Understanding the Morphogenesis of Hierarchically Porous Graphitic Aerogels Produced from Protein Precursors

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Shaharyar Wani, Bridget Denzer, Nick Caggiano, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4940554/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Hierarchically porous graphitic sheet-based aerogels with high surface area and ultra-low density have drawn massive attention for applications in catalysis, energy storage/conversion, water purification, and beyond. Single-step pyrolysis of protein precursors for creating hierarchically porous monolithic graphitic aerogels with exceptional electrical, electrochemical, and mechanical properties has emerged as a sustainable and green approach. However, the formation mechanisms underlying the transformation of proteins into these aerogel structures remain to be understood. Herein, we demonstrate a self-foaming mechanism for the green and scalable synthesis of hierarchically porous monolithic sheet and fiber-structured graphitic aerogels using protein as a precursor. Rather than creating a solid phase and then exchanging the sacrificial component for a gas phase, we create a gas phase and then convert the liquid into a solid phase. The controlled heating of the protein precursors induces an intrinsic foaming action via initial softening followed by gas evolution and graphitization, resulting in a hierarchically porous graphitic aerogel composed of an integrated sheet and fiber framework. Our systematic exploration into the processing-structure-property relationships that govern aerogel optimization elucidates the interplay between tunable synthesis variables and resultant aerogel properties and structure, enabling deliberate control over microstructural features. Physical sciences/Materials science/Nanoscale materials/Graphene/Synthesis of graphene Physical sciences/Materials science/Nanoscale materials/Synthesis and processing Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Graphitic sheet-based aerogels are hierarchically porous, ultra-low density materials that have drawn massive attention across diverse fields, including catalysis, electromagnetic shielding, energy storage/conversion, water purification, gas storage/separation, and biomedical applications 1–9 . Their widespread applicability arises from their exceptional electrical, electronic, electrochemical, and mechanical properties. As with any aerogel, structurally, graphitic aerogels can be conceptualized as biphasic systems comprising solid and gas phases. Conventional synthesis methods for these materials typically involve a multi-step process. Initially, a composite solid phase is created consisting of a precursor (such as graphene oxide), and a supporting/sacrificial phase, which can be either solid (e.g., templates) or liquid (e.g., solvents). The sacrificial component is subsequently removed through procedures such as etching or freeze-drying, leaving behind voids 2,10-13 . However, these approaches require multi-step processing, rely on using chemical reagents, and can produce harsh chemical waste, making them complicated and environmentally non-sustainable. Furthermore, the lack of seamless integration of graphitic sheets in these methods often compromises their properties. Using biobased precursors, such as proteins, presents a promising opportunity for the sustainable, bottom-up synthesis of hierarchically porous graphitic aerogel (HGA) structures. This approach leverages proteins' inherent capacity to form supramolecular aggregates and their unique thermal degradation behavior 1,18 . In contrast to traditional approaches for creating HGAs, which typically require the creation of a composite solid phase, we hypothesize that an intrinsic gaseous phase can be generated through a self-foaming effect within the protein precursor during pyrolysis. This process templates the formation of a hierarchical structure in HGAs, thus eliminating the need for an external supporting or sacrificial phase. While some studies have demonstrated similar effects through chemically induced foaming 10,14-17 , these approaches often involve multiple steps and rely on the addition of external foam-inducing chemicals, raising sustainability concerns. Furthermore, non-uniform mixing of the precursor and foaming agent may result in uneven gas evolution within the precursor structure, potentially impeding uniform structure formation and limiting the scalability of this approach. In our previous work, we have shown that the pyrolysis of albumin protein results in the formation of 3D-HGA, displaying high water purification and desalination efficiencies 1 . However, to effectively implement proteins for developing HGA structures, it is necessary to investigate the underlying transformation mechanisms from precursor to final hierarchically porous morphology. Further, unraveling the intricate processing-structure-property relationships that dictate material optimization is critical for enabling the scalable production of high-performance, environmentally benign HGA. In this study, we develop an understanding of a self-foaming mechanism for the synthesis of HGAs using protein precursors. The controlled heating of protein precursors during a one-step pyrolysis process initiates a cascade of transformations. This sequence begins with protein softening and melting, followed by gas release due to thermal decomposition, which causes intrinsic foaming. These liberated gases facilitate bubble expansion and coalescence, while the foamed structure simultaneously cures and becomes immobilized. This process results in a carbonaceous porous structure that is subsequently graphitized, yielding an HGA composed of an integrated framework of sheets and fibers. To elucidate this self-foaming formation mechanism, we implemented in situ characterization techniques, including optical microscopy, time-resolved thermal analysis, and mass spectrometry. These methods allowed us to track the structural and chemical evolution during the protein's transformation into HGA, providing insights into the intricate foaming process underlying the structural transition and revealing critical processing-structure relationships, such as the impact of heating rate on porosity and density, and the influence of temperature on conductivity. These findings demonstrate a self-foaming mechanism for creating HGA using protein precursors, offering advantages in terms of sustainability, scalability, and process simplification compared to traditional methods. Results Synthesis and characterization of HGA HGA is synthesized by pyrolytic conversion of freeze-dried egg-white protein as depicted in Fig. 1a 1 . The sample is first freeze-dried to get rid of the solvent from the precursor, leaving behind dried protein. Subsequently, the freeze-dried protein is heated to a peak temperature of 900°C under a nitrogen atmosphere. Pyrolytic treatment induces thermal decomposition and reduction of the freeze-dried precursor, resulting in the formation of the HGA exhibiting ultralow density, and hierarchical porosity extending over multiple length scales (Fig. 1b). The surface morphology and microstructure of HGA consists of a monolithic sheet and fiber as shown by SEM micrographs (Fig. 1c, d). High-resolution transmission electron microscopy (Supplementary Fig. S1) is performed to examine the atomic arrangement in HGA. The material possesses regions of both crystalline and amorphous phases. Generally, the