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2020 Volume 41 Issue 5
2020, 41(5):
Abstract:
2020, 41(5): 731-731
doi: 10.1016/S1872-2067(20)63558-6
Abstract:
2020, 41(5): 732-738
doi: S1872-2067(19)63325-5
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2020, 41(5): 739-755
doi: S1872-2067(19)63407-8
Abstract:
Proton exchange membrane fuel cells (PEMFCs) are considered a promising power source for electric vehicles and stationary residential applications. However, current PEMFCs have several problems that require solutions, including high cost, insufficient power density, and limited performance durability. A kinetically sluggish oxygen reduction reaction (ORR) is primarily responsible for these issues. The development of advanced Pt-based catalysts is crucial for solving these problems if the large-scale application of PEMFCs is to be realized. In this review, we summarize the recent progress in the development of PtM alloy (M=Fe, Co, Ni, etc.) catalysts with an emphasis on ordered PtM intermetallic catalysts, which exhibit significantly enhanced activity and stability. In addition to exploring the intrinsic catalytic performance in traditional aqueous electrolytes via engineering nanostructures, morphologies, and crystallinity of PtM particles, we highlight recent efforts to study catalysts under real fuel cell environments by the membrane electrode assembly (MEA).
Proton exchange membrane fuel cells (PEMFCs) are considered a promising power source for electric vehicles and stationary residential applications. However, current PEMFCs have several problems that require solutions, including high cost, insufficient power density, and limited performance durability. A kinetically sluggish oxygen reduction reaction (ORR) is primarily responsible for these issues. The development of advanced Pt-based catalysts is crucial for solving these problems if the large-scale application of PEMFCs is to be realized. In this review, we summarize the recent progress in the development of PtM alloy (M=Fe, Co, Ni, etc.) catalysts with an emphasis on ordered PtM intermetallic catalysts, which exhibit significantly enhanced activity and stability. In addition to exploring the intrinsic catalytic performance in traditional aqueous electrolytes via engineering nanostructures, morphologies, and crystallinity of PtM particles, we highlight recent efforts to study catalysts under real fuel cell environments by the membrane electrode assembly (MEA).
2020, 41(5): 756-769
doi: S1872-2067(19)63404-2
Abstract:
It is undisputed that hydrogen will play a great role in our future energetic mix, because it enables the storage of renewable electricity (power-to-H2) and the reversible conversion into electricity in fuel cell, not to speak of its wide use in the (petro)chemical industry. Whereas in these applications, pure hydrogen is required, today's hydrogen production is still largely based on fossil fuels and can therefore not be considered pure. Therefore, purification of hydrogen is mandatory, at a large scale. In addition, hydrogen being the lightest gas, its volumetric energy content is well-below its competing fuels, unless it is compressed at high pressures (typically 70 MPa), making compression unavoidable as well. This contribution will detail the means available today for both purification and for compression of hydrogen. It will show that among the available technologies, the electrochemical hydrogen compressor (EHC), which also enables hydrogen purification, has numerous advantages compared to the classical technologies currently used at the industrial scale. EHC has their thermodynamic and operational advantages, but also their ease of use. However, the deployment of EHCs will be viable only if they reach sufficient performances, which implies some specifications that their base materials should stick to. The present contribution will detail these specifications.
It is undisputed that hydrogen will play a great role in our future energetic mix, because it enables the storage of renewable electricity (power-to-H2) and the reversible conversion into electricity in fuel cell, not to speak of its wide use in the (petro)chemical industry. Whereas in these applications, pure hydrogen is required, today's hydrogen production is still largely based on fossil fuels and can therefore not be considered pure. Therefore, purification of hydrogen is mandatory, at a large scale. In addition, hydrogen being the lightest gas, its volumetric energy content is well-below its competing fuels, unless it is compressed at high pressures (typically 70 MPa), making compression unavoidable as well. This contribution will detail the means available today for both purification and for compression of hydrogen. It will show that among the available technologies, the electrochemical hydrogen compressor (EHC), which also enables hydrogen purification, has numerous advantages compared to the classical technologies currently used at the industrial scale. EHC has their thermodynamic and operational advantages, but also their ease of use. However, the deployment of EHCs will be viable only if they reach sufficient performances, which implies some specifications that their base materials should stick to. The present contribution will detail these specifications.
