Engineering Atomically Dispersed FeN[subscript 4] Active Sites for CO[subscript 2] Electroreduction
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Adli, Nadia Mohd↗
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Engineering Atomically Dispersed FeN 4 Active Sites for CO 2 Electroreduction
Atomically dispersed FeN 4 active sites have exhibited exceptional catalytic activity and selectivity for the electrochemical CO 2 reduction reaction (CO2RR) to CO. However, the understanding behind the intrinsic and morphological factors contributing to the catalytic properties of FeN 4 sites is still lacking. By using a Fe-N-C model catalyst derived from the ZIF-8, we deconvoluted three key morphological and structural elements of FeN 4 sites, including particle sizes of catalysts, Fe content, and Fe-N bond structures. Furthermore, their respective impacts on the CO2RR were comprehensively elucidated. Engineering the particle size and Fe doping is critical to control extrinsic morphological factors of FeN 4 sites for optimal porosity, electrochemically active surface areas, and the graphitization of the carbon support. In contrast, the intrinsic activity of FeN 4 sites was only tunable by varying thermal activation temperatures during the formation of FeN 4 sites, which impacted the length of the Fe-N bonds and the local strains. The structural evolution of Fe-N bonds was examined at the atomic level. First-principles calculations further elucidated the origin of intrinsic activity improvement associated with the optimal local strain of the Fe-N bond.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Tuning Structural and Electronic Configuration of FeN 4 via External S for Enhanced Oxygen Reduction Reaction
The Fe–N–C material represents an attractive oxygen reduction reaction electrocatalyst, and the FeN 4 moiety has been identified as a very competitive catalytic active site. Fine tuning of the coordination structure of FeN 4 has an essential impact on the catalytic performance. Herein, we construct a sulfur‐modified Fe–N–C catalyst with controllable local coordination environment, where the Fe is coordinated with four in‐plane N and an axial external S. The external S atom affects not only the electron distribution but also the spin state of Fe in the FeN 4 active site. The appearance of higher valence states and spin states for Fe demonstrates the increase in unpaired electrons. With the above characteristics, the adsorption and desorption of the reactants at FeN 4 active sites are optimized, thus promoting the oxygen reduction reaction activity. This work explores the key point in electronic configuration and coordination environment tuning of FeN 4 through S doping and provides new insight into the construction of M–N–C‐based oxygen reduction reaction catalysts.
25 ENERGY STORAGE↗
BOREAS TF-10 NSA-Fen Tower Flux and Meteorological Data
The BOREAS TF-10 team collected tower flux and meteorological data at two sites, a fen and a young jack pine forest, near Thompson, Manitoba, Canada, as part of BOREAS. A preliminary data set was assembled in August 1993 while field testing the instrument packages, and at both sites data were collected from 15-Aug to 31-Aug. The main experimental period was in 1994, when continuous data were collected from 08-Apr to 23-Sep at the fen site. A very limited experiment was run in the spring/ summer of 1995, when the fen site tower was operated from 08-Apr to 14-Jun in support of a hydrology experiment in an adjoining feeder basin. Upon examination of the 1994 data set, it became clear that the behavior of the heat, water, and carbon dioxide fluxes throughout the whole growing season was an important scientific question, and that the 1994 data record was not sufficiently long to capture the character of the seasonal behavior of the fluxes. Thus, the fen site was operated in 1996 in order to collect data from spring melt to autumn freeze-up. Data were collected from 29-Apr to 05-Nov at the fen site. All variables are presented as 30-minute averages. The data are stored in tabular ASCII files.
