Brown algae-functionalized clays as a novel biosupports for lead-free bismuth halide perovskites in the CO 2 reduction

One of the main environmental problems is air pollution due to high CO 2 emissions, a greenhouse gas that contributes to climate change because of the excessive use of fossil fuels. For this reason, CO 2 reduction emerges as a promising solution by converting it into renewable fuels using sunlight and advanced semiconductor materials. Recently, hybrid systems based on artificial leaves composed of lead-free halide perovskites and porous support materials have been demonstrated to be highly efficient for CO 2 reduction. In addition, the recycling and utilization of natural sources such as the brown algae, considered a plague in the Caribbean, represents an additional advantage for the pollution reduction, carbon sequestration, and social and economic impacts. This research proposes an innovative solution to address this environmental problem by demonstrating that hybrid systems based on bismuth halide perovskites (K 3 Bi 2 I 9) and brown algae-functionalized clay biosupports are promising for the reduction of CO 2 with high efficiencies for formic acid production (2.5 mmol h -1) under visible light. The content of the brown algae was investigated to find the best load that promotes higher and stable CO 2 reduction efficiencies. The presence of the brown algae enhanced light absorption by its chlorophyll, provided free electrons to the semiconductor and highly reactive species (•OH), that favored the formation of C1-C3 products, e.g., HCOOH, CH 3 COOH, and CH 3 (CH 2) 2 OH, with efficiencies in the order of >1 mmol. In addition, the stability of the hybrid systems was demonstrated after five hours of continuous visible light irradiation in liquid phase, which analysis of the medium showed a minimal leaching of potassium. The addition of 5 wt.% brown algae in the clays promoted both high efficiency and stability of the hybrid system by preventing cracking, while promoting a porous framework that maintained effective CO 2 adsorption. This enhanced effect was attributed to efficient perovskite encapsulation and the presence of chlorophyll (from algae) acting as an electron donor, enhancing light absorption and charge transfer. This synergistic effect enabled efficient CO 2 conversions to C 1 –C 3 value-added products. In conclusion, this work demonstrated that the utilization of abundant natural materials such as clays and sargassum supports an ecological and scalable approach while addressing global and local environmental problems.

The CO 2 reduction by artificial photosynthesis represents a sustainable option to produce alternative fuels, such as light C1 and C2 hydrocarbons (Chen H.-L. et al., 2024). In this process, the artificial leaves ideally should have an interconnected structure favoring light harvesting for gas diffusion and conversion (Kumar et al., 2022). In addition, a highly porous large surface area and a well-packed structure mimicking the mesophyll cells, which are suitable for inward diffusion of CO 2 through concentration gradients to internal structures, e.g., chloroplasts. Several research groups have designed artificial leaves with interesting properties and promising efficiencies to achieve profitable efficiencies in this process. The first attempts focused on attaining an efficient mass flow and light harvesting using biological systems as “architecture-directing agents” to create hybrid systems (Zhou et al., 2013; Zhang et al., 2025). In those works, the authors proposed the use of semiconductor materials with perovskite structure, e.g., ABO 3 (A = Li, Na, K, Sr, and B=Ta, Ti), as photocatalyst agents. Perovskite materials favored higher efficiencies to photoreduce the CO 2 molecule into carbon monoxide (CO) (up to 800 μmol g -1 h -1) that the use of the traditional photocatalyst in artificial leaves, e.g., TiO 2 (<1 μmol CO g -1 h -1 and 30 μmol CH4 g -1 h -1) (Hashemizadeh et al., 2018; Chen et al., 2018), Cu 2 O (<1 μmol CO g -1 h -1) (Yu et al., 2016), or ZnO (10 μmol CO g -1 h -1 dm -3) (Morawski et al., 2023). The subsequent efforts consisted of enhanced CO 2 adsorption on the leaves by the addition of porous supports based on g-C 3 N 4 foams (Sun et al., 2020) and metal–salen-incorporated conjugated microporous polymers (Wu et al., 2025), promoting the photocatalytic conversion of pure or low-concentration CO 2. Alternatively, the construction of artificial leaves on porous substrates, based on lead halide perovskites, has promoted promising efficiencies for CO 2 reduction to CO and other products such as alcohols (ethanol and n-propane) and formate (HCOO −) in batch systems or methane (CH 4) in the gas phase (Rahaman et al., 2023; Andrei et al., 2022; Chen Q. et al., 2024). However, the use of lead-free halide perovskite is preferred due to the controversial question about the lead (Pb) presence. Instead, low-dimensional perovskites with the formula of A 3 M 2 X 9 (A = Cs, K, and M = Bi, Sb) have been developed to improve stability by replacing Pb 2+ (in APbX 3 stoichiometry) with cations of lower toxicity, e.g., Bi 3+ (Getachew et al., 2023). Recently, our research group proposed the synthesis and application of potassium bismuth halide perovskite (K 3 Bi 2 I 9) materials for CO 2 reduction with outstanding efficiencies for formic acid (HCOOH) production (Quintero-Lizárraga et al., 2023; Luévano-Hipólito et al., 2024a). The efficiency of the CO 2 reduction to HCOOH was enhanced after the encapsulation of K 3 Bi 2 I 9 on ceramic porous supports (Luévano-Hipólito et al., 2024a); however, to propose new, affordable, and low-cost strategies for the application of this material at a larger scale, this work proposed the use of earth-abundant materials based on easily moldable clays.

