Polymer electrolyte membrane fuel cells (PEMFCs) running on hydrogen are attractive alternative power supplies for a range of applications, with in situ release of the required hydrogen from a stable liquid offering one way of ensuring its safe storage and transportation before use. The use of methanol is particularly interesting in this regard, because it is inexpensive and can reform itself with water to release hydrogen with a high gravimetric density of 18.8 per cent by weight. But traditional reforming of methanol steam operates at relatively high temperatures (200-350 degrees Celsius), so the focus for vehicle and portable PEMFC applications has been on aqueous-phase reforming of methanol (APRM). This method requires less energy, and the simpler and more compact device design allows direct integration into PEMFC stacks. There remains, however, the need for an efficient APRM catalyst. Here we report that platinum (Pt) atomically dispersed on α-molybdenum carbide (α-MoC) enables low-temperature (150-190 degrees Celsius), base-free hydrogen production through APRM, with an average turnover frequency reaching 18,046 moles of hydrogen per mole of platinum per hour. We attribute this exceptional hydrogen production-which far exceeds that of previously reported low-temperature APRM catalysts-to the outstanding ability of α-MoC to induce water dissociation, and to the fact that platinum and α-MoC act in synergy to activate methanol and then to reform it.
the-state-of-the-art ORR catalysts but their uses are largely restricted by the prohibitive cost and limited activity/stability. [3][4][5][6][7] In this regard, the development of non-Pt group metal (non-PGM) catalysts derived from earth-abundant elements for ORR is the fundamental solution for the widespread applications of PEMFCs. [8][9][10] Among various non-PGM ORR catalysts developed in last decade, transition metalnitrogen-carbon (M-N-C) catalysts with M-N x coordination active sites embedded in the basal planes of carbon matrixes were the most promising ones due to their decent activity in both acidic and alkaline media and ease of scale-up production. [11][12][13][14][15] The ORR performance of M-N-C electrocatalysts in alkaline electrolyte has been well demonstrated notwithstanding, [9,[16][17][18][19][20] their performance in acidic environment is still deficient and often degrades rapidly due to the etching of metal species and/or decomposition of active sites. [21,22] Generally, the M-N-C catalysts are prepared via high-temperature (T > 800 °C) pyrolysis process of transition metal (e.g., Fe, Co, Ni), nitrogen, and carbon precursors, during which the metal atoms are very easy to agglomerate into large particles. The aggregated metal and metal oxide/carbide particles will hinder the accessibility of M-N x /C active sites and lower the utilization of M atoms seriously, thus compromising their ORR activity. Furthermore, metal aggregates will be easily etched away in acid, leading to The development of high-performance oxygen reduction reaction (ORR) catalysts derived from non-Pt group metals (non-PGMs) is urgent for the wide applications of proton exchange membrane fuel cells (PEMFCs). In this work, a facile and cost-efficient supramolecular route is developed for making non-PGM ORR catalyst with atomically dispersed Fe-N x /C sites through pyrolyzing the metal-organic polymer coordinative hydrogel formed between Fe 3+ and α-L-guluronate blocks of sodium alginate (SA). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) verify that Fe atoms achieve atomic-level dispersion on the obtained SA-Fe-N nanosheets and a possible fourfold coordination with N atoms. The best-performing SA-Fe-N catalyst exhibits excellentORR activity with half-wave potential (E 1/2 ) of 0.812 and 0.910 V versus the reversible hydrogen electrode (RHE) in 0.5 m H 2 SO 4 and 0.1 m KOH, respectively, along with respectable durability. Such performance surpasses that of most reported non-PGM ORR catalysts. Density functional theory calculations suggest that the relieved passivation effect of OH* on Fe-N 4 /C structure leads to its superior ORR activity to Pt/C in alkaline solution. The work demonstrates a novel strategy for developing high-performance non-PGM ORR electrocatalysts with atomically dispersed and stable M-N x coordination sites in both acidic and alkaline media.
HIGHLIGHTS • Flexible sensitive carbon nanotubes/polydimethylsiloxane (CNTs/PDMS) nanocomposite with novel double-side rough porous structure was proposed by simple manufacturing methods.
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