formation of crystalline phases takes place at very high temperatures (above 2000°C) however studies have shown that graphitization of thin oriented polymer sheets requires less energy for ordering of graphitic layers 15,21,22 . A detailed account of the graphitization of these materials at similar temperatures is given by Ozden et. al 1 . During pyrolysis, there are various depolymerization transformations taking place in the protein precursor, which are indicated by the disappearance of the characteristic FTIR peaks at ~ 3200 cm -1 (corresponding to OH/NH stretching), ~ 2900 cm -1 (due to C-H stretching), ~ 1690 cm -1 and ~ 1455 cm -1 (attributed to C=O/C=N stretching modes in aromatic species) (Supplementary Fig.2). However, there are some traces of oxygen and nitrogen retained in the HGA structure, which is indicated by the presence of a small peak at ~1631 cm -1 and ~1400 cm -120 . Raman spectra display G (~1591 cm -1 ) and D (~1362 cm -1 ) peaks as shown in Supplementary Fig.3. The G-band arises from the E 2g vibrational mode of sp 2 hybridized carbon within the conjugated system, while the D-band originates from vibrations of sp 3 hybridized defect sites disrupting the hexagonal lattice periodicity. Similar Raman features have been reported in a previous study where the defect peak is expected to be due to the presence of atomic defects and the presence of impurities such as nitrogen and oxygen in the structure 1 . Formation Mechanism of HGA The formation of unique sheet and fiber-based microstructure in HGA occurs through a self-foaming mechanism. This process involves the generation of a biphasic material at high temperature, wherein gas pockets form within a molten polymer, which subsequently crosslinks to produce a hierarchically porous structure. As the temperature further increases to higher values, the material undergoes carbonization/graphitization, resulting in the formation of HGA (Fig. 2a.). To elucidate the mechanisms governing the transformation of egg white protein into the hierarchical porous architecture, the pyrolysis process is investigated using a high-definition camera, as shown in Fig. 2b-g and Supplementary Movie 1. Fig. 2b shows the freeze-dried samples and Fig. 2c-g shows samples at different time intervals during the pyrolysis process. As the temperature of freeze-dried egg white precursor is increased there is a substantial volume expansion observed in the sample. The sample volume becomes more than fourfold compared to the freeze-dried samples. In-situ optical microscopy was employed to elucidate the structural evolution during protein pyrolysis in the transformation of protein to HGA (Supplementary Fig.4 and Supplementary Movie 2). The process was observed to occur in three distinct stages: protein melting, decomposition/gas evolution, and carbonization/graphitization. Initially, as the temperature of the freeze-dried protein precursor increased, a brownish coloration developed (Supplementary Fig. S2b-d), indicative of Maillard reactions 15 . This transition, representing the first phase of HGA formation, reflects dehydration and the onset of decomposition reactions. Upon surpassing 200 °C, the precursor began decomposing, transforming into a highly viscous black intermediate (Supplementary Fig. 4d-h). Thermal decomposition of this intermediate resulted in depolymerization reactions, generating gases that induced foaming. The initial foaming stage was characterized by violent bubble generation and rupture (Supplementary Movie 2), attributed to the mismatch between extensive gas release and crosslinking kinetics of molten intermediates 2 . This self-foaming step is crucial for HGA synthesis, as the generated gases expand the bubbles, which subsequently stabilize due to ongoing self-assembly, resulting in the formation of sheet and fiber structures 2 . At temperatures of ~350°C and beyond, a balance between gas generation rates and curing of decomposed precursor intermediates was attained, enabling bubble expansion and stabilization. The bubbles assembled into interconnected polygons, primarily pentagons and hexagons, as material redistribution towards the cell walls occurred due to surface tension 15,17 . This surface energy-directed mass flow templated the morphogenesis of graphitic sheets, seamlessly interconnected by fiber-like networks. Further heating and isothermal treatment at 900°C induced carbonization and graphitization, yielding the final hierarchically porous aerogel architecture in HGA. This visualized progression provides direct evidence substantiating the hypothesized formation mechanism, wherein heating of the sample results in depolymerization reactions generating gases that cause self-foaming action. This process generates a gas phase around which the molten protein is redistributed as the components in the depolymerized precursor crosslink and subsequent high-temperature exposure leads to carbonization and graphitization. To further examine the formation mechanism, time-resolved thermal analysis is paired with mass spectrometry to trace the compositional evolutions occurring throughout the pyrolytic transformation of the protein precursor. Differential scanning calorimetry (DSC) reveals an initial endothermic peak at ~ 120 – 150 °C attributable to water evaporation (Fig. 3a). A second, endothermic feature manifests at ~250°C, correlating with the thermal decomposition of polypeptide chains 23 . Moreover, an exothermic peak is expected to show up around 250°C due to concurrent crystallization and crosslinking, forming stable aromatic ring structures. Given the simultaneous occurrence of endothermic bond dissociation and exothermic conjugated ring formation reactions, only minor net endothermic enthalpy changes are observed, likely obscuring the visualization of distinct exothermic peaks in the DSC spectrum 18 . To elucidate the relationship between structural transformations observed during in-situ optical examination and gas evolution during depolymerization, thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS) was employed to analyze the pyrolytic conversion and trace released gaseous species. Fig. 3b illustrates the TGA and derivative of mass loss (DTG) profiles of the protein decomposition, revealing three distinct stages. The first stage, spanning from approximately 30 to 200 °C, involves mass loss attributed to desorption of water and protein denaturation. During this phase, no visible structural transformation occurs in the precursor beyond the initiation of color change, as evidenced by in situ optical microscopy results (Supplementary Fig.S4). The second phase initiates as the temperature surpasses ~200 °C, marked by the onset of protein decomposition and peak mass loss. This region coincides with the endothermic peak observed in the DSC profile. The DTG curve indicates peak mass loss at approximately 340 °C, where depolymerization reactions occurring during precursor decomposition liberate various gaseous species including CO 2 , H 2 S, SO 2 , and NH 3 (Fig. 3d), resulting in precursor foaming. Specific reactions, such as carbonyl group scission, drive the evolution of CO 2 and CO 2 , while cysteine breakdown contributes to H 2 S formation 4,17 . This gas generation acts as the foaming agent, as observed during the in-situ examinations (Supplementary Fig.S4). The synchronization of the molten precursor's peak gas release and viscous properties facilitates effective foaming, generating the sheet and fiber structure. Upon