Special Issue "Emerging Nanocatalysts for Efficient Oxygen Reduction, Oxygen Evolution and Hydrogen Evolution"

A special issue of Catalysts (ISSN 2073-4344). This special issue belongs to the section "Nanostructured Catalysts".

Deadline for manuscript submissions: 30 June 2021.

Special Issue Editor

Dr. Bo Hou
Website
Guest Editor
Lecturer, Cardiff University, UK
Interests: solution-processed optoelectronics; electron microscopy (TEM) and electrochemical energy conversion and storage

Special Issue Information

Dear Colleagues,

The problem of global warming and global climate change cannot be ignored in any discussion regarding long-term environment policy—and the solution, as part of a comprehensive sustainable energy plan, is reducing or stopping greenhouse gas emissions which are generated through the release of naturally sequestered carbon via the human practice of burning fossil fuels. The ever-increasing detrimental effects of traditional fuels on the environment have stimulated extensive efforts worldwide to develop green and renewable energy technologies, including fuel cells, metal-air batteries and water-splitting systems. The fundamental electrochemical reaction mechanisms behind these renewable energy technologies are oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Indeed, the ORR is the heart of fuel cells, while the OER and HER are of paramount importance to metal-air batteries and electrochemical water-splitting systems. However, the large intrinsic overpotential associated with sluggish OER on anode and HER on the cathode make these electrochemical reactions inefficient. Thus, it is imperative to develop highly active catalysts to reduce the overpotential dramatically. So far, numerous promising nanocatalysts have been demonstrated, including i) multimetal oxide compound nanomaterials such as layered hydroxide, spinel and amorphous metal oxides; ii) metal-free nanocatalysts such as carbon nanotubes (CNTs), graphene, graphite, carbon nitride and 3D carbon architectures; iii) metallic transition–metal dichalcogenide (TMDC) nanocatalysts, where M represents a transition metal atom typically from group IVB to VIIB in the periodic table and X is a chalcogen atom such as S, Se, or Te; iv) Perovskite oxides (POs) such as ABO3±δ (where, A is alkaline or rare earth cations, B is first- row transition metal cations and δ is oxygen non-stoichiometry), Ruddlesden–Popper (An+ 1BnO3n + 1), Dion–Jacobson (AnBnO3n + 1), Aurivillius (Bi2An-1BnO3n + 3) homologous series, double- (A2BB’O6) and triple- (A2A’B2B’O9) layered POs, etc.

This Special Issue will focus on experimental and theoretical investigations into new nanocatalysts for the ORR, OER and HER, with a particular interest in the noble-metal-free or metal-free nanomaterials. Fundamental, applied studies, theoretical studies and advanced characterizations (structure, morphology, or interface charge transfer) are of interest. Additionally relevant are reports that detail new nanocatalysts for photoelectrolysis for water splitting, photocathodic protection and electrochemical organic degradation. The hope is to compile a set of manuscripts that inform the field of the state-of-the-art in electrochemical nanocatalysis.

Dr. Bo Hou
Guest Editor

Manuscript Submission Information

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Keywords

  • oxygen reduction reaction (ORR)
  • oxygen evolution reaction (OER)
  • hydrogen evolution reaction (HER)
  • bifunctional electrocatalyst
  • metal-air batteries
  • water splitting
  • carbon materials
  • perovskite oxide
  • photocathodic protection
  • electrochemical organic degradation
  • electrochemical supercapacitor
  • fuel cells
  • solar fuels
  • photoelectrolysis
  • density functional theory
  • molecular dynamics
  • electron microscopy
  • electrochemical impedance spectroscopy

Published Papers (7 papers)

