Ultralow Density Research Articles

Perovskite Single Crystals by Vacuum Evaporation Crystallization.

Perovskite single crystals have attracted tremendous attention owing to their excellent optoelectronic properties and stability compared to typical multicrystal structures. However, the growth of high-quality perovskite single crystals (PSCs) generally relies on temperature gradients or the introduction of additives to promote crystal growth. In this study, a vacuum evaporation crystallization technique is developed that allows PSCs to be grown under extremely stable conditions at constant temperature and without requiring additives to promote crystal growth. The new method enables the growth of PSCs of unprecedented quality, that is, MAPbBr3 single crystals that exhibit an ultranarrow full width at half maximum of 0.00701°, which surpasses that of all previously reported values. In addition, the MAPbBr3 single crystals deliver exceptional optoelectronic performance, including a long carrier lifetime of 1006ns, an ultralow trap-state density of 3.67 × 109 cm-3, and an ultrahigh carrier mobility of 185.86 cm2 V-1 s-1. This method is applicable to various types of PSCs, including organic-inorganic hybrids, fully inorganic structures, and low-dimensional structures.

Three-dimensional nanostructures of crosslinked aramid nanofibers with exceptional mechanical and thermal insulation characteristics

Despite the remarkable attributes of aramid nanofibers, which include a high aspect ratio, elevated strength and modulus, as well as exceptional chemical and thermal stabilities, their utilization in three-dimensional porous nanostructures remains constrained due to the inherent challenge of agglomeration between adjacent fibers. This study presents a novel approach involving citric acid-assisted crosslinking to mitigate aramid nanofiber agglomeration. By facilitating the formation of covalent bonds between aramid nanofibers and citric acid, this method leads to the development of a three-dimensional open-porous structure. The chemically crosslinked aramid nanofibers/citric acid composite demonstrated notable enhancements in physical, mechanical, and thermal insulation properties, including an ultra-low density of 8.7 mg cm−3, exceptional compressive and tensile strengths at 3.04 MPa and 287 kPa, respectively, and a low thermal conductivity of 40 mW m−1 K−1. These advancements can be attributed to the preservation of the porous structure achieved through established covalent bonds. These findings carry significant implications for unlocking the potential of aramid nanofibers, particularly in advancing fields like thermal maintenance, insulation, construction of ultra-light structures, and the mitigation of vibration or impact energy in demanding environments.

An ultra-lightweight and hydrophobic piezoresistive foam with super-wide strain and pressure detection range

Achieving a piezoresistive sensor with extensive strain and pressure response range for subtle and large motion detection remains a formidable challenge. Furthermore, an advanced parameter (waterproof capability), which is critical for wearable sensors, has received insufficient attention in current research. Herein, macroscopically segregated polyolefin elastomer (POE)/carbon nanostructures (CNS) foams with a super-low density and superior waterproof were fabricated via supercritical CO2 foaming, revealing a facile and eco-friendly process. Compared to piezoresistive foams with randomly distributed structure, POE/CNS foam sensors with macroscopically segregated structure displayed an ultra-low density (∼0.1 g/cm3), and apparently improved piezoresistive behavior. Additionally, the segregated POE/CNS foams exhibited a remarkably lower percolation threshold of around 0.011 vol%, enabling stable piezoresistive capability even with an ultra-low CNS content of approximately 0.0043 vol%. Therefore, the prepared POE/CNS foam sensors (FCNS0.1 and FCNS0.5) demonstrated an exceptional strain and pressure response range (0.5%–90% compression strain and approximately 0.5–3700 kPa), and the foam sensor (FCNS0.5) displayed a broad linear strain response range from 10% to 90% compression strain. Moreover, the segregated POE/CNS foams exhibited good hydrophobicity, enabling the piezoresistive foam (FCNS0.5) to maintain its sensing ability underwater. These outstanding piezoresistive performances enabled the foam sensors to monitor diverse human motions including subtle and superhigh pressure. Consequently, piezoresistive foams with exceptional performance and versatile applications lay the foundation for the development of the next generation of foam sensors.

Carbon Nanofibrous Aerogels Derived from Electrospun Polyimide for Multifunctional Piezoresistive Sensors.

