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2024 Volume 40 Issue 8
2024, 40(8): 230702
doi: 10.3866/PKU.WHXB202307024
Abstract:
Conjugated ladder polymers (CLPs) have attracted broad interest due to their intriguing optical and electronic properties. While many of these CLPs have been synthesized using solution-based reactions, on-surface synthesis under high vacuum conditions has gradually gained prominence in recent decades. This new approach holds promise for overcoming some of the limitations of conventional solution-based methods, such as the low solubility and stability of newly formed large π-conjugated systems. Azulene derivatives are attractive precursors for on-surface synthesis of CLPs that incorporate non-benzenoid moieties. The use of alkyl-substituted azulene precursors shows potential in providing CLPs with more complex backbone structures and modulated electronic properties compared to traditional CLPs that containing solely of six-membered rings. However, this strategy has been scarcely explored to date. In this study, we report on the thermal reactions of 3,3'-dibromo-2,2'-dimethyl-1,1'-biazulenyl (DBMA) on Au(111) surfaces. At room temperature, we observed that the deposited molecules formed amorphous aggregates in the fcc (face center cubic) regions of the reconstructed Au(111) surface, remaining unchanged below 100 ℃. Debromination of DBMA was induced above 150 ℃, leading to the formation of 1,1'-biazulenyl-2,2'-dimethyl-3,3'-diyls-Au organometallic polymers. These polymers exhibited complex stereostructures and distinct imaging features. At higher temperatures, the organometallic polymer underwent C―C coupling, followed by dehydrocyclization between the methyl groups and the adjacent azulene units, resulting in the ladder polymer containing benzo[a]azulene units. Interestingly, we observed that the formation of hexagonal rings between the methyl groups and the adjacent azulene units caused the polymeric chain to bend, increasing the distance between the corresponding reaction sites (methyl group and azulene) on the other side of the polymer chain. Due to the ring strain, the second ring closure did not occur within the azulene dimer as expected. Instead, this methyl group cyclized toward the other azulene unit, resulting in CLPs with a chevron shape and the absence of long-range periodicity. The evolution of related chemical species and the structures of CLPs were analyzed using scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (BR-AFM), and the reaction mechanism was discussed. This study thus demonstrates the feasibility of utilizing alkyl-substituted azulenic precursors in the synthesis of non-benzenoid carbon nanostructures on surfaces and suggests the possibility of developing two-dimensional nanostructures containing non-benzenoid units through on-surface azulene chemistry. ![]()
Conjugated ladder polymers (CLPs) have attracted broad interest due to their intriguing optical and electronic properties. While many of these CLPs have been synthesized using solution-based reactions, on-surface synthesis under high vacuum conditions has gradually gained prominence in recent decades. This new approach holds promise for overcoming some of the limitations of conventional solution-based methods, such as the low solubility and stability of newly formed large π-conjugated systems. Azulene derivatives are attractive precursors for on-surface synthesis of CLPs that incorporate non-benzenoid moieties. The use of alkyl-substituted azulene precursors shows potential in providing CLPs with more complex backbone structures and modulated electronic properties compared to traditional CLPs that containing solely of six-membered rings. However, this strategy has been scarcely explored to date. In this study, we report on the thermal reactions of 3,3'-dibromo-2,2'-dimethyl-1,1'-biazulenyl (DBMA) on Au(111) surfaces. At room temperature, we observed that the deposited molecules formed amorphous aggregates in the fcc (face center cubic) regions of the reconstructed Au(111) surface, remaining unchanged below 100 ℃. Debromination of DBMA was induced above 150 ℃, leading to the formation of 1,1'-biazulenyl-2,2'-dimethyl-3,3'-diyls-Au organometallic polymers. These polymers exhibited complex stereostructures and distinct imaging features. At higher temperatures, the organometallic polymer underwent C―C coupling, followed by dehydrocyclization between the methyl groups and the adjacent azulene units, resulting in the ladder polymer containing benzo[a]azulene units. Interestingly, we observed that the formation of hexagonal rings between the methyl groups and the adjacent azulene units caused the polymeric chain to bend, increasing the distance between the corresponding reaction sites (methyl group and azulene) on the other side of the polymer chain. Due to the ring strain, the second ring closure did not occur within the azulene dimer as expected. Instead, this methyl group cyclized toward the other azulene unit, resulting in CLPs with a chevron shape and the absence of long-range periodicity. The evolution of related chemical species and the structures of CLPs were analyzed using scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (BR-AFM), and the reaction mechanism was discussed. This study thus demonstrates the feasibility of utilizing alkyl-substituted azulenic precursors in the synthesis of non-benzenoid carbon nanostructures on surfaces and suggests the possibility of developing two-dimensional nanostructures containing non-benzenoid units through on-surface azulene chemistry.