Clays are widely used to fabricate support for catalysts due to their appropriate texture and chemical composition, i.e., mainly aluminosilicates. The clays enable easy extrusion and related advantages, which may help advance toward industrial application (Chafik, 2021; Sen et al., 2023). Of interest also is the possibility of tuning the catalytic behavior through the incorporation of appropriate active phases and promoters. In the case of artificial photosynthesis, there are several promoters for CO 2 conversion, such as metals (Ce, Mn, Ga, La, Li, Na, Ca, K, Cu, and Ni) (Sholeha et al., 2023; Zhang et al., 2022), CaCO 3 (Hu and Liu, 2024), and some complex light absorbers {Ru (bpy) 3 Cl 2, Re(NN) (CO) 3 Cl, ([Ir (btp) 2 (bpy-X2)] +, Cu(II) tetra(4-carboxylphenyl)porphyrin, chlorophyll (Chl), copper chlorophyllin (ChlCu) (Yang et al., 2020; Müller et al., 2022; Wang et al., 2018; Yu et al., 2023; Maimaitizi et al., 2022). Among them, Chlorophyll, a natural pigment, favors the efficiency of the artificial leaves via an enhancement effect in light harvesting (Maimaitizi et al., 2022; Joshi et al., 2009) since it can provide energized electrons to an acceptor material to initiate the photoinduced process. Also, it is worth to mention that this molecule can be unstable in solution, but when it is adsorbed on a solid support, the degree of denaturation decreases effectively (Joshi et al., 2009). It is therefore essential to immobilize chlorophyll on a suitable substrate such as the clays mentioned before.

Chlorophyll is found in many organisms, including plants, algae, and cyanobacteria. One type of brown algae that contains this bioactive compound is Phaeophyceae, e.g., Sargassum sp., Lobophora sp., Ectocarpus sp., Colpomenia sp., Zonaria sp., Padina sp., Dictyota sp. (Ridwan et al., 2017; Fraga and Robledo, 2022; Daza et al., 2023; García-García et al., 2021), which recently has arrived in the Caribbean Sea, converting them into a plague. Despite the massive arrival of brown algae requires urgent and large-scale solutions, there are few proposals for its utilization (Lee et al., 2023). Regarding artificial photosynthesis, the harnessing of brown algae has been limited to the synthesis of photocatalytic materials as templates (Anwar et al., 2021; Anand et al., 2019), and its use as a promoter in clay supports has not been explored. Therefore, this work proposes the utilization of brown algae to fabricate hybrid photosystems based on clays, brown algae, and perovskite materials to photoconvert the CO 2 molecule into value-added compounds or solar fuels. The advantage of using this natural resource lies mainly in two reasons: (i) it gives added value to the pest of brown algae that currently affects marine ecosystems and tourism, (ii) it improves light absorption in the hybrid system through the chlorophyll it contains, and (iii) it would provide porosity in the clays support to enhance the CO 2 affinity of the artificial leaves to generate value-added C1-C3 products. In addition, this strategy will help to the encapsulation of the halide perovskites as a host in the pores of the biosupports with the additional advantages mentioned before. This work evaluates the harnessing of brown algae (Lobophora sp. and Sargassum sp.) collected in the Mexican Caribbean Sea to fabricate hybrid biosupports from earth-abundant clays for host bismuth halide perovskites to induce the CO 2 conversion to different solar fuels under visible light irradiation with improved efficiencies.