2020, 41(5): 770-782
doi: S1872-2067(19)63438-8
Abstract:
Hydrogen will be at the basis of the World's energy policy in forthcoming decades, owing to its decarbonized nature, at least when produced from renewables. For now, hydrogen is still essentially produced from fossil feedstock (and to a minor extent from biomass); in consequence the present hydrogen gas on the market is containing non-negligible amounts of impurities that prevent its immediate usage in specialty chemistry or as an energy carrier in fuel cells, e.g. in transportation applications (cars, buses, trains, boats, etc.) that gradually spread on the planet. For these purposes, hydrogen must be of sufficient purity but also sufficiently compressed (at high pressures, typically 70 MPa), rendering purification and compression steps unavoidable in the hydrogen cycle. As shown in the first part of this contribution "Electrochemical hydrogen compression and purification versus competing technologies: Part I. pros and cons", electrochemical hydrogen compressors (EHCs), which enable both hydrogen purification and compression, exhibit many theoretical (thermodynamic) and practical (kinetics) advantages over their mechanical counterparts. However, in order to be competitive, EHCs must operate in very intensive conditions (high current density and low cell voltage) that can only be reached if their core materials, e.g. the membrane and the electrodes/electrocatalysts, are optimized. This contribution will particularly focus on the properties electrocatalysts must exhibit to be used in EHCs:they shall promote (very) fast hydrogen oxidation reaction (HOR) in presence of impurities, which implies that they are (very) tolerant to poisons as well. This consists of a prerequisite for the operation of the anode of an EHC used for the purification-compression of hydrogen, and the materials developed for poison-tolerance in the vast literature on low-temperature fuel cells, may not always satisfy these two criteria, as this contribution will review.
Hydrogen will be at the basis of the World's energy policy in forthcoming decades, owing to its decarbonized nature, at least when produced from renewables. For now, hydrogen is still essentially produced from fossil feedstock (and to a minor extent from biomass); in consequence the present hydrogen gas on the market is containing non-negligible amounts of impurities that prevent its immediate usage in specialty chemistry or as an energy carrier in fuel cells, e.g. in transportation applications (cars, buses, trains, boats, etc.) that gradually spread on the planet. For these purposes, hydrogen must be of sufficient purity but also sufficiently compressed (at high pressures, typically 70 MPa), rendering purification and compression steps unavoidable in the hydrogen cycle. As shown in the first part of this contribution "Electrochemical hydrogen compression and purification versus competing technologies: Part I. pros and cons", electrochemical hydrogen compressors (EHCs), which enable both hydrogen purification and compression, exhibit many theoretical (thermodynamic) and practical (kinetics) advantages over their mechanical counterparts. However, in order to be competitive, EHCs must operate in very intensive conditions (high current density and low cell voltage) that can only be reached if their core materials, e.g. the membrane and the electrodes/electrocatalysts, are optimized. This contribution will particularly focus on the properties electrocatalysts must exhibit to be used in EHCs:they shall promote (very) fast hydrogen oxidation reaction (HOR) in presence of impurities, which implies that they are (very) tolerant to poisons as well. This consists of a prerequisite for the operation of the anode of an EHC used for the purification-compression of hydrogen, and the materials developed for poison-tolerance in the vast literature on low-temperature fuel cells, may not always satisfy these two criteria, as this contribution will review.
2020, 41(5): 783-798
doi: 10.1016/S1872-2067(20)63536-7
Abstract:
Developing sustainable and clean electrochemical energy conversion technologies is a crucial step in addressing the challenges of energy shortage and environmental pollution. Exploring and developing new electrocatalysts with excellent performance and low cost will facilitate the commercial use of these energy conversion technologies. Recently, dual-atom catalysts (DACs) have attracted considerable research interest since they exhibit higher metal atom loading and more flexible active sites compared to single-atom catalysts (SACs). In this paper, the latest preparation methods and characterization techniques of DACs are systematically reviewed. The advantages of homonuclear and heteronuclear DACs and the catalytic mechanism and identification technologies between the two DACs are highlighted. The current applications of DACs in the field of electrocatalysis are summarized. The development opportunities and challenges of DACs in the future are prospected. The ultimate goal is to provide new ideas for the preparation of new catalysts with excellent properties by customizing diatomic catalysts for electrochemical applications.