2024, 40(8): 230704
doi: 10.3866/PKU.WHXB202307049
Abstract:
The social development model relying on coal, oil, natural gas, and other fossil fuels as the primary energy sources has not only hastened the depletion of non-renewable resources but also led to a continuous increase in atmospheric CO2 concentration. As human society's understanding of energy structures deepens and environmental consciousness grows, the pursuit of effective clean CO2 capture and catalytic conversion technologies has become a research priority. This is essential for promoting adjustments to the energy mix and achieving global carbon neutrality through artificial carbon cycling. Among the various CO2 capture and catalytic conversion technologies, electrochemical catalytic CO2 reduction (CO2RR) at ambient temperature and pressure holds promise for advancing artificial carbon cycling, carbon storage, and mitigating environmental degradation. This technology can be driven by intermittent renewable energy sources such as solar energy, wind energy, tidal power, geothermal energy, etc. Furthermore, using water as a clean proton source, a wide array of chemicals can be synthesized. While recent studies have made significant progress in CO2RR within aqueous solutions, there remains untapped potential in generating other important small organic molecules like urea, amides, amines, derivatives, and even amino acids. These compounds are of great interest due to their widespread applications in fertilizers, chemical synthesis, pharmaceuticals, and the aerospace industry. The electrocatalytic synthesis of organonitrogen compounds through N-integrated CO2RR (NCR) is considered crucial for improving the practical applications and offering a reference for biological small molecules. However, NCR involves multi-step electron and proton transfer processes, leading to current challenges, including slow kinetics and a complex reaction mechanism. In this review, we delve into the detailed reaction pathways and the rational design of catalysts for different NCR products, which are vital for developing highly efficient electrocatalysts. Although some progress has been made through various strategies, there are still challenges to overcome, limiting their large-scale practical applications. The discussion concludes by addressing these existing limitations and outlining potential avenues for future improvements. We hope that this feature article will be instrumental in the development of novel electrocatalysts for NCR. ![]()
The social development model relying on coal, oil, natural gas, and other fossil fuels as the primary energy sources has not only hastened the depletion of non-renewable resources but also led to a continuous increase in atmospheric CO2 concentration. As human society's understanding of energy structures deepens and environmental consciousness grows, the pursuit of effective clean CO2 capture and catalytic conversion technologies has become a research priority. This is essential for promoting adjustments to the energy mix and achieving global carbon neutrality through artificial carbon cycling. Among the various CO2 capture and catalytic conversion technologies, electrochemical catalytic CO2 reduction (CO2RR) at ambient temperature and pressure holds promise for advancing artificial carbon cycling, carbon storage, and mitigating environmental degradation. This technology can be driven by intermittent renewable energy sources such as solar energy, wind energy, tidal power, geothermal energy, etc. Furthermore, using water as a clean proton source, a wide array of chemicals can be synthesized. While recent studies have made significant progress in CO2RR within aqueous solutions, there remains untapped potential in generating other important small organic molecules like urea, amides, amines, derivatives, and even amino acids. These compounds are of great interest due to their widespread applications in fertilizers, chemical synthesis, pharmaceuticals, and the aerospace industry. The electrocatalytic synthesis of organonitrogen compounds through N-integrated CO2RR (NCR) is considered crucial for improving the practical applications and offering a reference for biological small molecules. However, NCR involves multi-step electron and proton transfer processes, leading to current challenges, including slow kinetics and a complex reaction mechanism. In this review, we delve into the detailed reaction pathways and the rational design of catalysts for different NCR products, which are vital for developing highly efficient electrocatalysts. Although some progress has been made through various strategies, there are still challenges to overcome, limiting their large-scale practical applications. The discussion concludes by addressing these existing limitations and outlining potential avenues for future improvements. We hope that this feature article will be instrumental in the development of novel electrocatalysts for NCR.
2024, 40(8): 230604
doi: 10.3866/PKU.WHXB202306048
Abstract:
With the advancement of science and technology, traditional energy sources such as oil and coal have been extensively depleted, leading to the emission of greenhouse gases like CO2. Consequently, issues such as energy scarcity and drastic environmental changes have emerged as pressing concerns that threaten human survival and development. Photocatalysis offers a promising solution by harnessing solar energy for chemical energy conversion, yielding clean and sustainable products. It is widely regarded as an emerging approach to address the energy crisis and environmental challenges. To achieve high-efficiency photocatalytic reactions, the selection of appropriate catalysts and co-catalysts plays a pivotal role. However, conventional photocatalysts such as TiO2, CdS, and g-C3N4 suffer from inherent limitations, including high charge recombination rates, low light utilization efficiency, poor stability, and sluggish charge transfer kinetics, which hinder the enhancement of photocatalytic efficiency. In this context, two-dimensional (2D) materials known as MXenes have gained prominence. These materials exhibit unique structural flexibility, diverse elemental compositions, superior conductivity, excellent carrier mobility, and abundant active sites, making them valuable co-catalysts in photocatalysis. MXenes accelerate interfacial charge transfer kinetics and mitigate charge recombination, enhancing the overall photocatalytic performance. This review provides a comprehensive overview of various methods employed to prepare high-quality MXenes under different conditions, such as water solution etching, water-free etching, and other physical methods. It also explores diverse strategies for constructing MXene-based composite photocatalytic systems, including in situ growth synthesis, in situ oxidation synthesis, and electrostatic self-assembly. Additionally, the review discusses various MXenes-based photosystems, such as MXene/TiO2, MXene/CdS, MXene/g-C3N4, MXene/WO3, and BiOBr/MXene/MMTex, and their applications in photocatalytic processes, including hydrogen production, CO2 reduction, environmental remediation, nitrogen fixation, and sterilization. The critical role of MXenes as reduction co-catalysts in these photoredox catalysis reactions is thoroughly examined, along with an elucidation of the relationship between MXene electronic structure and charge transfer characteristics. Furthermore, the review addresses the challenges related to the stability of MXenes in photocatalytic reactions and offers insights into potential strategies to mitigate this issue. Finally, the development prospects and future challenges of MXene-based composites in the field of photocatalysis are presented, taking into consideration the inherent limitations of MXenes and the requirements for industrialization. It is expected that this review will provide valuable insights into the physicochemical properties of MXenes and inspire innovative approaches to the rational design of diverse MXene-based photosystems for heterogeneous photocatalysis across various applications. ![]()