Fabrication of biosupports

The fabrication of biosupports consisted in the impregnation of the brown algae with commercial clays. In the first step of this research, Lobophora sp. (Lp) was selected as a bioadditive in the clays. This alga was collected in Quintana Roo, Mexico in January 2024; after collection, it was washed several times and ground in a ball mill to obtain powders, followed the procedure specified in a previous report (Mohamed et al., 2023). The clays used for this study consisted mainly of CaO, SiO 2, SO 3, and MgO; meanwhile, the alga consisted mainly of CaCO 3, as shown Table 1.

The reference support (CL) was fabricated with 5 g of clay and 1 mL of distilled water to form a paste that was molded into circles of 3.5 cm of diameter. For the fabrication of the biosupports different amounts (5–15%wt.) of brown algae pulverized was added into the mixture. The supports were left to dry at room temperature for 24 h. The identification of the biosupports was CL x, where x refers to the species of algae used: Lobophora Lp. (Lp) or Sargassum sp. (Sg).

Synthesis of hybrid systems (perovskite/biosupport)

The impregnation of the perovskite in the supports was made by a drop-casting method, followed the procedure that recently was reported (Quintero-Lizárraga et al., 2023). For this purpose, the precursor ink of the perovskite K 3 Bi 2 I 9 (KBI) was synthesized by mixing 1.5 M KI and 1 M BiI 3 (Aldrich, 99%) in 1 mL of N, N-dimethylformamide (Aldrich, 99%). The ink was sonicated for 15 min and stirred in a magnetic stirrer at 70 °C for 2 h, and then, it was filtered using a PTFE syringe (pore size = 0.2 mm). 50 μL of ink was dropped-casted onto the biosupports and dried at 80 °C for 6 h. The mass of the perovskite on each system was around 0.003 g. Figure 1 shows a summary of the methodology used to fabricate the hybrid systems noted as CLx-KBI.

Characterization

The crystal phase of the samples was characterized by X-ray powder diffraction (XRD) using a PANalytical Empyrean diffractometer with CuKα radiation. The functional groups were monitored by FTIR spectrophotometry in a Nicolet IS50. The morphology was investigated by Scanning Electronic Microscopy (SEM) using a JEOL microscope JSM-6490LV. The surface area of the samples was measured by the Brunauer-Emmet-Taller (BET) method in a Quantachrome Instrument model Nova 2000e. The spectra of photoluminescence were registered in a Cary Eclipse spectrophotometer using an excitation wavelength of 400 nm. Inductively coupled plasma-mass spectrometry was used to follow the possible lixiviation of the potassium from the perovskite into the liquid phase using ICP-MS Thermo Electron Modelo X Series II.

CO 2 reduction (CO2RR) assays

The photoactivity of the hybrid systems was assessed at room temperature in distilled water (without sacrificial agents) under a continuous flow of CO 2 (1 L min -1) in a borosilicate reactor (V = 0.2 L). The hybrid system was placed at the bottom of the reactor in front of the lamp (λ > 400 nm, 20 W, 1800 lumens). The spectrum of the lamp was previously reported by our group (Luévano-Hipolito and Torres-Martínez, 2017). The distance of the lamp to the reactor was adjusted to 5 cm. The exhaust gas flow was reabsorbed with a scrubber to prevent its release into the environment. A previous report showed the schematic representation of this set-up (Quintero-Lizárraga et al., 2023). Liquid products were monitored with High-Resolution Liquid Chromatography (HPLC) in a Shimadzu Prominence-i LC-2030C using a Shimadzu Nexcol C18 column as stationary phase delivering 0.6 mL min -1 of a mixture of H 3 PO 4 0.1% - acetonitrile 85:15 as mobile phase, detecting formic acid in 210 nm. Besides, acetic acid and n-propanol were also quantified with HPLC using an Aminex® HPX-87H column, delivering 0.6 mLmin -1 of H 2 SO 4 5 mM as mobile phase, detecting acetic acid at 210 nm and n-propanol in the Refractive index detector (RID). Electron paramagnetic resonance (EPR) was obtained by the spin-trapping reagent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were investigated on a JEOL JES-TE300 after 10 min of irradiation.

