Development of Magnetic Sponges Using Steel Melting on 3D Carbonized Spongin Scaffolds Under Extreme Biomimetics Conditions

Abstract

This study presents a novel approach to fabricating magnetic sponge-like composites by melting various types of steel onto three-dimensional (3D) carbonized spongin scaffolds under extreme biomimetic conditions. Spongin, a renewable marine biopolymer with high thermal stability, was carbonized at 1200 °C to form a turbostratic graphite matrix capable of withstanding the high-temperature steel melting process (1450-1600 °C). The interaction between molten steel vapors and the carbonized scaffolds resulted in the formation of nanostructured iron oxide (primarily hematite) coatings, which impart magnetic properties to the resulting composites. Detailed characterization using SEM-EDX, HRTEM, FT-IR, and XRD confirmed the homogeneous distribution of iron oxides on and within the carbonized fibrous matrix. Electrochemical measurements further demonstrated the electrocatalytic potential of the composite, particularly the sample modified with stainless steel 316L-for the hydrogen evolution reaction (HER), offering promising perspectives for green hydrogen production. This work highlights the potential of extreme biomimetics to create functional, scalable, and sustainable materials for applications in catalysis, environmental remediation, and energy technologies.

Keywords: HER; bioinspired materials; carbonization; composite materials; extreme biomimetics; spongin scaffold; steel melting; water splitting.

Figure 1

(A) The spongin-based skeleton of a H. communis marine sponge. Digital microscopy imagery: (B) dried skeleton treated with 3 M HCl; (C,D) skeleton after carbonization at 1200 °C for 1 h in an oxygen-free environment.

Figure 2

Schematic representation of the methodological approach to creating composites in the form of magnetic sponges.

Figure 3

(A) Photograph of the experimental setup taken prior to the “steel on carbon” melting test shows that the cylindrical steel sample exhibited no visible surface deformation on the carbonized spongin scaffold. (B,C) The metallic phase that remains after steel melting can be observed within the carbonized spongin scaffold. (D,E) SEM images show the microarchitecture of the steel fragment built within the fibrous carbonized scaffold after melting.

Figure 4

Transmission electron microscopic (TEM) image and electron diffraction pattern of nanoparticles obtained from carbonized spongin microfibers after the steel melting process. (A) TEM image of one hematite nanoparticle of ~20 nm in size, (B) electron diffraction pattern identified as hematite.

Figure 5

Digital optical microscope images of carbonized spongin (CS) samples after melting of selected steel samples at 1450 °C: (A) CS + construction steel EN S235JRG2 (AISI 1015); (B) CS + carbon steel C45; (C) CS + no. 172/1 low alloy cast iron; (D) CS + stainless steel 316 L powder.

Figure 6

Digital microscopy image (A) of the carbonized spongin construct after carbon steel C45 melting on its surface shows fragments with a characteristic metallic luster. TEM images (B,C) represent structural peculiarities of crystals of iron oxide origin (EDX data, (D)).

Figure 7

SEM images with EDS or elemental mapping analyses of carbonized spongin (CS) samples under study: (A) control sample CS (carbonized spongin scaffold at 1200 °C); (B) CS + construction steel EN S235JRG2 (AISI 1015) melted at 1450 °C; (C) CS + stainless steel 316 L powder melted at 1450 °C; (D) CS + carbon steel C45 melted at 1450 °C with (E) elemental mapping analysis; (F) CS + no. 172/1 low alloy cast iron melted at 1450 °C with (G) elemental mapping analysis.

Figure 8

SEM images (A,B) with elemental mapping analyses (C) of spongin treated with 3 M HCl and next 40% HF carbonized at 1200 °C after the carbon steel C45 melting process at 1600 °C. The average diameter of iron nanoparticles in image B is 90.40 nm (see also Supplementary Materials, Figures S2–S5).

Figure 9

(A) SEM image and (B) elemental mapping analysis of the phenomenon of microfiber rupture followed by metallization of the inner surface of carbonized at 1200 °C spongin previously treated with 3 M HCl and next 40% HF after carbon steel C45 melting process at 1600 °C (see also Supplementary Materials, Figures S6 and S7).

Figure 10

SEM image (A) with elemental mapping analysis (B) of carbonized spongin at 1200 °C previously treated with 3 M HCl and 40% HF after carbon steel C45 melting at 1600 °C. The iron-oxide-based microspheres are clearly visible as they are covered with a carbon layer (Supplementary Materials, Figure S8).

Figure 11

FT-IR spectra of control sample and samples after steel melting process (all spongin samples after 3 M HCl treatment before carbonization): (A) carbonized spongin (CS); (B) CS + stainless steel 316L, 1450 °C; (C) CS + construction steel EN S235JRG2 (AISI 1015), 1450 °C; (D) CS + carbon steel C45, 1450 °C; (E) CS + no. 172/1 low alloy cast iron, 1450 °C.

Figure 12

XRD patterns for the control sample (carbonized spongin) and samples after the steel melting process (samples after 3 M HCl treatment). (A) carbonized spongin (CS); (B) CS + stainless steel 316L, 1450 °C; (C) CS + no. 172/1 low alloy cast iron, 1450 °C; (D) CS + carbon steel C45, 1450 °C; (E) CS + construction steel EN S235JRG2 (AISI 1015), 1450 °C.

Figure 13

Samples attracted to the neodymium magnet: (A) CS + stainless steel 316 L, 1450 °C; (B) CS—control sample carbonized at 1200 °C.

Figure 14

Normalized hysteresis loops for all investigated materials: reference steel (red) and coated carbon sponge (black): (A) no. 172/1 low alloy cast iron; (B) stainless steel 316 L powder; (C) construction steel EN S235JRG2 (AISI 1015); (D) carbon steel C45.

Figure 15

Results of the electrochemical characterization: (A) LSV curves; (B) the Tafel plots extracted from the corresponding LSV curves.