With the advancement of science and technology, traditional energy sources such as oil and coal have been extensively depleted, leading to the emission of greenhouse gases like CO2. Consequently, issues such as energy scarcity and drastic environmental changes have emerged as pressing concerns that threaten human survival and development. Photocatalysis offers a promising solution by harnessing solar energy for chemical energy conversion, yielding clean and sustainable products. It is widely regarded as an emerging approach to address the energy crisis and environmental challenges. To achieve high-efficiency photocatalytic reactions, the selection of appropriate catalysts and co-catalysts plays a pivotal role. However, conventional photocatalysts such as TiO2, CdS, and g-C3N4 suffer from inherent limitations, including high charge recombination rates, low light utilization efficiency, poor stability, and sluggish charge transfer kinetics, which hinder the enhancement of photocatalytic efficiency. In this context, two-dimensional (2D) materials known as MXenes have gained prominence. These materials exhibit unique structural flexibility, diverse elemental compositions, superior conductivity, excellent carrier mobility, and abundant active sites, making them valuable co-catalysts in photocatalysis. MXenes accelerate interfacial charge transfer kinetics and mitigate charge recombination, enhancing the overall photocatalytic performance. This review provides a comprehensive overview of various methods employed to prepare high-quality MXenes under different conditions, such as water solution etching, water-free etching, and other physical methods. It also explores diverse strategies for constructing MXene-based composite photocatalytic systems, including in situ growth synthesis, in situ oxidation synthesis, and electrostatic self-assembly. Additionally, the review discusses various MXenes-based photosystems, such as MXene/TiO2, MXene/CdS, MXene/g-C3N4, MXene/WO3, and BiOBr/MXene/MMTex, and their applications in photocatalytic processes, including hydrogen production, CO2 reduction, environmental remediation, nitrogen fixation, and sterilization. The critical role of MXenes as reduction co-catalysts in these photoredox catalysis reactions is thoroughly examined, along with an elucidation of the relationship between MXene electronic structure and charge transfer characteristics. Furthermore, the review addresses the challenges related to the stability of MXenes in photocatalytic reactions and offers insights into potential strategies to mitigate this issue. Finally, the development prospects and future challenges of MXene-based composites in the field of photocatalysis are presented, taking into consideration the inherent limitations of MXenes and the requirements for industrialization. It is expected that this review will provide valuable insights into the physicochemical properties of MXenes and inspire innovative approaches to the rational design of diverse MXene-based photosystems for heterogeneous photocatalysis across various applications.
2024, 40(8): 230802
doi: 10.3866/PKU.WHXB202308020
Abstract:
Capacitive deionization (CDI) has been considered one of the most promising desalination technologies in the past decade. However, it faces challenges related to low salt removal efficiency in high salinity water. To address this issue, ion intercalation materials have been developed as anodes for CDI due to their abundant electroactive sites capable of accommodating large salty ions. V2O3, a typical intercalation host, has garnered significant attention in the field of metal-ion batteries and appears to be a suitable candidate for CDI. Nevertheless, structural instability and slow ion diffusion, resulting from large volume expansion and low intrinsic electron/ion conductivity, present obstacles to its commercial application. Given their high specific surface area, abundant ion diffusion channels, and excellent conductivity, derivatives of metal-organic frameworks (MOFs) have become highly attractive in the electrochemical research community. In this study, 2D V2O3@porous carbon (V2O3@PC) nanosheets were prepared using homologous metal V2CFx MXene as a precursor for CDI anodes, aiming to enhance salt removal capacity. The structure, crystallinity, wettability, graphitization degree, and electrochemical behavior of V2O3@PC were investigated by adjusting carbonization temperatures. The findings reveal that V2O3@PC exhibits a typical 2D nanosheet structure, with highly crystalline V2O3 nanoparticles securely enveloped by graphitized PC. The electronic coupling between PC and V2O3 ensures high electron conductivity. This unique structure demonstrates excellent interfacial wettability and high conductivity, facilitating electrolyte penetration, accelerating interfacial charge transfer, and enhancing salt ion diffusion. Additionally, the PC effectively restricts the volume expansion of V2O3. Moreover, reversible electrochemical conversion between V3+/V4+ contributes to Na+ storage, aiding the desalination/regeneration process. Notably, X-ray diffraction (XRD) analysis revealed the preferential growth of V2O3 crystal planes at different carbonization temperatures. Consequently, the optimized V2O3@PC-850 electrode exhibits remarkable desalination performance, including a desalination capacity of 2.20 mmol·g−1, desalination rate of 0.13 mmol·g−1·min−1, water recovery rate of 62%, and energy consumption of 24.0 Wh·m−3 at 1.2 V in 1000 μS·cm−1 NaCl solutions. Compared to V2O3@PC-750 and V2O3@PC-950, the superior performance of V2O3@PC-850 can be attributed to its enhanced interfacial wettability, which promotes charge transfer and improves salt ion diffusion kinetics. Additionally, the preferential growth of the (110) crystal plane in V2O3@PC-850 enhances ion storage capacity, contributing to its superior desalination performance. This study offers new insights into the synergistic enhancement of electrochemical ion removal characteristics through the utilization of metal oxide and carbon nanomaterials. ![]()
Capacitive deionization (CDI) has been considered one of the most promising desalination technologies in the past decade. However, it faces challenges related to low salt removal efficiency in high salinity water. To address this issue, ion intercalation materials have been developed as anodes for CDI due to their abundant electroactive sites capable of accommodating large salty ions. V2O3, a typical intercalation host, has garnered significant attention in the field of metal-ion batteries and appears to be a suitable candidate for CDI. Nevertheless, structural instability and slow ion diffusion, resulting from large volume expansion and low intrinsic electron/ion conductivity, present obstacles to its commercial application. Given their high specific surface area, abundant ion diffusion channels, and excellent conductivity, derivatives of metal-organic frameworks (MOFs) have become highly attractive in the electrochemical research community. In this study, 2D V2O3@porous carbon (V2O3@PC) nanosheets were prepared using homologous metal V2CFx MXene as a precursor for CDI anodes, aiming to enhance salt removal capacity. The structure, crystallinity, wettability, graphitization degree, and electrochemical behavior of V2O3@PC were investigated by adjusting carbonization