further heating to ~350 °C, the foam scaffold begins to attain structural stability due to crosslinking. The peak temperature, defined as the maximum temperature to which the sample is exposed, plays a critical role in structure evolution. To assess the impact of peak temperature, samples were prepared at different peak temperatures (250°C, 350°C, 450°C) as shown in Fig.3e, f. Exposure to peak temperatures below the maximum gas release point (corresponding to ~340°C) results in insignificant or no foaming, even with extended duration at these temperatures. This may be attributed to insufficient protein degradation, where depolymerization reactions fail to yield adequate gas release to induce foaming in the softened protein. Moreover, thermal analysis of the protein precursors is carried out at different peak temperatures (250, 300, 350, 375, and 400°C) (Supplementary Fig. S5). For samples processed at 250 and 300°C, the DTG curves exhibit sharp mass losses at the highest temperatures. In contrast, for samples processed at 350, 375, and 400°C, the peak mass loss occurs at approximately 340°C. Notably, this temperature coincides with the peak mass loss observed in the sample prepared at a peak temperature of 900°C, suggesting that the peak mass loss occurs at a similar temperature regardless of the peak processing temperature. The third phase follows, during which concurrent carbonization and graphitization of the carbonaceous residue occur, transforming the foamed intermediate into the final HGA architecture. The sample is then held at a peak temperature of 900°C for the synthesis of HGA. This comprehensive analysis of the thermal decomposition process provides crucial insights into the formation mechanism of the HGA structure, offering a foundation for process optimization and property tailoring. Optimization of processing conditions for tuning HGA structure The understanding of the formation mechanism provides an opportunity to examine modifications to the synthesis protocol for property optimization, such as density, porosity, and degree of graphitization, which governs performance in target applications. By tuning key process parameters such as heating rate and peak processing temperature, fine control over fiber width, porosity, and graphitization degree in the aerogel structure can be achieved (Fig.4.). SEM micrographs reveal morphological evolution in graphitic aerogels as a function of temperature ramp rate (Fig. 4a-e), where slower heating results in thicker fiber widths compared to faster heating protocol (Fig. 4f). The aerogel density exhibits an order of magnitude decrease (~ 3.62 mg/cm 3 ) with increasing heating rate (Fig. 4g). This significant reduction in density correlates to a complementary enhancement in aerogel porosity, as revealed in Fig. 4h. Structural evolution mechanisms provide rationale for heating rate effects on ultimate aerogel morphology. Slow thermal ramps allow sufficient gas diffusion periods reducing bubble expansion and providing sufficient time for maximum mass distribution towards the edges, yielding thicker fiber widths with largely spherical geometries. Faster heating impedes gas escape to foster inflated sheet structures, consistent with observed finer fiber networks. The peak processing temperature critically influences graphitization extent with major impacts on properties like electrical conductivity. Raman analysis shows decreasing defect density (I D /I G ) from 600 to 900 °C (Fig. 4i), reflecting improved order in the HGA structure under rising synthesis temperatures. Unexpectedly, a slight increase in I D /I G ratio is observed at 1000 °C, despite predictions of further structural enhancement at even higher temperatures. This result may be indicative of the simultaneous occurrence of crystal defect formation and thermally-induced enhancement of nanostructural ordering, both processes being facilitated by the elevated processing temperatures employed during the synthesis. 25,26 . The conductivity measurements display a comparable trend to the Raman defect density analysis (Supplementary Fig.S6). Conductivity markedly increases from 600 to 900 °C as the temperatures promote graphitization. However, a fall in conductivity occurs at 1000 °C indicating that increased structural disorder limits conductivity despite further thermal treatment. This drop coincides with the rise in defect concentration observed in the 1000 °C Raman spectrum. While higher process temperatures generally enhance graphitization kinetics, they likely concomitantly stimulate the generation of conductivity-impairing crystal imperfections 25,26 . Strategic tuning and selection of thermal ramp intensity and peak processing temperatures enable precise tailoring of fiber width, packing densities, conductivities, and other attributes to align with application requirements. Such understanding crucially unlocks viable pathways for customizable material enhancement compatible with diverse operational demands, via manipulation of heat treatment conditions to engineer structure and properties 16,27–30 . Conclusion In summary, the controlled pyrolysis of protein-based bio-precursor initiates a self-foaming action within a molten protein precursor which leads to the formation of a monolithic sheet and fiber-based hierarchically porous graphitic aerogel. During this process, gas bubbles generated from polymer decomposition percolate throughout the system, forming a distinct gaseous phase. As these bubbles expand, they drive the molten protein toward their peripheries, ultimately creating three-dimensional polygonal structures composed of interconnected sheets and fibers. As the pyrolysis temperature increases, the foamed structure undergoes carbonization and graphitization, culminating in the formation of HGA. Notably, the properties of the resulting HGA, including density, porosity, and conductivity, can be tailored by controlling the heating rate and peak temperatures during the pyrolysis process. The facile scalability and customizability of this single-step pyrolysis process present a promising pathway for the creation of HGA from bio-based precursors, particularly proteins. This approach not only offers a sustainable alternative to traditional synthetic methods but also provides a versatile platform for the development of advanced carbon materials with tunable properties. Methods Materials Commercially sourced pasteurized egg white protein was purchased from a local grocery store. Preparation of graphitic sheet-based aerogel Liquid egg white protein was freeze-dried to remove moisture and volatiles. The dried protein was then pyrolyzed under a nitrogen atmosphere at 900°C using heating rates of 1.25, 2.5, 5, 7.5, and 35°C/min. Upon reaching 900°C, an isothermal hold was maintained for 24 hours before allowing the samples to cool naturally to ambient temperature resulting in the formation of hierarchically porous graphitic aerogels. In a separate experimental set, pyrolysis was also carried out to final temperatures of 250, 350, 400, 450, 600, 800, 900, and 1000°C using a fixed heating rate of 35°C/min and an identical 24-hour isothermal hold at the peak temperature before cooling. Characterization The structural morphology and microstructure of the aerogel were examined using an environmental scanning electron microscope (ESEM) (FEI Quanta 200 FEG ESEM) and transmission electron microscope (Talos F200X, Scanning/Transmission Electron microscope S/TEM), respectively. Raman spectra