The nature of high-silica H–Y acid sites in dimethoxymethane carbonylation is investigated by NH3 poisoning with a combination of reaction results, diffuse reflection infrared Fourier transform spectra, and NH3 temperature-programmed desorption. After NH3 poisoning of H–Y, ammonia desorption of zeolites NH4–Y was conducted at different temperatures to obtain catalysts HNH4–Y that gradually exposed different acid sites. The zeolites thus produced catalyzed dimethoxymethane carbonylation with a linear dependence of the conversion on the ammonia desorption temperature before the plateau stage. It was observed that strong Brønsted acids in zeolite Y catalyzed the reaction more selectively toward the main product methyl methoxyacetate, and weak Brønsted acids catalyzed the reaction more selectively toward the byproduct methyl formate. Furthermore, the turnover frequency of Brønsted acids varied significantly among the Brønsted acids and decreased with acid strength. This finding suggested that only 10.1–18.3% of the total Al in zeolite Y with Si/Al ratios of 15, 30, and 40 contributed to the total framework Brønsted acids.
Ammonia as a hydrogen enrichment medium has tremendous advantages over pure hydrogen in terms of transportation and utilization. The high-efficiency anti-ammonia-poisoning membrane electrode assembly (MEA) is pivotal for the practical implementation of ammonia-resistant proton exchange membrane fuel cells (PEMFCs). As for the polymer electrolyte, which is an integral part of the proton exchange membrane (PEM) and catalyst layer (CL) adhesion agents in the MEA, structural design is an effective way to improve its resistance to ammonia poisoning. Herein, an in-depth study of sulfonimides with superacidic structures reveals that the proton dissociation energies can be altered by tuning the terminal groups with different electronegativity and enhancing the competitive adsorption of protons on Pt. Four perfluorinated backbone sulfonimides (PFSI-x (x = TSM, MSA, ESA, and iPSA)) with distinct charge delocalization were fabricated by grafting different electronegative monomers onto the perfluorosulfonyl fluoride (PFSF). The results show that PFSI-TSM possesses a higher electrostatic potential (2.69 eV) and the lowest activation energy (12.1 kJ/mol). Meanwhile, the performance of the PFSI-TSM PEMFC is 40.6% higher than the perfluorosulfonic acid (PFSA) at 80 °C, 100% relative humidity (RH) under 100 ppm NH 3 , with a maximum power density of 635.2 mW/cm 2 . The underlying reason for enhanced performance is in virtue of the more electronegative end groups render the superacid electrolyte with higher acidity and p K a values, which change the direction of the acid dissociation equilibrium reaction on the Pt surface, enhanced competitive adsorption of protons in MEAs elevated oxygen reduction reaction (ORR) activity and proton concentration in PEM and reduced the probability of ammonia adsorption poisoning. The insights in charge delocalization and proton ionization construct the structure-property relationship, which would guide a rational design of PEM for ammonia-resistant fuel cells.
The ammonia crossover that poisons the conventional cathode catalysts and damages the cell performance and efficiency is one of the paramount bottlenecks limiting the development of direct ammonia fuel cells (DAFCs). Synthetizing the ammonia tolerance catalysts has been proven to be an effective avenue to solve the above-mentioned problems, however, the mass and charge transports within the anti-ammonia poisoning electrodes has not been fully elucidated. Herein, an alkaline DAFC with anti-ammonia poisoning Fe-N-C cathode is reported, and the main effort is expended on investigating the impact of mass and charge transports on cell performance, internal resistance, and operating stability. The alkaline DAFC exhibits a thermodynamic efficiency as high as 88.6 %, higher than that of hydrogen-based fuel cells. The preliminary experimental results indicate that the performance of DAFC with anti-ammonia poisoning Fe-N-C cathode is higher than that of conventional Pt/C cathode under the same catalyst loading. The impressive open-circuit voltage of 0.74 V and peak power density of 107 mW cm −2 are obtained under atmospheric pressure and 80 °C, which is primarily ascribed to both the anti-ammonia poisoning of Fe-N-C catalyst and the facilitated mass and charge transports. Furthermore, the voltage curve of DAFC is quite stable during the constant-current discharging at 100 mA cm −2 for 10-h, demonstrating the good durability of DAFC with anti-ammonia poisoning Fe-N-C cathode.
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