temperatures. The findings reveal that V2O3@PC exhibits a typical 2D nanosheet structure, with highly crystalline V2O3 nanoparticles securely enveloped by graphitized PC. The electronic coupling between PC and V2O3 ensures high electron conductivity. This unique structure demonstrates excellent interfacial wettability and high conductivity, facilitating electrolyte penetration, accelerating interfacial charge transfer, and enhancing salt ion diffusion. Additionally, the PC effectively restricts the volume expansion of V2O3. Moreover, reversible electrochemical conversion between V3+/V4+ contributes to Na+ storage, aiding the desalination/regeneration process. Notably, X-ray diffraction (XRD) analysis revealed the preferential growth of V2O3 crystal planes at different carbonization temperatures. Consequently, the optimized V2O3@PC-850 electrode exhibits remarkable desalination performance, including a desalination capacity of 2.20 mmol·g−1, desalination rate of 0.13 mmol·g−1·min−1, water recovery rate of 62%, and energy consumption of 24.0 Wh·m−3 at 1.2 V in 1000 μS·cm−1 NaCl solutions. Compared to V2O3@PC-750 and V2O3@PC-950, the superior performance of V2O3@PC-850 can be attributed to its enhanced interfacial wettability, which promotes charge transfer and improves salt ion diffusion kinetics. Additionally, the preferential growth of the (110) crystal plane in V2O3@PC-850 enhances ion storage capacity, contributing to its superior desalination performance. This study offers new insights into the synergistic enhancement of electrochemical ion removal characteristics through the utilization of metal oxide and carbon nanomaterials.
2024, 40(8): 230703
doi: 10.3866/PKU.WHXB202307037
Abstract:
Single-component organic solar cells (SCOSCs) have emerged as promising candidates for renewable energy applications due to their simplified film fabrication process and well-controlled morphology. High-performance SCOSCs typically employ active layer materials comprising block copolymers and double-cable conjugated polymers. Among these, double-cable conjugated polymers have attracted a lot of interest in SCOSCs due to their precisely defined structure and easily controllable microphase morphology. In the early stages of double-cable conjugated polymers, most of them contain the polythiophene backbone and fullerene side units, severely limiting the development of SCOSCs. Fortunately, the emergence of novel materials has progressively led to the development of new types of double-cable conjugated polymers. Double-cable conjugated polymers based on acylimide compound have exhibited device performances exceeding 8%. Nevertheless, acylimide-type electron acceptors exhibit a limited photo-response range, resulting in lower photocurrents in SCOSCs. The utilization of A-D-A-type electron acceptors (where D represents electron-donating groups and A represents electron-withdrawing groups) have effectively broadened the absorption spectra of materials due to induced intramolecular charge transfer. Double-cable polymers using A-D-A-type electron acceptors as the side units have achieved efficiencies exceeding 10%. However, significant voltage losses have hampered further improvements in their performance. Chlorine atoms play a crucial role in organic solar cells due to enhanced crystallinity in both chlorine-substituted donor polymers and acceptor molecules, and they can also adjust material energy levels and optimize film morphology. Nevertheless, their role in SCOSCs has been scarcely explored. This limitation arises from the increased complexity of morphology control in double-cable conjugated polymers, where the donor and acceptor segments are covalently linked in one molecule making their crystalline behavior more complicated on account of their mutual restraint. In this study, we have designed and synthesized chlorine-substituted double-cable conjugated polymers, denoted as as-DCPIC-Cl and as-DCPIC-2Cl. The results indicate that the introduction of chlorine atoms into the conjugated backbone reduces energy losses in the devices, resulting in an enhancement of open-circuit voltage (VOC). However, the introduction of chlorine atoms also leads to unbalanced charge transport and increased trap-assisted charge recombination, causing a decrease in the fill factor (FF) and short-circuit current density (JSC). Meanwhile, Grazing-incidence wide-angle X-ray scattering (GIWAXS) tests demonstrate that the introduction of chlorine atoms does not affect the aggregation/crystallization behavior of acceptor units. SCOSCs based on as-DCPIC-Cl achieved a power conversion efficiency (PCE) of 10.14%, which is among the best PCEs reported for SCOSCs based on non-fused electron acceptors. ![]()
Single-component organic solar cells (SCOSCs) have emerged as promising candidates for renewable energy applications due to their simplified film fabrication process and well-controlled morphology. High-performance SCOSCs typically employ active layer materials comprising block copolymers and double-cable conjugated polymers. Among these, double-cable conjugated polymers have attracted a lot of interest in SCOSCs due to their precisely defined structure and easily controllable microphase morphology. In the early stages of double-cable conjugated polymers, most of them contain the polythiophene backbone and fullerene side units, severely limiting the development of SCOSCs. Fortunately, the emergence of novel materials has progressively led to the development of new types of double-cable conjugated polymers. Double-cable conjugated polymers based on acylimide compound have exhibited device performances exceeding 8%. Nevertheless, acylimide-type electron acceptors exhibit a limited photo-response range, resulting in lower photocurrents in SCOSCs. The utilization of A-D-A-type electron acceptors (where D represents electron-donating groups and A represents electron-withdrawing groups) have effectively broadened the absorption spectra of materials due to induced intramolecular charge transfer. Double-cable polymers using A-D-A-type electron acceptors as the side units have achieved efficiencies exceeding 10%. However, significant voltage losses have hampered further improvements in their performance. Chlorine atoms play a crucial role in organic solar cells due to enhanced crystallinity in both chlorine-substituted donor polymers and acceptor molecules, and they can also adjust material energy levels and optimize film morphology. Nevertheless, their role in SCOSCs has been scarcely explored. This limitation arises from the increased complexity of morphology control in double-cable conjugated polymers, where the donor and acceptor segments are covalently linked in one molecule making their crystalline behavior more complicated on account of their mutual restraint. In this study, we have designed and synthesized chlorine-substituted double-cable conjugated polymers, denoted as as-DCPIC-Cl and as-DCPIC-2Cl. The results indicate that the introduction of chlorine atoms into the conjugated backbone reduces energy losses in the devices, resulting in an enhancement of open-circuit voltage (VOC). However, the introduction of chlorine atoms also leads to unbalanced charge transport and increased trap-assisted charge recombination, causing a decrease in the fill factor (FF) and short-circuit current density (JSC). Meanwhile, Grazing-incidence wide-angle X-ray scattering (GIWAXS) tests demonstrate that the introduction of chlorine atoms does not affect the aggregation/crystallization behavior of acceptor units. SCOSCs based on as-DCPIC-Cl achieved a power conversion efficiency (PCE) of 10.14%, which is among the best PCEs reported for SCOSCs based on non-fused electron acceptors.
