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Floatable photocatalytic hydrogel nanocomposites for large-scale solar hydrogen production

Nature Nanotechnology volume 18, pages 754–762 (2023)Cite this article

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Abstract

Storing solar energy in chemical bonds aided by heterogeneous photocatalysis is desirable for sustainable energy conversion. Despite recent progress in designing highly active photocatalysts, inefficient solar energy and mass transfer, the instability of catalysts and reverse reactions impede their practical large-scale applications. Here we tackle these challenges by designing a floatable photocatalytic platform constructed from porous elastomer–hydrogel nanocomposites. The nanocomposites at the air–water interface feature efficient light delivery, facile supply of water and instantaneous gas separation. Consequently, a high hydrogen evolution rate of 163 mmol h–1 m–2 can be achieved using Pt/TiO2 cryoaerogel, even without forced convection. When fabricated in an area of 1 m2 and incorporated with economically feasible single-atom Cu/TiO2 photocatalysts, the nanocomposites produce 79.2 ml of hydrogen per day under natural sunlight. Furthermore, long-term stable hydrogen production in seawater and highly turbid water and photoreforming of polyethylene terephthalate demonstrate the potential of the nanocomposites as a commercially viable photocatalytic system.

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Fig. 1: Overview of the floatable photocatalytic nanocomposites.
Fig. 2: Material design and characterization of the nanocomposites.
Fig. 3: H2 production of the nanocomposites.
Fig. 4: Practical applications of the nanocomposites.
Fig. 5: Scale up of the nanocomposites.

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Source data are provided with this paper. All other data that support this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This research was supported by the Institute for Basic Science under grant numbers IBS-R006-A1 (W.H.L., G.D.C., H.P., S.-H.S. and D.-H.K.) and IBS-R006-D1 (C.W.L., B.-H.L., J.H., M.S.B., J.H.K. and T.H.).

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Author notes

  1. These authors contributed equally: Wang Hee Lee, Chan Woo Lee, Gi Doo Cha.

Authors and Affiliations

  1. Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, Republic of Korea

    Wang Hee Lee, Chan Woo Lee, Gi Doo Cha, Byoung-Hoon Lee, Hyunseo Park, Junhyeok Heo, Megalamane S. Bootharaju, Sung-Hyuk Sunwoo, Jeong Hyun Kim, Dae-Hyeong Kim & Taeghwan Hyeon

  2. School of Chemical and Biological Engineering, and Institute of Chemical Processes, Seoul National University, Seoul, Republic of Korea

    Wang Hee Lee, Chan Woo Lee, Gi Doo Cha, Byoung-Hoon Lee, Jae Hwan Jeong, Hyunseo Park, Junhyeok Heo, Megalamane S. Bootharaju, Sung-Hyuk Sunwoo, Kyung Hyun Ahn, Dae-Hyeong Kim & Taeghwan Hyeon

  3. Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea

    Dae-Hyeong Kim

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Contributions

W.H.L., C.W.L., G.D.C., D.-H.K. and T.H. conceived the ideas, designed the experiments and wrote the paper. W.H.L., C.W.L., G.D.C., B.-H.L., H.P., J.H., M.S.B., S.-H.S., J.H.K., D.-H.K. and T.H. performed the experiments and data analysis. J.H.J. and K.H.A. performed the computer simulations.

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Correspondence to Dae-Hyeong Kim or Taeghwan Hyeon.

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Nature Nanotechnology thanks Mingshan Zhu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Characterization of backbone polymers and bonding process.

a, Chemical structures of hydrophilic polyurethane (HPU; left) and poly(propylene glycol) (PPG; right). Functional groups for hydrogen bonding, which is a driving force in the formation of the elastomer-hydrogel composite, are marked with blue boxes. The hard segment and hydrophilic soft segments of HPU are marked with purple and green boxes, respectively. b, c, The molecular weight distribution (b) and the cumulative percent curve (c) of HPU reveal that the average molecular weight of HPU is ~ 474,000 g/mol. d, A calibration curve of the GPC measurement. e, HPU-PPG elastomer-hydrogel composites were dyed with Rhodamine B (orange) and Evans Blue (dark blue). Two dyed samples were bonded together with ethanol (EtOH). f-i, Firm bonding of the HPU-PPG composites could be made, which was maintained despite various deformations, such as (f) bending, (g) stretching, and (h) twisting. i, Scanning electron microscopy (SEM) image that shows the interface between two HPU-PPG nanocomposites bonded to each other. Seamless integration of a photocatalytic layer and a supporting layer can be observed.

