An official website of the United States government
Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN 4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs). In this work, we carefully tuned the doped Fe content in zeolitic imidazolate framework (ZIF)-8 precursors and achieved complete atomic dispersion of FeN 4 sites, the sole Fe species in the catalyst based on Mößbauer spectroscopy data. The Fe–N–C catalyst with the highest density of active sites achieved respectable ORR activity in rotating disk electrode (RDE) testing with a half-wave potential ( E 1/2 ) of 0.88 ± 0.01 V vs. the reversible hydrogen electrode (RHE) in 0.5 M H 2 SO 4 electrolyte. The activity degradation was found to be more significant when holding the potential at 0.85 V relative to standard potential cycling (0.6–1.0 V) in O 2 saturated acid electrolyte. The post-mortem electron microscopy analysis provides insights into possible catalyst degradation mechanisms associated with Fe–N coordination cleavage and carbon corrosion. High ORR activity was confirmed in fuel cell testing, which also divulged the promising performance of the catalysts at practical PEFC voltages. We conclude that the key factor behind the high ORR activity of the Fe–N–C catalyst is the optimum Fe content in the ZIF-8 precursor. While too little Fe in the precursors results in an insufficient density of FeN 4 sites, too much Fe leads to the formation of clusters and an ensuing significant loss in catalytic activity due to the loss of atomically dispersed Fe to inactive clusters or even nanoparticles. Advanced electron microscopy was used to obtain insights into the clustering of Fe atoms as a function of the doped Fe content. The Fe content in the precursor also affects other key catalyst properties such as the particle size, porosity, nitrogen-doping level, and carbon microstructure. Thanks to using model catalysts exclusively containing FeN 4 sites, it was possible to directly correlate the ORR activity with the density of FeN 4 species in the catalyst.
more »
« less
Electrodeposited palladium and tin bimetallic catalysts for efficient ethanol oxidation to acetate in alkaline electrolyte
Acetic acid (AA), an important commodity chemical, is produced via methanol carbonylation, emitting one ton of CO₂ per ton of product. As a sustainable alternative, we report the electrochemical oxidation of bioethanol to selectively produce AA using a novel Pdsingle bondSn alloy catalyst with nanodendritic morphology supported on nickel foam (PdSn@NF). The catalyst was synthesized via electrodeposition, and the presence of ammonium chloride in the deposition bath was found to critically affect the Pd-to-Sn ratio and, consequently, the catalyst performance. The vital role of catalyst structure, surface composition, and morphology on the activity and selectivity of PdSn@NF towards the EOR was revealed by X-ray diffractometry, emission spectroscopy, and electron microscopy. Specifically, the nanodendritic morphology of the PdSn@NF resulted in the formation of highly active undercoordinated sites, while in situ Raman spectroscopy suggested that Sn helps mitigate CO poisoning – likely a result of a lowered d-band center. Due to the strong synergy between the structural and electronic properties of PdSn@NF, ~100 % faradaic efficiency (FE) to AA at 400 mA cm−2 was achieved with lab-grade ethanol (LGE) in an H-type cell. In continuous flow operation, the FE declined due to product accumulation on active sites; however, this was mitigated by employing current pulses to remove surface-bound products. An optimized pulsing protocol restored ~100 % FE of AA for LGE and achieved ~94 % FE with bioethanol at 400 mA cm−2 despite the presence of fermentation impurities. This study underscores the promise of PdSn@NF as a highly selective and industrially relevant electrocatalyst for sustainable AA production.