The fabrication of carbon aerogels with ultralow density, high electrical conductivity, and ultraelasticity still remains substantial challenges. This study utilizes electrospun polyimide aerogel as the source to fabricate flexible carbon nanofibrous aerogel (PI-CNA) capable of multifunctional applications. The lightweight PI-CNA based piezoresistive sensor shows a wide linear range (0-217 kPa), rapid response/recovery time, and fatigue resistance (12,000 cycles). More importantly, the superior pressure sensing enables the PI-CNA for all-range healthcare sensing, including pulse monitoring, physiological activity detection, speech recognition, and gait recognition. Moreover, the EMI SE and the A coefficient of the PI-CNA reach 45 dB and 0.62, respectively, indicating the outstanding absorption dominated EMI shielding effects due to the multiple reflections and absorption. Furthermore, PI-CNA exhibits satisfying Joule heating performance up to 120 °C with rapid response time (10-30 s) under low supply voltages (1.5-5 V) and possesses sufficient heating reliability and repeatability in long-term repeated heating/cooling cycles. The fabricated PI-CNA shows significant potential applications in wearable technologies, energy conversion, electronic skin, and artificial intelligence.

Silica aerogel-PVA dough: A high internal phase composite with superior thermal insulation and gas barrier

Silica aerogel (SA) has shown great promise in constructing thermal insulation composites in various forms. However, it is still challenging to disperse a huge amount of SA particles into other matrices for fabricating superior thermal insulation materials or highly filled masterbatch, owing to its high specific surface area and ultra-low density. Herein, inspired by high internal phase emulsion (HIPE), we successfully prepare processable SA/polyvinyl alcohol (PVA) composites (SA-PVA dough) with over 74 vol% dispersed SA, using a PVA aqueous solution as a binder. This is achieved by gradually mixing a large amount of hydrophobic SA filler into a small amount of PVA aqueous solution. The resulting SA-PVA dough exhibits an internal structure resembling that of HIPEs, with hydrophobic SA particles stabilized in the water phase by amphiphilic PVA chains, tightly packed within the thin continuous PVA aqueous phase. Benefiting from such a HIPE structure, the SA-PVA dough can be easily shaped into various forms and exhibits good applicability in further processing. The dried dough inherits the excellent properties of SA, including ultra-low density, superhydrophobicity (water contact angle: 153°), thermal insulation (thermal conductivity: 0.03 W m−1 K−1), and flame resistance. Moreover, the continuous PVA network structure, combined with the nanoscale pore structure of SA, provides the dough with excellent gas barrier properties, greatly expanding the application scope of SA-based composites.

Reverse-engineered borohydride hydrolysis: Vacancy-rich cobalt aerogel nanowafers as high-efficiency electrode materials for supercapacitor applications

Transition metal oxide aerogels (AGLs) have garnered considerable attention over the past decade due to their unique and exceptional properties, including high porosity, extensive surface area, and ultra-low density. To meet the rising energy requirements and the need for renewable energy storage solutions, there is a growing demand for devices offering high energy and power densities. To realize such a device, a reverse borohydride hydrolysis method was employed in this study to synthesize oxygen vacancy-rich cobalt-based AGL (CoAGL) nanowafers with Co2+/Co3+ defect chemistry. Additionally, we annealed CoAGL samples at 400 °C (CoAGL@400) to successfully design and fabricate three-electrode supercapacitors. The characterization techniques encompassed X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron studies, shedding light on the X-ray properties of these AGLs. CoAGL demonstrated a specific capacitance (Csp) of 42.3 F/g in a device setup, displaying excellent cyclability and capacitance retention. Furthermore, it exhibited an energy density of 14.4 Wh/kg and a power density of 800 W/kg at a current density of 1 A/g, respectively. The practical application of the aerogel was demonstrated by fabricating a two-electrode device employing CoAGL as the key component.

Facile fabrication of microfibrillated cellulose-based aerogels incorporated with nisin/β-cyclodextrin microcapsule for channel catfish preservation

In this study, a hydrophobic and antibacterial pad was prepared to preserve Channel Catfish (Ictalurus punctatus). The pad composite the microfibrillated cellulose and β-cyclodextrin/nisin microcapsules. The hydrophobic pad ensures a dry surface in contact with the fish, reducing microbial contamination. The pad has a low density and high porosity, making it lightweight and suitable for packaging applications, while also providing a large surface area for antibacterial activity. Results demonstrated that this antibacterial pad exhibits an ultralow density of 9.0 mg/cm3 and an ultrahigh porosity of 99.10%. It can extend the shelf life of Channel Catfish fillets to 9 days at 4 °C, with a total volatile base nitrogen below 20 mg/100 g. The study proposes a novel solution for preserving aquatic products by combining antibacterial substances with the natural base material aerogel. This approach also extends the utilization of aerogel and nisin in food packaging.