2024, 40(8): 230801
doi: 10.3866/PKU.WHXB202308015
Abstract:
Electrocatalysts play a pivotal role in the electrochemical water splitting process to produce hydrogen fuel. The advancement of this technology relies on the development of efficient, cost-effective, and readily available electrocatalysts. Two-dimensional (2D) MXene materials have garnered significant attention due to their unique physicochemical properties, rendering them promising candidates for electrocatalytic applications. While there are numerous types of MXene materials available, only a few possess intrinsic hydrogen evolution reaction (HER) catalytic activity. However, MXene materials can serve as excellent platforms for enhancing catalytic HER activity by combining them with other substances, owing to their large specific surface area, high conductivity, and abundant surface functional groups. In this study, we initially conducted a predictive analysis using density functional theory (DFT) to assess the potential of combining CoP with Ti3C2Tx MXene materials (where Tx represents ―F and ―OH functional groups) in reducing the adsorption free energy of hydrogen (ΔGH*). The results indicated that the CoP-Ti3C2Tx nanocomposites exhibited a ΔGH* value approaching 0, suggesting promising HER performance. Following this theoretical prediction, we synthesized the CoP-Ti3C2Tx MXene nanocomposites. Comprehensive characterization of the synthesized nanocomposites was performed using various techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These analyses confirmed the successful decoration of CoP on the MXene nanosheets and provided insights into the structural and compositional properties of the nanocomposites. Furthermore, we evaluated the electrochemical performance of the CoP-Ti3C2Tx nanocomposites through linear sweep voltammetry and chronoamperometry measurements. The results demonstrated superior catalytic activity and stability for the HER compared to pure Ti3C2Tx and CoP catalysts. Specifically, the as-synthesized CoP-Ti3C2Tx MXene nanocomposites exhibited remarkable electrocatalytic HER kinetics, featuring a low overpotential of 135 mV at a current density of 10 mA∙cm−2 and a small Tafel slope of 48 mV∙dec−1 in a 0.5 mol∙L−1 H2SO4 solution, with the electrocatalyst maintaining stability for up to 50 h. Subsequent theoretical calculations were conducted to elucidate the factors contributing to the exceptional electrocatalytic performance of the CoP-Ti3C2Tx MXene nanocomposites. It was determined that the metallic conductivity of Ti3C2Tx MXene materials, well-structured interface charge transfer, and optimized electronic structure of CoP played significant roles in enhancing catalytic activity. In conclusion, this study underscores the potential of CoP-decorated Ti3C2Tx MXene nanocomposites as promising electrocatalysts for efficient HER in various energy conversion and storage devices. These findings represent a significant contribution to the development of robust and efficient catalysts for hydrogen generation, a critical component of renewable energy applications and sustainable development.![]()
Electrocatalysts play a pivotal role in the electrochemical water splitting process to produce hydrogen fuel. The advancement of this technology relies on the development of efficient, cost-effective, and readily available electrocatalysts. Two-dimensional (2D) MXene materials have garnered significant attention due to their unique physicochemical properties, rendering them promising candidates for electrocatalytic applications. While there are numerous types of MXene materials available, only a few possess intrinsic hydrogen evolution reaction (HER) catalytic activity. However, MXene materials can serve as excellent platforms for enhancing catalytic HER activity by combining them with other substances, owing to their large specific surface area, high conductivity, and abundant surface functional groups. In this study, we initially conducted a predictive analysis using density functional theory (DFT) to assess the potential of combining CoP with Ti3C2Tx MXene materials (where Tx represents ―F and ―OH functional groups) in reducing the adsorption free energy of hydrogen (ΔGH*). The results indicated that the CoP-Ti3C2Tx nanocomposites exhibited a ΔGH* value approaching 0, suggesting promising HER performance. Following this theoretical prediction, we synthesized the CoP-Ti3C2Tx MXene nanocomposites. Comprehensive characterization of the synthesized nanocomposites was performed using various techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These analyses confirmed the successful decoration of CoP on the MXene nanosheets and provided insights into the structural and compositional properties of the nanocomposites. Furthermore, we evaluated the electrochemical performance of the CoP-Ti3C2Tx nanocomposites through linear sweep voltammetry and chronoamperometry measurements. The results demonstrated superior catalytic activity and stability for the HER compared to pure Ti3C2Tx and CoP catalysts. Specifically, the as-synthesized CoP-Ti3C2Tx MXene nanocomposites exhibited remarkable electrocatalytic HER kinetics, featuring a low overpotential of 135 mV at a current density of 10 mA∙cm−2 and a small Tafel slope of 48 mV∙dec−1 in a 0.5 mol∙L−1 H2SO4 solution, with the electrocatalyst maintaining stability for up to 50 h. Subsequent theoretical calculations were conducted to elucidate the factors contributing to the exceptional electrocatalytic performance of the CoP-Ti3C2Tx MXene nanocomposites. It was determined that the metallic conductivity of Ti3C2Tx MXene materials, well-structured interface charge transfer, and optimized electronic structure of CoP played significant roles in enhancing catalytic activity. In conclusion, this study underscores the potential of CoP-decorated Ti3C2Tx MXene nanocomposites as promising electrocatalysts for efficient HER in various energy conversion and storage devices. These findings represent a significant contribution to the development of robust and efficient catalysts for hydrogen generation, a critical component of renewable energy applications and sustainable development.