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Research

Open AccessArticle
Enhanced Hydrogen Evolution Reaction in Surface Functionalized MoS2 Monolayers
Catalysts 2021, 11(1), 70; https://doi.org/10.3390/catal11010070 - 06 Jan 2021
Abstract
Monolayered, semiconducting MoS2 and their transition metal dichalcogenides (TMDCs) families are promising and low-cost materials for hydrogen generation through electrolytes (HER, hydrogen evolution reaction) due to their high activities and electrochemical stability during the reaction. However, there is still a lack of [...] Read more.
Monolayered, semiconducting MoS2 and their transition metal dichalcogenides (TMDCs) families are promising and low-cost materials for hydrogen generation through electrolytes (HER, hydrogen evolution reaction) due to their high activities and electrochemical stability during the reaction. However, there is still a lack of understanding in identifying the underlying mechanism responsible for improving the electrocatalytic properties of theses monolayers. In this work, we investigated the significance of controlling carrier densities in a MoS2 monolayer and in turn the corresponding electrocatalytic behaviors in relation to the energy band structure of MoS2. Surface functionalization was employed to achieve p-doping and n-doping in the MoS2 monolayer that led to MoS2 electrochemical devices with different catalytic performances. Specifically, the electron-rich MoS2 surface showed lower overpotential and Tafel slope compared to the MoS2 with surface functional groups that contributed to p-doping. We attributed such enhancement to the increase in the carrier density and the corresponding Fermi level that accelerated HER and charge transfer kinetics. These findings are of high importance in designing electrocatalysts based on two-dimensional TMDCs. Full article
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Open AccessCommunication
Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction
Catalysts 2021, 11(1), 61; https://doi.org/10.3390/catal11010061 - 05 Jan 2021
Abstract
Solution-processed perovskite quantum dots (QDs) have been intensively researched as next-generation photocatalysts owing to their outstanding optical properties. Even though the intrinsic physical properties of perovskite QDs have been significantly improved, the chemical stability of these materials remains questionable. Their low long-term chemical [...] Read more.
Solution-processed perovskite quantum dots (QDs) have been intensively researched as next-generation photocatalysts owing to their outstanding optical properties. Even though the intrinsic physical properties of perovskite QDs have been significantly improved, the chemical stability of these materials remains questionable. Their low long-term chemical stability limits their commercial applicability in photocatalysis. In this study, we investigated the photodegradation mechanisms of perovskite QDs and their hybrids via photoluminescence (PL) by varying the excitation power and the ultraviolet (UV) exposure power. Defects in perovskite QDs and the interface between the perovskite QD and the co-catalyst influence the photo-stability of perovskite QDs. Consequently, we designed a stable perovskite QD film via an in-situ cross-linking reaction with amine-based silane materials. The surface ligand comprising 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)) and 5-hexynoic acid improved the interface between the Ni co-catalyst and the perovskite QD. Then, ultrathin SiO2 was fabricated using 3-aminopropyltriethoxy silane (APTES) to harness the strong surface binding energy of the amine functional group of APTES with the perovskite QDs. The Ni co-catalyst content was further increased through Ni doping during purification using a short surface ligand (3-butynoic acid). As a result, stable perovskite QDs with rapid charge separation were successfully fabricated. Time-correlated single photon counting (TCSPC) PL study demonstrated that the modified perovskite QD film exhibited slow photodegradation owing to defect passivation and the enhanced interface between the Ni co-catalyst and the perovskite QD. This interface impeded the generation of hot carriers, which are a critical factor in photodegradation. Finally, a stable red perovskite QD was synthesized by applying the same strategy and the mixture between red and green QD/Ni(ppy)/SiO2 displayed an CO2 reduction capacity for CO (0.56 µmol/(g∙h)). Full article
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Open AccessArticle
Redox-Mediated Polymer Catalyst for Lithium-Air Batteries with High Round-Trip Efficiency
Catalysts 2020, 10(12), 1479; https://doi.org/10.3390/catal10121479 - 17 Dec 2020
Abstract
Lithium-air batteries (LABs) continue to receive attention as a promising power source because they possess a high theoretical energy density of 3436 Wh L−1. However, the remaining Li2O2 resulting from the irreversible decomposition of Li2O2 [...] Read more.
Lithium-air batteries (LABs) continue to receive attention as a promising power source because they possess a high theoretical energy density of 3436 Wh L−1. However, the remaining Li2O2 resulting from the irreversible decomposition of Li2O2 during the charge process is one of the key challenges so as to address the deterioration of the cycling performance of LABs. In this study, we propose and report a redox-mediated polymer catalyst (RPC) as a cathode catalyst being composed of LiI and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with multi-wall carbon nanotubes (MWCNTs) as the cathode material. In the RPC, iodine molecules are chemically combined with the PVDF-HFP chain. The as-prepared RPC exhibits increased cycling performance by 194% and decreased overpotential by 21.1% at 0.1 mA cm−2 compared to the sample without LiI molecules. Furthermore, these results suggest that the RPC consisting of a polymer chain and redox mediators will be extensively utilized as highly efficient catalysts of LABs. Full article
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Open AccessArticle
Accelerating the Oxygen Reduction Reaction and Oxygen Evolution Reaction Activities of N and P Co-Doped Porous Activated Carbon for Li-O2 Batteries
Catalysts 2020, 10(11), 1316; https://doi.org/10.3390/catal10111316 - 13 Nov 2020
Abstract
Rechargeable lithium–oxygen (Li-O2) batteries represent state-of-the-art electrochemical energy storage devices that provide high energy densities. However, their commercialization is challenging owing to their low charging/discharging efficiencies, short battery lives, high overpotentials, and high cathode manufacturing costs. In this study, we prepared [...] Read more.