Developing sustainable and clean electrochemical energy conversion technologies is a crucial step in addressing the challenges of energy shortage and environmental pollution. Exploring and developing new electrocatalysts with excellent performance and low cost will facilitate the commercial use of these energy conversion technologies. Recently, dual-atom catalysts (DACs) have attracted considerable research interest since they exhibit higher metal atom loading and more flexible active sites compared to single-atom catalysts (SACs). In this paper, the latest preparation methods and characterization techniques of DACs are systematically reviewed. The advantages of homonuclear and heteronuclear DACs and the catalytic mechanism and identification technologies between the two DACs are highlighted. The current applications of DACs in the field of electrocatalysis are summarized. The development opportunities and challenges of DACs in the future are prospected. The ultimate goal is to provide new ideas for the preparation of new catalysts with excellent properties by customizing diatomic catalysts for electrochemical applications.
2020, 41(5): 799-806
doi: S1872-2067(19)63405-4
Abstract:
Introducing catalytically-active Fe and N into carbon materials results in promising FeNC catalysts for oxygen reduction reaction. However, the doped Fe and N species are frequently subject to heavy loss in a traditional carbonization process owing to Fe agglomeration and evaporation of N-contained small molecules. Besides, pyrolysis may make materials sintering which embeds a large number of active sites in the bulk phase and impedes direct exposure of reactive centers to the reactants. We here report that when calcinations, the addition of ZnCl2, an ordinary salt with very wide melting temperature range well covering the carbonization process of the precursor iron porphyrin, can significantly enhance the doping level of the active species and simultaneously create highly porous structures for FeNC catalysts. The obtained FeNC demonstrates ultrahigh catalytic activities even significantly better than Pt/C in oxygen reduction reaction.
Introducing catalytically-active Fe and N into carbon materials results in promising FeNC catalysts for oxygen reduction reaction. However, the doped Fe and N species are frequently subject to heavy loss in a traditional carbonization process owing to Fe agglomeration and evaporation of N-contained small molecules. Besides, pyrolysis may make materials sintering which embeds a large number of active sites in the bulk phase and impedes direct exposure of reactive centers to the reactants. We here report that when calcinations, the addition of ZnCl2, an ordinary salt with very wide melting temperature range well covering the carbonization process of the precursor iron porphyrin, can significantly enhance the doping level of the active species and simultaneously create highly porous structures for FeNC catalysts. The obtained FeNC demonstrates ultrahigh catalytic activities even significantly better than Pt/C in oxygen reduction reaction.
2020, 41(5): 807-812
doi: S1872-2067(19)63451-0
Abstract:
Pt monolayer-based core-shell catalysts have garnered significant interest for the application of low temperature fuel cell technology as their use may enable a decreased loading of Pt while still providing sufficient current density to meet volumetric requirements. One promising candidate in this class of materials is a Pd@Pt core-shell catalyst, which shows enhanced activity toward oxygen reduction reaction (ORR). One concern with the use of Pd@Pt, however, is the durability of the core-shell structure as Pd atoms are thermodynamically favored to migrate to the surface. The pathway of the migration has not been systematically studied. The current study explores the stability of this structure to thermal annealing and probes the effect of this heat treatment on the catalyst surface structure and its oxygen reduction activity. It was found that surface alloying between Pd and Pt occurs at temperatures as low as 200℃, and significantly alters the structure and ORR catalytic activity in the range of 200-300℃. Our results shed lights on the thermal induced interatomic diffusion in all core-shell and thin film structures.
Pt monolayer-based core-shell catalysts have garnered significant interest for the application of low temperature fuel cell technology as their use may enable a decreased loading of Pt while still providing sufficient current density to meet volumetric requirements. One promising candidate in this class of materials is a Pd@Pt core-shell catalyst, which shows enhanced activity toward oxygen reduction reaction (ORR). One concern with the use of Pd@Pt, however, is the durability of the core-shell structure as Pd atoms are thermodynamically favored to migrate to the surface. The pathway of the migration has not been systematically studied. The current study explores the stability of this structure to thermal annealing and probes the effect of this heat treatment on the catalyst surface structure and its oxygen reduction activity. It was found that surface alloying between Pd and Pt occurs at temperatures as low as 200℃, and significantly alters the structure and ORR catalytic activity in the range of 200-300℃. Our results shed lights on the thermal induced interatomic diffusion in all core-shell and thin film structures.
