# Highly open 3D carbon structures enhance solar steam generation

R. Fillet, Vincent Nicolas, Pauline Blyweert, J. Castro-Gutiérrez, Benoit Rousseau, A. Celzard, V. Fierro

► To cite this version:

R. Fillet, Vincent Nicolas, Pauline Blyweert, J. Castro-Gutiérrez, Benoit Rousseau, et al. Highly open 3D carbon structures enhance solar steam generation. Chemical Engineering Journal, 2025, 518, pp.164684. 10.1016/j.cej.2025.164684. hal-05220971

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Submitted on 24 Aug 2025

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# Highly open 3D carbon structures enhance solar steam generation

R. Fillet<sup>a</sup>, V. Nicolas<sup>a,*</sup>, P. Blyweert<sup>a</sup>, J. Castro-Gutiérrez<sup>a</sup>, B. Rousseau<sup>b</sup>, A. Celzard<sup>a,c</sup>, V. Fierro<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

3D carbon

Bio-based materials

Solar evaporation

Heat and mass transfers

Water production

## ABSTRACT

3D bio-based carbon structures, characterized by high specific surface area and open macroporosity, were used to improve solar steam generation. These 3D structures were printed by laser stereolithography using an acrylate-tannin resin. After pyrolysis and eventual physical activation, the resulting 3D carbon structures were used as solar evaporators. Activation with CO₂ and water vapour resulted in 3D carbon structures with high specific surface areas and enhanced oxygen content, which in turn increased water sorption. Solar evaporation performances were as high as 2.45 kg m² h⁻¹ under 1 sun illumination and low convection flow, 0.3 m s⁻¹, and increased to 7.32 kg m² h⁻¹ when using forced convection flow, 4.5 m s⁻¹. These results show that 3D-printed carbons are promising materials for solar evaporation, especially in a complete water distillation system with forced air flow.

## 1. Introduction

Solar evaporation of water to produce steam is a topic of interest, as it aims at tackling the crucial issue of water scarcity, since water steam can be a source of freshwater. It is estimated that one third of the global urban population faces a severe water shortage, and by 2050 this figure is expected to reach half of the global urban population [1]. A wide variety of materials have been tested for solar evaporation, such as biomass materials [2], metallic nanostructures, inorganic semiconductor nanostructures, polymers or carbons [3]. Tests usually involve an open distillation process using a black material to absorb energy from solar rays. Among the various parameters, thermal conversion efficiency has been the most thoroughly investigated, while the effect of other system characteristics has been largely overlooked [4]. For instance, ambient conditions in a closed system differ significantly, as the air becomes saturated with steam. In addition, material scalability is a challenge, as many evaporation materials are tested using small samples, which can artificially increase the evaporation rate [5].

Under standard temperature and pressure conditions, 2D evaporators typically achieve an evaporation rate of around 1.47 kg m² h⁻¹ under 1 sun illumination (equivalent to 1 kW m⁻²) [6], which represents the theoretical limit. To maximise evaporation, insulating foam is used both as a support structure and as thermal insulator, effectively

minimizing heat transfer to the bulk water. To ensure a continuous water supply to the evaporation surface, highly hydrophilic materials are used. At the interface exposed to solar flux, materials with high light absorption properties, such as carbon, metal nanoparticles and specialized coatings, are commonly used. For instance, wood combined with carbon nanotubes achieved 1.63 kg m² h⁻¹ under 1 sun illumination [7]. Similar performance was observed with materials such as graphene aerogel [8], gourd-based material with polypyrrole coating [9], carbonized balsawood [10], coal-based carbon membrane [11], or a porous PDMS-CNPs sponge [12], which achieved evaporation rates of 1.52, 1.62, 1.55, 1.92 and 1.36 kg m² h⁻¹, respectively. These materials are chosen for their ability to efficiently transport water to the surface. After coating or treatment to enhance radiation absorption, their performance is rather similar, approaching the theoretical limit.

To further enhance the evaporation rate, 3D materials have been developed to increase the surface area available for evaporation compared with flat surfaces. Usually, the solar input is distributed over the entire structure, ensuring that the surface temperature of the material remains lower than that of the surrounding air. Therefore, heat exchange between the air and the material provides additional energy for the evaporation process. For instance, Tian et al. (2024) fabricated an array of carbon fibres capable of achieving an evaporation rate of 2.21 kg m² h⁻¹ [13], and Song et al. (2024) produced a double-sided

<footnote><sup>a</sup> Université de Lorraine, CNRS, IIL, F-88000 Epinal, France</footnote>

<footnote><sup>b</sup> Université de Nantes, Laboratoire de Thermique et Energie de Nantes (UMR CNRS 6607), 44306, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), F-75231 Paris, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: vincent.nicolas@univ-lorraine.fr (V. Nicolas), vanessa.fierro@univ-lorraine.fr (V. Fierro).</footnote>
graphene-based multilayer material which, when tilted, can harness solar absorption from both sides, resulting in an impressive evaporation rate of 3.15 kg m² h⁻¹ [14]. Jiang et al. (2024) [15] developed a bio-based 3D material from calcined bamboo leaves, achieving an evaporation rate of 2.29 kg m² h⁻¹. These results show that it is possible to improve evaporation by increasing the evaporation area [13–15]. These materials often exhibit relatively rather simple shapes, such as cubes or cylinders, due to the challenges associated with fabricating complex geometries. However, intricate geometries could further enhance water diffusion to the surface and potentially improve overall evaporation performance.