2024, 40(8): 230800
doi: 10.3866/PKU.WHXB202308003
Abstract:
Surface reconstruction inevitably occurs during pre-catalysis for the oxygen evolution reaction (OER); however, obtaining OER electrocatalysts with high performance and stability remains a challenge. In this study, we have developed a bimetallic leaching-induced surface reconstruction strategy to fabricate efficient electrocatalysts for water oxidation. Microcolumn arrays consisting of α-CoMoO4, K2Co2(MoO4)3, Co3O4, and CoFe2O4 four-phase oxides were integrated as pre-catalyst by a hydrothermal, ion-exchange, and subsequent annealing process. In situ Raman spectroelectrochemical and ex situ X-ray diffraction (XRD) studies revealed that the rapid dissolution of the unstable component K2Co2(MoO4)3 triggered the adaptive leaching of Mo and K, which accelerated the transformation of the surface-enriched α-Co(OH)2 to the active phase of CoOOH at low voltage. Furthermore, the stable CoFe2O4 component couples the reconfigured new phase CoO with the amorphous layer CoOOH to form a compact hierarchical structure of CoFe2O4@CoO@CoOOH, which plays the role of a nanofence and effectively prevents the catalyst from over-reconstruction, thus achieving excellent catalytic stability. This work provides a novel idea for designing OER catalysts with excellent activity and stability at high current densities. ![]()
Surface reconstruction inevitably occurs during pre-catalysis for the oxygen evolution reaction (OER); however, obtaining OER electrocatalysts with high performance and stability remains a challenge. In this study, we have developed a bimetallic leaching-induced surface reconstruction strategy to fabricate efficient electrocatalysts for water oxidation. Microcolumn arrays consisting of α-CoMoO4, K2Co2(MoO4)3, Co3O4, and CoFe2O4 four-phase oxides were integrated as pre-catalyst by a hydrothermal, ion-exchange, and subsequent annealing process. In situ Raman spectroelectrochemical and ex situ X-ray diffraction (XRD) studies revealed that the rapid dissolution of the unstable component K2Co2(MoO4)3 triggered the adaptive leaching of Mo and K, which accelerated the transformation of the surface-enriched α-Co(OH)2 to the active phase of CoOOH at low voltage. Furthermore, the stable CoFe2O4 component couples the reconfigured new phase CoO with the amorphous layer CoOOH to form a compact hierarchical structure of CoFe2O4@CoO@CoOOH, which plays the role of a nanofence and effectively prevents the catalyst from over-reconstruction, thus achieving excellent catalytic stability. This work provides a novel idea for designing OER catalysts with excellent activity and stability at high current densities.