2020, 41(5): 813-819
doi: S1872-2067(19)63310-3
Abstract:
Bimetallic Pt-based catalysts have been extensively investigated to enhance the performance of direct methanol fuel cells (DMFCs) because CO, a by-product, reduces the activity of the pure Pt catalysts. Herein, we synthesized Pt-Pb hexagonal nanoplates as a model catalyst for the methanol oxidation reaction (MOR) and further controlled the Pt and Pb distributions on the surface of the nanoplates through acetic acid (HAc) treatment. As a result, we obtained Pt-Pb nanoplates and HAc-treated Pt-Pb nanoplates with homogeneous and heterogeneous distributions of the Pt-Pb alloy surfaces, respectively. We showed that the MOR activity and stability of the Pt-Pb nanoplates improved compared to those of the HAc-treated Pt-Pb nanoplates, mainly due to the enhanced CO tolerance and the modified electronic structure of Pt under the influence of the oxophilic Pb.
Bimetallic Pt-based catalysts have been extensively investigated to enhance the performance of direct methanol fuel cells (DMFCs) because CO, a by-product, reduces the activity of the pure Pt catalysts. Herein, we synthesized Pt-Pb hexagonal nanoplates as a model catalyst for the methanol oxidation reaction (MOR) and further controlled the Pt and Pb distributions on the surface of the nanoplates through acetic acid (HAc) treatment. As a result, we obtained Pt-Pb nanoplates and HAc-treated Pt-Pb nanoplates with homogeneous and heterogeneous distributions of the Pt-Pb alloy surfaces, respectively. We showed that the MOR activity and stability of the Pt-Pb nanoplates improved compared to those of the HAc-treated Pt-Pb nanoplates, mainly due to the enhanced CO tolerance and the modified electronic structure of Pt under the influence of the oxophilic Pb.
2020, 41(5): 820-829
doi: S1872-2067(19)63456-X
Abstract:
A solvothermal assisted ethylene glycol reduction method is a common technology for Pt/C catalysts preparation. Here, the coordination mechanism of the Pt-containing species is deeply studied by innovatively adopting the ultraviolet-visible spectroscopy technology and H+ concentration detector. Moreover, the amount of NaOH that effectively coordinates Pt4+ has been tentatively qualified and the heating parameters during the preparation process of Pt/C have also been optimized. As investigated, the optimized 20-(1/22)-140-2 Pt/C (20 wt%Pt; m(Pt):m(NaOH)=1/22; heating temperature:140℃, heating time:2 h) exhibits higher electrocatalytic activity towards oxygen reduction reaction (ORR) than the commercial 20 wt% Pt/C (E-TEK) in acidic media. This work provides a theoretical reserve and technical accumulation for industrialized mass production of highly efficient Pt/C catalysts for ORR in proton exchange membrane fuel cells.
A solvothermal assisted ethylene glycol reduction method is a common technology for Pt/C catalysts preparation. Here, the coordination mechanism of the Pt-containing species is deeply studied by innovatively adopting the ultraviolet-visible spectroscopy technology and H+ concentration detector. Moreover, the amount of NaOH that effectively coordinates Pt4+ has been tentatively qualified and the heating parameters during the preparation process of Pt/C have also been optimized. As investigated, the optimized 20-(1/22)-140-2 Pt/C (20 wt%Pt; m(Pt):m(NaOH)=1/22; heating temperature:140℃, heating time:2 h) exhibits higher electrocatalytic activity towards oxygen reduction reaction (ORR) than the commercial 20 wt% Pt/C (E-TEK) in acidic media. This work provides a theoretical reserve and technical accumulation for industrialized mass production of highly efficient Pt/C catalysts for ORR in proton exchange membrane fuel cells.
2020, 41(5): 830-838
doi: S1872-2067(19)63485-6
Abstract:
The electrochemical reduction of CO2 (CO2RR) can substantially contribute to the production of useful chemicals and reduction of global CO2 emissions. Herein, we presented N and S dual-doped high-surface-area carbon materials (SZ-HCN) as CO2RR catalysts. N and S were doped by one-step pyrolysis of a N-containing polymer and S powder. ZnCl2 was applied as a volatile porogen to prepare porous SZ-HCN. SZ-HCN with a high specific surface area (1510 m2 g-1) exhibited efficient electrocatalytic activity and selectivity for CO2RR. Electrochemical measurements demonstrated that SZ-HCN showed excellent catalytic performance for CO2-to-CO reduction with a high CO Faradaic efficiency (~93%) at -0.6 V. Furthermore, SZ-HCN offered a stable current density and high CO selectivity over at least 20 h continuous operation, revealing remarkable electrocatalytic durability. The experimental results and density functional theory calculations indicated that N and S dual-doped carbon materials required lower Gibbs free energy to form the COOH* intermediate than that for single-N-doped carbon for CO2-to-CO reduction, thereby enhancing CO2RR activity.