McCaughey, J. Harry↗
Methane emission hotspots in a boreal forest-fen mosaic potentially linked to deep taliks
Permafrost thaw is transforming boreal forests into mosaics of wetlands and drier uplands. Topographic controls on hydrological and ecological conditions impact methane (CH 4 ) fluxes, contributing to uncertainty in local and regional CH 4 budgets and underlying drivers. The objective of this study was to explore CH 4 fluxes and their drivers in a transitioning boreal forest-fen ecosystem (Goldstream Valley, Alaska, USA). This landscape is characterized by thawing discontinuous permafrost and heterogeneous mosaics of fens, collapse-scar channels, and small mounds of permafrost soils. From a survey in July 2021, observed chamber CH4 fluxes included fen areas with intermediate to very high emissions (29.8–635.3 mg CH 4 m −2 d −1 ), clustered locations with CH 4 uptake (−2.11 to −0.7 mg CH 4 m −2 d −1 ), and three anomalous emission hotspots (342.4–772.4 mg CH 4 m −2 d −1 ) that were located near samples with lower emissions. Some surface and near-surface variables partially explained the spatial variation in CH 4 flux. Log-transformed CH 4 flux had a positive linear relationship with soil moisture at 20 cm depth ( R 2 = 0.31, p -value < 1e-5) and negative linear relationships with microtopography ( R 2 = 0.13, p -value < 0.006) and slope ( R 2 = 0.28, p -value < 2e-5). Methane emissions generally occurred in flat, wet, graminoid-dominated fens, whereas CH 4 uptake occurred on permafrost mounds dominated by feather mosses and woody vegetation. However, the CH 4 hotspots occurred on drier, slightly sloped locations with low or undetectable near-surface methanogen abundance, suggesting that CH 4 was produced in deeper soils. When the hotspot samples were omitted, log-transformed CH 4 flux had a positive linear relationship with near-surface methanogen abundance ( R 2 = 0.29, p -value = 0.0023), and stronger linear relationships with soil moisture, slope, and soil macronutrient concentrations. Our findings suggest that some CH 4 emission hotspots could arise from CH 4 in deep taliks. The inference that methanogenesis occurs in deep taliks was strengthened by the identification of intrapermafrost taliks across the study area using low-frequency geophysical induction. This study assesses surface spatial heterogeneity in the context of subsurface permafrost conditions and highlights the complexity of CH 4 flux patterns in transitioning forest-wetland ecosystems. To better inform regional CH 4 budgets, further research is needed to understand the spatial distribution of terrestrial CH 4 hotspots and to resolve their surface, near-surface, and subsurface drivers.
boreal↗
Identification of durable and non-durable FeN x sites in Fe–N–C materials for proton exchange membrane fuel cells
We report that while Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe–N–C catalysts initially comprising two distinct FeN x sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure–activity correlations drawn from end-of-test 57 Fe Mössbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ 57 Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeN x moieties in different electronic states, we identify S1 to be a high-spin FeN 4 C 12 moiety and S2 a low- or intermediate-spin FeN 4 C 10 moiety. These insights lay the groundwork for rational approaches towards Fe–N–C cathodes with improved durability in acidic fuel cells.
36 MATERIALS SCIENCE↗
Hierarchically Porous Carbons with Highly Curved Surfaces for Hosting Single Metal FeN 4 Sites as Outstanding Oxygen Reduction Catalysts
Iron–nitrogen–carbon (Fe₋N₋C) materials have emerged as a promising alternative to platinum-group metals for catalyzing the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells. However, their low intrinsic activity and stability are major impediments. Herein, an Fe₋N–C electrocatalyst with dense FeN 4 sites on hierarchically porous carbons with highly curved surfaces (denoted as FeN 4 - hc C) is reported. The FeN 4 - hc C catalyst displays exceptional ORR activity in acidic media, with a high half-wave potential of 0.85 V (versus reversible hydrogen electrode) in 0.5 m H 2 SO 4 . When integrated into a membrane electrode assembly, the corresponding cathode displays a high maximum peak power density of 0.592 W cm -2 and demonstrates operating durability over 30 000 cycles under harsh H 2 /air conditions, outperforming previously reported Fe–N₋C electrocatalysts. These experimental and theoretical studies suggest that the curved carbon support fine-tunes the local coordination environment, lowers the energies of the Fe d-band centers, and inhibits the adsorption of oxygenated species, which can enhance the ORR activity and stability. This work provides new insight into the carbon nanostructure–activity correlation for ORR catalysis. It also offers a new approach to designing advanced single-metal-site catalysts for energy-conversion applications.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Unravelling biogeochemical drivers of methylmercury production in an Arctic fen soil and a bog soil