were obtained using a 532 nm excitation laser on a Horiba Raman spectrometer. Fourier transform infrared spectroscopy (FTIR) spectra were recorded in reflection mode on a Nicolet iN10 MX spectrometer (Thermo Scientific). In-situ optical microscopy was done using an upright light microscope (The Zeiss Axio Scope A1) while heating the freeze-dried sample using a Linkam stage at a heating rate of 35 °C/min under nitrogen conditions to capture the morphological transition happening in the protein during pyrolysis. Phase transition was analyzed with a differential scanning calorimeter (DSC-8500, PerkinElmer). Thermal decomposition behavior and gas composition were examined using a thermogravimetric analyzer (TGA) coupled with a mass spectrometer (MS) (PerkinElmer TGA-GC/MS: TGA-8000 TGA, Clarus 680 Gas Chromatography, Clarus SQ 8 T MS hyphenated system). Electrical conductivity measurements were performed via a standard two-probe method. Declarations Acknowledgments This work was supported by the Department of Mechanical and Aerospace Engineering, Princeton University, and the National Science Foundation (NSF). The authors acknowledge the use of Princeton’s Imaging and Analysis Center, which is partially supported through the Princeton Center for Complex Materials (PCCM); an NSF-MRSEC program (DMR2011750). The authors are thankful to Maddie Armstrong for helping with the freeze-drying process. Author Contributions M.S.W. and C.B.A. conceived the project ideas. M.S.W. conducted the experiments and performed material characterization. M.S.W. performed data analysis and wrote the manuscript with support from C.B.A. M.S.W. and N.J.C. performed freeze drying. B.D. was a part of the discussions and reviewing the manuscript. All the authors discussed and approved the final manuscript. Competing interests The authors declare no competing interests. Supplementary information Supplementary information is provided with the manuscript on the Nature Communications website. References Ozden, S. et al. Egg protein derived ultralightweight hybrid monolithic aerogel for water purification. Mater. 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Supplementary Files SIMechanismpaperv61.docx mechanismpapersuppmovie1.mp4 Supplementary Movie 1: Movie showing the self foaming action during the pyrolysis of protein mechanismpaperSuppmovie2.mp4 Supplementary Movie 2: Movie showing the structural evolution of protein precursor during the pyrolysis - gas bubble formation Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4940554","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":350141650,"identity":"679f372c-4266-4832-9077-3276f44db498","order_by":0,"name":"Craig Arnold","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6UlEQVRIiWNgGAWjYDACCTBpwdgPJA/ARXkIa5FgnNkA0SJBvJYNB5D5+LTwz25+9rmgQkJ28+3eh4cLamzq+KUbGB+8bcNjyZ1jxrNnnJEw3nbnuMHhGcfSJCTnHGA2nItHi4FEgjEzb5tE4rYbaQyHeRsOSxjcSGCT5sWrJf0zWMvmGWAt/0Fa2H/j15IDsWWDBFjLAbAtzPi0SNw5U8zMA/TLjDvHGA7zHEuWnDkjsVlyzjncWvhnt29m5qmwke2f3cb8mafGjp9fIvnghzdluLUg2QdnMTYQox5FyygYBaNgFIwCVAAAiY9NVbpG7sAAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-0293-5411","institution":"Princeton University","correspondingAuthor":true,"prefix":"","firstName":"Craig","middleName":"","lastName":"Arnold","suffix":""},{"id":350141651,"identity":"e139f207-6f8e-4e40-97bc-772e5c9a650c","order_by":1,"name":"M. Shaharyar Wani","email":"","orcid":"","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"M.","middleName":"Shaharyar","lastName":"Wani","suffix":""},{"id":350141652,"identity":"e9914816-1bf6-4f58-bc56-302c624e9311","order_by":2,"name":"Bridget Denzer","email":"","orcid":"https://orcid.org/0000-0003-4054-1584","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"Bridget","middleName":"","lastName":"Denzer","suffix":""},{"id":350141653,"identity":"4705b6f5-f46e-4e55-9ed4-f802a0acc708","order_by":3,"name":"Nick Caggiano","email":"","orcid":"","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"Nick","middleName":"","lastName":"Caggiano","suffix":""},{"id":350141654,"identity":"0baee2dc-4724-46bd-9edc-1f02e032aa40","order_by":4,"name":"Robert Prudhomme","email":"","orcid":"https://orcid.org/0000-0003-2858-0097","institution":"Princeton University","correspondingAuthor":false,"prefix":"","firstName":"Robert","middleName":"","lastName":"Prudhomme","suffix":""}],"badges":[],"createdAt":"2024-08-19 19:05:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4940554/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4940554/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":64079461,"identity":"d12a7621-009d-4018-a926-c7509e888d1d","added_by":"auto","created_at":"2024-09-06 09:45:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":729873,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis of HGA and its characterization\u003c/strong\u003e (\u003cstrong\u003ea) \u003c/strong\u003eSchematic showing key steps to prepare HGA aerogel. (\u003cstrong\u003eb) \u003c/strong\u003ePhotograph of HGA (\u003cstrong\u003ec) \u003c/strong\u003eScanning electron microscopy image of HGA. (\u003cstrong\u003ed)\u003c/strong\u003e Monolithic sheet and fiber configuration.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/6e5e3040c52e3b5b51ca7bd3.png"},{"id":64079048,"identity":"1fb2b7d1-0e4d-420b-9b30-55dacbd0e10f","added_by":"auto","created_at":"2024-09-06 09:37:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1101634,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTransformation of protein precursor into HGA. (a)\u003c/strong\u003e Schematic showing the formation mechanism of HGA. \u003cstrong\u003e(b)\u003c/strong\u003e Sample before pyrolysis. \u003cstrong\u003e(c-g) \u003c/strong\u003eSample during and after pyrolysis, showing expansion in the volume of the precursor,\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/020145b43ba8e12b2a8c689d.png"},{"id":64079050,"identity":"fb2833dc-d575-4aa6-9bdd-4747438e8479","added_by":"auto","created_at":"2024-09-06 09:37:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":607256,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTime-resolved thermal analysis. (a)\u003c/strong\u003e Differential Scanning Calorimetry analysis (\u003cstrong\u003eb) \u003c/strong\u003eThermogravimetric analysis. Weight loss and the derivative of weight loss as a function of temperature. (\u003cstrong\u003ec)\u003c/strong\u003e Mass spectrum representing the gas evolution during the pyrolysis of freeze-dried protein. (\u003cstrong\u003ed) \u003c/strong\u003eEvolution of different gasses over time. \u003cstrong\u003e(e and f) \u003c/strong\u003eSamples prepared at different peak temperatures (250°C, 350°C, and 450°C) - (e) Top view (f) Front view.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/57b3412221560e8f98376095.png"},{"id":64079462,"identity":"abd1cd9b-ca0d-4d7b-99f4-a1ff6c5424b5","added_by":"auto","created_at":"2024-09-06 09:45:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":709098,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTuning structural properties of HGA. (a-e) \u003c/strong\u003eSEM images of carbon aerogel prepared at different heating rates of 1.25 °C/min, 2.5\u003cstrong\u003e \u003c/strong\u003e°C/min, 5 °C/min, 7.5 °C/min, and 35 °C/min respectively. (\u003cstrong\u003ef) \u003c/strong\u003eThe average\u003cstrong\u003e \u003c/strong\u003efiber width estimated from the SEM images is represented in Fig.4. a-e respectively. (\u003cstrong\u003eg)\u003c/strong\u003e Density of HGA prepared at different heating rates (\u003cstrong\u003eh)\u003c/strong\u003e Trends in porosity of HGA prepared at different heating rates. (\u003cstrong\u003ei) \u003c/strong\u003eI\u003csub\u003ed\u003c/sub\u003e/I\u003csub\u003eg\u003c/sub\u003e ratios of MSFG prepared at different peak temperatures.