The electrochemical reduction of CO2 (CO2RR) can substantially contribute to the production of useful chemicals and reduction of global CO2 emissions. Herein, we presented N and S dual-doped high-surface-area carbon materials (SZ-HCN) as CO2RR catalysts. N and S were doped by one-step pyrolysis of a N-containing polymer and S powder. ZnCl2 was applied as a volatile porogen to prepare porous SZ-HCN. SZ-HCN with a high specific surface area (1510 m2 g-1) exhibited efficient electrocatalytic activity and selectivity for CO2RR. Electrochemical measurements demonstrated that SZ-HCN showed excellent catalytic performance for CO2-to-CO reduction with a high CO Faradaic efficiency (~93%) at -0.6 V. Furthermore, SZ-HCN offered a stable current density and high CO selectivity over at least 20 h continuous operation, revealing remarkable electrocatalytic durability. The experimental results and density functional theory calculations indicated that N and S dual-doped carbon materials required lower Gibbs free energy to form the COOH* intermediate than that for single-N-doped carbon for CO2-to-CO reduction, thereby enhancing CO2RR activity.
2020, 41(5): 839-846
doi: S1872-2067(19)63488-1
Abstract:
Although carbon-supported platinum (Pt/C) is still considered the most active electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), its applications in metal-air batteries as a cathode catalyst, or for oxygen generation via water splitting electrolysis as an anode catalyst is mainly constrained by the insufficient kinetic activity and stability in the oxygen evolution reaction (OER). Here, MOF-253-derived nitrogen-doped carbon (N/C)-confined Pt single nanocrystals (Pt@N/C) have been synthesized and shown to be efficient catalysts for the OER. Even with low Pt mass loading of 6.1 wt% (Pt@N/C-10), the catalyst exhibits greatly improved activity and long-time stability as an efficient OER catalyst. Such high catalytic performance is attributed to the core-shell structure relationship, in which the active N-doped-C shell not only provides a protective shield to avoid rapid Pt nanocrystal oxidation at high potentials and inhibits the Pt migration and agglomeration, but also improves the conductivity and charge transfer kinetics.
Although carbon-supported platinum (Pt/C) is still considered the most active electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), its applications in metal-air batteries as a cathode catalyst, or for oxygen generation via water splitting electrolysis as an anode catalyst is mainly constrained by the insufficient kinetic activity and stability in the oxygen evolution reaction (OER). Here, MOF-253-derived nitrogen-doped carbon (N/C)-confined Pt single nanocrystals (Pt@N/C) have been synthesized and shown to be efficient catalysts for the OER. Even with low Pt mass loading of 6.1 wt% (Pt@N/C-10), the catalyst exhibits greatly improved activity and long-time stability as an efficient OER catalyst. Such high catalytic performance is attributed to the core-shell structure relationship, in which the active N-doped-C shell not only provides a protective shield to avoid rapid Pt nanocrystal oxidation at high potentials and inhibits the Pt migration and agglomeration, but also improves the conductivity and charge transfer kinetics.
2020, 41(5): 847-852
doi: S1872-2067(19)63356-5
Abstract:
We report a facile way to prepare sulfur (S) doped Ni4/5Fe1/5-layered double hydroxide (LDH) electrocatalysts for oxygen evolution reaction (OER). The influence of S doping amount on the OER activity of the resulted NiFe-LDHs was studied and the optimal surface S content was ca. 0.43 at%. The developed S-doped NiFe-LDH exhibits excellent OER catalyst activity in 1.0 M KOH with overpotential of only 257 mV at the current density of 10 mA cm-2. Moreover, the catalyst could maintain high activity after 30 h stability test. The high activity of the S-doped NiFe-LDH catalysts may originate from the synergistic effect between S and the Fe sites. This work provides a simple but efficient way to improve the OER performance of transition metal oxides/(oxy)hydroxides.