To address this challenge, 3D printing offers a promising solution due to its ability to produce any desired geometry. With 3D-printed structures, the entire evaporation-condensation system can take on various shapes, and the design can be tailored to specific needs. For instance, airflow within the system can be enhanced by incorporating open geometries that allow dry air to pass through, thereby increasing its humidity and improving the efficiency of the process. Indeed, previous work has demonstrated that open 3D structures have the potential to provide high evaporation performance, even under conditions of low air convection [16]. An evaporation rate of 1.91 kg m² h⁻¹ was achieved under an air flow of 0.3 m s⁻¹, using a 3D-printed skeleton filled with activated carbon. The skeleton was fabricated using stereolithography (SLA), a technique that uses laser to selectively cure a liquid resin, enabling the creation of intricate, precise geometries for enhanced performance. A subsequent numerical study demonstrated that adding perforations to the structure further improved performance under both low and high convective flow conditions [17]. Moreover, 3D printing can be used to create complex surfaces, which a precision typically below 1 mm (depending on the printing technique). For example, a conical seashell-like evaporator was developed with a 3D-printed support and a polypyrrole coating, achieving an evaporation rate of 1.73 kg m² h⁻¹ [18]. Another example of 3D-printed evaporator is a porous cellulose/alginate/carbon black hydrogel, which achieved an evaporation rate of 1.33 kg m² h⁻¹ [19]. Compared with a simple conical shape under the same conditions, the properties provided by the 3D-printed surface allow better diffusion of water through the material.

Additionally, another 3D-printed evaporator inspired by a bird ratchet was created using the same technique, achieving an evaporation rate of 4.02 kg m² h⁻¹ under 1 sun [20]. Compared with the aforementioned conical material, this new design features a high height-to-diameter ratio, which could explain the higher evaporation rate. However, a direct comparison is challenging, as errors are easily made when measuring evaporation rates [5]. Overall, these two 3D-printed materials were fabricated using the same printing method, Fused Deposition Modelling (FDM), which consists of depositing molten filament layer by layer to build the structure. Another printing method using liquid resin was employed to create a Janus evaporator [21]. Here, 3D printing allowed to build a substrate containing an aerogel with a hydrophilic bottom surface and a hydrophobic top surface. Another printing technology, known as Digital Light Processing (DLP), which uses a photosensitive liquid resin, was also used to create a Janus evaporator. The latter achieved an average evaporation rate of 2.8 kg m² h⁻¹ when evaporating seawater for 10 cycles. Although all these studies incorporate 3D printing into their processes, the printed material primarily serves as a support structure.

In this work, a novel approach of 3D carbon printing approach was employed to fabricate highly open carbon structures that facilitate efficient water diffusion. The solar evaporation performance of three 3D-printed structures was investigated. Activations with CO₂ and water vapour were used to enhance the properties of the structures, and characterization was performed to select the activation parameters that give the material the best performance when used in a solar evaporation system under forced convection.

## 2. Materials and methods

### 2.1. Material preparation

The tannin-acrylate resin for 3D printing was prepared according to a process described elsewhere [22]. Hexanediol diacrylate (HDDA, SR238), pentaerythritol tetra-acrylate (PETA, SR295) and acrylate oligomer (CN154 CG), the materials for the acrylate monomer, were kindly supplied by Sartomer (Arkema Group, France). The photo-initiator Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) was supplied by Lambson (Arkema Group, England). Mimosa (Acacia Mearnsii) tannin extract, commercially available as Fintan OP, was kindly provided by SilvaChimica (St. Michele Mondovi, Italy). Acrylate monomer (HDDA 14.9 wt%, PETA 29.9 wt% and CN154 GG 29.9 wt%) and photoinitiator (BAPO 0.3 % wt%) were mixed at 50 °C using a BRANSON ultrasonic tip for 25 min. Then, mimosa tannin (25 wt%) was added and mixed by hand.

Each 3D array was first drawn on COMSOL Multiphysics® and designed using CAD software NAUTA (DWS, Italy), and then printed using a DWS J28 high-resolution 3D printer (desktop SLA) equipped with a class 1 laser (λ = 405 nm, 32 mW) of spot size 17 µm. The printed arrays were cleaned using isopropanol in a VWR ultrasonic bath for 15 min to remove uncured resin. Next, the arrays were post-cured in an UV oven (DWS, λ = 405 nm) for 20 min at room temperature.

To obtain 3D carbon structures suitable for solar evaporation, the printed and cleaned arrays were pyrolyzed at 900 °C for 1 h under a flow (100 mL min⁻¹) of nitrogen in a tubular furnace with a heating rate of 1 °C min⁻¹. The cooling rate was 2.5 °C min⁻¹ and the process can be seen in Fig. 1a. Three types of cylindrical geometries, shown in Fig. 1b were designed: one with a straight square grid, named Square (Sq); one with a radial grid with a tube in the centre, named Radial (Ra); and another radial geometry named Radial-Rotated (RR), where each level is rotated to ensure that the entire projected area effectively receives sunlight.