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/804d38aa63d0f455a856d096.png"},{"id":65934347,"identity":"6ac22f88-71bf-40e4-9bfe-6a6cc4705ca1","added_by":"auto","created_at":"2024-10-04 14:45:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4030572,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/4f6a1c20-88ea-4966-83ee-e36f5fa5f43d.pdf"},{"id":64079046,"identity":"ec6bd644-6bad-48b2-87d0-09e1f9fa8075","added_by":"auto","created_at":"2024-09-06 09:37:00","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7194404,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SIMechanismpaperv61.docx","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/20c448f22fc37b9c61fc5e47.docx"},{"id":64079047,"identity":"b9b0ad9f-f3ec-4b94-931e-8b5450bd05db","added_by":"auto","created_at":"2024-09-06 09:37:00","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":8776659,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Movie 1: Movie showing the self foaming action during the pyrolysis of protein\u003c/p\u003e","description":"","filename":"mechanismpapersuppmovie1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/47fc8c3eacfc309c7d559fde.mp4"},{"id":64079052,"identity":"0b0e6a18-8660-457f-a1be-f2b27d168b5c","added_by":"auto","created_at":"2024-09-06 09:37:00","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":33103596,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Movie 2: Movie showing the structural evolution of protein precursor during the pyrolysis - gas bubble formation\u003c/p\u003e","description":"","filename":"mechanismpaperSuppmovie2.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4940554/v1/c1a88f6469b66588f521a6ea.mp4"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Understanding the Morphogenesis of Hierarchically Porous Graphitic Aerogels Produced from Protein Precursors","fulltext":[{"header":"Introduction","content":"\u003cp\u003eGraphitic sheet-based aerogels are hierarchically porous, ultra-low density materials that have drawn massive attention across diverse fields, including catalysis, electromagnetic shielding, energy storage/conversion, water purification, gas storage/separation, and biomedical applications\u003csup\u003e1\u0026ndash;9\u003c/sup\u003e. Their widespread applicability arises from their exceptional electrical, electronic, electrochemical, and mechanical properties. As with any aerogel, structurally, graphitic aerogels can be conceptualized as biphasic systems comprising solid and gas phases. Conventional synthesis methods for these materials typically involve a multi-step process. Initially, a composite solid phase is created consisting of a precursor (such as graphene oxide), and a supporting/sacrificial phase, which can be either solid (e.g., templates) or liquid (e.g., solvents). The sacrificial component is subsequently removed through procedures such as etching or freeze-drying, leaving behind voids\u003csup\u003e2,10-13\u003c/sup\u003e. However, these approaches require multi-step processing, rely on using chemical reagents, and can produce harsh chemical waste, making them complicated and environmentally non-sustainable. Furthermore, the lack of seamless integration of graphitic sheets in these methods often compromises their properties.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eUsing biobased precursors, such as proteins, presents a promising opportunity for the sustainable, bottom-up synthesis of hierarchically porous graphitic aerogel (HGA) structures. This approach leverages proteins\u0026apos; inherent capacity to form supramolecular aggregates and their unique thermal degradation behavior\u003csup\u003e1,18\u003c/sup\u003e. In contrast to traditional approaches for creating HGAs, which typically require the creation of a composite solid phase, we hypothesize that an intrinsic gaseous phase can be generated through a self-foaming effect within the protein precursor during pyrolysis. This process templates the formation of a hierarchical structure in HGAs, thus eliminating the need for an external supporting or sacrificial phase. While some studies have demonstrated similar effects through chemically induced foaming\u003csup\u003e10,14-17\u003c/sup\u003e, these approaches often involve multiple steps and rely on the addition of external foam-inducing chemicals, raising sustainability concerns. Furthermore, non-uniform mixing of the precursor and foaming agent may result in uneven gas evolution within the precursor structure, potentially impeding uniform structure formation and limiting the scalability of this approach. In our previous work, we have shown that the pyrolysis of albumin protein results in the formation of 3D-HGA, displaying high water purification and desalination efficiencies\u003csup\u003e1\u003c/sup\u003e. However, to effectively implement proteins for developing HGA structures, it is necessary to investigate the underlying transformation mechanisms from precursor to final hierarchically porous morphology. Further, unraveling the intricate processing-structure-property relationships that dictate material optimization is critical for enabling the scalable production of high-performance, environmentally benign HGA.\u003c/p\u003e\n\u003cp\u003eIn this study, we develop an understanding of a self-foaming mechanism for the synthesis of HGAs using protein precursors. The controlled heating of protein precursors during a one-step pyrolysis process initiates a cascade of transformations. This sequence begins with protein softening and melting, followed by gas release due to thermal decomposition, which causes intrinsic foaming. These liberated gases facilitate bubble expansion and coalescence, while the foamed structure simultaneously cures and becomes immobilized. This process results in a carbonaceous porous structure that is subsequently graphitized, yielding an HGA composed of an integrated framework of sheets and fibers. To elucidate this self-foaming formation mechanism, we implemented in situ characterization techniques, including optical microscopy, time-resolved thermal analysis, and mass spectrometry. These methods allowed us to track the structural and chemical evolution during the protein\u0026apos;s transformation into HGA, providing insights into the intricate foaming process underlying the structural transition and revealing critical processing-structure relationships, such as the impact of heating rate on porosity and density, and the influence of temperature on conductivity. These findings demonstrate a self-foaming mechanism for creating HGA using protein precursors, offering advantages in terms of sustainability, scalability, and process simplification compared to traditional methods.