Rechargeable lithium–oxygen (Li-O2) batteries represent state-of-the-art electrochemical energy storage devices that provide high energy densities. However, their commercialization is challenging owing to their low charging/discharging efficiencies, short battery lives, high overpotentials, and high cathode manufacturing costs. In this study, we prepared a metal-free, N,P co-doped, porous activated carbon (N,P-PAC) electrode via KOH activation and P doping for application as a Li-O2 battery cathode. When used in a rechargeable Li-O2 battery, the N,P-PAC cathode showed a high specific discharge capacity (3724 mA h g−1 at 100 mA g−1), an excellent cycling stability (25 cycles with a limit capacity of 1000 mA h g−1), and a low charge/discharge voltage gap (1.22 V at 1000 mA h g−1). The N,P-PAC electrode showed a low overpotential (EOER-ORR) of 1.54 V. The excellent electrochemical performance of the N,P-PAC electrode can mainly be attributed to its large active area and oxygen-containing functional groups generated via KOH activation and P-doping processes. Therefore, the N,P-PAC prepared in this study was found to be a promising eco-friendly and sustainable metal-free cathode material for Li-O2 batteries. Full article
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Open AccessArticle
Simple and Facile Fabrication of Anion-Vacancy-Induced MoO3−X Catalysts for Enhanced Hydrogen Evolution Activity
Catalysts 2020, 10(10), 1180; https://doi.org/10.3390/catal10101180 - 14 Oct 2020
Abstract
Advanced catalysts for clean hydrogen generation and storage offer an attractive possibility for developing a sustainable and ecofriendly future energy system. Transition metal oxides (TMO) are appealing candidates to be largely considered as electrode catalysts. However, for practical applications, there are still challenges—the [...] Read more.
Advanced catalysts for clean hydrogen generation and storage offer an attractive possibility for developing a sustainable and ecofriendly future energy system. Transition metal oxides (TMO) are appealing candidates to be largely considered as electrode catalysts. However, for practical applications, there are still challenges—the intrinsic catalytic properties of TMOs should be further improved and TMOs should be synthesized by practical routes for cost-effective and scalable production of catalysts. Therefore, finding promising ways to fabricate highly active TMOs with outstanding electrochemical hydrogen evolution performance is required. Here, we present a direct and facile synthetic approach to successfully provide highly efficient MoO3−X catalysts with electrochemically active oxygen vacancies through a one-step thermal activation process on a Mo metal mesh. Variations in the oxidation states of molybdenum oxides can significantly increase the active sites of the catalysts and improve the electrochemical activity, making these oxide compounds suitable for hydrogen evolution reaction (HER). Compared to the bare Mo mesh and fully oxidized Mo (MoO3) electrodes, the fabricated MoO3−X electrode exhibits better electrochemical performance in terms of overpotentials and Tafel slope, as well as the electrochemical 1000 cycling stability, confirming the improved HER performance of MoO3−X. This provides new insight into the simple procedure suitable for the large-production supply. Full article
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Open AccessArticle
Thermal Profiles of Carbon Fiber Based Anisotropic Thin-Films: An Emerging Heat Management Solution for High-Current Flow Electrocatalysis and Electrochemical Applications
Catalysts 2020, 10(10), 1172; https://doi.org/10.3390/catal10101172 - 12 Oct 2020
Abstract
Carbon fiber has been extensively used in the photocatalysis, electrocatalysis and energy storage fields as supporting platform and conductive media. However, less attention has been paid with regards to its function in phonon transport and thermal management. We have investigated the effect of [...] Read more.
Carbon fiber has been extensively used in the photocatalysis, electrocatalysis and energy storage fields as supporting platform and conductive media. However, less attention has been paid with regards to its function in phonon transport and thermal management. We have investigated the effect of current flow direction on the heat management performance of carbon fiber based thin film heaters (CFTFHs) with anisotropic percolation network of carbon fibers (CFs). The anisotropic percolation network of carbon fibers (CFs) formed by roll-to-roll spray coating leads to the anisotropic electrical properties of CFs. As a result, CFs based thin films (CFTFs) have lower sheet resistance when measured parallel to the CFs alignment, compared to when they are aligned perpendicular. Because connectivity and current flow in CFs are critically dependent on the direction alignment of CFs, the saturation temperature (106.4 °C) of CFTFH with parallel aligned carbon fiber is higher than that (117.3 °C) of CFTFH with perpendicular alignment. Therefore, current flow in the same direction as the alignment of CFs is very important to achieve high-performance. Moreover, our study on thermal profile of anisotropic CFTFs under high current flows illustrates that carbon fiber thin films have great potential in thermal management solution for electrocatalytic and electrochemical energy storage applications. Full article
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Open AccessArticle
Synthesis of Lanthanide-Functionalized Carbon Quantum Dots for Chemical Sensing and Photocatalytic Application
Catalysts 2020, 10(8), 833; https://doi.org/10.3390/catal10080833 - 24 Jul 2020
Cited by 1
Abstract
Tunable photoluminescent-functionalized carbon quantum dots [email protected] (TFA)3 (Ln = Eu, Tb; TFA: trifluoroacetylacetone) were designed and synthesized by introducing lanthanide complexes into the modified CQDs surface through the carboxyl group. The as-prepared [email protected] (TFA)3 emit strong blue–green light with the peak [...] Read more.
Tunable photoluminescent-functionalized carbon quantum dots [email protected] (TFA)3 (Ln = Eu, Tb; TFA: trifluoroacetylacetone) were designed and synthesized by introducing lanthanide complexes into the modified CQDs surface through the carboxyl group. The as-prepared [email protected] (TFA)3 emit strong blue–green light with the peak at 435 nm and simultaneously show the characteristic emission of Ln3+ under irradiation of 365 nm light in aqueous solution. Moreover, these functionalized CQDs exhibit excellent photoluminescence properties. In addition, a white luminescent solution [email protected]/Tb (TFA)3 was obtained by adjusting the ratio of Eu3+/Tb3+ and the excitation wavelengths. Moreover, [email protected] (TFA)3 can be utilized as a fluorescent probe for the sensitive and selective detection of MnO4 without interference from other ions in aqueous solution. These results provide the meaningful data for the multicomponent assembly and the photoluminescent-functionalized materials based on the modified CQDs and lanthanide, which can be expected to have potential application in photocatalytic or sensors. Full article
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: BiVO4 photocatalyst with controllable wettability and its enhanced catalytic activity for degradation of 17α-ethinylestradiol
Authors: Fengzhi JIANG
Affiliation: School of Chemical Science and Technology, Yunnan University, 2 North Cuihu Road, 650091 Kunming, P.R.China