CO₂ activation was performed in a tubular oven with a CO₂ flow of 50 mL min⁻¹ at 900 °C for 30, 120, 180 and 240 min. The heating rate up to 900 °C was set to 3 °C min⁻¹ under a N₂ flow of 50 mL min⁻¹. Once the target temperature was reached, the gas flow was switched to CO₂ for a given time. After the activation period, the gas flow was returned to N₂ at 50 mL min⁻¹, with a cooling rate of 2 °C min⁻¹. The square grid was chosen to test different activation times, and the samples were named Sq-X-C, where X stands for the activation time in minutes and C for CO₂. As a reference, Sq-0 represents the pyrolyzed structure without activation, and Sq-30-C stands for the pyrolyzed structure activated for 30 min with CO₂. Activation with water vapour at 900 °C was also investigated, using the same tubular oven and the same heating and cooling rates, both under a N₂ flow of 200 mL min⁻¹. The water vapour flow was obtained by saturating a 200 mL min⁻¹ nitrogen flow by passing it through a water-filled hot balloon (80 °C). Steam activation was thus carried out for 120, 180, 240 and 300 min on 3D-printed carbon structures, which were labelled Sq-X-V, where X stands for the activation time in minutes and V for water vapour. After pyrolysis and activation, the structures were black, as shown in Fig. 1c, and exhibited a volume shrinkage of approximately 25 %.

### 2.2. Gas adsorption measurements

The textural properties and water affinity of the carbon materials were investigated by: (i) acquiring nitrogen (N₂) and hydrogen (H₂) adsorption-desorption isotherms at -196 °C, using an ASAP 2020 (Micromeritics) automated adsorption apparatus; and (ii) water vapour (H₂O) adsorption isotherms at 20 °C, using a Belsorp Max II (Microtrac), respectively. Prior to analysis, the materials were heated to 110 °C while outgassing under secondary vacuum for 24 h. BET area (m² g⁻¹) was calculated using N₂ adsorption isotherm at -196 °C and following Rouquerol's criteria, in accordance with IUPAC recommendations [23].
<figure><img src="image_4.png" /><figcaption>Fig. 1. a) Schematic of the material preparation process; b) 3D structures modelled using COMSOL Multiphysics®, including Square, Radial and Radial-rotated geometries; c) Activated carbon structures after 3D printing and activation with CO₂, including Square (Sq-120-C), Radial (Ra-120-C) and Radial-Rotated (RR-120-C) geometries.</figcaption></figure>

On the other hand, 2D non-local density functional theory for heterogeneous surface (2D-NLDFT-HS) was applied simultaneously to the N₂ and H₂ adsorption isotherms at -196 °C. This analysis was used to determine pore size distributions (PSDs) from which surface area (S<sub>NLDFT</sub>) and pore volumes across different diameter ranges were calculated [24].

## 2.3. Water uptake at saturation

The water uptake at saturation was obtained by first submerging the samples in water overnight under low vacuum. The saturated weight was then measured using a balance accurate to the nearest milligram. Subsequently, the samples were dried in an oven at 103 °C and weighted again to obtain their dry weight.

## 2.4. Spectral reflectance measurements

Normal hemispherical reflectance spectra were measured at room temperature on a Fourier Transform Visible/Infrared Bruker 80v spectrometer into which two A-652 integrating spheres were successively plugged. This combination covers a spectral range going from 500 to 17,500 cm⁻¹, i.e. from 0.57 to 20 µm. The first sphere contains a gold-based inner coating and its detection port is connected to a LN₂-cooled HgCdTe sensor. The second sphere has a Teflon coating and is connected to a silicon diode detector. Measurements were performed with an instrumental resolution of 4 cm⁻¹. Since the 3D-printed carbon structures are optically thick (i.e. their normal hemispherical transmittance spectra are very close to 0), it can be proved that, at thermal equilibrium and by virtue of Kirchhoff's law, A<sub>NN</sub>(σ, 20) = 1 - R<sub>NH</sub>(σ, 20) where A<sub>NN</sub>(σ, 20) is the normal spectral absorptance at 20 °C, and R<sub>NH</sub>(σ, 20) is the normal hemispherical spectral reflectance.

## 2.5. X-ray photoelectron spectroscopy (XPS)

XPS was carried out using an ESCAPlus OMICRON spectrometer equipped with a non-monochromatic Mg Kα X-ray source. Spectra were

processed by first applying a Shirley background, and element quantification was achieved using CasaXPS® software. Deconvolution of C1s, O1s and N1s peaks was performed by fitting Gaussian Lorentz curves using the least-squares method.

## 2.6. Elemental analysis

An Elementar Vario EL Cube device was used to obtain the elemental composition (C, H, S, N and O) of the 3D structures. The material was first ground to a fine powder, then dried to remove any trace of water. A precise quantity was burnt in an oven at over 1200 °C, leading to the production of gases (CO₂, H₂O, SO₂ and N₂). These gases were then trapped in an adsorption column for analysis with a thermal conductivity detector, which gives information on C, H, and N contents. An infrared sensor was used to detect S, and O was measured using a different combustion column.

