Oxygen Vacancy-Engineered Ni:Co₃O₄/Attapulgite Photothermal Catalyst from Recycled Spent Lithium-Ion Batteries for Efficient CO₂ Reduction

Abstract

Accelerated industrialization and surging energy demands have led to continuously rising atmospheric CO₂ concentrations. Developing sustainable methods to reduce atmospheric CO₂ levels is crucial for achieving carbon neutrality. Concurrently, the rapid development of new energy vehicles has driven a significant increase in demand for lithium-ion batteries (LIBs), which are now approaching an end-of-life peak. Efficient recycling of valuable metals from spent LIBs represents a critical challenge. This study employs conventional hydrometallurgical processing to recover valuable metals from spent LIBs. Subsequently, Ni-doped Co₃O₄ (Ni:Co₃O₄) supported on the natural mineral attapulgite (ATP) was synthesized via a sol–gel method. The incorporation of a small amount of Ni into the Co₃O₄ lattice generates oxygen vacancies, inducing a localized surface plasmon resonance (LSPR) effect, which significantly enhances charge carrier transport and separation efficiency. During the photocatalytic reduction of CO₂, the primary product CO generated by the Ni:Co₃O₄/ATP composite achieved a high production rate of 30.1 μmol·g⁻¹·h⁻¹. Furthermore, the composite maintains robust catalytic activity even after five consecutive reaction cycles.

1. Introduction

Accelerated industrialization has driven a substantial increase in global energy demand, leading to rising atmospheric CO₂ concentrations. A key strategy in current energy conservation and emission reduction efforts involves harnessing renewable energy sources, such as solar power, to efficiently convert CO₂ and H₂O into high-value-added chemical products and fuels [1]. Utilizing photothermal technology for efficient CO₂ conversion represents a promising approach to addressing energy utilization challenges. Spinel-based catalysts have been extensively studied due to their efficient photothermal conversion capabilities. As a typical cobalt-based spinel oxide, Co₃O₄ and its composite nanomaterials demonstrate outstanding catalytic performance in CO₂ reduction. Furthermore, in the field of photocatalysis, its narrow band gap and conduction band position render CO₂ reduction thermodynamically feasible [2]. Studies indicate that Co₃O₄ can effectively degrade organic pollutants and reduce CO₂ under ultraviolet or visible light irradiation. However, the photocatalytic activity of pure Co₃O₄ is limited by its high charge carrier recombination rate. To overcome this limitation, integrating Co₃O₄ into photothermal synergistic catalytic systems has been shown to significantly enhance catalytic efficiency through photothermal coupling effects. Currently, Co₃O₄ has demonstrated promising applications in areas such as photothermal removal of volatile organic compounds (VOCs) [3,4]. For instance, Ren et al. [5] synthesized hierarchical Co₃O₄ nanosheets by calcining novel cobalt MOF nanosheets prepared via a light oil bath method, which exhibited a high CO production rate during visible light photocatalytic CO₂ reduction. Gu et al. [6] prepared a series of Co₃O₄ catalysts for photothermal CO₂ reduction. The results demonstrated that the synthesized Co₃O₄ catalysts with porous nanosheet structures exhibited superior catalytic activity.

Meanwhile, the surging demand for lithium-ion batteries (LIBs) has led to a corresponding increase in the generation of spent LIBs. Spent LIBs contain higher concentrations of valuable metals, such as cobalt (Co) and nickel (Ni), compared to natural ores. Therefore, the recovery of spent LIBs is imperative [7,8]. Utilizing spent LIBs to prepare environmental functional materials presents a promising recycling route due to its low energy consumption and simplified separation-purification steps. For example, Maroufi et al. [9] reported a core–shell structured Co₃O₄@substrate photocatalyst derived from spent LIBs, which demonstrated excellent photocatalytic activity for methylene blue degradation. However, reports on converting recycled spent batteries into supported catalysts for photothermal catalytic CO₂ conversion remain limited.