Arctic tundra soils store a globally significant amount of mercury (Hg), which could be transformed to the neurotoxic methylmercury (MeHg) upon warming and thus poses serious threats to the Arctic ecosystem. However, our knowledge of the biogeochemical drivers of MeHg production is limited in these soils. Using substrate addition (acetate and sulfate) and selective microbial inhibition approaches, we investigated the geochemical drivers and dominant microbial methylators in 60-day microcosm incubations with two tundra soils: a circumneutral fen soil and an acidic bog soil, collected near Nome, Alaska, United States. Results showed that increasing acetate concentration had negligible influences on MeHg production in both soils. However, inhibition of sulfate-reducing bacteria (SRB) completely stalled MeHg production in the fen soil in the first 15 days, whereas addition of sulfate in the low-sulfate bog soil increased MeHg production by 5-fold, suggesting prominent roles of SRB in Hg(II) methylation. Without the addition of sulfate in the bog soil or when sulfate was depleted in the fen soil (after 15 days), both SRB and methanogens contributed to MeHg production. Analysis of microbial community composition confirmed the presence of several phyla known to harbor microorganisms associated with Hg(II) methylation in the soils. Lastly, the observations suggest that SRB and methanogens were mainly responsible for Hg(II) methylation in these tundra soils, although their relative contributions depended on the availability of sulfate and possibly syntrophic metabolisms between SRB and methanogens.
54 ENVIRONMENTAL SCIENCES↗
AmeriFlux FLUXNET-1F US-BZF Bonanza Creek Rich Fen
This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site US-BZF Bonanza Creek Rich Fen. This is the FLUXNET version of the carbon flux data for the site US-BZF Bonanza Creek Rich Fen produced by applying the standard ONEFlux (1F) software. Site Description - The rich fen is in the boreal peatland lowlands of the Tanana Flats in interior Alaska. Lacks near-surface permafrost even though the surrounding landscape is marked by the presence of permafrost. Peat depth is 1-2m. Primary wind direction in the summer is WSW.
Euskirchen, Eugenie↗
AmeriFlux FLUXNET-1F CA-CF1 Churchill Fen Site 1
This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site CA-CF1 Churchill Fen Site 1. This is the FLUXNET version of the carbon flux data for the site CA-CF1 Churchill Fen Site 1 produced by applying the standard ONEFlux (1F) software. Site Description - Fen site dominated by sedges. Flat, high water table. Frozen for long periods each year. Close to Arctic Ocean coast so affected by land/sea breezes and coastal weather.
Papakyriakou, Tim↗
AmeriFlux FLUXNET-1F US-KPL Lily Lake Fen
This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site US-KPL Lily Lake Fen. This is the FLUXNET version of the carbon flux data for the site US-KPL Lily Lake Fen produced by applying the standard ONEFlux (1F) software. Site Description - fen
Sullivan, Patrick [University of Alaska Anchorage]↗
AmeriFlux FLUXNET-1F CA-CF2 Churchill Fen Site 2
This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site CA-CF2 Churchill Fen Site 2. This is the FLUXNET version of the carbon flux data for the site CA-CF2 Churchill Fen Site 2 produced by applying the standard ONEFlux (1F) software. Site Description - Fen site dominated by sedges. Flat, high water table. Frozen for long periods each year. Close to Arctic Ocean coast so affected by land/sea breezes and coastal weather.
Tenuta, Mario [University of Manitoba]↗
AmeriFlux FLUXNET-1F CA-CF3 Churchill Fen Site 3
This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site CA-CF3 Churchill Fen Site 3. This is the FLUXNET version of the carbon flux data for the site CA-CF3 Churchill Fen Site 3 produced by applying the standard ONEFlux (1F) software. Site Description - Fen site dominated by sedges. Flat, high water table. Frozen for long periods each year. Close to Arctic Ocean coast so affected by land/sea breezes and coastal weather.