## 2.7. Mechanical properties measurement

The opposite faces of samples were inserted between two poly (methyl methacrylate) (PPMA) plates, which were used to evenly distribute the force applied to the material, and glued using epoxy resin. Compression tests were conducted using an Instron 5944 universal testing machine, equipped with a 2 kN load cell. The compression speed was set to 2 mm min⁻¹ and maintained throughout the testing process.

## 2.8. Evaporation measurements

The experimental setup is shown in Fig. 2a. The experiment involves illuminating the material, placed on a beaker (internal diameter 58 mm), using a solar light simulator (SOLAR LIGHT LS-1000). To minimize convection and radiative heat loss to the surroundings, a layer of insulation was wrapped around the beaker, which was then surrounded by aluminium foil. Distilled water was used to measure the evaporation rate. Polystyrene foam, wrapped in cotton, was employed to supply water to the top of the beaker, where the material under study was
<figure><img src="image_5.png" /><figcaption>Fig. 2. a) Schematic of the experimental setup and b) photo of the RR-120-C 3D-printed carbon during an evaporation experiment.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. a) Photos of 3D-printed resin, pyrolyzed sample and samples activated with CO₂ and H₂O; b) N₂ and c) H₂ adsorption isotherms for the different CO₂-activated structures; d) specific surface area (S<sub>NLDFT</sub>); e) N₂ and f) H₂ adsorption isotherms for the different H₂O-activated structures; and g) solar spectrum absorption for a non-activated sample and CO₂- and H₂O-activated samples for 120 min on a flat surface (solid line) and a grid surface (dashed line).</figcaption></figure>
placed. The evaporation rate was determined by measuring the mass change using a Sartorius ENTRIS weighing scale, accurate to the nearest milligram. T-type thermocouples were used to measure air temperature at various points. To minimize unwanted evaporation from the side of the beaker (i.e. not due to the material studied), a paraffin film was applied around the polystyrene foam and the upper side of the beaker. Additionally, a limited aperture was used to control the solar flux, to avoid any additional diffusive solar input to the material. These key features are shown in the schematic and picture in Fig. 2.

All these measurements were conducted under a simulated solar input of 1 kW m⁻¹, with ambient temperatures ranging from 15 °C to 20 °C, and relative humidity of around 50 %. These ambient conditions are not ideal for solar evaporation, but in previous studies, a regression method was used to compare results at 25 °C [5]. However, this method is not applicable when air velocity varies, as is the case in the present study. Therefore, the evaporation performance presented here may be slightly lower than that obtained in a confined solar evaporation system.

# 3. Results and discussion

## 3.1. Characterization

After printing, the 3D structures are made of a non-porous resin that cannot transport water (see Fig. 3a). The pyrolysis process transforms these structures into predominantly microporous carbon materials, with a carbon yield close to 20 wt% [22]. The porosity of the material, both in terms of macro- and microporosity, was determined using the 2D-NLDFT-HS method applied simultaneously to N₂ and H₂ gas adsorption data. The resulting cumulative pore volume as a function of pore diameter is shown in Fig. S2. As pyrolysis alone produces insufficient porosity, an additional gas activation treatment was employed to further enhance pore texture. This activation process introduces microporosity and can also generate mesoporosity with longer activation durations. Therefore, to further enhance the porosity of these carbon structures, additional treatments such as activation with CO₂ or H₂O can be applied.

This section aims to investigate the impact of these activation processes on the properties of the carbon materials, focusing on various treatment durations. While SEM and TEM are valuable for visualizing surface morphology and micro/nanostructure, these techniques have limitations when it comes to quantitatively characterizing the pore texture, especially in terms of pore size distribution, pore volume, and connectivity across micro- and mesoporous ranges. Given the complex 3D porous architecture of the pyrolyzed and activated carbon structures, more comprehensive and quantitative analysis is required, best achieved through gas adsorption techniques. Accordingly, H₂O, N₂, and H₂ adsorption isotherms were measured. The results for CO₂ activation are presented in Fig. 3b and Fig. 3c, respectively, and those for H₂O activation in Fig. 3e and Fig. 3f, respectively. Each series of activated samples shows an increase in gas uptake with longer activation times. However, the sample Sq-300-V showed a lower quantity of adsorbed gas compared to those activated for shorter durations, such as Sq-240-V and Sq-180-V, suggesting that the activation process is excessive and counterproductive at this longer duration.

Activation is a partial gasification process that alters the materials surface, introducing oxygen functional groups and causing weight loss, known as burn-off. Both CO₂ and H₂O activation resulted in darker-looking structures, whose characterization and discussion are detailed below. Six cubic samples are represented in Fig. 3a. The first on the left is the printed resin, which is brownish due to tannin, followed by five treated samples. In the centre is the pyrolyzed one. As expected, the H₂O-activated samples show increasing burn-off with activation time (7 wt% burn-off for 60 min, and 23 wt% burn-off for 120 min). Similarly, the CO₂-activated samples show burn-offs of 22 wt% and 29 wt% for 60 min and 120 min activation times, respectively. The sample activated with CO₂ for 120 min is notably darker than the others. Additionally, H₂O activation, as conducted in this study, leads to lower burn-off than

CO₂ activation for the same activation time.

