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The electrochemical CO2 reduction reaction (CO2RR), which converts CO2 into value-added feedstocks and renewable fuels, has been increasingly studied as a next-generation energy and environmental solution. Here, we report that single-atom metal sites distributed around active materials can enhance the CO2RR performance by controlling the Lewis acidity-based local CO2 concentration. By utilizing the oxidation Gibbs free energy difference between silver (Ag), zinc (Zn), and carbon (C), we can produce Ag nanoparticle-embedded carbon nanofibers (CNFs) where Zn is atomically dispersed by a one-pot, self-forming thermal calcination process. The CO2RR performance of AgZn–CNF was investigated by a flow cell with a gas diffusion electrode (GDE). Compared to Ag–CNFs without Zn species (53% at −0.85 V vs. RHE), the faradaic efficiency (FE) of carbon monoxide (CO) was approximately 20% higher in AgZn–CNF (75% at −0.82 V vs. RHE) with 1 M KOH electrolyte.

Electrocatalysts for CO2 electroreduction require not only high-performance active materials to control the series reaction but also conductive and durable supports to ensure long-term stability under harsh operating conditions. Instead of conventional heterogeneous catalysts made by attaching metal on supports, we manufactured a self-formed tandem catalyst designed for a cascade electroreduction of CO2 to C2H4. Using oxygen partial pressure-controlled calcination, electrospun copper acetate/polyacrylonitrile nanofibers were successfully transformed into porous carbon nanofibers consisting of doped N and metallic Cu particles. Doped nitrogen atoms adjacent to Cu atoms trigger the reaction by increasing the amount of CO* on the Cu surfaces, which lowers the energy required for CO dimerization that is used for C2H4 production. The Cu-embedded N-doped carbon nanofibers exhibit a C2H4 faradaic efficiency of 62% at a potential of −0.57 V vs. RHE with high current density of 600 mA cm−2 and excellent long-term stability. DFT calculations suggest that the lowered overpotential originates from the decreased CO dimerization energy barrier due to the doped N triggering CO production around the Cu particles.

It has previously been predicted [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12, 14699] and observed [E. Kim, et al., Solid State Ionics, 2013, 235, 22] that the open-circuit voltage U of a galvanic cell, involving a ternary or higher compound with more than one kind of mobile ionic carrier, is path- and time-dependent upon imposition or removal of the mobile components’ chemical potential differences, in contradistinction to the cell involving a binary compound. This has been attributed [H.-I. Yoo and M. Martin, Phys. Chem. Chem. Phys., 2010, 12 14699; J.-Y. Yoon, et al., Solid State Ionics, 2012, 213, 22] to the decoupled redistributions of multiple mobile components or multi-fold relaxation. We hereby experimentally demonstrate with SrTi0.982Al0.018O3−Δ, known to have an appreciable water solubility depending on temperature, that introduction of a secondary ionic carrier H+ in addition to the native O2− indeed renders the otherwise time-independent U time-dependent; and that this phenomenon may, thus, be employed to probe the presence of a secondary ionic carrier, e.g., H+ in addition to the primary O2− in BaTi0.982Al0.018O3−Δ whose water solubility is yet to be known. The temporal behavior of U of SrTi0.982Al0.018O3−Δ subjected to the two fixed chemical potential differences, ΔμO and ΔμH, is precisely delineated in terms of two-fold relaxation of H and O, yielding their chemical diffusivity values, and consequently, the ambiguity with the EMF-method to determine the ionic transference numbers of a multinary compound is cleared away.

Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO2 reduction in an aqueous condition to compete with the reduction of water. The phase of copper oxide nanofibers was controlled by predictive synthesis, which controls the nanoscale gas–solid reaction by considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu2O in which monophase Cu2O can be fabricated in a reasonable reaction time scale is discovered. Herein, we report the monophase Cu2O nanofiber photocathode, which photoelectrochemically converted CO2 into methanol with over 90% selectivity in an aqueous electrolyte, and a hierarchical structure is developed to optimize the photoactivity and stability of the electrode. Our work suggests...