Arndt, Kyle [Woodwell Climate Research Center]↗
A season of heat, water vapor, total hydrocarbon, and ozone fluxes at a subarctic fen
High-latitude environments are thought to play several critical roles in the global balance of radiatively active trace gases. Adequate documentation of the source and sink strengths for trace gases requires long time series of detailed measurements, including heat and moisture budgets. A fen near Schefferville, Quebec, was instrumented during the summer of 1990 for the measurement of the surface energy, radiation, and moisture balances as well as for eddy correlation estimates of ozone and methane flux. Despite the limited fetch at this site, analysis of the tower flux 'footprint' indicates that at least 80% of the flux observed originates from sources within the fen. Sensible heat fluxes averaged 25% of the daytime net radiation at the site, while the latent heat flux, determined from the energy balance, was 63%; the Bowen ratio varied from 0.2 to 0.8 from day to day, without a seasonal trend to the variation. The competing effects of rooted macrophyte development (with concomitant effects on roughness and transpiration) and the normal shift in synoptic pattern around day 200 to warm, dry conditions results in a lack of net seasonal effect on the energy partitioning. Over the period from days 170 to 230, the evaporation (167 mm) was double the rainfall, while the decline in water level was 107 mm, leaving a net runoff of 0.44 mm/d. The total hydrocarbon flux was 75-120 mg m(exp -2)/d, following a diurnal pattern similar to heat or moisture flux, while the daytime ozone flux was about -1.11 x 10(exp 11) molecules cm(exp -2)/s. A period near the end of the experiment, during week 30, produced the strongest total hydrocarbon flux, associated with warmer deep (1 m) soil temperatures, lower fen water levels, and the late summer shift in wind direction at that time. An early summer 'flush' of total hydrocarbon was not observed.
Moore, Kathleen E.↗
Materials Data on Zn(FeN)2 by Materials Project
Zn(FeN)2 crystallizes in the tetragonal P-4m2 space group. The structure is two-dimensional and consists of one Zn(FeN)2 sheet oriented in the (0, 0, 1) direction. Fe2+ is bonded in a linear geometry to two equivalent N3- atoms. Both Fe–N bond lengths are 1.77 Å. Zn2+ is bonded to four equivalent N3- atoms to form corner-sharing ZnN4 tetrahedra. All Zn–N bond lengths are 2.18 Å. N3- is bonded in a see-saw-like geometry to two equivalent Fe2+ and two equivalent Zn2+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on FeN by Materials Project
FeN crystallizes in the monoclinic C2 space group. The structure is one-dimensional and consists of two FeN ribbons oriented in the (1, 1, 0) direction. Fe3+ is bonded in a linear geometry to two equivalent N3- atoms. Both Fe–N bond lengths are 1.72 Å. N3- is bonded in a linear geometry to two equivalent Fe3+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on Ca(FeN)2 by Materials Project
Ca(FeN)2 crystallizes in the tetragonal P-4m2 space group. The structure is two-dimensional and consists of one Ca(FeN)2 sheet oriented in the (0, 0, 1) direction. Ca2+ is bonded to four equivalent N3- atoms to form distorted corner-sharing CaN4 tetrahedra. All Ca–N bond lengths are 2.46 Å. Fe2+ is bonded in a linear geometry to two equivalent N3- atoms. Both Fe–N bond lengths are 1.76 Å. N3- is bonded in a rectangular see-saw-like geometry to two equivalent Ca2+ and two equivalent Fe2+ atoms.
36 MATERIALS SCIENCE↗
AmeriFlux CA-CF2 Churchill Fen Site 2
This is the AmeriFlux version of the carbon flux data for the site CA-CF2 Churchill Fen Site 2. Site Description - Fen site dominated by sedges. Flat, high water table. Frozen for long periods each year. Close to Arctic Ocean coast so affected by land/sea breezes and coastal weather.
Tenuta, Mario↗