Another important criterion for evaluating activating carbons is their specific surface area. Fig. 3d shows the NLDFT surface area (S_NLDFT) calculated from N₂ and H₂ isotherms, and Fig. S1 shows the BET area (A_BET). Both figures show that with longer activation time, the specific surface area increases. The surface area generally increases with burn-off, although this increase slows down at higher levels of burn-off. Moreover, H₂O activation results in a slightly smaller increase in surface area than CO₂ activation. Among the two types, S_NLDFT provides a more accurate measure of the specific surface area, especially for microporous materials, where A_BET can sometimes be either underestimated or overestimated.

Additionally, it is important to note that, after reaching a certain level of burn-off, the surface area starts to decrease due to pore widening and merging. The highest S_NLDFT achieved was 1037 m² g⁻¹ by CO₂ activation after 180 min and 23 wt% burn-off. This is attributed to the greater development of microporosity by the CO₂ activation process compared to H₂O activation, as can be seen in Figs. S2a-c. Indeed, for H₂O activation, the surface area only reaches a maximum S_NLDFT of 804 m² g⁻¹ after 240 min and 15 wt% burn-off. In conclusion, both activation methods are capable of increasing the specific surface area of the structure and therefore the amount of water that can be sorbed on its surface. For reference, the pyrolyzed structure exhibits a relatively low S_NLDFT of 11 m² g⁻¹. Increasing the extent of the surface is advantageous, as it provides more area for water sorption, thus enhancing the material's overall capacity for solar evaporation.

Fig. 3g shows the visible near-infrared spectra of a non-activated carbon structure, as well as samples activated for 120 min with CO₂ and H₂O. The spectra reveal that all samples exhibit strong absorption across the entire spectral band, typical of disordered carbon structures [25]. The non-activated and H₂O-activated samples absorbed an average of 77 % and 78 % at wavelengths ranging from 570 to 2500 nm, respectively, while the CO₂-activated sample absorbed at 80 %, on average. All samples exhibited increasing absorption capacities with wavelength, indicating that absorption is lowest in the visible part of the spectrum, where solar intensity is the highest (as shown on the right-hand axis in Fig. 3g). This suggests that the materials have better absorption in the infrared region, which is beneficial for capturing and converting solar energy.

Additionally, the dashed lines in Fig. 3g represent samples with a single row of grid structures, similar to the Sq-120-C sample in Fig. 1b. The flat and grid sides of the same sample are shown in Fig. S3. Light absorption on both sides was measured under the same experimental conditions as those described in the corresponding section. Absorption inside these grid structures was higher than that of samples with flat surfaces. This improvement is due to the ability of the grid structure to trap light inside its geometry, which suggests that samples like Sq-120-C may have superior solar flux harnessing capabilities, potentially improving solar evaporation performance. Since the walls of the carbon-based grid are opaque and highly absorbing, a few scattering events are enough to significantly increase the effective normal spectral absorption. The non-activated grid structure absorbed 16 % more than the flat structure, reaching an average of 89 % absorption at wavelengths ranging from 570 to 2500 nm. Both H₂O and CO₂ activation further increased the average absorption to 90 % and 92 %, respectively. Moreover, the absorption curves for the grid structures are flatter, indicating an increase in light absorption over a broader range of the spectrum, including lower wavelengths. This suggests that grid structures are more efficient at capturing a wider spectrum of solar radiation. Overall, while the activated structures did not reach the absorptance levels of black bodies, they demonstrated excellent solar light-harvesting capabilities. CO₂ activation contributes to a slight increase in absorption (about 5 %), particularly when applied to the grid structures, improving the efficiency of solar energy capture for applications such as solar evaporation.

Fig. 4a and Fig. 4b illustrate the beginning of the H₂O adsorption
<figure><img src="image_7.png" /><figcaption>Fig. 4. a) H₂O adsorption isotherms for the different CO₂-activated structures; b) H₂O adsorption isotherms for the different H₂O-activated structures; c) water affinity; d) maximum liquid water uptake; e) oxygen content of the different activated structures as a function of burn-off; and f) maximum stress before rupture. The labels next to the points represent the activation time in minutes.</figcaption></figure>

isotherm, between 0 and 0.4 P/P₀, while Fig. 4c shows the water affinity of the samples, represented by the slope of the water vapour isotherm between 0 and 0.2 P/P₀. This slope gives a good indication of whether a material is hydrophobic or hydrophilic [26]. Activation of the structures increases water affinity, as all activated samples show a higher affinity than the non-activated Sq-0. However, beyond a certain point, excessive activation causes a reduction in water affinity. This drop occurs at 15 % burn-off for H₂O activation and 24 % burn-off for CO₂ activation. This phenomenon can be attributed to pore widening, leading to a reduction in surface area, and which has a negative impact on water affinity. In general, the shape of the isotherms, and consequently the affinity for water, are similar for both activation methods, indicating that both CO₂ and H₂O activation can improve the material's ability to attract and retain water, but that this effect diminishes if activation exceeds an optimum level.