CO 2 reduction using hybrid systems

In the beginning of this research, different amounts of brown algae (Lobophora Lp.) were incorporated into biosupports to find what is the adequate load into the clays to achieve both (i) higher CO 2 reduction and (ii) assure the stability of the hybrid system. Figure 2 shows the results of the CO 2 reduction assays using the hybrid system CLLp-KBI with different loads of alga after 1 h of visible reaction. As shown, apparently the samples with 10%wt. of Lp shows the highest activity, evidenced by comparing the means of each system by Tukey tests in Supplementary Table S1. Meanwhile, the samples with 5, 7, and 15% showed comparable efficiencies; however, the incorporation of ≥7% of algae in the hybrid system after CO2RR promoted the formation of cracks, which is not desirable for the stability of the perovskite (Supplementary Figure S1). Thus, the sample with 5 %wt. of Lp was selected for further examination.

The formic acid production over time with references and the hybrid system is shown in Figure 3a. As shown, it is evident that immobilize the perovskite on the clays significantly enhanced (5.5x) its activity to produce HCOOH. The hybrid system exhibited stable activity to reduce the CO 2 molecule into HCOOH with efficiencies up to 2.5 mmol during 5 h under visible light. After the test, ICP-MS analysis was carried out in the liquid phase to investigate the presence of K. The reference experiment with the use of the support (CL) and biosupports showed 10.08 and 8.11 ppm of potassium, respectively. Instead, the analysis of the liquid phase after the 5 h of testing of the CLLp-KBI sample revealed 13.63 ppm of potassium; this slight increase indicates minimal leaching, confirming that the perovskite remained largely stable during the reaction and that the encapsulation strategy effectively limited its dissolution.

The comparison of the efficiency of the CL-KBI and CLLp-KBI demonstrated that both samples exhibited similar behavior to produce HCOOH (Figure 3b). However, to find additional differences between the perovskite supported onto the clays and in the hybrid system, other reaction products were monitored; the results are shown in Supplementary Figure S2. Both samples produced acetic acid (6.5 μmol) and n-propanol in different amounts. The production of this late value-added compound was double enhanced (up to 1,538 μmol after 5 h) after adding the alga in the hybrid system, which confirms that the utilization of this natural material is highly beneficial to designing artificial leaves. In addition, control experiments under dark and Ar atmosphere were carried out; the results indicated a formic acid production of 19 and 42 μmol after 1 h, respectively.

The reason behind the activity of these samples will be further discussed in the characterization section.

Characterization of the hybrid system

SEM analysis was carried out on the reference and hybrid system to observe the alga and the perovskite dispersion in the support. The morphology of the reference clays consisted of flake-like particles, some of them with hexagonal shape, as shown in Figure 4a. This morphology is typical of natural clays; meanwhile, the microstructure of the alga consisted in irregular particles, as was recently reported (Mohamed et al., 2023). The alga addition in the supports resulted in agglomerated particles surrounding the flake particles of the clay (Figure 4b), whose morphology was compacted after the impregnation of the perovskite (Figure 4c). A more open view of the microstructure of the hybrid system evidenced the presence of the KBI perovskite (as long rods) distributed on the biosupports. The identification of each type of morphology in the system was confirmed by EDS analysis, as shown in Supplementary Figure S3. After the reaction, the micrographs revealed that the KBI particles remained well distributed and strongly attached to the clay–algae flakes (Figure 4e, f). This morphological stability confirms that the perovskite was successfully encapsulated within the biosupport, preventing its detachment during the CO 2 photoreduction.

Fourier transform infrared (FTIR) spectroscopy was used to identify the functional groups on the samples (Figure 5). The spectrum of reference support evidenced the presence of characteristic bands of clays. For example, the bands around 3,600, 3,390, and 1,615 cm -1 are related to the stretching (ν(OH)) and bending (δ(OH)) hydroxyl groups located in octahedral and tetrahedral layers of clays (Louati et al., 2016). Other bands at 990 and 690 cm -1 are associated with Si-O and Si-O-Si (Al) stretching vibrations from clay (Chetverikova et al., 2022); meanwhile the signal of carbonates (ν(C=O)) is present around 1700; while a low intensity band at 750 cm -1 was observed, associated with out-of-plane bending (δ(C–O)). The vibration related to the sulfate groups (ν(S-O)) was identified at approximately 1,115 cm -1 (Li et al., 2020).

The addition of the algae did not show significant variations in the spectrum, only a small band around 3,700 cm -1 was observed, which was present in previous reports of algae (López-Sosa et al., 2020; Abdelwahab et al., 2023).