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eSynthesis and characterization of HGA\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHGA is synthesized by pyrolytic conversion of freeze-dried egg-white protein as depicted in Fig. 1a\u003csup\u003e1\u003c/sup\u003e. The sample is first freeze-dried to get rid of the solvent from the precursor, leaving behind dried protein. Subsequently, the freeze-dried protein is heated to a peak temperature of 900\u0026deg;C under a nitrogen atmosphere. Pyrolytic treatment induces thermal decomposition and reduction of the freeze-dried precursor, resulting in the formation of the HGA exhibiting ultralow density, and hierarchical porosity extending over multiple length scales (Fig. 1b). The surface morphology and microstructure of \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;HGA consists of a monolithic sheet and fiber as shown by SEM micrographs (Fig. 1c, d). High-resolution transmission electron microscopy (Supplementary Fig. S1) is performed to examine the atomic arrangement in HGA. \u0026nbsp;The material possesses regions of both crystalline and amorphous phases. Generally, the formation of crystalline phases takes place at very high temperatures (above 2000\u0026deg;C) however studies have shown that graphitization of thin oriented polymer sheets requires less energy for ordering of graphitic layers\u003csup\u003e15,21,22\u003c/sup\u003e. \u0026nbsp;A detailed account of the graphitization of these materials at similar temperatures is given by Ozden et. al\u003csup\u003e1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eDuring pyrolysis, there are various depolymerization transformations taking place in the protein precursor, which are indicated by the disappearance of the characteristic FTIR peaks at ~ 3200 cm\u003csup\u003e-1\u003c/sup\u003e (corresponding to OH/NH stretching), ~ 2900 cm\u003csup\u003e-1\u003c/sup\u003e (due to C-H stretching), ~ 1690 cm\u003csup\u003e-1\u003c/sup\u003e and ~ 1455 cm\u003csup\u003e-1\u003c/sup\u003e (attributed to C=O/C=N stretching modes in aromatic species) (Supplementary Fig.2). However, there are some traces of oxygen and nitrogen retained in the HGA structure, which is indicated by the presence of a small peak at ~1631 cm\u003csup\u003e-1\u003c/sup\u003e and ~1400 cm\u003csup\u003e-120\u003c/sup\u003e. Raman spectra display G (~1591 cm\u003csup\u003e-1\u003c/sup\u003e) and D (~1362 cm\u003csup\u003e-1\u003c/sup\u003e) peaks as shown in Supplementary Fig.3. The G-band arises from the E\u003csub\u003e2g\u003c/sub\u003e vibrational mode of sp\u003csup\u003e2\u003c/sup\u003e hybridized carbon within the conjugated system, while the D-band originates from vibrations of sp\u003csup\u003e3\u003c/sup\u003e hybridized defect sites disrupting the hexagonal lattice periodicity. Similar Raman features have been reported in a previous study where the defect peak is expected to be due to the presence of atomic defects and the presence of impurities such as nitrogen and oxygen in the structure\u003csup\u003e1\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFormation Mechanism of HGA\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe formation of unique sheet and fiber-based microstructure in HGA occurs through a self-foaming mechanism.\u0026nbsp;This process involves the generation of a biphasic material at high temperature, wherein gas pockets form within a molten polymer, which subsequently crosslinks to produce a hierarchically porous structure. As the temperature further increases to higher values, the material undergoes carbonization/graphitization, resulting in the formation of HGA (Fig. 2a.). To elucidate the mechanisms governing the transformation of egg white protein into the hierarchical porous architecture, the pyrolysis process is investigated using a high-definition camera, as shown in Fig. 2b-g and Supplementary Movie 1. Fig. 2b shows the freeze-dried samples and Fig. 2c-g shows samples at different time intervals during the pyrolysis process. As the temperature of freeze-dried egg white precursor is increased there is a substantial volume expansion observed in the sample. The sample volume becomes more than fourfold compared to the freeze-dried samples. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn-situ optical microscopy was employed to elucidate the structural evolution during protein pyrolysis in the transformation of protein to HGA (Supplementary Fig.4 and Supplementary Movie 2). The process was observed to occur in three distinct stages: protein melting, decomposition/gas evolution, and carbonization/graphitization. Initially, as the temperature of the freeze-dried protein precursor increased, a brownish coloration developed (Supplementary Fig. S2b-d), indicative of Maillard reactions\u003csup\u003e15\u003c/sup\u003e. This transition, representing the first phase of HGA formation, reflects dehydration and the onset of decomposition reactions. Upon surpassing 200 \u0026deg;C, the precursor began decomposing, transforming into a highly viscous black intermediate (Supplementary Fig. 4d-h). Thermal decomposition of this intermediate resulted in depolymerization reactions, generating gases that induced foaming. The initial foaming stage was characterized by violent bubble generation and rupture (Supplementary Movie 2), attributed to the mismatch between extensive gas release and crosslinking kinetics of molten intermediates\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThis self-foaming step is crucial for HGA synthesis, as the generated gases expand the bubbles, which subsequently stabilize due to ongoing self-assembly, resulting in the formation of sheet and fiber structures\u003csup\u003e2\u003c/sup\u003e. At temperatures of ~350\u0026deg;C and beyond, a balance between gas generation rates and curing of decomposed precursor intermediates was attained, enabling bubble expansion and stabilization. The bubbles assembled into interconnected polygons, primarily pentagons and hexagons, as material redistribution towards the cell walls occurred due to surface tension\u003csup\u003e15,17\u003c/sup\u003e. This surface energy-directed mass flow templated the morphogenesis of graphitic sheets, seamlessly interconnected by fiber-like networks. Further heating and isothermal treatment at 900\u0026deg;C induced carbonization and graphitization, yielding the final hierarchically porous aerogel architecture in HGA. This visualized progression provides direct evidence substantiating the hypothesized formation mechanism, wherein heating of the sample results in depolymerization reactions generating gases that cause self-foaming action. This process generates a gas phase around which the molten protein is redistributed as the components in the depolymerized precursor crosslink and subsequent high-temperature exposure leads to carbonization and graphitization.\u003c/p\u003e\n\u003cp\u003eTo further examine the formation mechanism, time-resolved thermal analysis is paired with mass spectrometry to trace the compositional evolutions occurring throughout the pyrolytic transformation of the protein precursor. Differential scanning calorimetry (DSC) reveals an initial endothermic peak at ~ 120 \u0026ndash; 150 \u0026deg;C attributable to water evaporation (Fig. 3a). A second, endothermic feature manifests at ~250\u0026deg;C, correlating with the thermal decomposition of polypeptide chains\u003csup\u003e23\u003c/sup\u003e. Moreover, an exothermic peak is expected to show up around 250\u0026deg;C due to concurrent crystallization and crosslinking, forming stable aromatic ring structures. Given the simultaneous occurrence of endothermic bond dissociation and exothermic conjugated ring formation reactions, only minor net endothermic enthalpy changes are observed, likely obscuring the visualization of distinct exothermic peaks in the DSC spectrum\u003csup\u003e18\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo elucidate the relationship between structural transformations observed during in-situ optical