The additional porosity, whose characteristics are detailed in Figs. S2a-c, significantly improves the structure's ability to retain water. Indeed, Fig. 4d shows that the uptake of liquid water at saturation increases as activation time and burn-off increase. The pyrolyzed structure, without activation, can retain approximately 80 % of its weight in water. In contrast, activated structures show a linear increase in water uptake as burn-off progresses, reaching up to 142 % and 133 % of their weight for H₂O- and CO₂-activated samples, respectively. However, it is noteworthy that the structure with the highest burn-off after CO₂ activation is not the best in terms of water uptake. This can be attributed to the excessive widening of the PSD, which decrease the water retention capacity. This suggests that the development of porosity through activation needs to be carefully controlled to optimize the material's ability to retain water.

A key criterion associated with water affinity is the oxygen content at the surface, as this influences the hydrophilic or hydrophobic properties of the material [27]. Fig. 4e shows the surface oxygen content measured by XPS for the different structures, with additional details provided in

Table S1. For samples subjected to CO₂ activation, the oxygen content increased with the burn-off, indicating a higher level of surface oxidation. In contrast, the oxygen content of H₂O-activated samples remained relatively higher, even at low burn-off levels, suggesting that water activation introduces more surface oxygen groups than CO₂ activation. This trend was confirmed by elemental analysis, which measures the bulk composition of materials, as shown in Table S2.

Elemental analysis indicates a slight increase in oxygen content as activation time increases. It is important to note that the sample labelled Sq-120-V is not shown in Fig. 4e as its oxygen content was exceptionally high. This anomaly may be attributed to the size of the sample, which could have influenced the measurement results. Indeed, XPS is performed on a small fraction of the structure, which may not be fully representative of the material as a whole, whereas the other analyses are conducted using several pieces from the same structure and may provide a more comprehensive result. Ultimately, increasing the activation time can increase the amount of oxygen on the surface through the addition of functional groups, which play a major role in triggering water vapour adsorption [28].

However, one of the main drawbacks of longer activation times is the reduced strength of the structure. As shown in Fig. 4f, the maximum compressive stress the structure can withstand decreases as the activation time increases for CO₂ activation. In contrast, for H₂O activation, longer activation times do not appear to have a significant effect on the maximum stress, as it is already lower than that observed for CO₂ activation. This suggests that H₂O activation considerably weakens the structure, even at low burn-off levels, which could limit the material's overall durability and strength for certain applications.

Activation of the 3D carbon structures results in an increase in open porosity, primarily by improving microporosity (see again Fig. S2c), which leads to an increase in the specific surface area of the material, whether expressed as A<sub>BET</sub> or S<sub>NLDFF</sub>. Additionally, water affinity is also increased by activation, although longer activation times do not seem to
bring any further improvement in this respect. However, activation also weakens the structure, with a more pronounce effect for steam activation. Given this trade-off between performance and structural integrity, a CO₂ activation time of 120 min was chosen for subsequent water uptake and affinity measurements, as it provides a balanced improvement in porosity and surface properties without excessively compromising the structural strength of the material.

## 3.2. Water evaporation results

In this section, the performance of the three types of activated, 3D-printed structures shown in Fig. 1c, is investigated. Each structure features a distinct geometry and was subjected to pyrolysis and CO₂ activation for 2 h. The Sq structure had a burn-off of 17.5 wt%, while the Ra structure and the RR structure had higher burn-offs of 28.3 wt% 24.1 wt%, respectively. The higher burn-offs of the latter two structures may be attributed to their geometries, which potentially create different flow paths within the tubular furnace. Despite these differences, the geometries had very similar areas of 46,890, 46,822 and 46,859 mm² for Sq, Ra and RR respectively.

Fig. 5a shows the mass change of the structures as well as that of a flat 2D material composed of filter paper with activated carbon (AC) on top, under low convection flow (0.3 m s⁻¹). Among the structures, the Sq structure showed the highest evaporation rate at 2.45 kg m² h⁻¹,

followed by the Ra structure at 2.01 kg m² h⁻¹, and the RR structure at 1.82 kg m² h⁻¹. This variation can be attributed to the Sq's more open longitudinal axis, which allows water-saturated air to escape more easily, unlike the centripetal design of radial structures. In comparison, the non-activated, non-pyrolyzed Sq structure had a lower evaporation rate of 0.31 kg m² h⁻¹, while the non-activated, pyrolyzed Sq structure (Sq-0) evaporated at 0.95 kg m² h⁻¹.

The low evaporation rate of the non-pyrolyzed Sq structure can be explained by the fact that the non-pyrolyzed resin structure is neither black nor porous. Activation of the Sq structure resulted in a 172 % improvement in evaporation rate, highlighting the critical role of enhancing the porosity in 3D-printed structures to increase water transport. For further comparison, the performance of AC on a cellulose filter (2D AC), used in previous studies, is also presented along with the evaporation rate of bulk water [5]. The 2D AC material showed an evaporation rate of 1.02 kg m² h⁻¹, while bulk water evaporated at a much lower rate of 0.24 kg m² h⁻¹. It is worth noting that the 2D AC material showed poorer evaporation performance than in a previous study, which can be attributed to the lower ambient air temperature at the time of measurement, as explained in section 2.8. Still, the activated structures improved evaporation by at least 500 % compared with the material-free case.