In this study, Ni, Co, and Li elements were first recovered from spent LIBs using an environmentally friendly and efficient separation process. Subsequently, a Ni-doped Co₃O₄ (Ni:Co₃O₄) composite supported on the natural mineral attapulgite (ATP) was successfully synthesized via a sol–gel method, achieving uniform loading of the Ni-Co oxides. While mitigating the environmental issues associated with spent LIBs, the prepared Ni:Co₃O₄/ATP composite effectively drives the photothermal catalytic conversion of CO₂ to CO, aligning with a “waste control by waste” strategy.

2. Results and Discussion

2.1. XRD Analysis

XRD measurements were performed to determine the crystal structures of ATP and the Ni:Co₃O₄/ATP composites. As shown in Figure 1a, the characteristic diffraction peaks observed for all Ni:Co₃O₄/ATP samples, prepared with varying mass ratios of LIBs to ATP, can be indexed to Co₃O₄ (JCPDS PDF#43-1003) and attapulgite (JCPDS PDF#21-0958). Specifically, the diffraction peaks of the Ni:Co₃O₄ component located at 31.2°, 36.7°, and 44.6° are assigned to the (220), (311), and (400) crystal planes of the spinel structure, respectively. The peak at 8.49° corresponds to the (110) plane of ATP. The intensity of the (220), (311), and (400) diffraction peaks, attributed to the Ni:Co₃O₄ phase within the Ni:Co₃O₄/ATP composites, decreases with an increase in Ni:Co₃O₄ loading (i.e., increasing LIBs:ATP mass ratio). Figure 1b presents a magnified view of the XRD patterns focusing on the (311) diffraction peak. Analysis reveals that as Ni:Co₃O₄ loading increases, the (311) peak position shifts towards lower angles ascribed to the structural stain in the samples [10], accompanied by a broadening of its full width at half maximum. This observation suggests a reduction in the crystallinity of the Ni:Co₃O₄ spinel and potentially an increase in the concentration of oxygen vacancies [11].

2.2. TEM Analysis

TEM images revealing the structural features of ATP and the Ni:Co₃O₄/ATP composites are presented in Figure 2. Figure 2a shows that ATP exhibits its characteristic rod-like morphology. The Ni:Co₃O₄/ATP composites prepared with LIBs:ATP mass ratios of 0.8:1 and 0.6:1 (Figure 2b and Figure 2c, respectively) display significant nanoparticle aggregation. In contrast, the 0.4:1 Ni:Co₃O₄/ATP sample (Figure 2d) demonstrates well-dispersed Ni:Co₃O₄ nanoparticles without noticeable aggregation or detachment. The relative content of Ni:Co₃O₄ is comparatively lower in the 0.2:1 Ni:Co₃O₄/ATP sample (Figure 2e). Figure 2f provides a HRTEM image of the 0.4:1 Ni:Co₃O₄/ATP nanocomposite. The image reveals ordered lattice fringes with an interplanar spacing of 0.24 nm, corresponding to the (311) plane of Co₃O₄. This observation is consistent with the XRD results.

Figure 3a clearly displays a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, revealing a highly dispersed loading structure of Ni:Co₃O₄ nanoparticles within the ATP nanorod support. The nanoparticles exhibit a size distribution ranging from 5 to 8 nm, appearing as bright spots corresponding to regions of higher atomic number. Furthermore, Figure 3b–g presents STEM-EDS elemental mapping images. Figure 3b–e demonstrates the homogeneous distribution of elements characteristic of ATP, including O, Mg, Al, and Si. Figure 3f,g reveals the uniform distribution of Co and the successful doping of trace amounts of Ni throughout the Ni:Co₃O₄/ATP composite.