Robust micro-nano hybrid aerogel derived from waste resources for thermal management and high-efficiency adsorption

Ultra-low density nanoaerogels as the lightest solid material have aroused extensive attention. However, lacking crack extension resistance limited their further development and application. Here, aramid and cellulose wastes were recycled to prepare high value-added nanofibers, which were further self-assembled to construct multilfunctional hybrid aerogels. In-situ growth of zeolitic imidazolate frameworks-8 (ZIF-8) on the cellulose nanofiber (CNF) created a robust skeleton. Flexible aramid nanofibers (ANF), as burgeoning building blocks were employed to inlay into the microscopic skeleton structure for establishing micro-nano double networks. Benefiting from the existence of multiscale networks, the maximum value in the compressive stress of S0, S1, S2 and S3 increased from 2.74 to 4.63, 5.19 and 6.61 MPa, respectively.Meanwhile, ANF-ZIF-8-CNF hybrid aerogels exhibited low thermal conductivity of 3.58–3.97 × 10-2 W/(m⋅K). The significant increase in the specific surface area (481.46 m2/g) endowed hybrid aerogels with high adsorption abilities for nitrogen (280.83 cm3/g), prussian blue (34.12 mg⋅g−1), rhodamine B (37.30 mg⋅g−1), DMF (25.13 g⋅g−1) and DMSO (15.48 g⋅g−1), respectively. This multiscale structural engineering strategy opens an avenue toward to upcycle waste resources into the next-generation high-performance nanoaerogels, showing huge application potential in thermal management, air purification and sewage treatment.

Scalable Synthesis of Bilayer Graphene at Ambient Temperature.

In this work, we develop for the first time a facile chemical lithiation-assisted exfoliation approach to the controllable and scalable preparation of bilayer graphene. Biphenyl lithium (Bp-Li), a strong reducing reagent, is selected to realize the spontaneous Li-intercalation into graphite at ambient temperature, forming lithium graphite intercalation compounds (Li-GICs). The potential of Bp-Li (0.11 V vs Li/Li+), which is just lower than the potential of stage-2 lithium intercalation (0.125 V), enables the precise lithiation of graphite to stage-2 Li-GICs (LiC12). Intriguingly, the exfoliation of LiC12 leads to the bilayer-favored production of graphene, giving a high selectivity of 78%. Furthermore, the mild intercalation-exfoliation procedure yields high-quality graphene with negligible structural deterioration. The obtained graphene exhibits ultralow defect density (ID/IG ∼ 0.14) and a considerably high C/O ratio (∼29.7), superior to most current state-of-the-art techniques. This simple and scalable strategy promotes the understanding of chemical Li-intercalation methods for preparing high-quality graphene and shows great potential for layer-controlled engineering.

Ultralight and Robust Covalent Organic Framework Fiber Aerogels.

Shaping covalent organic frameworks (COFs) into macroscopic objects with robust mechanical properties and hierarchically porous structure is of great significance for practical applications but remains formidable and challenging. Herein, a general and scalable protocol is reported to prepare ultralight and robust pure COF fiber aerogels (FAGs), based on the epitaxial growth synergistic assembly (EGSA) strategy. Specifically, intertwined COF nanofibers (100-200nm) are grown in situ on electrospinning polyacrylonitrile (PAN) microfibers (≈1.7µm) containing urea-based linkers, followed by PAN removal via solvent extraction to obtain the hollow COF microfibers. The resultant COF FAGs possess ultralow density (14.1-15.5mg cm-3) and hierarchical porosity that features both micro-, meso-, and macropores. Significantly, the unique interconnected structure composed of nanofibers and hollow microfibers endows the COF FAGs with unprecedented mechanical properties, which can fully recover at 50% strain and be compressed for 20 cycles with less than 5% stress degradation. Moreover, the aerogels exhibit excellent capacity for organic solvent absorption (e.g., chloroform uptake of >90g g-1). This study opens new avenues for the design and fabrication of macroscopic COFs with excellent properties.