2024, 40(8): 230802
doi: 10.3866/PKU.WHXB202308027
Abstract:
The electrochemical carbon dioxide reduction reaction (eCO2RR) can convert CO2 into valuable chemicals, achieving a carbon cycle. Copper-based catalysts have demonstrated a unique ability to produce C2+ products in eCO2RR, which is often limited by the scaling relationship of the reaction intermediates, complex reaction mechanism and competitive H2 evolution. Organic functionalization is a promising strategy for regulating the activity and selectivity of eCO2RR toward C2+ products. However, the mechanism behind such regulation of eCO2RR, especially at the molecular level, remains elusive. In this study, Cu nanoparticles were prepared and functionalized with a set of amine derivatives, including hexadecylamine (HDA), N-methylhexadecylamine (N-MHDA), hexadecyldimethylamine (HDDMA), and palmitamide (PMM). The impact of the molecular structure of the amine surfactants on the selectivity and activity toward eCO2RR was systematically explored through both experiments and theoretical calculations. X-ray photoelectron spectroscopy and density functional theory calculations revealed that HDA functionalization of the Cu catalyst surface resulted in negative charge transfer from amine molecules to Cu. ECO2RR was examined in a 1.0 mol∙L−1 KOH aqueous electrolyte. HDA functionalization of the Cu catalyst achieved the highest Faradaic efficiency (FE) of 73.5% for C2 products and 46.4% for C2H4, respectively. It also provided the highest C2 partial current density of 131.4 mA∙cm−2 at −0.9 Ⅴ vs. reversible hydrogen electrode (RHE) among these amine derivatives functionalized Cu catalysts. In contrast, the highest FE and partial current density for C2 products achieved with pristine Cu catalysts were only 27.0% and 50.5 mA·cm−2, respectively. Theoretical studies demonstrated that hydrogen bonding interactions of HDA with CO2 and eCO2RR intermediates enriched CO2, CO, and other intermediates, lowered the kinetic energy barrier of CO―CHO coupling and thereby promoted eCO2RR to C2 products. Replacing the H atoms of the amine group with methyl groups in N-MHDA and HDDMA resulted in dominant hydrogen evolution reaction (HER) in eCO2RR. PMM bonding with the Cu surface through a Cu―O bond, instead of Cu―N bonding as in HDA, N-MHDA and HDDMA, resulted in preferred ethanol production. In situ Raman spectroscopy indicated CO adsorption on Cu at atop sites for HDA-capped Cu catalysts, instead of bridge site CO adsorption on clean Cu surfaces, possibly due to the enriched CO in the former case. HDA also increased the local pH relative to pristine Cu catalysts. The Cu-HDA-based rechargeable Zn-CO2 battery exhibited a superior maximum power density of 6.48 mW∙cm–2 at a discharge current density of 16 mA∙cm–2 and remarkable rechargeable durability for 60 h, outperforming most of the reported catalysts in the literature. This work enhances CO2-C2 conversion by tuning the eCO2RR activity and selectivity of Cu-based materials, unravels the reaction mechanism at the molecular level, and provides new insights for promoting C2 products in eCO2RR through surface functionalization with organic molecules. ![]()
The electrochemical carbon dioxide reduction reaction (eCO2RR) can convert CO2 into valuable chemicals, achieving a carbon cycle. Copper-based catalysts have demonstrated a unique ability to produce C2+ products in eCO2RR, which is often limited by the scaling relationship of the reaction intermediates, complex reaction mechanism and competitive H2 evolution. Organic functionalization is a promising strategy for regulating the activity and selectivity of eCO2RR toward C2+ products. However, the mechanism behind such regulation of eCO2RR, especially at the molecular level, remains elusive. In this study, Cu nanoparticles were prepared and functionalized with a set of amine derivatives, including hexadecylamine (HDA), N-methylhexadecylamine (N-MHDA), hexadecyldimethylamine (HDDMA), and palmitamide (PMM). The impact of the molecular structure of the amine surfactants on the selectivity and activity toward eCO2RR was systematically explored through both experiments and theoretical calculations. X-ray photoelectron spectroscopy and density functional theory calculations revealed that HDA functionalization of the Cu catalyst surface resulted in negative charge transfer from amine molecules to Cu. ECO2RR was examined in a 1.0 mol∙L−1 KOH aqueous electrolyte. HDA functionalization of the Cu catalyst achieved the highest Faradaic efficiency (FE) of 73.5% for C2 products and 46.4% for C2H4, respectively. It also provided the highest C2 partial current density of 131.4 mA∙cm−2 at −0.9 Ⅴ vs. reversible hydrogen electrode (RHE) among these amine derivatives functionalized Cu catalysts. In contrast, the highest FE and partial current density for C2 products achieved with pristine Cu catalysts were only 27.0% and 50.5 mA·cm−2, respectively. Theoretical studies demonstrated that hydrogen bonding interactions of HDA with CO2 and eCO2RR intermediates enriched CO2, CO, and other intermediates, lowered the kinetic energy barrier of CO―CHO coupling and thereby promoted eCO2RR to C2 products. Replacing the H atoms of the amine group with methyl groups in N-MHDA and HDDMA resulted in dominant hydrogen evolution reaction (HER) in eCO2RR. PMM bonding with the Cu surface through a Cu―O bond, instead of Cu―N bonding as in HDA, N-MHDA and HDDMA, resulted in preferred ethanol production. In situ Raman spectroscopy indicated CO adsorption on Cu at atop sites for HDA-capped Cu catalysts, instead of bridge site CO adsorption on clean Cu surfaces, possibly due to the enriched CO in the former case. HDA also increased the local pH relative to pristine Cu catalysts. The Cu-HDA-based rechargeable Zn-CO2 battery exhibited a superior maximum power density of 6.48 mW∙cm–2 at a discharge current density of 16 mA∙cm–2 and remarkable rechargeable durability for 60 h, outperforming most of the reported catalysts in the literature. This work enhances CO2-C2 conversion by tuning the eCO2RR activity and selectivity of Cu-based materials, unravels the reaction mechanism at the molecular level, and provides new insights for promoting C2 products in eCO2RR through surface functionalization with organic molecules.