2.3. UV–Vis Analysis

Figure 4a displays the UV–Vis diffuse reflectance spectra of ATP and the Ni:Co₃O₄/ATP composites with varying Ni:Co₃O₄ loading amounts. Compared to ATP, the light absorption capability of the Ni:Co₃O₄/ATP composites initially strengthened and then weakened with an increase in Ni:Co₃O₄ loading. The 0.4:1 Ni:Co₃O₄/ATP sample exhibited the strongest light absorption probably due to the best dispersion status according to the TEM results. The composites showed enhanced absorption peaks within the range of 600–800 nm thanks to the highly exposed area. This phenomenon is attributed to the generation of oxygen vacancies induced by loading Ni:Co₃O₄ onto the ATP support, which subsequently triggered a localized LSPR effect [12]. The Tauc plots for ATP and Ni:Co₃O₄ are presented in Figure 4b (judging the bandgap energy from the red dashed tangent line). The optical band gap energies Eg of the semiconductors were calculated using the Kubelka–Munk equation [13], as follows:

(αhν)^n/2 = K(hν − Eg)

where α, h, ν, K, and Eg represent the absorption coefficient, Planck’s constant, optical frequency, a proportionality constant, and the band gap energy, respectively. Furthermore, n = 1 and n = 4 correspond to semiconductors with direct and indirect band gaps, respectively. The optical absorption characteristics of Co₃O₄ align with an indirect transition model (n = 4) [14]. The calculated band gap energy for Ni:Co₃O₄ was 1.36 eV judged by the red dashed line.

2.4. Thermal Imaging Analysis

Figure 5a shows that after 5 min of irradiation under a 300 W Xenon lamp, the surface temperature of ATP reached 107.6 °C, while that of pure Ni:Co₃O₄ reached 151.3 °C. Furthermore, the surface temperature of the composites was significantly enhanced compared to ATP with an increase in Ni:Co₃O₄ loading. The 0.4:1 Ni:Co₃O₄/ATP sample exhibited the highest surface temperature, peaking at 260.1 °C. The surface temperature of the Ni:Co₃O₄/ATP samples exceeded that of pure Ni:Co₃O₄. This enhancement is likely attributed to the plasmon resonance effect induced by oxygen vacancies generated when Ni:Co₃O₄ is supported on ATP, resulting in localized heating at the sample surface. Figure 5b illustrates the temporal evolution of the surface temperature for the 0.4:1 Ni:Co₃O₄/ATP composite, rising from 24.6 °C to 260.1 °C within 5 min under illumination. The infrared thermal imaging results demonstrate that the Ni:Co₃O₄/ATP nanocomposite exhibits a pronounced thermal response triggered by the plasmon resonance effect.

2.5. XPS Analysis

XPS was employed to further evaluate the surface chemical composition of the synthesized Ni:Co₃O₄ and the 0.4:1 Ni:Co₃O₄/ATP composite. As shown in the survey spectra (Figure 6a), elements including Co, Ni, O, and C were detected for Ni:Co₃O₄. Additional elements Fe, Si, and Al were identified in the survey spectrum of the 0.4:1 Ni:Co₃O₄/ATP composite, attributable to the incorporation of ATP. In the high-resolution O 1s spectrum (Figure 6b), the peak at ~529.6 eV corresponds to lattice oxygen (O_L), while the peaks near ~530.9 eV and ~532.3 eV are ascribed to surface-adsorbed oxygen species (O_A) and hydroxyl groups or chemisorbed water, respectively. An increased proportion of adsorbed oxygen species was observed for the 0.4:1 Ni:Co₃O₄/ATP composite. This is likely due to lattice expansion or distortion in Ni:Co₃O₄ upon combination with ATP, leading to the formation of more defects and oxygen vacancies, consistent with the XRD and EPR results. The Co 2p spectrum (Figure 6c) exhibits characteristic Co 2p_3/2 and Co 2p_1/2 peaks, each resolved into three component peaks. The peaks at ~780.0 eV and ~794.9 eV are assigned to Co³⁺, while those at ~781.3 eV and ~796.6 eV belong to Co²⁺. The peaks at ~788.7 eV and ~803.8 eV are satellite features. Upon forming the composite with ATP, an increase in the molar ratio of Co²⁺ was observed, indicating the formation of more low-valence cobalt species (Co²⁺) within the Ni:Co₃O₄ catalyst [15]. Figure 6d displays the Si 2p spectra. For pristine ATP, two peaks are observed at binding energies (BE) of ~104.5 eV and ~102.8 eV. In contrast, the Si 2p peak for Ni:Co₃O₄/ATP appears at ~102.6 eV, exhibiting a noticeable negative shift compared to ATP. This shift arises because Co possesses a slightly higher electronegativity than Si. When Ni:Co₃O₄ is supported on ATP, Si atoms tend to donate electrons to the surrounding Co atoms via chemical bonding. This electron transfer increases the electron density around the Si atoms, reducing the effective nuclear charge experienced by their core electrons [16]. Since binding energy (BE) is inversely proportional to electron density, the Si 2p XPS peak consequently shifts towards lower binding energy.