Sputtering-grown undoped GeSn/Ge multiple quantum wells on n-Ge for low-cost visible/shortwave infrared dual-band photodetection

Multispectral photodetectors (PDs) operating in visible (VIS) to shortwave infrared (SWIR) wavelength ranges are highly desired for various civil and military applications. Traditional vertical p-i-n SWIR PDs lack the detection ability for VIS light due to photon dissipation in the neutral regions. Herein, an i-n type PD was proposed and demonstrated with sputtering-grown undoped Ge0.92Sn0.08/Ge multiple quantum wells (MQWs) on n-Ge substrate to realize VIS/SWIR dual-band detection from 400 to 2000 nm. Three response peaks resulted from the resonate cavity effect at 430, 660 and 1530 nm, respectively, were observed. Benefiting from the defect-free coherent GeSn/Ge MQWs and low-temperature surface plasma treatment, a remarkably low bulk leakage current density of 5.9 mA/cm2 and a low surface leakage current of 37.5 μA/cm were achieved at room temperature under −1 V, resulting in an ultralow dark current density of 13.36 mA/cm2 for a 200-μm-diameter PD. Temperature-dependent electrical characterizations revealed that the dark current was ideally dominated by carrier diffusion mechanism. Eminent detectivities of 5.2 × 108, 7.1 × 108 and 5.6 × 109 Jones at 410, 660 and 1530 nm, respectively, under −1 V were achieved simultaneously at room temperature. This work provides a path way for fabricating low-cost, VIS/SWIR dual-band PDs with complementary metal–oxide–semiconductor (CMOS) compatible technology.

Ultralight, Robust, Thermal Insulating Silica Nanolace Aerogels Derived from Pickering Emulsion Templates.

Synthesis of silica aerogel insulators with ultralight weight and strong mechanical properties using a simplified technique remains challenging for functional soft materials. This study introduces a promising method for the fabrication of mechanically reinforced ultralight silica aerogels by employing attractive silica nanolace (ASNLs)-armored Pickering emulsion templates. For this, silica nanolaces (SiNLs) are fabricated by surrounding a cellulose nanofiber with necklace-shaped silica nanospheres. In order to achieve amphiphilicity, which is crucial for the stabilization of oil-in-water Pickering emulsions, hydrophobic alkyl chains and hydrophilic amine groups are grafted onto the surface of SiNLs by silica coupling reactions. Freeze-drying of ASNLs-armored Pickering emulsions has established a new type of aerogel system. The ASNLs-supported mesoporous aerogel shows 3-fold greater compressive strength, 4-fold reduced heat transfer, and a swift heat dissipation profile compared to that of the bare ASNL aerogel. Additionally, the ASNL aerogel achieves an ultralow density of 8 mg cm-3, attributed to the pore architecture generated from closely jammed emulsion drops. These results show the potential of the ASNL aerogel system, which is ultralight, mechanically stable, and thermally insulating and could be used in building services, energy-saving technologies, and the aerospace industry.

Real-Time Compact Digital Processing Chain for the Detection and Sorting of Neural Spikes from Implanted Microelectrode Arrays

Implantable microelectrodes arrays are used to record electrical signals from surrounding neurons and have led to incredible improvements in modern neuroscience research. Digital signals resulting from conditioning and the analog-to-digital conversion of neural spikes captured by microelectrodes arrays have to be elaborated in a dedicated DSP core devoted to a real-time spike-sorting process for the classification phase based on the source neurons from which they were emitted. On-chip spike-sorting is also essential to achieve enough data reduction to allow for wireless transmission within the power constraints imposed on implantable devices. The design of such integrated circuits must meet stringent constraints related to ultra-low power density and the minimum silicon area, as well as several application requirements. The aim of this work is to present real-time hardware architecture able to perform all the spike-sorting tasks on chip while satisfying the aforementioned stringent requirements related to this type of application. The proposed solution has been coded in VHDL language and simulated in the Cadence Xcelium tool to verify the functional behavior of the digital processing chain. Then, a synthesis and place and route flow has been carried out to implement the proposed architecture in both a 130 nm and a FD-SOI 28 nm CMOS process, with a 200 MHz clock frequency target. Post-layout simulations in the Cadence Xcelium tool confirmed the proper operation up to a 200 MHz clock frequency. The area occupation and power consumption of the proposed detection and clustering module are 0.2659 mm2/ch, 7.16 μW/ch, 0.0168 mm2/ch, and 0.47 μW/ch for the 130 nm and 28 nm implementation, respectively.