Extended Data Fig. 2 Characterization of photocatalysts.

a, Schematic illustrations of the fabrication process of Pt/TiO2 cryoaerogel including the (i) flash-freezing and (ii) freeze-drying steps. In detail, first, Pt and TiO2 NPs in an as-prepared colloidal solution are assembled into a Pt-TiO2 network through the flash-freezing technique. The ice crystals serve as a template for the final structure of the cryoaerogel. The subsequent freeze-drying step produces the Pt/TiO2 cryoaerogel, which has a porous and voluminous structure composed of interconnected thin sheets. b, Optical image of Pt/TiO2 cryoaerogel. c, Transmission electron microscopy (TEM) image of a Pt nanoparticle (NP) (left), TiO2 NPs (middle), and Pt/TiO2 cryoaerogel (right). d, Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping data (insets) of Pt NP (left), TiO2 NP (middle), and Pt/TiO2 cryoaerogel (right). e, The absorption spectrum of Pt/TiO2 cryoaerogel. f, X-ray photoelectron spectroscopy (XPS) spectrum of Pt 4 f of Pt/TiO2 cryoaerogel. g, Brunaeur-Emmett-Teller (BET) surface area data of Pt/TiO2 cryoaerogel. h, Optical image of Cu-SA/TiO2 NPs. i, TEM image (left) and EDS mapping data (other frames) of Cu-SA/TiO2 NPs. j, k, (j) X-ray absorption near edge structure (XANES) and (k) extended X-ray absorption fine structure (EXAFS) data of Cu-SA/TiO2 NPs.

Extended Data Fig. 3 Material characterization of the elastomer-hydrogel nanocomposites.

a, Contact angle measurement (n = 3 for each group) at various locations of the hydrogel nanocomposite: the top surface (Photo. top) and the bottom surface (Photo. bottom) of the photocatalytic layer and the bottom surface of the supporting layer (Support. bottom). b, Schematic illustration for the measurement of the equilibrium density. The equilibrium density was determined by the density of the acetonitrile solution at which the hydrogel nanocomposites sink. The solution density is controlled by adding acetonitrile to water. c, Optical images of the nanocomposites on which a twofold amount of silica aerogel is added. The equilibrium density of the nanocomposites is 0.885 mg/mL. d, The normalized absorbance spectrum of hydrogel nanocomposites with (red) and without (black) silica aerogel. e, The mass transfer rate of the hydrogel nanocomposite was evaluated by measuring the amount of remaining water after 36 hours of evaporation at 60 °C (bottom). The open system (middle) and closed system (top) correspond to the positive and negative control group, respectively. f, The ratio of remaining water of open (n = 3), closed (n = 4), and bilayer (n = 3) systems after 36 hours of evaporation at 60 °C. g, Optical images of photocatalytic/supporting bilayer (left) and supporting/supporting bilayer (right) that absorb water including dye (Rhodamine B). The dye reaches the top layer regardless of the components of the bilayers, corroborating the facile water transfer through the hydrogel nanocomposites. The insets show the photocatalytic/supporting bilayer (left) and supporting/supporting bilayer (right) before absorbing the dye solution. h, Leaching percentage of Pt/TiO2 cryoaerogel (red, n = 4) and silica aerogel (blue, n = 4) from the HPU-PPG composite under harsh environments such as strong alkaline solutions and seawater with organic solvents. i, Swelling ratio (Q) of the supporting layer (Support., n = 8), photocatalytic layer (Photo., n = 4), bilayer elastomer-hydrogel nanocomposite (Bilayer, n = 4), and polyacrylamide (PAM) hydrogel. (NS; p > 0.05, ***p < 0.001 by independent samples t-test). All data in bar charts are presented as mean values plus standard deviation.

Extended Data Fig. 4 Measurement of hydrogen production of the nanocomposites.

a, Optical images of the nanocomposites in a quartz cell connected to a gas chromatography (GC) line before (left) and while (right) the light (122 mW/cm2) is illuminated on the nanocomposites. Insets show infrared camera images of the top surface of nanocomposites before (left) and after (right) one hour of simulated sunlight irradiation. b, GC peaks during photocatalytic HER on the floatable photocatalytic hydrogel nanocomposites. c, H2 peaks detected by GC during photocatalytic HER on the floatable photocatalytic hydrogel nanocomposites. d-f, Captured images from Supplementary Movie 1 that show HER on the floatable photocatalytic hydrogel nanocomposites. (d) The hydrogel nanocomposites floated on a solution in a quartz cell connected to GC. (e) Light (122 mW/cm2) was illuminated on the top surface of the nanocomposites. (f) In a few minutes, H2 bubbles were generated by the photocatalytic HER on the nanocomposites. g, Optical images of the nanocomposites in which 2 (top; left), 4 (top; right), 8 (bottom; left), and 17 (bottom; right) wt% Pt/TiO2 cryoaerogel is embedded. h, i, (h) Time course and (i) evolution rate of H2 of the nanocomposites (under 100 mW/cm2) in which 2 (blue), 4 (green), and 8 (red) wt% Pt/TiO2 cryoaerogel is embedded. j, Time course of H2 for the floatable (blue circle) and sunken (red circle) nanocomposites (both with 17 wt% Pt/TiO2) under different intensities of simulated sunlight. k, Light intensity decreases as the light travels deep into the water due to scattering and absorption.