The grid structures are very open, with material accounting for only 13.3 % and 12.4 % of the total volume for Sq and radial structures,

<figure><img src="image_8.png" /><figcaption>Fig. 5. a) Mass change of different cases under low convection (without fan, 0.3 m s⁻¹); b) two evaporation cycles with 3 h illumination and 3 h darkness, for cases without forced convection (without fan, 0.3 m s⁻¹ air flow); c) evaporation rates of various 3D-printed materials in the literature [16,21,29–37]; and d-g) thermal imaging at different times during the evaporation experiment for different structures: d) non-activated Sq-0, e) activated Sq-120-C, f) activated Ra-120-C and g) activated RR-120-C.</figcaption></figure>
respectively. In other words, the structures are approximately 85 % empty, allowing air to pass freely throughout. This openness also means that water has a limited area to diffuse upwards through the material, making water diffusion a critical performance factor. To evaluate the sustainability of water evaporation after repeated cycles of evaporation, a test was conducted simulating daytime (1 sun illumination with fan) and night-time (no light without fan) conditions. Fig. 5b shows the evaporation rate over two cycles, each considering 3-h simulated day and night. The “day” phase was simulated by switching on a 1000 W m⁻² light source, while the “night” phase involved switching off the light. These tests were conducted under laboratory conditions, with ambient temperatures ranging from 15 °C to 20 °C, as detailed in Section 2.8. The evaporation rate was initially high, decreased during the simulated night period after 3 h, and increased again after 6 h, demonstrating that the structures can recover their performance after a cycle under low convection conditions.

Although the difference in evaporation rates between day and night is moderate, the trend is consistent. Among the samples, Sq-120-C exhibited the highest evaporation rate, followed by Ra-120-C and then RR-120-C, which can be attributed to differences in geometry. Both radial geometries include an inner cylinder designed to enhance structural robustness; however, its inner surface accounts for 8 % of the total surface area, potentially increasing evaporation performance. Since this inner cylinder is only open at its top, the air inside quickly becomes saturated, reducing evaporation efficiency from the surface. Moreover, activation significantly improves performance, as shown by the approximately twofold increase in evaporation rate between Sq-120-C and Sq-0, highlighting the beneficial impact of activation on evaporation performance. The literature on 3D-printed structures generally reports evaporation rates ranging from 1 to 4 kg m⁻² h⁻¹, as shown in Fig. 5c. With an average evaporation rate of 2.37 kg m⁻² h⁻¹, the Sq 3D-printed carbon structure shows comparable performance to other materials documented in the literature. However, as discussed in reference [5], evaporation rate alone should not be the only criterion for comparing materials, structural designs and overall performance of solar evaporators. In some cases, the high evaporation rates reported may result from experimental conditions that inadvertently enhance the measured performance.

To better explain the differences seen between the grid configurations, Fig. 5d to Fig. 5g show thermal images of each activated structure and the non-activated Sq structure at various stages of evaporation. The images capture the structures when they are initially wet after being immersed in water, then after 40 min of evaporation, and finally after 8 h, representing the end of a full day of solar evaporation. The thermal images reveal that the non-activated structure dries over time, as evidenced by the warm surface visible after 40 min. This suggests minimal ongoing evaporation and highlights the critical role of the activation

process in enhancing textural properties. In contrast, the activated Sq structure remained mostly cool throughout the experiment, with only a small portion of the upper edges drying due to localized heating from the solar input. Even at the end of the experiment, the structure maintained a cool surface, indicating ongoing evaporation. This pattern is only partially observed in the Ra and RR structures, which show approximately half of their lengths dried at the end of the experiment. This could be attributed to their higher burn-off levels, which may slightly reduce their affinity for water, as discussed in section 3.1. These observations align with the evaporation rate results, confirming that the Sq structure performs best under low convection conditions.

Like Fig. 5b, Fig. 6a presents the evaporation rate over two cycles, each consisting of 3 h of illumination and forced air convection (4.5 m s⁻¹) followed by 3 h of simulated night conditions in a laboratory environment. In this case, the difference in evaporation rate between day and night is more pronounced. However, the structures are still able to recover their initial performance after the first cycle. The high evaporation rate observed at the start of the first cycle is attributed to the surface water film formed when the structure is initially immersed in water before being placed on the cotton. This surface water film is not renewed during the second cycle, as the material remains inside the system. Additionally, the differences between the 3D-printed structures and the 2D flat case widened and, in particular, the differences in performance between the various 3D geometries were also more pronounced. In this case, the RR structure achieved the highest evaporation rate. Furthermore, it should be noted that the evaporation rate of the 3D-printed structures fluctuated and was less linear than that of the 2D material. This variability is attributed to changes in air temperature caused by localized heating in the room, emphasizing the significant influence of air temperature on evaporation performance. Moreover, the curves in Fig. 6a exhibit more noise than those in Fig. 5b, which is due to the fan blowing on the balance. However, this does not affect the accuracy of evaporation rate measurements.