Direct Observation of Room-Temperature Magnetic Skyrmion Motion Driven by Ultra-Low Current Density in Van Der Waals Ferromagnets.

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe3GaTe2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

Tuning Shortwave‐Infrared J‐aggregates of Aromatic Ring‐Fused Aza‐BODIPYs by Peripheral Substituents for Combined Photothermal and Photodynamic Therapies at Ultralow Laser Power

AbstractAchieving photothermal therapy (PTT) at ultralow laser power density is crucial for minimizing photo‐damage and allowing for higher maximum permissible skin exposure. However, this requires photothermal agents to possess not just superior photothermal conversion efficiency (PCE), but also exceptional near‐infrared (NIR) absorptivity. J‐aggregates, exhibit a significant redshift and narrower absorption peak with a higher extinction coefficient. Nevertheless, achieving predictable J‐aggregates through molecular design remains a challenge. In this study, we successfully induced desirable J‐aggregation (λabsmax: 968 nm, ϵ: 2.96×105 M−1 cm−1, λemmax: 972 nm, ΦFL: 6.2 %) by tuning electrostatic interactions between π‐conjugated molecular planes through manipulating molecular surface electrostatic potential of aromatic ring‐fused aza‐BODIPY dyes. Notably, by controlling the preparation method for encapsulating dyes into F‐127 polymer, we were able to selectively generate H‐/J‐aggregates, respectively. Furthermore, the J‐aggregates exhibited two controllable morphologies: nanospheres and nanowires. Importantly, the shortwave‐infrared J‐aggregated nanoparticles with impressive PCE of 72.9 % effectively destroyed cancer cells and mice‐tumors at an ultralow power density of 0.27 W cm−2 (915 nm). This phototherapeutic nano‐platform, which generates predictable J‐aggregation behavior, and can controllably form J‐/H‐aggregates and selectable J‐aggregate morphology, is a valuable paradigm for developing photothermal agents for tumor‐treatment at ultralow laser power density.

Tuning Shortwave-Infrared J-aggregates of Aromatic Ring-Fused Aza-BODIPYs by Peripheral Substituents for Combined Photothermal and Photodynamic Therapies at Ultralow Laser Power.

Achieving photothermal therapy (PTT) at ultralow laser power density is crucial for minimizing photo-damage and allowing for higher maximum permissible skin exposure. However, this requires photothermal agents to possess not just superior photothermal conversion efficiency (PCE), but also exceptional near-infrared (NIR) absorptivity. J-aggregates, exhibit a significant redshift and narrower absorption peak with a higher extinction coefficient. Nevertheless, achieving predictable J-aggregates through molecular design remains a challenge. In this study, we successfully induced desirable J-aggregation (λabs max : 968 nm, ϵ: 2.96×105 M-1 cm-1 , λem max : 972 nm, ΦFL : 6.2 %) by tuning electrostatic interactions between π-conjugated molecular planes through manipulating molecular surface electrostatic potential of aromatic ring-fused aza-BODIPY dyes. Notably, by controlling the preparation method for encapsulating dyes into F-127 polymer, we were able to selectively generate H-/J-aggregates, respectively. Furthermore, the J-aggregates exhibited two controllable morphologies: nanospheres and nanowires. Importantly, the shortwave-infrared J-aggregated nanoparticles with impressive PCE of 72.9 % effectively destroyed cancer cells and mice-tumors at an ultralow power density of 0.27 W cm-2 (915 nm). This phototherapeutic nano-platform, which generates predictable J-aggregation behavior, and can controllably form J-/H-aggregates and selectable J-aggregate morphology, is a valuable paradigm for developing photothermal agents for tumor-treatment at ultralow laser power density.

Ultralow Electric Current-Assisted Magnetization Switching due to Thermally Engineered Magnetic Anisotropy.

