biomass. These bio-based carbon materials are integrated into hybrid supports with reduced graphene oxide (rGO), aiming to enhance the performance and reduce the production cost of the Pt electrocatalyst. Electrochemical and physicochemical characterizations of the synthesized catalysts, including Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2, were conducted. X-ray diffraction analysis revealed crystallite sizes ranging from 1.18 nm to 1.68 nm. High-resolution transmission electron microscopy (HRTEM) images with average particle sizes ranging from 1.91 nm to 2.72 nm demonstrated spherical dispersion of Pt nanoparticles with some agglomeration across all catalysts. The electrochemical active surface area (ECSA) was determined, with Pt-rGO/GAC-1:1 exhibiting the highest ECSA of 73.53 m2 g-1. Despite its high ECSA, Pt-rGO/GAC-1:1 displayed the lowest methanol oxidation reaction (MOR) current density, indicating active sites with poor catalytic efficiency. Pt-rGO/PAC-1:1 and Pt-rGO/PAC-1:2 exhibited the highest MOR current densities of 0.77 mA*cm-2 and 0.74 mA*cm-2, respectively. Moreover, Pt-rGO/PAC-1:2 and Pt-rGO/PAC-1:1 demonstrated superior electrocatalytic mass (specific) activities of 7.55 mA/mg (0.025 mA*cm-2) and 7.25 mA/mg (0.021 mA*cm-2), respectively. Chronoamperometry tests revealed Pt-rGO/PAC-1:2 and Pt-rGO/PAC-1:1 as the most stable catalysts. Additionally, they exhibited the lowest charge transfer resistances and highest MOR current densities after durability tests, highlighting their potential for DMFC applications. The synthesized Pt supported on PACs hybrids demonstrated remarkable catalytic performance, stability, and CO tolerance, highlighting their potential for enhancing DMFC efficiency.

Keywords: Biowaste; Direct methanol fuel cell; Green chemistry; Macadamia nuts; Pt electrocatalysts.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
XRD patterns of (a) GO and (b) rGO.
Fig. 2
Fig. 2
XRD patterns of (a) rGO, (b) PAC, (c) rGO/PAC-1:1, and (d) rGO/PAC-1:2.
Fig. 3
Fig. 3
XRD patterns of (a) rGO, (b) GAC, (c) rGO/GAC-1:1, and (d) rGO/GAC-1:2.
Fig. 4
Fig. 4
XRD patterns of (a) Pt-rGO/PAC-1:1, (b) Pt-rGO/PAC-1:2, (c) Pt-rGO/GAC-1:1 and (d) Pt-rGO/GAC-1:2 electrocatalysts.
Fig. 5
Fig. 5
FTIR spectra of GO (black) and rGO (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6
Fig. 6
FTIR spectra of (a) granular activated carbon (GAC) and (b) powdered activated carbon (PAC) derived from macadamia nut shells.
Fig. 7
Fig. 7
FTIR spectra of (a) rGO (black), (b) rGO/PAC-1:1 (red) and (c) rGO/PAC-1:2 (blue) hybrid supports. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8
Fig. 8
FTIR spectra of (a) rGO (black), (b) rGO/GAC-1:1 (red) and (c) rGO/GAC-1:2 (blue) hybrid supports. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9
Fig. 9
FTIR spectra of (a) Pt-rGO/PAC-1:1 and Pt-rGO/PAC-1:2 electrocatalysts.
Fig. 10
Fig. 10
FTIR spectra of (a) Pt-rGO/GAC-1:1 and Pt-rGO/GAC-1:2 electrocatalysts.
Fig. 11
Fig. 11
Raman spectra of GO and rGO.
Fig. 12
Fig. 12
Raman spectra of (a) rGO, (b) PAC, (c) rGO/PAC-1:1 and (d) rGO/PAC-1:2.
Fig. 13
Fig. 13
Raman spectra of a) rGO, (b) GAC, (c) rGO/GAC-1:1 and (d) rGO/GAC-1:2.