Increased air convection significantly improved the evaporation performance of the structures. Fig. 6b shows the evaporation rates of all of them, along with the 2D material and bulk water without material. Under high-velocity airflow, the RR structure achieved the highest evaporation rate at 7.32 kg m² h⁻¹, followed by the Ra structure at 5.72 kg m² h⁻¹, and Sq at 4.79 kg m² h⁻¹. The superior performance of radial geometries under conditions of high convection can be attributed to their design, which creates more obstacles to airflow. This forced airflow interacts extensively with the internal surfaces, improving thermal and mass transfer throughout the 3D structure. In contrast, the Sq-120-C square grid structure features larger gaps that allow air to pass through more easily, limiting the improvement of mass transfer and resulting in lower evaporation rates. Compared with the 2D material, which evaporates at 1.85 kg m² h⁻¹, the Sq, Ra, and RR structures

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** a) Evaporation rates for 2 cycles, each comprising 3 h of illumination and 3 h of darkness, under forced convection (with fan, 4.5 m s⁻¹); b) evaporation rates for the different cases under 1 sun illumination with and without fan, as well as without illumination or fan; c) evaporation rates as a function of air velocity for the different cases.</figcaption></figure>
increase evaporation rates by 159 %, 209 %, and 296 %, respectively. Moreover, the structures maintained efficient evaporation under night-time conditions, characterized by low convection (0.3 m s⁻¹) without solar input. In this configuration, the Sq structure achieved an evaporation rate of 1.07 kg m² h⁻¹, followed by the Ra and RR structures at 0.99 kg m² h⁻¹ and 0.71 kg m² h⁻¹, respectively. These results underscore the ability of open 3D structures to enhance evaporation by harnessing both solar and ambient air energy, making them efficiency passive materials for water evaporation applications.

Finally, Fig. 6c shows the evaporation rates for the different cases under various air velocities, ranging from 0.3 m s⁻¹ (without a fan) to 4.5 m s⁻¹ (with the fan set to maximum). In addition to these two cases, two intermediate airflow velocities, 2.25 m s⁻¹ and 3.38 m s⁻¹, are also included. For both the 2D material and water, the evaporation rate increases linearly with air flow velocity, with the two 2D cases showing similar trends, their curves remaining largely parallel. In contrast, while the increase in evaporation rate with air velocity remains linear for the activated structures, the effect is much more pronounced for the RR structure than for the other two 3D structures. This suggests that the RR geometry benefits more from higher air flow velocities than the Sq and Ra structures.

It is important to note that the structures have similar total evaporation areas, while the total evaporation area of the 2D material is 1590 mm. Typically, the total evaporation area is a key factor when comparing similar 3D materials [16]; however, this is not the case under high air flow conditions, as the areas are comparable in the 3D structures. The higher evaporation rate of the RR structure can be attributed to its geometry, where the entire structure collects radiation energy, unlike the Sq and Ra structures, where a significant part of the energy is absorbed by the bottom part. In the RR design, the solar energy input is distributed over the multiple blades, instead of passing through and down to the bottom, where the structure rests. Overall, the RR activated carbon structure shows great potential for solar evaporation under high convection flow. These results align with those of similar systems; for instance, Cheng et al. achieved an evaporation rate of 3.91 kg m² h⁻¹ in a closed system with an air flow rate of 40 mL min⁻¹ [38].

## 4. Conclusion

This study presents significant advances in solar evaporation using activated, 3D-printed carbon structures. By optimizing material activation and geometry, the research highlights the potential of these structures for efficient water evaporation. A 120-min CO₂ activation increased microporosity and specific surface area to 943 m² g⁻¹, improving water affinity and storage capacity while balancing structural integrity. A radially rotated grid geometry proved to be a key innovation, designed to evenly distribute solar radiation across its branches. It demonstrated superior performance under strong convection, achieving a peak evaporation rate of 7.32 kg m² h⁻¹ at an airflow of 4.5 m s⁻¹ and maintaining its performance across day-night cycles. In contrast, a square grid excelled at low convection, with an evaporation rate of 2.45 kg m² h⁻¹. The study underscores the potential of forced convection to overcome the limitations of closed systems, enabling independent design of evaporation and condensation processes. These findings pave the way for advanced and scalable solar desalination systems. Future work should focus on enhancing condensation efficiency to fully capitalize on this approach, and on refining geometric designs to mitigate salt-ion crystallization during evaporation.

### CRediT authorship contribution statement

R. Fillet: Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. V. Nicolas: Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Data curation, Conceptualization. P. Blyweert: Investigation. J. Castro-Gutiérrez: Writing – review & editing, Investigation. B. Rousseau:

Writing – review & editing, Resources, Investigation, Formal analysis. A. Celzard: Writing – review & editing, Formal analysis. V. Fierro: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Formal analysis.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by the French Ministry of the Armed Forces - Defence Innovation Agency. We thank Sartomer France and Elkem Silicone France for providing us with materials for the 3D printing resin and the resin tanks.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcej.2025.164684.

### Data availability

Data will be made available on request.

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