Control of magnetic anisotropy in thin films with perpendicular magnetic anisotropy is of paramount importance for the development of spintronics with ultralow-energy consumption and high density. Traditional magnetoelectric heterostructures utilized the synergistic effect of piezoelectricity and magnetostriction to realize the electric field control of magnetic anisotropy, resulting in additional fabrication and modulation processes and a complicated device architecture. Here, we have systematically investigated the electric current tuning of the magnetic properties of the metallic NiCo2O4 film with intrinsic perpendicular magnetic anisotropy. Ferrimagnetic-to-paramagnetic phase transition has been induced through Joule heating, resulting in a rapid decrease of both magnetic coercivity and moment. An ultralow current density of 2.5 × 104 A/cm2, which is 2 to 3 orders magnitude lower than that of conventional spin transfer torque devices, has been verified to be effective for the control of the magnetic anisotropy of NiCo2O4. Successful triggering of magnetic switching has been realized through the application of a current pulse. These findings provide new perspectives toward the electric control of magnetic anisotropy and design of spintronics with an ultralow driving current density.

Strongly Confined and Spectrally Tunable CsPbBr3 Quantum Dots for Deep Blue QD‐LEDs

AbstractDespite recent advances, it is still difficult to fabricate deep blue‐emitting perovskite light‐emitting diodes (LEDs) that are immune to the color instability caused by halide mixing. This is largely because it is still challenging to achieve bright, stable, pure bromide perovskite materials with deep blue emission. Here, a novel strategy is reported for synthesizing ultrasmall CsPbBr3 quantum dots (QDs) under ambient conditions. Precise size control is used to tune the peak emission wavelengths, via the confinement effect, from 433 to 501 nm. These ultrasmall QDs, with sizes of 2–6 nm, exhibit ultralow trap density and stable photoluminescence quantum yields up to 94%, particularly in the deep blue spectral region (440–465 nm). QD‐LEDs fabricated with these materials achieve an external quantum efficiency of 1.0% at 459 nm. The exceptional control of the emission properties demonstrated here suggests a new strategy for building stable, efficient blue QD‐LEDs.

A new strategy for the preparation of wood-epoxy resin composites reinforced with controllable osmotic interfaces

As a multifunctional material, wood is widely used in interior decoration and construction engineering. The full-cell process of impregnating monomers into wood pores cavity in situ is one of the most common wood modification methods. However, this approach is time-consuming, material-wasting, and not environmentally friendly. In this study, we proposed a simple strategy to directly convert wood into high-performance composites by controlling wood surface penetration through resin pre-curing. A slight amount of resin used was sufficient to penetrate the wood surfaces, and the weight percent gain (WPG) of 11.45 %. High energy efficiency was achieved by simple temperature control (60 °C/120 °C). In addition, this method can rapidly produce wood composites with ultra-low density (0.51 g/cm3) and ultra-high specific bending strength (284 MPa·cm3g−1) in a very short time (4 h and 30 min). The results showed that the problem of excessive resin penetration was effectively solved by the pre-curing process, which enriched the gradient distribution of resin from the surface to the bottom of the wood and significantly improved the material properties. This efficient and environmentally friendly preparation strategy makes natural wood a low-cost, low-density, high-strength engineering material, which broadens the application of wood composites in construction engineering and interior decoration.

Tuning the performance of Fe-porphyrin aerogel-based PGM-free oxygen reduction reaction catalysts in proton exchange membrane fuel cells.

Fe-N-C catalysts are currently the leading candidates to replace Pt-based catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells. To maximize their activity, it is necessary to optimize their structure to allow high active site density on one hand, and hierarchical porous structure that will allow good mass transport of reactants and products to and from the active sites on the other hand. Hence, the hierarchical structure of the catalyst plays an important role in the balance between the electrochemical active site density and the mass transport resistance. Aerogels were synthesized in this work to study the interplay between these two parameters. Aerogels are covalent organic frameworks with ultra-low density, high porosity, and large surface area. The relative ease of tuning the composition and pore structure of aerogels make them prominent candidates for catalysis. Herein, we report on a tunable Fe-N-C catalyst based on an Fe porphyrin aerogel, which shows high electrocatalytic oxygen reduction reaction activity with tunable hierarchical pore structure and studied the influence of the porous structure on the overall performance in proton exchange membrane fuel cells.