The electronic transitions of the samples were investigated by UV-Vis spectroscopy. Figure 6 shows the absorption spectra obtained. The reference shows an absorption band around 270 nm, commonly found in clays (Hariram et al., 2021). Meanwhile, the biosupports exhibited a band characteristic of the Chlorophyll A at 665 nm (Yusprianto et al., 2021), which is result from an electronic transition π - π* originating from the porphyrin ring found in the Q band region in algae. The presence of chlorophyll in the photo system could be beneficial since it acts as an electron donor, which improves the photoactivity of the materials (Worathitanon et al., 2019). In the hybrid system, there are two absorption bands: (i) the main absorption edge related to the perovskite band gap (2.0 eV) from 600 nm and (ii) the chlorophyll absorption at 665 nm. The coexistence of both transitions could benefit the photoreaction since the chlorophyll can transfer electrons to the conduction band of KBI, promoting more radical species to reduce the CO 2 molecule.

Photoluminescence spectroscopy (PL) is an important technique to investigate and establish a link between photoemission and charge generation in catalysts sensitized with different pigments, e.g., chlorophyll. Figure 7 shows the photoluminescence emission spectra of the substrate with and without alga and the supported perovskite. The CLLp sample exhibited four bands: (i) 570, (ii) 607, (iii) 643, and (iv) 680 nm. The first bands can be associated with clays in the substrate since they were present in reference (at lower intensities); however, the origin of these bands is not clearly understood. The bands at 643 and 680 nm are related to the presence of chlorophyll (Lopez-Delgado et al., 2019; Ayudhya et al., 2015), which agrees with the UV-Vis spectroscopy. However, in the hybrid system, these bands are not clearly seen, probably due to the impregnation of the perovskite. The first bands were more intense at 520–560 nm, which could be related to the overlap emission generated with the perovskite in the same region (Kundu et al., 2020; Ali et al., 2019). Also, this higher PL emission in the hybrid system could be associated with the chemisorbed chlorophyll on the perovskite acts as an electron donor through the carboxylic acid binding site, increasing the emission observed by PL (Zheng et al., 2024).

Figure 8a shows the adsorption–desorption N 2 isotherms obtained at 77 K for references and hybrid system. According to the IUPAC taxonomy, type-II isotherms were observed in the CL and CLLp-KBI samples. In contrast, type-IV isotherms with an H2 hysteresis loop were identified in the CLLp sample. The H2 hysteresis ascribes materials with bottleneck constrictions (ALOthman, 2012); this pore shape could be beneficial to encapsulate materials to avoid its degradation in water or under environmental conditions. As shown in Figure 8a, after the impregnation of the CLLp sample with the perovskite, the pores tended to be occupied by this material and the taxonomy of the isotherm changed from IV to II in the hybrid system (CLLp-KBI), which confirms its encapsulation in the clay framework. Also, the analysis of the pore radius by the Barrett–Joyner–Halenda (BJH) method in Figure 8b evidenced the higher porosity in the CLLp, which was filled with the perovskite.

The surface area was determined by the BET method from the N 2 adsorption isotherm; meanwhile, the average volume and pore size of samples were obtained by the BJH method, the results of which are presented in Table 2. The addition of the alga resulted in an increased in porosity, which was further reduced in the hybrid system due to the fill pores by the perovskite, which suggests a partial pore blocking and a structural rearrangement of the mesoporous network after perovskite deposition. Regarding the pore radius, the CLLp-KBI system exhibited up to four times higher values than CLLp, probably by the perovskite surface properties. This restructuring indicates that the pore network was not completely collapsed but reorganized into larger mesopores, maintaining open channels for gas transport. Therefore, even though the BET surface area decreased after KBI incorporation, the improved interfacial contact between the perovskite and the biosupports could facilitate charge transfer and CO 2 affinity or adsorption. The remaining mesoporosity and interconnected channels favor CO 2 diffusion, mainly when the surface areas are not high (>500 m 2 /g) as in this case (Song et al., 2015).