examination and gas evolution during depolymerization, thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS) was employed to analyze the pyrolytic conversion and trace released gaseous species. Fig. 3b illustrates the TGA and derivative of mass loss (DTG) profiles of the protein decomposition, revealing three distinct stages. The first stage, spanning from approximately 30 to 200 \u0026deg;C, involves mass loss attributed to desorption of water and protein denaturation. During this phase, no visible structural transformation occurs in the precursor beyond the initiation of color change, as evidenced by in situ optical microscopy results (Supplementary Fig.S4). The second phase initiates as the temperature surpasses ~200 \u0026deg;C, marked by the onset of protein decomposition and peak mass loss. This region coincides with the endothermic peak observed in the DSC profile. The DTG curve indicates peak mass loss at approximately 340 \u0026deg;C, where depolymerization reactions occurring during precursor decomposition liberate various gaseous species including CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eS, SO\u003csub\u003e2\u003c/sub\u003e, and NH\u003csub\u003e3\u003c/sub\u003e (Fig. 3d), resulting in precursor foaming. Specific reactions, such as carbonyl group scission, drive the evolution of CO\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e, while cysteine breakdown contributes to H\u003csub\u003e2\u003c/sub\u003eS formation\u003csup\u003e4,17\u003c/sup\u003e. This gas generation acts as the foaming agent, as observed during the in-situ examinations (Supplementary Fig.S4). The synchronization of the molten precursor\u0026apos;s peak gas release and viscous properties facilitates effective foaming, generating the sheet and fiber structure. Upon further heating to ~350 \u0026deg;C, the foam scaffold begins to attain structural stability due to crosslinking.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe peak temperature, defined as the maximum temperature to which the sample is exposed, plays a critical role in structure evolution. To assess the impact of peak temperature, samples were prepared at different peak temperatures (250\u0026deg;C, 350\u0026deg;C, 450\u0026deg;C) as shown in Fig.3e, f. Exposure to peak temperatures below the maximum gas release point (corresponding to ~340\u0026deg;C) results in insignificant or no foaming, even with extended duration at these temperatures. This may be attributed to insufficient protein degradation, where depolymerization reactions fail to yield adequate gas release to induce foaming in the softened protein.\u0026nbsp;Moreover, thermal analysis of the protein precursors is carried out at different peak temperatures (250, 300, 350, 375, and 400\u0026deg;C) (Supplementary Fig. S5). For samples processed at 250 and 300\u0026deg;C, the DTG curves exhibit sharp mass losses at the highest temperatures. In contrast, for samples processed at 350, 375, and 400\u0026deg;C, the peak mass loss occurs at approximately 340\u0026deg;C. Notably, this temperature coincides with the peak mass loss observed in the sample prepared at a peak temperature of 900\u0026deg;C, suggesting that the peak mass loss occurs at a similar temperature regardless of the peak processing temperature. \u0026nbsp;The third phase follows, during which concurrent carbonization and graphitization of the carbonaceous residue occur, transforming the foamed intermediate into the final HGA architecture. The sample is then held at a peak temperature of 900\u0026deg;C for the synthesis of HGA. This comprehensive analysis of the thermal decomposition process provides crucial insights into the formation mechanism of the HGA \u0026nbsp;structure, offering a foundation for process optimization and property tailoring.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOptimization of processing conditions for tuning HGA structure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe understanding of the formation mechanism provides an opportunity to examine modifications to the synthesis protocol for property optimization, such as density, porosity, and degree of graphitization, which governs performance in target applications. By tuning key process parameters such as heating rate and peak processing temperature, fine control over fiber width, porosity, and graphitization degree in the aerogel structure can be achieved (Fig.4.). SEM micrographs reveal morphological evolution in graphitic aerogels as a function of temperature ramp rate (Fig. 4a-e), where slower heating results in thicker fiber widths compared to faster heating protocol (Fig. 4f). The aerogel density exhibits an order of magnitude decrease (~ 3.62 mg/cm\u003csup\u003e3\u003c/sup\u003e) with increasing heating rate (Fig. 4g). This significant reduction in density correlates to a complementary enhancement in aerogel porosity, as revealed in Fig. 4h. Structural evolution mechanisms provide rationale for heating rate effects on ultimate aerogel morphology. Slow thermal ramps allow sufficient gas diffusion periods reducing bubble expansion and providing sufficient time for maximum mass distribution towards the edges, yielding thicker fiber widths with largely spherical geometries. Faster heating impedes gas escape to foster inflated sheet structures, consistent with observed finer fiber networks.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe peak processing temperature critically influences graphitization extent with major impacts on properties like electrical conductivity. Raman analysis shows decreasing defect density (I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e) from 600 to 900 \u0026deg;C (Fig. 4i), reflecting improved order in the HGA structure under rising synthesis temperatures. Unexpectedly, a slight increase in I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e ratio is observed at 1000 \u0026deg;C, despite predictions of further structural enhancement at even higher temperatures.\u0026nbsp;This result may be indicative of the simultaneous occurrence of crystal defect formation and thermally-induced enhancement of nanostructural ordering, both processes being facilitated by the elevated processing temperatures employed during the synthesis.\u003csup\u003e25,26\u003c/sup\u003e.\u0026nbsp;The conductivity measurements display a comparable trend to the Raman defect density analysis (Supplementary Fig.S6). Conductivity markedly increases from 600 to 900 \u0026deg;C as the temperatures promote graphitization. However, a fall in conductivity occurs at 1000 \u0026deg;C indicating that increased structural disorder limits conductivity despite further thermal treatment. This drop coincides with the rise in defect concentration observed in the 1000 \u0026deg;C Raman spectrum. While higher process temperatures generally enhance graphitization kinetics, they likely concomitantly stimulate the generation of conductivity-impairing crystal imperfections\u003csup\u003e25,26\u003c/sup\u003e. Strategic tuning and selection of thermal ramp intensity and peak processing temperatures enable precise tailoring of fiber width, packing densities, conductivities, and other attributes to align with application requirements. Such understanding crucially unlocks viable pathways for customizable material enhancement compatible with diverse operational demands, via manipulation of heat treatment conditions to engineer structure and properties\u003csup\u003e16,27\u0026ndash;30\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, the