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Among many types of low-dimensional inorganic nanostructures, nanostructured MnO2 provides unique advantages in designing and synthesizing efficient electrode and catalyst materials for novel energy storage technologies. The low cost, high redox potential, theoretically high energy storage capacity, environmentally friendliness, and rich deposits provide MnO2 nanomaterials with a high level of economic feasibility for various renewable energy-related applications, like electrodes for metal-ion batteries/metal–sulfur batteries/metal–O2 batteries/supercapacitors and electrocatalysts for oxygen evolution/oxygen reduction/CO2 reduction/N2 fixation. In this review, a wide spectrum of the energy-related applications of diverse nanostructured MnO2 materials is systematically surveyed along with versatile synthetic methods of these metal oxides and chemical design strategies to improve their electrochemical functionalities. Future research perspectives for nanostructured MnO2 materials are provided to offer insightful directions for the exploration of next-generation energy storage/conversion systems.

A highly efficient CO2 electrolysis system could be created by introducing biomass oxidation as an alternative anodic reaction to the sluggish oxygen evolution reaction in a CO2-saturated and near-neutral electrolyte. Here, we successfully demonstrate anodic biomass oxidation by synthesizing 5 nm nickel oxide nanoparticles (NiO NPs). NiO NPs show a unique electrocatalytic activity for 5-hydroxymethylfurfural (HMF) oxidation under near-neutral conditions, exhibiting an anodic current onset (1 mA cm-2) at 1.524 V versus the reversible hydrogen electrode and a total Faradaic efficiency of ≤70%. Electrokinetic and in situ ultraviolet-visible spectroscopic analyses suggest that a redox active nickel hydroxide species is formed on the surface of NiO electrocatalysts during HMF oxidation, and this oxidation of Ni(II) hydroxide to Ni(III) oxyhydroxide could be the rate-determining step. This mechanistic study of biomass oxidation in a CO2-saturated electrolyte provides insight into constructing a highly efficient system for the paired electrolysis of CO2 reduction and biomass oxidation.

Electrochemical water oxidation is considered as one of the most important reactions for a sustainable future. However, in heterogeneous water oxidation, atomistic understanding of this four-electr...

Electrochemical water oxidation is a key counter reaction in obtaining value-added chemicals by reduction in aqueous solution. However, slow kinetics is a problem in this process, so the quantitati...

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co3O4 nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co3O4 nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO2-based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.

A highly efficient CO

The electrochemical reduction of carbon dioxide (CO

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co

Among diverse nanostructured materials, two-dimensional (2D) inorganic nanosheets have attracted intense research interest because of their unique characteristics such as high morphological anisotropy, expanded 2D surface area with many surface active sites, and unique electronic structures. The existence of many surface- and edge-exposed active sites as well as tunable chemical composition with large concentration of redoxable component ions renders 2D inorganic nanosheets as promising electrocatalysts for diverse electrochemical reactions. Additionally, highly anisotropic 2D inorganic nanosheet with high surface-to-volume ratio can be used as efficient hybridization matrix to optimize the electrocatalyst performance of nanostructured material via strong interfacial interaction and synergistic coupling between hybridized species. For the electrocatalyst application, 2D inorganic nanosheets can play crucial roles as main active components, substrates, additives, and hybrid building blocks in hybrid-type electrocatalysts. In this review, unique advantages of 2D inorganic nanosheets in the design and synthesis of electrocatalysts are discussed with in-depth discussion about their critical roles in 2D nanosheet-based electrocatalysts. Depending on target electrocatalysis reactions, diverse examples of 2D inorganic nanosheet-based electrocatalysts are surveyed along with their versatile characterization techniques. Future research perspectives for the exploration of 2D inorganic nanosheet-based electrocatalysts are presented to provide valuable insights for the development of high-performance electrocatalysts applicable for emerging fields of renewable energy technologies.

The electrochemical reduction of carbon dioxide (CO2 ) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO2 to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO2 to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO2 in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.

Water electrolysis has been regarded an attractive approach to produce sustainable energy without undesirable byproducts. The anodic half reaction, oxygen evolving reaction (OER), is a major bottleneck for overall water electrolysis process because of its complex four electron transfer steps and high activation energy for O-O bond formation. Thus, transition-metal-based film-type electrocatalysts have been extensively reported with moderate efficiency due to their low price and abundance. Interestingly, transition metal based OER catalyst film generally exhibited optimal catalytic performance at considerable thickness (~ hundreds nanometers). The overall catalytic performance for the film-type catalysts are determined by various electrochemical processes, such as surface catalysis and electron transport. Therefore, to establish rational design rule for efficient OER electrocatalyst, it is necessary to extract well-defined kinetic parameters describing each electrochemical processes. In this regard, electrochemical impedance spectroscopy (EIS) is effective analysis to decouple and parametrize complicated electrochemical behavior in the catalyst film during OER. Here, we conducted EIS analysis for sub 10 nm-sized partially oxidized MnO nanoparticles (Mn-oxide NPs) operating with superb activity under neutral condition. Furthermore, we proposed a transmission line model with Havriliak-Negami capacitors to obtain reliable kinetic parameters for OER on the Mn-oxide NPs film. From this analysis, we explained the rationale for the optimum thickness. Also, we revealed that protons are involved in transport on the surface of p-MnO NPs, which directly proved the ongoing hypothesis of the oxo-hopping mechanism during catalysis. Furthermore, from correlation between the reaction rate constant (22.1 s-1 at 1.35 V vs. NHE) and TOF values, only small ratio of Mn sites served as factual active sites for OER. We believe that this mechanistic investigation using EIS analysis with our circuit model could be an effective platform to investigate comprehensive electrochemical processes in general film-type electrocatalysts. Figure 1