more »
« less
- PAR ID:
- 10654663
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Chemical Engineering Journal
- Volume:
- 527
- ISSN:
- 1385-8947
- Page Range / eLocation ID:
- 171698
- Subject(s) / Keyword(s):
- Pulsed electrolysis, Electrocatalysis, Bimetallic alloyed catalyst, Bio upgrading, Nanodendrite morphology
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)A series of Al2O3-supported Fe-containing catalysts were synthesized by incipient wetness impregnation. The iron surface density was varied from 1 to 13 Fe atoms/nm2 spanning sub- and above-monolayer coverage. The resulting supported Fe-catalysts were characterized with N2 physisorption, ex situ XRD, PDF, XAS, AC-STEM and chemically probed by H2-TPR. The results suggest that over this entire range of loadings, Fe was present as dispersed species, with only a very small fraction of Fe2O3 aggregates, at the highest Fe loading. The in situ sulfidation of Fe/Al2O3 resulted in the formation of a highly active and selective PDH catalyst. The highest activity with 52% propane conversion and ~99% propylene selectivity at 560 °C was obtained for the 6.4 Fe/Al2O3 catalyst suggesting that this is the highest amount of Fe that could be fully dispersed on the support in sulfided form. XRD and AC-STEM indicated the absence of any crystalline iron sulfide aggregates after sulfidation and reaction. H2-TPR results indicated that the amount of the reducible Fe sites in the sulfided catalyst remained constant above monolayer coverage, and increasing loading did not increase the number of reducible Fe sites. Consistent with these results, the reactivity per gram of catalyst showed no increase with Fe loading above monolayer coverage, suggesting that additional Fe remains conformal to the alumina surface.more » « less
-
Dissolved iron (Fe) species are a pre-requisite for the most active catalyst sites for the oxygen evolution reaction in alkaline electrolytes, but the overall effects of dissolved Fe on energy- efficient advanced alkaline water electrolysis cells remain unclear. Here, we systematically control the concentration of Fe in a model zero-gap alkaline water electrolyzer to understand the interactions between Fe and high surface area catalyst coatings. Cells employing a platinum-group- metal-containing cathode and a high surface area, mixed-metal-oxide anode yielded an optimum voltage efficiency at elevated temperatures and in the presence of 6 ppm Fe, which reduced the cell voltage by ~100 mV compared to rigorously Fe-free electrolytes. Increasing concentrations of Fe led to a systematic increase in anode activity towards the oxygen evolution reaction and a reduction in the electrochemically active surface area at both the anode and cathode. Metallic Fe was not observed to electrodeposit at cathodes which operate at overpotentials ≤ 120 mV, but dissolved Fe does reduce the apparent number density of sites available for hydride adsorption. These findings suggest that the energy efficiency of advanced alkaline water electrolysis systems can be improved by managing the Fe concentration in recirculating KOH electrolytes.more » « less
-
Urea synthesis through the simultaneous electrocatalytic reduction of N2and CO2molecules under ambient conditions holds great promises as a sustainable alternative to its industrial production, in which the development of stable, highly efficient, and highly selective catalysts to boost the chemisorption, activation, and coupling of inert N2and CO2molecules remains rather challenging. Herein, by means of density functional theory computations, we proposed a new class of two‐dimensional nanomaterials, namely, transition‐metal phosphide monolayers (TM2P, TM = Ti, Fe, Zr, Mo, and W), as the potential electrocatalysts for urea production. Our results showed that these TM2P materials exhibit outstanding stability and excellent metallic properties. Interestingly, the Mo2P monolayer was screened out as the best catalyst for urea synthesis due to its small kinetic energy barrier (0.35 eV) for C–N coupling, low limiting potential (−0.39 V), and significant suppressing effects on the competing side reactions. The outstanding catalytic activity of the Mo2P monolayer can be ascribed to its optimal adsorption strength with the key *NCON species due to its moderate positive charges on the Mo active sites. Our findings not only propose a novel catalyst with high‐efficiency and high‐selectivity for urea production but also further widen the potential applications of metal phosphides in electrocatalysis.more » « less
-
Abstract The electrochemical reduction of nitrates (NO3−) enables a pathway for the carbon neutral synthesis of ammonia (NH3), via the nitrate reduction reaction (NO3RR), which has been demonstrated at high selectivity. However, to make NH3synthesis cost‐competitive with current technologies, high NH3partial current densities (jNH3) must be achieved to reduce the levelized cost of NH3. Here, the high NO3RR activity of Fe‐based materials is leveraged to synthesize a novel active particle‐active support system with Fe2O3nanoparticles supported on atomically dispersed Fe–N–C. The optimized 3×Fe2O3/Fe–N–C catalyst demonstrates an ultrahigh NO3RR activity, reaching a maximum jNH3of 1.95 A cm−2at a Faradaic efficiency (FE) for NH3of 100% and an NH3yield rate over 9 mmol hr−1cm−2. Operando XANES and post‐mortem XPS reveal the importance of a pre‐reduction activation step, reducing the surface Fe2O3(Fe3+) to highly active Fe0sites, which are maintained during electrolysis. Durability studies demonstrate the robustness of both the Fe2O3particles and Fe–Nxsites at highly cathodic potentials, maintaining a current of −1.3 A cm−2over 24 hours. This work exhibits an effective and durable active particle‐active support system enhancing the performance of the NO3RR, enabling industrially relevant current densities and near 100% selectivity.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.cej.2025.171698