2.6. Electrochemical Analysis

Electrochemical characterization was performed to investigate the charge carrier separation efficiency of Ni:Co₃O₄ and Ni:Co₃O₄/ATP composites with varying loading ratios (0.2:1 to 0.8:1). Figure 7a presents the transient photocurrent responses of Ni:Co₃O₄ and the Ni:Co₃O₄/ATP composites with different loading ratios. The Ni:Co₃O₄/ATP composites exhibit significantly stronger photocurrent responses compared to pure Ni:Co₃O₄. This enhancement indicates that the formation of the composite structure between Ni:Co₃O₄ and ATP facilitates more efficient charge carrier separation. Furthermore, the composite with the 0.4:1 loading ratio demonstrated the highest photocurrent intensity, signifying its superior charge separation efficiency among the tested samples [17]. Additionally, electrochemical impedance spectroscopy (EIS) measurements were conducted on Ni:Co₃O₄ and the Ni:Co₃O₄/ATP composites (0.2:1 to 0.8:1) to assess charge transfer kinetics. A smaller arc radius in the Nyquist plot corresponds to lower electrochemical impedance [18]. Figure 7b reveals that the Ni:Co₃O₄/ATP composite with the 0.4:1 loading ratio exhibits the smallest arc radius, while pure Ni:Co₃O₄ displays the largest impedance. This observation indicates that compositing Ni:Co₃O₄ with ATP significantly enhances charge carrier mobility.

2.7. In Situ DRIFTS Analysis

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to investigate the reaction intermediates and their transformation pathways during the photocatalytic reduction of CO₂ and H₂O over the Ni:Co₃O₄/ATP catalyst. Figure 8a depicts the dark adsorption process on Ni:Co₃O₄/ATP. Water vapor, carried by CO₂ gas flow via a bubbling apparatus, interacted with the catalyst surface. As time progressed, increasing amounts of CO₂ and H₂O were adsorbed, reaching peak adsorption intensity at 10 min, indicative of adsorption equilibrium. Subsequently, gradual desorption of CO₂ and H₂O started. Figure 8b displays the spectra obtained during the reaction of CO₂ and H₂O on the Ni:Co₃O₄/ATP composite surface under illumination. The absorption band observed between 1627 and 1618 cm⁻¹ is associated with unbound H₂O molecules on the Ni:Co₃O₄/ATP surface [19]. Key reaction intermediates were detected on the catalyst surface. The peak at 1670 cm⁻¹ is assigned to the CO₂⁻ intermediate formed during CO₂ photoreduction [20]. Peaks at 1339 cm⁻¹ and 1508 cm⁻¹ are attributed to monodentate carbonate (m-CO₃²⁻). Peaks at 1618 cm⁻¹ and 1457 cm⁻¹ belong to bidentate carbonate (b-CO₃²⁻). The peak at 1570 cm⁻¹ is ascribed to the *COOH intermediate [21]. Consequently, the mechanistic pathway for the photothermal reduction of CO₂ over Ni:Co₃O₄/ATP can be proposed, as illustrated by the following reaction equations:

$$\mathrm{Ni}:{\mathrm{Co}}_3{\mathrm{O}}_4/\mathrm{ATP} \to {\mathrm{e}}^{-} + {\mathrm{h}}^{+}$$