Developing alternatives to replace fossil fuels is an important issue for maintaining sustainable energy system in the future. Hydrogen fuel is an attractive alternative owing to its high energy density and environmentally friendly by-products. Electrical water splitting powered by a renewable energy source is a promising way to produce hydrogen fuel. However, anodic water oxidation is considered to be a bottleneck due to its high overpotential. Therefore, designing efficient catalyst for water splitting is regarded as an important issue. Previously, Ir and Ru-based catalysts have been shown to exhibit remarkable catalytic activity toward oxygen evolving reaction (OER) under acidic condition. Despite their high activity, it is difficult to use noble-metal-based catalysts for commercial applications because of the scarcity of precious metals and their costs. Therefore, non-precious transition metal oxide electrocatalysts were investigated for water oxidation reaction such as cobalt oxides, nickel oxides, iron oxides and manganese oxides.1,2 Their activities were only retained under basic conditions and advancement in electrocatalysts that show moderate activity at neutral pH has been relatively insufficient. Nature has highly efficient OER catalysts in photosystem II which is Mn4CaO5 cluster also called as oxygen evolving complex. Inspired by unique structure and catalytic mechanism of this cluster, Mn-based catalysts have been actively studied. Among various manganese oxide, nanoparticle system showed superior catalytic activity due to its totally different reaction mechanism compared to the bulk counterparts.3,4 However, after synthesis of manganese oxide nanoparticles, long alkyl organic ligands are attached on the surface of nanoparticles interrupting the approach of water molecules, which inhibits water oxidation. Previously, hydrophobic organic ligands were removed by thermal decomposition with sufficient annealing process. Alternative way to remove alkyl ligands is replacing it with different ligands through surface ligand treatment such as BF4 -, PF6 -, OH-, thiol, SCN- and PbCl3 -. Surface ligand exchange is beneficial in that it does not need high temperature annealing and possibility of fine surface control. We have successfully synthesized 6 nm Mn3O4 nanoparticles (NPs) and used for further surface treatments. Mn3O4 NPs were OH ligand exchanged as a first step and followed by methylamine treatment in aqueous solution. Identical TEM images and XRD patterns showed that the size of NPs was unchanged and the phase of NPs was maintained as Mn3O4. Electrokinetic study of the OER was analyzed by cyclic voltammetry (CV) at pH 7 and compared between OH ligand exchanged Mn3O4 NPs and further methylamine treated Mn3O4 NPs. Methylamine treatment reduced the overpotential for OER by nearly 100 mV at the current density 5 mA cm-2. Tafel slopes was also decreased after methylamine treatment to 83.1 mV dec-1. To understand the phenomenon of increased catalytic activity after methylamine treatment, further analysis of the surface of NPs were conducted using spectroscopic measurement. In the XPS O 1s spectra, surface hydroxyl group (Mn-OH) was increased up to 57.3 % after OH ligand exchange. Also, FTIR spectra showed increased ratio of Mn-OH to lattice oxygen (Mn-O-Mn) for OH ligand exchanged Mn3O4 NPs compared to Mn3O4 NPs which indicates successful OH ligand exchange on the surface of Mn3O4 NPs. After methylamine treatment of OH ligand exchanged Mn3O4 NPs, methylamine was not attached to the surface of NPs. XPS N 1s spectra did not show characteristic methylamine peak, FTIR did not show C-N vibration mode and elemental analysis indicates that N does not exist in the NPs. To figure out the role of methylamine on the surface of OH ligand exchanged Mn3O4 NPs, titration and zeta potential was measured. After methylamine treatment surface was more deprotonated. Deprotonation resulted in loss of positively charged proton which induced negative charge on the surface of Mn3O4 NPs and showed negative zeta potential. On the basis of nonexistence of methylamine on the surface and surface deprotonation after methylamine treatment, we hypothesized that methylamine work as Brønsted-Lowry base. Methylamine was replaced by ethylamine, ethylenediamine, dimethylamine, trimethylamine and ammonia water, then after reaction with different amines, overpotential was decreased for all cases. Interestingly, amines with larger pK a showed larger decrease in overpotential. Based on these results, we first proposed correlation between the surface charge and oxygen evolving activity. Deprotonating the surface of the catalyst through simple acid-base reaction induced a change in the surface state followed by catalytic activity improvement. This understanding can be applied for developing efficient nano-catalysts by deprotonating the nanoparticle surface to induce surface state change. [1] J. Am. Chem. Soc., 2014, 136, 4201-4211 [2] J. Am. Chem. Soc., 2014, 136, 7435-7443 [3] Sci. Rep., 2015, 5, 10279 [4] J. Am. Chem. Soc., 2017, 139, 2277-2285 Figure 1

In the wireless communication systems, the power consumption of a mobile device is very important issue due to its battery limitations. Hence most of the standards for wireless networks including a mobile WiMAX system are supporting their own power saving mode in way that a mobile device is able to reduce its energy usage while in the mode. However, those standards just define the arrangement of special time intervals, called a paging listening interval, during which the device needs to receive the paging-related control messages, and they do not specify how to effectively reduce the power in many different network environments. This means the amount of power spent by the device is very dependent on the implementations of individual device-vendors, and undesirable paging loss may happen according to the channel conditions. To reduce unnecessary power usage and the risk of paging loss, this paper proposes the effective frequency/BS selection algorithm applicable to a mobile device operating in the power saving mode, which serves the device with better BS based on the tracking for channel variation. This algorithm consists of the channel estimation phase during each paging listening interval, the tracking phase for the measured results, the frequency reselection phase based on the tracking activity, and the preferred BS reselection phase. Thus the proposed method can improve the paging performance while the device is moving in the network. Also the simulation result shows that the presented scheme is superior to other candidates in energy efficiency due to the channel-adaptive frequency/BS selection.