Extended Data Fig. 5 Computational simulation of the hydrogen production by the nanocomposites.

a, Schematic illustration of the lab-scale simulation domain. b, Schematic illustration of the sunken nanocomposites at the bottom of 10 cm-deep water (Scale bar: 1 cm). c, The concentration distribution of H2 during photocatalytic HER of the sunken nanocomposites at the bottom of 10 cm-deep water at various time points: 60, 400, 1000, and 1500 min (Scale bar: 1 cm). d, H2 concentration as a function of depth (z) in water at various time points: 60 min (black), 400 min (purple), 1000 min (blue), and 1500 min (green) of the sunken nanocomposites.

Extended Data Fig. 6 Floatable photocatalytic nanocomposites with Pt/g-C3N4 and Cu-SA/TiO2 photocatalysts.

a, Optical image of Pt/g-C3N4 NPs. b, The absorption spectrum of Pt/g-C3N4 nanoparticles. c, Optical image of the HPU-PPG-Pt/g-C3N4 NP elastomer-hydrogel nanocomposites in a quartz cell connected to GC while light (122 mW/cm2) is illuminated on the top surface of the nanocomposites. d, Time course of H2 of the HPU-PPG-Pt/g-C3N4 NP nanocomposites under photoirradiation. e, Optical images of the fabrication process of the HPU-PPG-Pt/g-C3N4 NP elastomer-hydrogel nanocomposites. (i) Pt/g-C3N4 NPs are embedded in the HPU-PPG-NaCl gel. (ii) Photocatalytic layer is formed after drying and swelling. (iii) Photocatalytic layer with a Janus structure is formed by adding silica aerogel. (iv) The bilayer nanocomposites with Pt/g-C3N4 NPs is fabricated by the integration of a photocatalytic layer and a supporting layer with ethanol. f, Optical images showing the photoactivation cycle of the Cu-SA/TiO2 nanocomposite. (i) Resting state: photo-excitation is implemented by irradiation of simulated sunlight on the nanocomposite in the resting state. (ii) Active state: the colour of the photocatalytic layer changes from light yellow to grey by irradiation of simulated sunlight. (iii) Resting state: the colour of the photocatalytic layer returns to light yellow by exposure to O2. g, Time course of H2 of the Cu-SA/TiO2 nanocomposites measured at various time points (for example, day 0, day 7, and day 14) during the long-term HER measurements.

Extended Data Fig. 7 Real seawater and waste reforming demonstrations.

a, Optical image of the nanocomposites after swelling in seawater for 14 days. b, c, Optical images of the photocatalytic HER of (b) the sunken Pt/TiO2 cryoaerogel and (c) the floatable Pt/TiO2 cryoaerogel nanocomposites in orange and blue dye solutions, which are imitating turbid seawater. d, Optical image of the nanocomposites after swelling in 1 M KOH solution for 14 days. e, Optical images of the PET photo-reforming experiment using floatable nanocomposites without (left) and with (right) illumination (122 mW/cm2).

Extended Data Fig. 8 Scalable hydrogen production of the nanocomposites.

a–c, (a) Top view, (b) bottom view, and (c) side view of the nanocomposites with a length of 11.5 cm and a width of 11.5 cm. d, Optical image of the floatable nanocomposites from the top view (left) and under the solar simulator (AM 1.5 G illumination; right) of the device in a quartz cell for scalable solar H2 production. e, Optical images that show the injection of the syringe into the quartz cell (left) and extraction of H2 from the headspace of the quartz cell (right) during the scalable solar H2 production demonstration. f, Time course of H2 during the scalable solar H2 production.

Extended Data Fig. 9 Hydrogen production and computational simulation of the 1 m2-scale nanocomposites.

a, b, (a) Top view and (b) side view of a single unit of the nanocomposites with a length of 25 cm and a width of 25 cm. c, Schematic illustration of the 1 m2-scale simulation domain. d, e, The H2 concentration distribution of the (d) floatable and (e) sunken nanocomposites at various time points (for example, 100, 1000, 5000, 20000, 40000, and 80000 min) during the photocatalytic HER (Scale bar: 0.5 m). f, g, H2 concentration as a function of depth (z) in water at various time points: 100 or 1000 min (black), 5000 min (purple), 20000 min (blue), 40000 min (green), and 80000 min (red) for the (f) floatable and (g) sunken nanocomposites.

Extended Data Fig. 10 Computational simulation for hydrogen production by the nanocomposites with a total area of 100 m2.

a, b, H2 concentration as a function of depth (z) in water at various time points: 1000 min or day 70 (black), 4000 min or day 1400 (purple), day 7000 (blue), day 28000 (green), and day 70000 (red) for the (a) floatable and (b) sunken nanocomposites.

Supplementary information

Supplementary Video 1

HER on the floatable photocatalytic hydrogel nanocomposites.

Source data

Source Data Fig. 2

Material characterization of the nanocomposites.

Source Data Fig. 3

HER performance of the nanocomposites.

Source Data Fig. 4

Performance of the nanocomposites for practical applications.

Source Data Fig. 5

Large-scale performance of the nanocomposites.

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Lee, W.H., Lee, C.W., Cha, G.D. et al. Floatable photocatalytic hydrogel nanocomposites for large-scale solar hydrogen production. Nat. Nanotechnol. 18, 754–762 (2023). https://doi.org/10.1038/s41565-023-01385-4

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