(1)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+} \to {\mathrm{H}}^{+}+·\mathrm{OH}$$

(2)

$${\mathrm{CO}}_2+{\mathrm{e}}^{-} \to ·{\mathrm{CO}}_2^{-}$$

(3)

$${\mathrm{CO}}_2^{-}+·\mathrm{OH} \to \mathrm{H}{\mathrm{CO}}_3^{-}$$

(4)

$${\mathrm{CO}}_2^{-}+·{\mathrm{CO}}_2^{-} \to {\mathrm{CO}}_3^{-} +\mathrm{CO}$$

(5)

$${\mathrm{CO}}_2^{-}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} \to *\mathrm{COOH}$$

(6)

$$*\mathrm{COOH} \to \mathrm{CO}+{\mathrm{OH}}^{-}$$

(7)

2.8. EPR Analysis

Electron paramagnetic resonance (EPR) spectroscopy was employed to analyze oxygen vacancies in ATP, Co₃O₄/ATP, and the 0.4:1 Ni:Co₃O₄/ATP composite. As shown in Figure 9, no distinct EPR signal was detected for pristine ATP. In contrast, both Co₃O₄/ATP and 0.4:1 Ni:Co₃O₄/ATP exhibited characteristic EPR signals at g = 2.007. Notably, the 0.4:1 Ni:Co₃O₄/ATP sample synthesized from spent LIBs demonstrated a significantly stronger EPR signal intensity compared to the Co₃O₄/ATP sample prepared from analytical-grade reagents. This enhanced signal intensity indicates a higher concentration of unpaired electrons in the Ni:Co₃O₄/ATP composite, attributable to an increased density of oxygen vacancies. When oxygen atoms are absent from the lattice, neighboring metal atoms can retain unpaired electrons, thereby forming paramagnetic centers. This result is consistent with the findings from XPS and XRD analyses [22].

2.9. Photothermal CO₂ Reduction Performance

The catalytic performance was evaluated through CO₂ reduction experiments under illumination. Besides CO, CH₄ and CH₃OH were also detected, but their yields were significantly lower than that of CO. As shown in Figure 10a,b, the CO yield over ATP remained very low and essentially unchanged with an increase in illumination time. The Co₃O₄/ATP catalyst synthesized from analytical-grade reagents exhibited a CO production rate of 8.99 μmol·g⁻¹, which is markedly lower than that of the Ni:Co₃O₄/ATP catalysts derived from spent LIBs. Under solar light irradiation, the CO production rate continuously increased over time. Among the catalysts, the 0.4:1 Ni:Co₃O₄/ATP composite demonstrated the most outstanding catalytic performance, achieving a CO production rate of 30.1 μmol·g⁻¹. Under infrared (IR) light irradiation, the overall CO production rates decreased. The Co₃O₄/ATP sample prepared from analytical-grade reagents showed a rate of 4.3 μmol·g⁻¹, still lower than those of the LIBs-derived Ni:Co₃O₄/ATP catalysts. The 0.4:1 Ni:Co₃O₄/ATP sample reached a CO production rate of 10.3 μmol·g⁻¹ under IR light. Figure 10c depicts the CO₂ adsorption capacities of ATP, Co₃O₄/ATP, and Ni:Co₃O₄/ATP at 25 °C and 1 bar. At 25 °C, ATP exhibited a CO₂ adsorption capacity of only 10.3 mmol·g⁻¹. In contrast, the 0.4:1 Ni:Co₃O₄/ATP sample demonstrated a significantly higher capacity of 33.9 mmol·g⁻¹, far exceeding the 21.5 mmol·g⁻¹ capacity of the Co₃O₄/ATP sample prepared from analytical-grade reagents. The superior catalytic performance of the LIBs-derived samples compared to those prepared from analytical-grade reagents can be attributed to two main factors. First, spent LIBs contain trace amounts of Ni, which can be doped into the Co₃O₄ lattice. This doping promotes the formation of a greater number of oxygen vacancies, leading to enhanced catalytic activity—consistent with the EPR analysis. Second, the LIBs were processed in acidic solution, and the subsequent loading of the active components onto ATP also occurred in acidic conditions. This acid treatment effectively opens up ATP’s inherent one-dimensional nanochannels and increases its specific surface area, thereby enhancing its physical CO₂ adsorption capacity. The recyclability of the optimal 0.4:1 Ni:Co₃O₄/ATP catalyst was assessed. After washing with deionized water, the catalyst underwent five consecutive photocatalytic CO₂ reduction cycles. As demonstrated in Figure 10d, the catalyst maintained high catalytic activity throughout these cycles, indicating excellent recyclability and stability.