X-ray powder diffraction analysis of the clay reference sample revealed the presence of three main phases: (i) Gympsum (CaSO 4 ·2H 2 O, 33–0311 card), (ii) Clinochlore [Mg 5 Al(Si,Al) 4 O 10 (OH) 8, 46–1,322 card], and (iii) Anhydrite (CaSO 4, 83–2270 card). These phases remain stable after the Lp incorporation, even after the perovskite impregnation; meanwhile, in the hybrid system, the crystallinity was enhanced, probably due to a decrease in the porosity of the sample. A small reflection around 2θ = 26° was identified in the hybrid system, which was related to the K 3 Bi 2 I 9 phase (Quintero-Lizárraga et al., 2023), as shown in Figure 9. It is worth noting that after the reaction, this reflection remains; however, its intensity decreases, as the rest of the phases. Other components detected by XRF analysis were not observable on the XRD pattern. This is a probable consequence of the low amount of these components in the clays.

Mechanism

To examine how the algae incorporated into the hybrid system influences the CO 2 reduction mechanism, the effect of each system on radical formation was analyzed. For this purpose, terephtalic acid (TP) was used to evidence the formation of hydroxyl radicals (· OH), as reactive species, during the illumination of the system. In this test, these radicals react with the terephthalic acid, giving rise to 2-hydroxytere-phthalic acid, which emits a single fluorescence signal around 425 nm (Alsalme et al., 2024). Supplementary Figure S4 shows the intensity growth as a function of the irradiation time using the perovskite supported in the hybrid system (CLLp-KBI) and without brown alga (CL-KBI). The increase in the signal is associated with a rise in the amount of hydroxyl radicals formed, which suggests a higher hydroxyl radical formation in the hybrid system. These radicals contribute directly to the conversion of the CO 2 molecule through their photooxidation, providing more electrons to the reaction (Kreft et al., 2019). In addition, these results were confirmed with EPR spin-trapping spectroscopy. This technique is based on the interaction of an externally applied magnetic field with the magnetic moments existing by any un-paired electrons in the studied samples (e.g., CL-KBI and CLLp-KBI), and the signals produced are a result of splitting the electron energy levels (Rasheed et al., 2021). In this context, the intensity of EPR signals is directly correlated to the number of free radicals in the sample. As shown in Figure 10, the EPR spectra of the samples with (CLLp-KBI) and without (CL-KBI) algae display clear differences in the radical signal intensity. The hybrid system incorporating brown algae exhibits significantly stronger EPR signals compared to CL-KBI, indicating a higher concentration of hydroxyl radicals (· OH) generated under illumination. This observation supports the fluorescence findings (Supplementary Figure S4), confirming that the presence of algae enhances the formation of reactive species. The increased radical generation in the CLLp-KBI system suggests a more efficient photocatalytic environment, likely due to synergistic effects between the perovskite and the biologically active components introduced by the algae. Such radicals play a crucial role in CO 2 photoreduction by improving the separation of photo-generated charges and promoting electron availability for the redox reactions involved.

Thus, considering these results it is proposed the mechanism shown in Figure 11. In this mechanism, the light is absorbed by two routes. First, chlorophyll (Chl) absorbs light, which excites an electron (e −) from a ground state to an excited singlet state (1 Chl), and some electrons undergo intersystem crossing (ISC) to their triplet state (3 Chl) (Phongamwong et al., 2015). In the excited state, the electrons from the LUMO of Chl (−0.67 V vs. NHE) (L et al., 2022) can transfer to the conduction band (CB) of K 3 Bi 2 I 9 (−0.44 V vs. NHE according to the theoretical potential (Luévano-Hipolito et al., 2022)) contributing to the CO 2 reduction. At the same time, the perovskite creates electron–hole pairs by absorbing visible light to reduce the CO 2 molecule. In this mechanism, H 2 O is oxidized by the holes (h +) of the valence band (VB) through the formation of H + and OH − ions, which later consecutively react with holes on K 3 Bi 2 I 9 to generate the · OH radicals. According to the results, it is proposed that the photooxidation of surface hydroxyl groups occurs, which acts as an additional electron source for CO 2 reduction (Kreft et al., 2019). Considering these results, the relatively high number of photogenerated charges in the hybrid system favored the formation of other products that required more H + /e − based on C 2 and C 3 products, e.g., CH 3 COOH and CH 3 H 7 OH. This was confirmed after the comparison of the efficiency of the C 3 products between CL-KBI and CLLp-KBI samples (Supplementary Figure S2).

Evaluation of other species of brown algae plague

To investigate if the hybrid system could be fabricated using another type of brown algae considered a plague, Sargassum sp. was collected in Cancun, Quintana Roo, Mexico. This alga belongs to other specie of the same family of brown algae Phaeophyceae. The alga was submitted using the same washing and grounding procedure as previously mentioned.