We report a facile way to prepare sulfur (S) doped Ni4/5Fe1/5-layered double hydroxide (LDH) electrocatalysts for oxygen evolution reaction (OER). The influence of S doping amount on the OER activity of the resulted NiFe-LDHs was studied and the optimal surface S content was ca. 0.43 at%. The developed S-doped NiFe-LDH exhibits excellent OER catalyst activity in 1.0 M KOH with overpotential of only 257 mV at the current density of 10 mA cm-2. Moreover, the catalyst could maintain high activity after 30 h stability test. The high activity of the S-doped NiFe-LDH catalysts may originate from the synergistic effect between S and the Fe sites. This work provides a simple but efficient way to improve the OER performance of transition metal oxides/(oxy)hydroxides.
2020, 41(5): 853-857
doi: 10.1016/S1872-2067(20)63538-0
Abstract:
All-inorganic and earth-abundant bi-/trimetallic hydr(oxy)oxides are widely used as oxygen evolution electrocatalysts owing to their remarkable performance. However, their atomically precise structures remain undefined, complicating their optimization and limiting the understanding of their enhanced performance. Here, the underlying structure-property correlation is explored by using a well-defined cobalt-phosphate polyoxometalate cluster[{Co4(OH)3(PO4)}4(SiW9O34)4]32- (1), which may serve as a molecular model of multimetal hydr(oxy)oxides. The catalytic activity is enhanced upon replacing Co by Fe in 1, resulting in a reduced overpotential (385 mV) for oxygen evolution (by 66 mV) compared to that of the parent 1 at 10 mA cm-2 in an acidic medium; this overpotential is comparable to that for the IrO2 catalyst. These abundant-metal-based polyoxometalates exhibit high stability, with no evidence of degradation even after 24 h of operation.
All-inorganic and earth-abundant bi-/trimetallic hydr(oxy)oxides are widely used as oxygen evolution electrocatalysts owing to their remarkable performance. However, their atomically precise structures remain undefined, complicating their optimization and limiting the understanding of their enhanced performance. Here, the underlying structure-property correlation is explored by using a well-defined cobalt-phosphate polyoxometalate cluster[{Co4(OH)3(PO4)}4(SiW9O34)4]32- (1), which may serve as a molecular model of multimetal hydr(oxy)oxides. The catalytic activity is enhanced upon replacing Co by Fe in 1, resulting in a reduced overpotential (385 mV) for oxygen evolution (by 66 mV) compared to that of the parent 1 at 10 mA cm-2 in an acidic medium; this overpotential is comparable to that for the IrO2 catalyst. These abundant-metal-based polyoxometalates exhibit high stability, with no evidence of degradation even after 24 h of operation.
2020, 41(5): 858-867
doi: S1872-2067(19)63507-2
Abstract:
The application of electrocatalysts for the oxygen reduction reaction (ORR) is vital in a variety of energy conversion technologies. Exploring low-cost ORR catalysts with high activity and long-term stability is highly desirable, although it still remains challenging. Herein, we report a facile and reliable route to convert ZIF-8 modified by Fe-phenanthroline into Fe-incorporated and N-doped carbon dodecahedron nanoarchitecture (Fe-NCDNA), in which carbon nanosheets are formed in situ as the building blocks with uniform Fe-N-C species decoration. Systematic electrochemical studies demonstrate that the as-synthesized Fe-NCDNA electrocatalyst possesses highly attractive catalytic features toward the ORR in terms of activity and durability in both alkaline and neutral media. The Zn-air battery with the optimal Fe-NCDNA catalyst as the cathode performs impressively, delivering a power density of 184 mW cm-2 and a specific capacity of 801 mAh g-1; thus, it exhibits great competitive advantages over those of the Zn-air devices employing a Pt-based cathode electrocatalyst.
