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 ABO_3±δ (where, A is alkaline or rare earth cations, B is first- row transition metal cations and δ is oxygen non-stoichiometry), Ruddlesden–Popper (A_{n+ 1}B_nO_{3n + 1}), Dion–Jacobson (A_nB_nO_{3n + 1}), Aurivillius (Bi₂A_n-1B_nO_{3n + 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

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• 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

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.