Title: Lanthanide Functionalized Carbon Quantum Dots for MnO4- Detection and White-Light Tuning
Authors: Ying Li
Affiliation: School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

Title: Emerging energy harvesting technology for photocatalytic water splitting
Authors: Yuljae Cho
Affiliation: UM–SJTU Joint Institute, Shanghai Jiao Tong University, Shanghai, China
Abstract: In recent years, we have experienced extreme climate changes due to the global warming, continuously impacting and changing our daily lives. To build sustainable environment and society, various energy technologies have been developed and introduced. Among them, energy harvesting, converting ambient environmental energy into electrical one, has emerged as one of the promising technologies for a variety of energy applications. In particular, a photocatalytic water splitting system, coupled with emerging energy harvesting technology, demonstrated high device performance, which showed its great social impact for the development of the new water splitting system. In this review article, we focus on emerging energy harvesting technology for photocatalytic water splitting applications. The article comprehensively includes i) fundamentals of photocatalysis and water splitting, ii) recent trend in materials for energy harvesting, iii) basic mechanisms of the energy harvesting, and iv) water splitting applications by coupling the emerging energy harvesting with the photocatalytic process. Lastly, future research directions for photocatalytic water splitting coupled with the emerging energy harvesting technology will be discussed.

Title: The Progress of NiFe-based Electrocatalysts for Water Splitting
Authors: Le Yu
Affiliation: College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Title: Platinum nanoparticles decorated with nitrogen-doped carbon quantum dots
Authors: Hyo-Jin Ahn; Hyun-gi Jo
Affiliation: Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea
Abstract: Platinum nanoparticles decorated with nitrogen-doped carbon quantum dots (N-CQDs) were synthesized for the oxygen reduction reaction (ORR) catalyst. They were prepared using hydrothermal and reduction methods, suggesting a concentration ratio adjustment of the N-CQDs. The optimized ratio of Pt/N-CQDs showed excellent electrochemical catalyst performance compared to commercial Pt/C. Additionally, the physical and chemical analyses of the prepared catalysts were investigated by transmission electron microscopy (TEM), FT-IR/Raman spectroscopy, and X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

Title: 3D networked MoS2/reduced graphene oxide aerogel with highly porous architecture for hydrogen evolution reaction
Authors: Young-Woo Lee
Affiliation: Department of Energy Systems, Soonchunhyang University,
Abstract: Designing and engineering the nanostructures with low cost material and high surface area, as non-precious metallic electrocatalysts, are most important strategy in the pursuit of high performance electroactive materials for hydrogen evolution reaction. Here, we introduce a facile method to design and synthesize the 3D networked MoS2/reduced graphene oxide aerogel with a highly porous structure for hydrogen evolution reaction. Such a 3D networked nanostructure not only facilitates the electrode-electrolyte interaction but also provides an efficient electron pathway within the graphene oxide network. Thus, we expect that the as-prepared 3D networked MoS2/reduced graphene oxide aerogel exhibit superior electrochemical properties for hydrogen evolution reaction.