The application of electrocatalysts for the oxygen reduction reaction (ORR) is vital in a variety of energy conversion technologies. Exploring low-cost ORR catalysts with high activity and long-term stability is highly desirable, although it still remains challenging. Herein, we report a facile and reliable route to convert ZIF-8 modified by Fe-phenanthroline into Fe-incorporated and N-doped carbon dodecahedron nanoarchitecture (Fe-NCDNA), in which carbon nanosheets are formed in situ as the building blocks with uniform Fe-N-C species decoration. Systematic electrochemical studies demonstrate that the as-synthesized Fe-NCDNA electrocatalyst possesses highly attractive catalytic features toward the ORR in terms of activity and durability in both alkaline and neutral media. The Zn-air battery with the optimal Fe-NCDNA catalyst as the cathode performs impressively, delivering a power density of 184 mW cm-2 and a specific capacity of 801 mAh g-1; thus, it exhibits great competitive advantages over those of the Zn-air devices employing a Pt-based cathode electrocatalyst.
2020, 41(5): 868-876
doi: S1872-2067(19)63500-X
Abstract:
A unique redox-coupled biomimetic system was developed, in which Fe-Anderson type polyoxometalates (POMs) were employed as electron transfer mediators (ETMs) and benzenesulfonic acid (BSA)-based deep eutectic solvents (DESs) were used as electron-donors for aerobic oxidative desulfurization (AODS) of diesel fuel. Different compositions of DESs were used and the polyethylene glycol 2000 (PEG2000)/2.5BSA system showed the highest desulfurization activity, with the removal of dibenzothiophene (DBT) at 60℃ reaching 95% in 60 min. The excellent desulfurization activity of the system is due to the in situ formation of peroxysulfonate via a biomimetic process. By constructing a coupled redox system, Fe-Anderson type POMs as ETMs reduce the activation energy of oxygen-activated sulfonate. The physical characteristics of four different DESs were tested. The results show that the conductivity of DESs is correlated with the composition of BSA-based DESs. However, there is no similar trend in viscosity testing at the same temperature, and the maximum viscosity value is obtained for the PEG2000/2.5BSA system at 60℃, which may be associated with the stronger hydrogen bonds. It is worth noting that the PEG2000/2.5BSA system also possesses the best desulfurization activity, which suggests that the activity of the desulfurization system is related to the strength of the hydrogen bond in DESs. Finally, the biomimetic desulfurization system exhibits excellent performance and good stability under mild reaction conditions (60℃, atmospheric pressure, oxygen as the oxidant).
A unique redox-coupled biomimetic system was developed, in which Fe-Anderson type polyoxometalates (POMs) were employed as electron transfer mediators (ETMs) and benzenesulfonic acid (BSA)-based deep eutectic solvents (DESs) were used as electron-donors for aerobic oxidative desulfurization (AODS) of diesel fuel. Different compositions of DESs were used and the polyethylene glycol 2000 (PEG2000)/2.5BSA system showed the highest desulfurization activity, with the removal of dibenzothiophene (DBT) at 60℃ reaching 95% in 60 min. The excellent desulfurization activity of the system is due to the in situ formation of peroxysulfonate via a biomimetic process. By constructing a coupled redox system, Fe-Anderson type POMs as ETMs reduce the activation energy of oxygen-activated sulfonate. The physical characteristics of four different DESs were tested. The results show that the conductivity of DESs is correlated with the composition of BSA-based DESs. However, there is no similar trend in viscosity testing at the same temperature, and the maximum viscosity value is obtained for the PEG2000/2.5BSA system at 60℃, which may be associated with the stronger hydrogen bonds. It is worth noting that the PEG2000/2.5BSA system also possesses the best desulfurization activity, which suggests that the activity of the desulfurization system is related to the strength of the hydrogen bond in DESs. Finally, the biomimetic desulfurization system exhibits excellent performance and good stability under mild reaction conditions (60℃, atmospheric pressure, oxygen as the oxidant).
2020, 41(5): 877-888
doi: 10.1016/S1872-2067(20)63532-X
Abstract:
To understand the effect of the doping amount of Cu2+ on the structure and reactivity of SnO2 in NOx-SCR with NH3, a series of Sn-Cu-O binary oxide catalysts with different Sn/Cu ratios have been prepared and thoroughly characterized. Using the XRD extrapolation method, the SnO2 lattice capacity for Cu2+ cations is determined at 0.10 g CuO per g of SnO2, equaling a Sn/Cu molar ratio of 84/16. Therefore, in a tetragonal rutile SnO2 lattice, only a maximum of 16% of the Sn4+ cations can be replaced by Cu2+ to form a stable solid solution structure. If the Cu content is higher, CuO will form on the catalyst surface, which has a negative effect on the reaction performance. For samples in a pure solid solution phase, the number of surface defects increase with increasing Cu content until it reaches the lattice capacity, as confirmed by Raman spectroscopy. As a result, the amounts of both active oxygen species and acidic sites on the surface, which critically determine the reaction performance, also increase and reach the maximum level for the catalyst with a Cu content close to the lattice capacity. A distinct lattice capacity threshold effect on the structure and reactivity of Sn-Cu binary oxide catalysts has been observed. A Sn-Cu catalyst with the best reaction performance can be obtained by doping the SnO2 matrix with the lattice capacity amount of Cu2+.