2024, 40(8): 230802
doi: 10.3866/PKU.WHXB202308024
Abstract:
Large-scale hydrogen production through the electrochemical water splitting technique is an important way for addressing the impending energy and environmental crisis. This approach requires highly efficient and robust bifunctional cost-effective electrocatalysts. Engineering amorphous and crystalline phases within electrocatalysts is a key method for enhancing the catalytic kinetics of water electrolysis, due to their unique physicochemical properties. The interface and amorphous regions constructed within heterostructures serve as highly active sites that play a crucial role in electrochemical reactions. On the other hand, highly crystalline regions within the heterostructure demonstrated high tolerance in harsh environments, which helps to improve the stability of the overall catalyst. However, effectively tailoring the crystalline state of catalysts within a microenvironment presents a significant challenge. Herein, construction of a novel CrS/CoS2 heterojunction with precise control over crystallinity were presented. The optimized amorphous CrS/highly crystalline CoS2 heterojunction (A-CrS/HC-CoS2) exhibits a low overpotential of 90.6 mV (at 10 mA∙cm−2) and 370.5 mV (at 50 mA∙cm−2) for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations reveal that charge redistribution induces variations in the d-band center value at the A-CrS/HC-CoS2 heterostructure interface, enhancing the catalytic activity for both HER and OER. The displacement of the d-band due to charge redistribution in the Cr―S―Co bond within A-CrS/HC-CoS2 contributes to the modulation of the adsorption capacity of H* and OOH* intermediates on the catalyst surface, thereby optimizing the rate-determining step for HER and OER. The amorphous/highly crystalline structure also facilitates the structural and compositional evolution of A-CrS/HC-CoS2 during water electrolysis, ensuring excellent stability. As a bifunctional catalyst in a methanol-assisted energy-saving hydrogen production device, A-CrS/HC-CoS2 operates at a low cell voltage of 1.51 Ⅴ to deliver a current density of 10 mA∙cm−2, making it a promising candidate among metal-based catalysts. The well-preserved amorphous/crystalline heterointerfaces in A-CrS/HC-CoS2, along with favorable changes in surface composition, contribute to robust HER and OER stability. This work provides valuable insights into the manipulation of catalytic activity through crystalline control within amorphous/crystalline heterojunctions for bifunctional transition metal compound electrocatalysts. ![]()
Large-scale hydrogen production through the electrochemical water splitting technique is an important way for addressing the impending energy and environmental crisis. This approach requires highly efficient and robust bifunctional cost-effective electrocatalysts. Engineering amorphous and crystalline phases within electrocatalysts is a key method for enhancing the catalytic kinetics of water electrolysis, due to their unique physicochemical properties. The interface and amorphous regions constructed within heterostructures serve as highly active sites that play a crucial role in electrochemical reactions. On the other hand, highly crystalline regions within the heterostructure demonstrated high tolerance in harsh environments, which helps to improve the stability of the overall catalyst. However, effectively tailoring the crystalline state of catalysts within a microenvironment presents a significant challenge. Herein, construction of a novel CrS/CoS2 heterojunction with precise control over crystallinity were presented. The optimized amorphous CrS/highly crystalline CoS2 heterojunction (A-CrS/HC-CoS2) exhibits a low overpotential of 90.6 mV (at 10 mA∙cm−2) and 370.5 mV (at 50 mA∙cm−2) for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations reveal that charge redistribution induces variations in the d-band center value at the A-CrS/HC-CoS2 heterostructure interface, enhancing the catalytic activity for both HER and OER. The displacement of the d-band due to charge redistribution in the Cr―S―Co bond within A-CrS/HC-CoS2 contributes to the modulation of the adsorption capacity of H* and OOH* intermediates on the catalyst surface, thereby optimizing the rate-determining step for HER and OER. The amorphous/highly crystalline structure also facilitates the structural and compositional evolution of A-CrS/HC-CoS2 during water electrolysis, ensuring excellent stability. As a bifunctional catalyst in a methanol-assisted energy-saving hydrogen production device, A-CrS/HC-CoS2 operates at a low cell voltage of 1.51 Ⅴ to deliver a current density of 10 mA∙cm−2, making it a promising candidate among metal-based catalysts. The well-preserved amorphous/crystalline heterointerfaces in A-CrS/HC-CoS2, along with favorable changes in surface composition, contribute to robust HER and OER stability. This work provides valuable insights into the manipulation of catalytic activity through crystalline control within amorphous/crystalline heterojunctions for bifunctional transition metal compound electrocatalysts.
2024, 40(8): 230803
doi: 10.3866/PKU.WHXB202308036
Abstract:
The production of renewable fuels through water splitting via photocatalytic hydrogen production holds significant promise. Nonetheless, the sluggish kinetics of hydrogen evolution and the inadequate water adsorption on photocatalysts present notable challenges. In this study, we have devised a straightforward hydrothermal method to synthesize Bi2O3 (BO) derived from metal-organic frameworks (MOFs), loaded with flower-like ZnIn2S4 (ZIS). This approach substantially enhances water adsorption and surface catalytic reactions, resulting in a remarkable enhancement of photocatalytic activity. By employing triethanolamine (TEOA) as a sacrificial agent, the hydrogen evolution rate achieved with 15% (mass fraction) ZIS loading on BO reached an impressive value of 1610 μmol∙h−1∙g−1, marking a 6.34-fold increase compared to that observed for bare BO. Furthermore, through density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations, we have identified the reactions occurring at the ZIS/BO S-scheme heterojunction interface, including the identification of active sites for water adsorption and catalytic reactions. This study provides valuable insights into the development of high-performance composite photocatalytic materials with tailored electronic properties and wettability. ![]()
The production of renewable fuels through water splitting via photocatalytic hydrogen production holds significant promise. Nonetheless, the sluggish kinetics of hydrogen evolution and the inadequate water adsorption on photocatalysts present notable challenges. In this study, we have devised a straightforward hydrothermal method to synthesize Bi2O3 (BO) derived from metal-organic frameworks (MOFs), loaded with flower-like ZnIn2S4 (ZIS). This approach substantially enhances water adsorption and surface catalytic reactions, resulting in a remarkable enhancement of photocatalytic activity. By employing triethanolamine (TEOA) as a sacrificial agent, the hydrogen evolution rate achieved with 15% (mass fraction) ZIS loading on BO reached an impressive value of 1610 μmol∙h−1∙g−1, marking a 6.34-fold increase compared to that observed for bare BO. Furthermore, through density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations, we have identified the reactions occurring at the ZIS/BO S-scheme heterojunction interface, including the identification of active sites for water adsorption and catalytic reactions. This study provides valuable insights into the development of high-performance composite photocatalytic materials with tailored electronic properties and wettability.