Title: Redox-mediated Polymer Catalysts for Lithium-oxygen Batteries with High Energy Densities
Authors: Jung Inn Sohn
Affiliation: Division of Physics and Semiconductor Science, Dongguk University-Seoul, Seoul, South Korea
Abstract: Lithium-oxygen (Li-O2) batteries have been considered as a next-generation power source because of their high theoretical energy density of 3,436 Wh L-1. However, the cycling performance of Li-O2 batteries can be degraded by an irreversible decomposition of Li2O2 during charge process where Li2O2 is formed and accumulated on the cathode electrode surface during discharge process. In this study, we prepare a redox-mediated polymer catalyst (RPC) consisting of LiI and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with multi-wall carbon nanotubes as a cathode catalyst. In the RPC, iodine is chemically combined at the end of PVDF-HFP chains. The as-prepared RPC exhibits cycling performance increased by 194% and overpotential decreased by 21.1% at 0.1 mA cm-2 compared to the sample prepared in the absence of LiI. Thus, we believe that the RPC consisting of a polymer chain and a redox mediator can be promising cathode catalysts for Li-O2 batteries with high energy densities.

Title: Reducing the photo-degradation of Perovskite QD as a photo-catalytic behavior in CO2 reduction
Authors: Hanleem Lee; Meeree Kim; Hyoyoung Lee
Affiliation: a Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea South Korea. b Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
Abstract: Solution-processed perovskite quantum dots (QDs) have been intensively researched for next-generation photo-catalyst owing to their outstanding optical properties (e.g., high color purity, tunable emission wavelength, broad light absorption range, and high photoluminescence (PL) quantum yield (PLQY)). Even though the intrinsic physical properties of perovskite QDs have tremendously improved, the chemical stability of these materials has been questionable. The low chemical stability of perovskite QD hinders operational and long-term stability as a photo-catalyst which satisfies the demand for commercialization. Here, we design the chemically stable perovskite QDs film via an in-situ cross-linking reaction. The surface ligand, which composes of the 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide with silane material, for perovskite QDs are carefully chosen to maintain the optoelectrical properties and to drive the in-situ cross-linking reaction. The transient PL study demonstrates that the modified perovskite QDs film exhibits slow photodegradation due to low dielectric coating, compared to bare silane coating. 4′-Hydroxy-4-biphenyl carboxylic acid act as a barrier in which polar silane reduces dielectric confinement of perovskite QDs. On the other hand, the ultrathin SiO2 via silanization effectively prevents the ion migration and vaporization of organic cation, resulting in better operational stability. As a result, our system obtains an effective CO2 reduction capacity to CO (1.2umol cm-2 h-1).

Title: Effect of current flow direction on the heating performance of carbon fiber based flexible thin film heaters
Authors: Sang-Hwi Lim; Han-Ki Kim
Affiliation: SungKyunKwan University
Abstract: We have investigated the effect of current flow direction on the heating performance of flexible carbon fiber based thin film heaters (CFTFHs). The anisotropic percolation network of carbon fibers (CFs) formed by roll-to-roll coating leads to the anisotropic electrical properties of CFs. Flexible CF based flexible thin films (CFTFs) have lower sheet resistance when measured parallel to the CFs alignment, compared to when they are aligned perpendicular. This alignment originates from the from the shear flow direction when CFs were ejected from the nozzle. Because connectivity and current flow in CFs are critically dependent on the direction alignment of CFs, the saturation temperature (104.9 C) of CFTFH with parallel aligned carbon fiber is higher than that (113.1 C) of CFTFH with perpendicular alignment. Therefore, current flow in the same direction as the alignment of CFs is very important to achieve high-performance flexible CFTFHs. In addition, due to the outstanding mechanical flexibility of CF, there was no sheet resistance change or structural change of CFTHs after severe flexibility tests. This indicated that the roll-to-roll coated CF based CFTFHs are promising flexible material for high-performance CFTFHs.

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