To understand the effect of the doping amount of Cu2+ on the structure and reactivity of SnO2 in NOx-SCR with NH3, a series of Sn-Cu-O binary oxide catalysts with different Sn/Cu ratios have been prepared and thoroughly characterized. Using the XRD extrapolation method, the SnO2 lattice capacity for Cu2+ cations is determined at 0.10 g CuO per g of SnO2, equaling a Sn/Cu molar ratio of 84/16. Therefore, in a tetragonal rutile SnO2 lattice, only a maximum of 16% of the Sn4+ cations can be replaced by Cu2+ to form a stable solid solution structure. If the Cu content is higher, CuO will form on the catalyst surface, which has a negative effect on the reaction performance. For samples in a pure solid solution phase, the number of surface defects increase with increasing Cu content until it reaches the lattice capacity, as confirmed by Raman spectroscopy. As a result, the amounts of both active oxygen species and acidic sites on the surface, which critically determine the reaction performance, also increase and reach the maximum level for the catalyst with a Cu content close to the lattice capacity. A distinct lattice capacity threshold effect on the structure and reactivity of Sn-Cu binary oxide catalysts has been observed. A Sn-Cu catalyst with the best reaction performance can be obtained by doping the SnO2 matrix with the lattice capacity amount of Cu2+.
2020, 41(5): 889-897
doi: S1872-2067(19)63499-6
Abstract:
Loading of cocatalysts can effectively inhibit the recombination of photogenerated carriers in photocatalysts and greatly improve the photocatalytic hydrogen production rate. Cocatalysts can be deposited at the outlet points of electrons using a photochemical method, which is beneficial for the following photocatalytic hydrogen production reaction. H2PO2- has been used in the photochemical reduction of transition metals because of its special properties. However, the particles formed in the presence of H2PO2- are very large and highly crystalline, which may inhibit the activity of photocatalysts. In this study, we designed a new method for synthesizing photocatalysts by photodeposition using some other phosphates, aiming to prepare controllable weakly crystalline and small-size cocatalysts to improve the hydrogen production activity. The cocatalyst prepared using H2PO3- as an inorganic sacrificial agent has an amorphous structure and an average size of about 10 nm. The optimal photocatalytic hydrogen production rate of the obtained Ni(OH)2/g-C3N4 (4.36 wt%) is 13707.86 μmol·g-1·h-1, which is even higher than the activity of Pt-4.36 wt%/g-C3N4 (11210.93 μmol·g-1·h-1). Mechanistic studies show that loading of Ni(OH)2 can efficiently accelerate the separation and transfer efficiency of photogenerated charge carriers.
Loading of cocatalysts can effectively inhibit the recombination of photogenerated carriers in photocatalysts and greatly improve the photocatalytic hydrogen production rate. Cocatalysts can be deposited at the outlet points of electrons using a photochemical method, which is beneficial for the following photocatalytic hydrogen production reaction. H2PO2- has been used in the photochemical reduction of transition metals because of its special properties. However, the particles formed in the presence of H2PO2- are very large and highly crystalline, which may inhibit the activity of photocatalysts. In this study, we designed a new method for synthesizing photocatalysts by photodeposition using some other phosphates, aiming to prepare controllable weakly crystalline and small-size cocatalysts to improve the hydrogen production activity. The cocatalyst prepared using H2PO3- as an inorganic sacrificial agent has an amorphous structure and an average size of about 10 nm. The optimal photocatalytic hydrogen production rate of the obtained Ni(OH)2/g-C3N4 (4.36 wt%) is 13707.86 μmol·g-1·h-1, which is even higher than the activity of Pt-4.36 wt%/g-C3N4 (11210.93 μmol·g-1·h-1). Mechanistic studies show that loading of Ni(OH)2 can efficiently accelerate the separation and transfer efficiency of photogenerated charge carriers.