2.10. Mechanism of Photothermal Catalytic CO₂ Reduction

Based on the aforementioned experimental results, a mechanism for the photothermal catalytic reduction of CO₂ over Ni:Co₃O₄/ATP is proposed. Figure 11a illustrates the proposed mechanism under full-spectrum solar irradiation. As for the Ni:Co₃O₄/ATP composite, ATP primarily acts as a support. The acid treatment during processing effectively opens up ATP’s inherent one-dimensional nanochannels and increases its specific surface area, thereby enhancing its CO₂ adsorption capacity. Under solar light irradiation, Ni:Co₃O₄ is photoexcited, generating electron–hole pairs. Concurrently, LSPR is induced in the Ni:Co₃O₄ nanoparticles supported on ATP, facilitated by oxygen vacancies. This LSPR effect operates through two synergistic pathways. First, hot electrons generated on the Ni:Co₃O₄ surface react with adsorbed CO₂ molecules. Second, the photothermal effect elevates the local temperature at the Ni:Co₃O₄/ATP interface. These synergistic effects enhanced charge carrier dynamics from plasmonic excitation along with localized heating, which may collectively promote the photothermal reduction of CO₂, leading to improved reaction efficiency and catalyst performance. Figure 11b depicts the reaction mechanism under IR light irradiation. Under IR light, Ni:Co₃O₄ cannot be photoexcited. Only the LSPR effect occurs, generating hot electrons. These hot electrons react with protons (H⁺) derived from water splitting to produce a limited amount of CO. Crucially, in this photocatalytic CO₂ reduction system, oxygen vacancies serve as charge carrier traps. They significantly promote interfacial charge transfer at the catalyst surface while effectively suppressing the recombination of photogenerated electrons and holes.

3. Experimental Section

3.1. Materials

ATP was obtained from Xuyi, China. Citric acid monohydrate (C₆H₈O₇·H₂O), oxalic acid dihydrate (C₂H₂O₄·2H₂O), and anhydrous glucose (C₆H₁₂O₆) were purchased from Aladdin Chemical Co., Ltd. Spent lithium-ion batteries (LiCoO₂ cathodes) were obtained from discarded Apple phone batteries manufactured by the Desay Corporation.

3.2. Recovery of Valuable Metals from Spent LIBs

The spent LIBs underwent a pre-treatment process involving discharge, dismantling, crushing, and calcination to obtain cathode powder. This cathode powder was subsequently subjected to acid leaching. Certain amount of the obtained cathode powder was transferred into 20 mL of an aqueous solution containing 6.6 g of citric acid monohydrate and 0.12 g of glucose. The resulting suspension was then vigorously stirred at 80 °C for 2 h. After the reaction, the leachate was obtained by filtration of the mixture. Lithium ions (Li⁺) were separated from the other two metals (Ni, Co) in the leachate using an oxalic acid-based selective precipitation method. The procedure was as follows: 50 mL of the leachate was added to a 400 mL beaker. Oxalic acid dihydrate was then added to this solution in a 1:1 molar ratio relative to the total moles of Ni and Co metals. The resulting suspension was vigorously stirred at room temperature for 0.5 h, followed by filtration. This yielded a Li⁺-containing solution and an oxalate precipitate containing Ni and Co.