controlled pyrolysis of protein-based bio-precursor initiates a self-foaming action within a molten protein precursor which leads to the formation of a monolithic sheet and fiber-based hierarchically porous graphitic aerogel. During this process, gas bubbles generated from polymer decomposition percolate throughout the system, forming a distinct gaseous phase. As these bubbles expand, they drive the molten protein toward their peripheries, ultimately creating three-dimensional polygonal structures composed of interconnected sheets and fibers. As the pyrolysis temperature increases, the foamed structure undergoes carbonization and graphitization, culminating in the formation of HGA. Notably, the properties of the resulting HGA, including density, porosity, and conductivity, can be tailored by controlling the heating rate and peak temperatures during the pyrolysis process. The facile scalability and customizability of this single-step pyrolysis process present a promising pathway for the creation of HGA from bio-based precursors, particularly proteins. This approach not only offers a sustainable alternative to traditional synthetic methods but also provides a versatile platform for the development of advanced carbon materials with tunable properties.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCommercially sourced pasteurized egg white protein was purchased from a local grocery store.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of graphitic sheet-based aerogel\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eLiquid egg white protein was freeze-dried to remove moisture and volatiles. The dried protein was then pyrolyzed under a nitrogen atmosphere at 900\u0026deg;C using heating rates of 1.25, 2.5, 5, 7.5, and 35\u0026deg;C/min. Upon reaching 900\u0026deg;C, an isothermal hold was maintained for 24 hours before allowing the samples to cool naturally to ambient temperature resulting in the formation of hierarchically porous graphitic aerogels. In a separate experimental set, pyrolysis was also carried out to final temperatures of 250, 350, 400, 450, 600, 800, 900, and 1000\u0026deg;C using a fixed heating rate of 35\u0026deg;C/min and an identical 24-hour isothermal hold at the peak temperature before cooling.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe structural morphology and microstructure of the aerogel were examined using an environmental scanning electron microscope (ESEM) (FEI Quanta 200 FEG ESEM) and transmission electron microscope (Talos F200X, Scanning/Transmission Electron microscope S/TEM), respectively. Raman spectra were obtained using a 532 nm excitation laser on a Horiba Raman spectrometer. Fourier transform infrared spectroscopy (FTIR) spectra were recorded in reflection mode on a Nicolet iN10 MX spectrometer (Thermo Scientific). In-situ optical microscopy was done using an upright light microscope (The Zeiss Axio Scope A1) while heating the freeze-dried sample using a Linkam stage at a heating rate of 35 \u0026deg;C/min under nitrogen conditions to capture the morphological transition happening in the protein during pyrolysis. \u0026nbsp;Phase transition was analyzed with a differential scanning calorimeter (DSC-8500, PerkinElmer). Thermal decomposition behavior and gas composition were examined using a thermogravimetric analyzer (TGA) coupled with a mass spectrometer (MS) (PerkinElmer TGA-GC/MS: TGA-8000 TGA, Clarus 680 Gas Chromatography, Clarus SQ 8 T MS hyphenated system). Electrical conductivity measurements were performed via a standard two-probe method.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Department of Mechanical and Aerospace Engineering, Princeton University, and the National Science Foundation (NSF). The authors acknowledge the use of Princeton\u0026rsquo;s Imaging and Analysis Center, which is partially supported through the Princeton Center for Complex Materials (PCCM); an NSF-MRSEC program (DMR2011750). The authors are thankful to Maddie Armstrong for helping with the freeze-drying process.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.S.W. and C.B.A. conceived the project ideas. M.S.W. conducted the experiments and performed material characterization. M.S.W. performed data analysis and wrote the manuscript with support from C.B.A. M.S.W. and N.J.C. performed freeze drying. B.D. was a part of the discussions and reviewing the manuscript. All the authors discussed and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information is provided with the manuscript on the Nature Communications website.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eOzden, S. \u003cem\u003eet al.\u003c/em\u003e Egg protein derived ultralightweight hybrid monolithic aerogel for water purification. \u003cem\u003eMater. 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Mater.\u003c/em\u003e \u003cstrong\u003e30\u003c/strong\u003e, 1\u0026ndash;17 (2020).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4940554/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4940554/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Hierarchically porous graphitic sheet-based aerogels with high surface area and ultra-low density have drawn massive attention for applications in catalysis, energy storage/conversion, water purification, and beyond. Single-step pyrolysis of protein precursors for creating hierarchically porous monolithic graphitic aerogels with exceptional electrical, electrochemical, and mechanical properties has emerged as a sustainable and green approach. However, the formation mechanisms underlying the transformation of proteins into these aerogel structures remain to be understood. Herein, we demonstrate a self-foaming mechanism for the green and scalable synthesis of hierarchically porous monolithic sheet and fiber-structured graphitic aerogels using protein as a precursor. Rather than creating a solid phase and then exchanging the sacrificial component for a gas phase, we create a gas phase and then convert the liquid into a solid phase. The controlled heating of the protein precursors induces an intrinsic foaming action via initial softening followed by gas evolution and graphitization, resulting in a hierarchically porous graphitic aerogel composed of an integrated sheet and fiber framework. Our systematic exploration into the processing-structure-property relationships that govern aerogel optimization elucidates the interplay between tunable synthesis variables and resultant aerogel properties and structure, enabling deliberate control over microstructural features.","manuscriptTitle":"Understanding the Morphogenesis of Hierarchically Porous Graphitic Aerogels Produced from Protein Precursors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-06 09:36:55","doi":"10.21203/rs.3.rs-4940554/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f43f3283-65b2-4569-b3cc-3f532a8f8fc9","owner":[],"postedDate":"September 6th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":37170803,"name":"Physical sciences/Materials science/Nanoscale materials/Graphene/Synthesis of graphene"},{"id":37170804,"name":"Physical sciences/Materials science/Nanoscale materials/Synthesis and processing"}],"tags":[],"updatedAt":"2024-10-04T14:36:53+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-06 09:36:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4940554","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4940554","identity":"rs-4940554","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}