Oxygen nonstoichiometry and electrical conductivity were measured on the system of γ-NaxCoO2, allegedly so far the best p-type thermoelectric oxide, against oxygen activity (aO2) across its widest ever range below aO2 = 1 for a fixed Na-content x = 0.706 at different temperatures in the range of 773–973 K, and at a fixed temperature 973 K for x = 0.664, 0.706 and 0.731, respectively. It has been deduced therefrom that as aO2 decreases, the majority disorder type shifts from [ V Na ′ ] ≈ p to either [ V Na ′ ] ≈ 2[ Co Na ⋅⋅ ] or [ V Na ′ ] ≈ 3[ Co i ⋯ ], however, exhibiting a positive deviation from the ideal defect behavior. The latter is attributed to the positive deviation of holes due to their degeneracy. By taking into appropriate account of the activity coefficient of holes in terms of the Fermi-Dirac integral of order 1/2, the nonstoichiometry and electrical conductivity have been precisely and consistently depicted to evaluate the defect-chemical parameters including the effective mass and mobility of holes and the redox equilibrium constant. The phase-stability limit of γ-NaxCoO2 is documented against temperature and Na-content.

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS2) polymorph conversion without cation insertion. Thermodynamic MoS2-CO-CO2 ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS2 structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS2 layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS2) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.

Both high activity and mass production potential are important for bifunctional electrocatalysts for overall water splitting. Catalytic activity enhancement was demonstrated through the formation of CoS2 nanoparticles with mono-phase and extremely porous structures. To fabricate porous structures at the nanometer scale, Co-based metal-organic frameworks (MOFs), namely a cobalt Prussian blue analogue (Co-PBA, Co3[Co(CN)6]2), was used as a porous template for the CoS2. Then, controlled sulfurization annealing converted the Co-PBA to mono-phase CoS2 nanoparticles with ~ 4 nm pores, resulting in a large surface area of 915.6 m2 g−1. The electrocatalysts had high activity for overall water splitting, and the overpotentials of the oxygen evolution reaction and hydrogen evolution reaction under the operating conditions were 298 mV and −196 mV, respectively, at 10 mA cm−2.

Due to the influence of human beings in the modern society, the concentration of CO2 in the atmosphere is rapidly increasing, which causes various environmental problems such as abnormal climate caused by global warming and destruction of ecosystem due to ocean acidification. Therefore, many institutional, scientific, and engineering efforts are being made to reduce the concentration of CO2 in the atmosphere. From the scientific point of view, to make the fuels and chemicals we need from the atmospheric CO2, electrochemical reduction of CO2 is actively being studied. The electrochemical CO2 reduction reaction can be divided into two categories, one for synthesizing multi-carbon products directly such as ethylene or ethanol and the other for producing syngas mixed with CO and H2. The former is mainly researched on Cu based material and has an advantage in that the desired product can be obtained immediately. In the latter case, by adjusting the ratio of CO and H2 in the synthesis gas, it is possible to synthesize multi-carbon products. The latter method can be said to have an advantage in the purity of the gas phase produced (reduction of separation cost). Research is underway in the form of co-catalysts of CO production catalyst like Au, Ag, Zn, Cu & H2 production catalysts. Because simply increasing the current density is not a good thing, but it must be able to maintain an industrially meaningful range of syngas ratios (1 to 3) while producing a high current density. Therefore, the potential window of studies on the rate-controlled syngas studied so far is typically high below -1.2V Vs RHE and high current density of ~ 30 mA/cm2. The reported lifetime at this level of current density is less than 10 hours, and for papers that report longer lifespans of tens to 100 hours, measured at 1 to 2 mA/cm2 levels. We fabricated Ag nanoparticle embedded in Zn dispersed carbon nanofiber to maintain syngas ratio stably in high potential window. In this catalyst carbon nanofiber produces H2 and Ag produces CO and dispersed Zn helps Ag in high potential. We annealed electrospun PAN/zinc nitrate&silver nitrate nanofiber at 800°C 3h in N2 ambient after stabilization in air. From this, we obtained Ag nanoparticle embedded in Zn & N doped carbon nanofiber. Ag tends to form nanoparticle and non-particle Zn was dispersed in carbon nanofiber. we obtained almost potential insensitive syngas ratio unlike conventional Ag catalyst in syngas production. Only N-doped carbon nanofiber sample produced increasing H2 with potential. Zn & N doped carbon nanofiber produced ratio of syngas higher than 2. Ag nanoparticle embedded carbon nanofiber produced ratio of syngas lower than 1 and sharply increased in higher potential (< -1.6V Vs. RHE). However, Ag nanoparticle embedded in Zn & N-doped carbon nanofiber maintained its low ratio of around 0.5 even in -2.2 V Vs. RHE and almost potential insensitive. We altered this insensitive syngas ratio with the mass ratio between carbon nanofiber and Ag/Zn-carbon nanofiber. It made syngas ratio increases slowly with potential and overall syngas ratio can be shifted with the mass of carbon nanofiber.