3.3. Synthesis of Ni:Co₃O₄/ATP Composite

The oxalate precipitate obtained in the previous step was dissolved in a 1.0 mol·L⁻¹ HNO₃ solution. Subsequently, 3 g of citric acid and 1 g of ATP were added to the solution. The mixture was vigorously stirred in a water bath at 80 °C to form a gel. The resulting gel was then subjected to overnight drying at 80 °C. Finally, the dried product was calcined at 400 °C for 4 h with a heating rate of 5 °C·min⁻¹ to yield the Ni:Co₃O₄/ATP composite. The mass ratio of the recovered cathode material (derived from spent LIBs) to ATP, denoted as LIBs:ATP, was adjusted in the range of 0.2–0.8:1 by varying the amount of cathode powder added. The process is shown in Figure 12.

3.4. Material Characterization

The microstructure of the samples was examined using an FEI Tecnai F20 transmission electron microscope (TEM) from FEI Company, Hillsboro, OR, USA. The crystal structure was characterized by X-ray powder diffraction (XRD) performed on a Bruker D8 Advance diffractometer from Bruker Corporation, Billerica, MA, USA. Chemical structure analysis was conducted using a Thermo Scientific Nicolet iS20 Fourier transform infrared (FT-IR) spectrometer from Thermo Fisher Scientific Inc., Waltham, MA, USA. Optical absorption properties were characterized by ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS) using a Shimadzu UV-3600i Plus spectrophotometer from Shimadzu Corporation, Kyoto, Japan. Surface elemental composition and chemical states were determined via X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific ESCALAB 250Xi spectrometer from Thermo Fisher Scientific Inc., Waltham, MA, USA.

3.5. Photocatalytic CO₂ Reduction Experiment

Photocatalytic CO₂ reduction experiments were conducted in a 100 mL photochemical reactor. First, 30 mL of deionized water and 50 mg of catalyst were added to the photochemical high-pressure autoclave. High-purity CO₂ gas was then purged into the reactor, and the pressure was maintained at 1.01 bar for 2 min. Subsequently, the gas within the reaction chamber was vented until the pressure equilibrated with atmospheric pressure. This purging procedure was repeated three times to ensure an atmosphere of pure CO₂ within the reactor. The reaction mixture was irradiated using a 300 W xenon lamp where a transparent window was located on the top of the autoclave. To ensure NIR irradiation, a NIR filter was put on the window to allow NIR light to pass. Gas samples were periodically withdrawn from the reactor headspace at designated time intervals and analyzed using a gas chromatograph (GC-7860 Plus from Shanghai Nuoxi instrument Co., LTD, Shanghai, China).

4. Conclusions

In summary, valuable metals were recovered from spent lithium cobalt oxide (LiCoO₂) batteries via hydrometallurgical processing. These recovered metals were subsequently transformed into catalyst active components and loaded onto ATP support using a sol–gel method. The Ni doping in Co₃O₄ led to a substantial increase in oxygen vacancy concentration triggering a LSPR effect, which enabled the material to absorb NIR light and release high-energy hot electrons. The resulting photothermal effect induced localized heating at the catalyst surface, significantly boosting the efficiency of photothermal CO₂ reduction. Under solar light irradiation, the Ni:Co₃O₄/ATP composite achieved a CO production rate of 30.1 μmol·g⁻¹·h⁻¹, representing a 3.3-fold enhancement compared to Co₃O₄/ATP prepared from cobalt nitrate. Moreover, the catalyst maintained excellent activity even after five consecutive reaction cycles. This work presents a novel strategy for the valorization of spent lithium-ion batteries and offers an effective approach for the efficient conversion of CO₂.