Fig. 14
Fig. 14
Raman spectra of (a) Pt-rGO/PAC-1:1, (b) Pt-rGO/PAC-1:2, (c) Pt-rGO/GAC-1:1, and (d) Pt-rGO/GAC-1:2.
Fig. 15
Fig. 15
TGA curves of (a) GO and (b) rGO under N2 atmosphere.
Fig. 16
Fig. 16
TGA curves of (a) rGO, (b) rGO/PAC-1:1, (c) PAC, and (d) rGO/PAC-1:2 under N2 atmosphere.
Fig. 17
Fig. 17
TGA curves of (a) rGO, (b) rGO/GAC-1:1, (c) rGO/GAC-1:2, and (d) GAC under N2 under N2 atmosphere.
Fig. 18
Fig. 18
TGA curves of (a) Pt-rGO/PAC-1:1, (b) Pt-rGO/PAC-1:2, (c) Pt-rGO/GAC-1:1 and (d) Pt-rGO/GAC-1:2 electrocatalysts under N2 atmosphere.
Fig. 19
Fig. 19
SEM images of (a & b) GO and (c & d) rGO at different magnifications, and EDS spectra of (e) GO and (f) rGO.
Fig. 20
Fig. 20
SEM-EDS micrographs at different magnifications for (a & b) PAC, (c & d) rGO/PAC-1:1, and (e & f) rGO/PAC-1:2.
Fig. 21
Fig. 21
SEM-EDS micrographs at different magnifications for (a & b) GAC, (c & d) rGO/GAC-1:1, and (e & f) rGO/GAC-1:2.
Fig. 22
Fig. 22
SEM-EDS micrographs of electrocatalysts at various magnifications for (a & b) Pt-rGO/PAC-1:1, (c & d) Pt-rGO/PAC-1:2, (e & f) Pt-rGO/GAC-1:1, and, (g & h) Pt-rGO/GAC-1:2.
Fig. 23
Fig. 23
HRTEM images of (a) GAC, (b) PAC, (c) rGO, (d) rGO/PAC-1:1, (e) rGO/PAC-1:2, (f) rGO/GAC-1:1, and (g) rGO/GAC-1:2 at different magnifications.
Fig. 24
Fig. 24
HRTEM images of (a) Pt-rGO/PAC-1:1, (b) Pt-rGO/PAC-1:2, (c) Pt-rGO/GAC-1:1, and (d) Pt-rGO/GAC-1:2 at 20 nm magnification, and corresponding size histograms of (e) Pt-rGO/PAC-1:1, (f) Pt-rGO/PAC-1:2, (g) Pt-rGO/GAC-1:1, and (h) Pt-rGO/GAC-1:2.
Fig. 25
Fig. 25
Cyclic voltammograms (CVs) of Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1 and Pt-rGO/GAC-1:2 electrocatalysts in 1 M HClO4 (scan rate at 0.03 V/s).
Fig. 26
Fig. 26
CV curves of Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2 electrocatalysts in 1 M HClO4 + CH3OH solution with a scan rate of 0.03 V/s.
Scheme 1
Scheme 1
Detailed mechanism for MOR on the Pt catalyst surface [89].
Fig. 27
Fig. 27
Chronoamperometry curves of methanol oxidation on Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2 electrocatalysts in 1 M CH3OH + 1 M HClO4.
Fig. 28
Fig. 28
CV curves of Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2 electrocatalysts in 1 M HClO4 + CH3OH solution with a scan rate of 0.03 V/s After chronoamperometry studies.
Fig. 29
Fig. 29
Nyquist plots for Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2 electrocatalysts in 1 M HClO4 + CH3OH at a saturated N2 atmosphere.
Fig. 30
Fig. 30
Equivalent circuits for EIS of methanol oxidation on Pt-rGO/PAC-1:1, Pt-rGO/PAC-1:2, Pt-rGO/GAC-1:1, and Pt-rGO/GAC-1:2 electrocatalysts in 1 M HClO4 + CH3OH at a saturated N2 atmosphere. Rp = Rct = Charge transfer resistance estimated from the equivalent circuit.
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