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Reactive metals hydrolysis offers significant advantages for hydrogen storage and production. However, the regeneration of common reactive metals (e.g., Mg, Al, etc.) is energy-intensive and produces unwanted byproducts such as CO2 and Cl2. Herein, we employ Zn as a reactive mediator that can be easily regenerated by electrolysis of ZnO in an alkaline solution with a Faradaic efficiency of >99.9 %. H2 is produced in the same electrolyte by constructing a Zn-H2O hydrolysis battery consisting of a Zn anode and a Raney-Ni cathode to unlock the Zn-H2O reaction. The entire two-step water splitting reaction with a net energy efficiency of 70.4 % at 80 °C and 50 mA cm-2. Additionally, the Zn-H2O system can be charged using renewable energy to produce H2 on demand and runs for 600 cycles only sacrificing 3.76 % energy efficiency. DFT calculations reveal that the desorption of H* on Raney-Ni (-0.30 eV) is closer to zero compared with that on Zn (-0.87 eV), indicating a faster desorption of H* at low overpotential. Further, a 24 Ah electrolyzer is demonstrated to produce H2 with a net energy efficiency of 65.5 %, which holds promise for its real application.

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The anode stability is critical for efficient and reliable seawater electrolyzers. Herein, a NiFe-based film catalyst was prepared by anodic oxidation to serve as a model electrode, which exhibited a satisfactory oxygen evolution performance in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) with an overpotential of 348 mV at 100 mA cm-2 and a long-term stability of over 100 h. After that, the effects of the current density and bulk pH of the electrolyte on its stability were evaluated. It was found that the electrode stability was sensitive to electrolysis conditions, failing at 20 mA cm-2 in 0.1 M KOH + 0.5 M NaCl but over 500 mA cm-2 in 0.5 M KOH + 0.5 M NaCl. The electrode dissolved, and some precipitates immediately formed at the region very close to the electrode surface during the electrolysis. This can be ascribed to the pH difference between the electrode/electrolyte interface and the bulk electrolyte under anodic polarization. In other words, the microzone acidification accelerates the corrosion of the electrode by Cl-, thus affecting the electrode stability. The operational performances of the electrode under different electrolysis conditions were classified to further analyze the degradation behavior, which resulted in three regions corresponding to the stable oxygen evolution, violent dissolution-precipitation, and complete passivation processes, respectively. Thereby increasing the bulk pH could alleviate the microzone acidification and improve the stability of the anode at high current densities. Overall, this study provides new insights into understanding the degradation mechanism of NiFe-based catalysts and offers electrolyte engineering strategies for the application of anodes.

RESUMEN

The interface interaction between deposited carbon and metallic electrode substrates in tuning the growth of CO2-derived products (e.g., amorphous carbon, graphite, carbide) is mostly unexplored for the high-temperature molten-salt electrolysis of CO2. Herein, the carbon deposition on different transition-metal cathodes was performed to reveal the role of substrate materials in the growth of cathodic products. At the initial stage of electrolysis, transition metals (e.g., Cr, Fe, Ni, and Co) that exhibit appropriate carbon-binding ability (in range of -30 to 60 kJ mol-1) allow carbon diffusing into and then dissociating from metal to form graphite, as the carbon-binding ability can be determined by the Gibbs free energy of formation of metallic carbides. The catalytic cathodes showing super strong (e.g., Ti, V, Mo, and W) or weak (e.g., Cu) carbon-binding ability produce stable carbides or amorphous carbon, respectively. However, the subsequent deposited carbon is immune to the catalysis of the substrate, forming amorphous carbon nanoparticles and nanofibers on the surface of carbides and graphite, respectively. This paper not only highlights the role of the catalytic cathodes for carbon deposition, but also offers a material selection principle for the controllable growth of CO2-derived products in molten salts.

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High-temperature molten-salt electrolyzers play a central role in metals, materials and chemicals production for their merit of favorable kinetics. However, a low-cost, long-lasting, and efficient high-temperature oxygen evolution reaction (HT-OER) electrode remains a big challenge. Here we report an iron-base electrode with an in situ formed lithium ferrite scale that provides enhanced stability and catalytic activity in both high-temperature molten carbonate and chloride salts. The finding is stemmed from a discovery of the ionic potential-stability relationship and a basicity modulation principle of oxide films in molten salt. Using the iron-base electrode, we build a kiloampere-scale molten carbonate electrolyzer to efficiently convert CO2 to carbon and oxygen. More broadly, the design principles lay the foundations for exploring cheap, Earth-abundant, and long-lasting HT-OER electrodes for electrochemical devices with molten carbonate and chloride electrolytes.

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Herein, we engineered the cobalt core size and carbon shell thickness of Co@C by molten salt electrolysis (MSE) to investigate the enhanced essence of decreasing core size as well as the shell thickness dependence-mediated transition of catalytic mechanisms. We found that the reaction activation energy (RAE) of Co@C/peroxymonosulfate (PMS) systems was intimately dependent on the core sizes for sulfamethoxazole (SMX) degradation. The smaller core size of 26 nm provided a lower RAE of 13.39 kJ mol-1. In addition, increasing carbon shell thicknesses of Co@C altered the catalytic mechanisms from a radical pathway of SO4•- and •OH to to a non-radical pathway of 1O2 and electron-transfer process (ETP), which were verified by experimental results and density functional theory (DFT) calculations. Interestingly, increasing carbon shell thicknesses promoted the charge transfer between Co metal slab and carbon shell, increased the adsorption energy of PMS molecule on the Co@C slab, and decreased the length of OO, which favoured the occurrence of non-free radical processes.