With growing demands for sustainable energy sources, hydrogen energy is receiving great attention due to their abundant, eco-friendly, and renewable nature. Although water electrolysis is most promising technologies for hydrogen production, it is currently obstructed by the sluggish kinetic of oxygen evolution reaction (OER) compared with hydrogen evolution reaction (HER). Furthermore, according to the demand for simplicity and cost effectiveness of electrolysis, the need for bifunctional electrocatalysts which operate in the same electrolyzer, is rapidly increasing. The cubic pyrite-phase transition metal dichalcogenides, such as FeS2, NiS2, and CoS2, have been proposed as candidates for bifuctional electrocatalysts. Among them, CoS2 has been reported to exhibit high conductivity and high activity for HER and OER in strong alkaline condition. In this study, while there are many phases of cobalt sulfide (e.g., CoS, Co9S8, and CoS2 etc.), it was successfully controlled cobalt sulfide phase according to the sulfur ratio through predicted synthesis based on thermodynamic. A highly porous CoS2 nanoparticles (NPs) was directly synthesized by controlling the ratio of sulfur and cobalt Prussian blue analogues (Co3[Co(CN)6]2), one of the metal organic frameworks (MOFs). MOF-driven CoS2 could lead to improvements in catalytic activity. The newly-designed MOF-driven CoS2 OER, HER catalysts show distinct strong points. First, CoS2 is as one of transition metal compounds and has high catalytic activity. Furthermore, the CoS2 of this work shows a highly porous and uniform particle size because it was synthesized through MOFs using precipitation. The Co3[Co(CN)6]2 was used as the starting material before sulfurization. The Co3[Co(CN)6]2 was prepared through a facile precipitation method. The CoS2 NPs were synthesized through gas-solid reaction in vacuum system. A 5 mL size ampoule containing total 45 mg of a 2:1 mixture of Co3[Co(CN)6]2 and sulfur powders was arranged, then the ampoule was vacuumed to 10-2 torr. The prepared ampoule was annealed at 500 ℃ for 2 hr in a furnace at ramping rate of 5 ℃ min-1. The average powder size of synthesized CoS2 NPs was approximately 30 nm with ~ 4nm pores, resulting in a large surface area of 915.6 m2 g-1. Compared to commercial IrO2, CoS2 NPs showed exceptional performance. The synthesized catalysts achieved a catalytic current density of 10 mA cm-2 at overpotentials as low as 300, 200 mV from oxygen, hydrogen evolution polarization curves through linear sweep voltammograms (LSVs). Furthermore, it has outstanding long-term durability under 10 mA cm-2. The electrochemical performance was conducted with three electrode cell using 1.0 M KOH electrolyte and Pt counter electrode, and working electrode is ink type on Ni foam. Furthermore, MOF-driven CoS2 was conducted through a full-cell test of overall water splitting for a practical two-electrode system in 1.0 M KOH. It exhibited an overpotential of 1.65 V at 10 mA cm-2. We introduced MOFs to a new methodology based on thermodynamic processes. It showed the possibility of replacing noble metals such as Ir, Ru in terms of catalytic performance.

Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO

The current (I) vs. voltage (U) behavior of an asymmetric ion-blocking cell, Pt , a O 2 o | B i 1.46 Y 0.54 O 3 | Pt was examined, at a fixed temperature of 700 °C, over a U range far exceeding the decomposition voltage U⁎ relative to the reference oxygen activity a O 2 o of the electrolytic compound Bi1.46Y0.54O3. It has been observed that until before decomposition, I vs. U behaves in a usual way, resulting in the partial electronic conductivity of the compound; decomposition starts always at a voltage a little over U⁎, indicating the presence of an energy barrier to decomposition; once decomposition starts at the blocking cathode, I vs. U immediately turns linear with a resistance much higher than the expectation, the total resistance of the electrolytic compound itself, indicating the presence of overwhelming anodic overpotential; decomposition of the compound proceeds as (Bi0.73Y0.27)2O3 → (Bi0.73 ‐ δY0.27)2O3 ‐ 3δ + δ(2Bi + 1.5O2); open-circuit voltage relaxation upon depolarization from - U(> - U⁎) always exhibits a time duration at − U⁎, indicating the reoxidation of reduced Bi. The I vs. U variation over the entire range of U as well as the decomposition kinetics of the compound are quantitatively described.