The micrograph of the Sargassum sp., shown in Figure 12a, shows an irregular morphology composed of semicircular particles, which are mainly composed of C, O, Cl, K, Ca, and Na. Once this alga was incorporated in the hybrid system, the perovskite tended to cover the particles, as similar with the Lp alga. The encapsulation of the K 3 Bi 2 I 9 in the hybrid clay, resulted in a high activity to reduce the CO 2 molecule as shown in Figure 12b. The production was similar than the obtained with the CLLp-KBI system, reaching a stable HCOOH production of 2.5 mmol with the CLSg-KBI composite. Further characterization by UV-Vis and Photoluminescence spectroscopy showed similar results with both systems (Supplementary Figure S5), which demonstrated that the introduction of the alga in the clays represents a valuable strategy for: Harnessing a natural residue to obtain value-added products sets a precedent for its incorporation into ceramic materials for different purposes.

Increase the CO 2 conversion to different C 1 -C 3 products that can be directly used as renewable fuels.

Provide more free charges in the photosystem to increase the efficiency of artificial photosynthesis.

Despite their slightly different elemental compositions and physical properties (Mellado-Lira et al., 2025), both systems exhibited almost identical performance, which results indicate that the beneficial effect of the algae is not species-dependent, but rather related to their common biochemical composition rich in polysaccharides and alkaline earth elements, which promote efficient perovskite encapsulation.

To better contextualize the photocatalytic activity of the synthesized systems, a comparison with other perovskite materials was carried out. Table 3 summarizes some representative examples of perovskite-based systems for CO 2 photoreduction under various conditions and catalyst configurations (powders and immobilized systems) (Quintero-Lizárraga et al., 2023; L et al., 2025; Guo et al., 2024; Dasireddy et al., 2022; Abarca et al., 2024; Lin et al., 2018; Ran et al., 2024; Chen et al., 2020; Tailor et al., 2024; Ding et al., 2025; Luévano-Hipólito et al., 2024b). As is shown, the hybrid system developed here exhibits one of the highest formic acid productions (2500 μmol h -1) from CO 2 photoreduction compared with other systems. This remarkable performance can be attributed to the presence of the brown algae matrix, which not only improves the dispersion and stability of the perovskite particles but also facilitates charge separation and transfer through its organic functional groups, e.g., Chlorophyll A. These characteristics enhance the CO 2 photoconversion efficiency by reducing charge recombination and promoting more efficient CO 2 activation on the catalyst surface.

Additionally, the obtained rates are comparable to those reported for photoelectrochemical systems, even though the present configuration operates under simple photochemical conditions, without applied bias or external co-catalysts. This highlights the potential of biosupports for immobilized perovskite catalyst as sustainable and efficient platforms for visible-light-driven CO 2 reduction under ambient conditions.

This work demonstrates the improved efficiency of lead-free bismuth halide perovskites encapsulated in brown algae-functionalized (Lobophora sp. and Sargassum sp.) clay supports with enhanced efficiencies compared to reference materials. This approach favored the design and fabrication of sustainable and low-cost hybrid systems for artificial photosynthesis. Some relevant findings found were: Adding 5 %wt. of algae maintains both high efficiency and stability of the hybrid system, avoiding cracks and saturation.

The addition of the alga resulted in an increased in porosity, which was further fill with the perovskite. This restructuring indicated that the pore network was not completely collapsed but reorganized into larger mesopores, maintaining open channels for CO 2 transport.

The enhanced effect (and stability) of the hybrid systems was related to a correct encapsulation of the perovskite (with minimal potassium leaching) in the porous support and to the chlorophyll from brown algae that participates as an electron donor and improves the light absorption, making CO 2 reduction more efficient.

A mechanism of the charge transfer was proposed according to the theoretical thermodynamic LUMO and CB potentials of chlorophyll A and K 3 Bi 2 I 9 perovskite.

The synergistic effect and efficient charge transfer favored the efficient formation of renewable value-added products, e.g., C 1 (HCOOH), C 2 (CH 3 COOH), and C 3 [(CH 3 (CH 2) 2 OH] in the order of mmol, which productions are one of the most competitive reported so far.

Overall, these hybrid systems represent a promising alternative for the capture and photoreduction of CO 2 while combating the sargassum plague in the Caribbean.