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RESUMEN

Despite the vital roles of Co nanoparticles catalytic oxidation in the Fenton-like system for eliminating pollutants, contributions of Co phases are typically overlooked. Herein, a biphase Co@C core-shell catalyst was synthesized by the electrochemical co-reduction of CaCO3 and Co3O4 in molten carbonate. Unlike the traditional pyrolysis method that is performed over 700 °C, the electrolysis was deployed at 450 °C, at which biphase structures, i.e., face-centered cubic (FCC) and hexagonal close-packed (HCP) structures, can be obtained. The biphase Co@C shows excellent catalytic oxidation performance of diethyl phthalate (DEP) with a high turnover frequency value (TOF, 28.14 min-1) and low catalyst dosage (4 mg L-1). Furthermore, density functional theory (DFT) calculations confirm that the synergistic catalytic effect of biphase Co@C is the enhancement for the breaking of the peroxide O-O bond and the charge transfer from catalysts to PMS molecule for the activation. Moreover, the results of radicals quenching experiments and electron paramagnetic resonance (EPR) tests confirm that SO4•-, •OH, O2•-, and 1O2 co-degrade DEP. Remarkably, 100% removals of three model contaminants, including DEP, sulfamethoxazole (SMX) and 2,4-dichlorophen (2,4-DCP), were achieved, either in pure water or actual river water. This paper provides an electrochemical pathway to leverage the phase of catalysts and thereby mediate their catalytic capability for remediating refractory organic contaminants.

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The reported mechanical strength of carbon nanocoils (CNCs) obtained from traditional preparation of catalytic acetylene pyrolysis is far below its theoretical value. Herein, we report a molten salt electrolysis method that employs CO32- as feedstock to grow CNCs without using metal catalyst. We meticulously mediate the alkalinity of molten carbonate to tune the electrochemical reduction of CO32- on graphite electrode to selectively grow CNCs in Li2CO3-Na2CO3-K2CO3-0.001 wt %Li2O. Graphite substrate, current density, and alkalinity of molten salt dictate the growth of CNCs. In addition, the electrolytic CNCs shows a spring constant of 1.92-39.41 N/m and a shear modulus of 21-547 GPa, which are 10-200 times that of CNCs obtained from catalyst-assisted gas-to-solid conversions. Overall, this paper opens up an electrochemical way to prepare CNCs through liquid-to-solid conversion without using catalysts and acetylene, providing new perspectives on green synthesis of 1D carbon nanomaterials with high mechanical strength.

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RESUMEN

Efficient water electrolyzers are constrained by the lack of low-cost and earth-abundant hydrogen evolution reaction (HER) catalysts that can operate at industry-level conditions and be prepared with a facile process. Here we report a self-standing MoC-Mo2C catalytic electrode prepared via a one-step electro-carbiding approach using CO2 as the feedstock. The outstanding HER performances of the MoC-Mo2C electrode with low overpotentials at 500 mA cm-2 in both acidic (256 mV) and alkaline electrolytes (292 mV), long-lasting lifetime of over 2400 h (100 d), and high-temperature performance (70 oC) are due to the self-standing hydrophilic porous surface, intrinsic mechanical strength and self-grown MoC (001)-Mo2C (101) heterojunctions that have a ΔGH* value of -0.13 eV in acidic condition, and the energy barrier of 1.15 eV for water dissociation in alkaline solution. The preparation of a large electrode (3 cm × 11.5 cm) demonstrates the possibility of scaling up this process to prepare various carbide electrodes with rationally designed structures, tunable compositions, and favorable properties.

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The wettability of molten carbonate on carbon determines the electrochemical performances of high-temperature direct carbon fuel cells (DCFCs). However, a universal method to measure the high-temperature wettability of molten carbonate is absent and the wetting kinetics is not well understood. Herein, we develop a dispensed drop (DD) method to measure the wetting kinetics of molten carbonate (Li2CO3-Na2CO3-K2CO3, 43.5:31.5:25.0, molar ratio) on the carbon substrate at 450-750 °C under controlled atmospheres (100%Ar, 100%CO2, and 1%O2-99%N2). The measured contact angles under different conditions reveal that increasing the O2- concentration in the gas-liquid-solid (GLS) interface decreases the contact angle. In addition, elevating the temperature, introducing O2 in the gas atmosphere, or pretreating the carbon substrate can enhance the wetting kinetics of molten carbonates. The molten carbonate completely wets the carbon substrate in 150 min in Ar gas atmosphere and in 30 min in 1%O2-99%N2 gas atmosphere at 600 °C. Further, it takes only 30 min to completely wet the pretreated carbon substrate in Ar atmosphere at 600 °C. Overall, this paper offers the DD method to study the wettability of molten carbonate on the carbon substrate, which is helpful to understand the underlying wetting mechanism and engineer the electrode design for DCFCs.

RESUMEN

Converting CO2 into value-added chemical fuels and functional materials by CO2 reduction reaction (CO2RR) is conducive to achieving a carbon-neutral energy cycle. However, it is still challenging to efficiently navigate CO2RR toward desirable products. Herein, we report a facile strategy to extend product species in borate-containing molten electrolyte at a positively shifted cathodic potential with a high current density (e.g. 100 mA/cm2), which can selectively electro-transform CO2 into desired products (either CO or solid carbon nanofibers, respectively reaching a high selectivity of ∼90%). The borates can act as a controller of electrolyte alkalinity to buffer the concentration of sequentially generated O2- during CO2RR, positively shifting the reduction potential of the captured CO2 and concurrently extending the product species. The sustainable buffering effect is available under CO2 atmosphere. Compared with borate-free electrolyte, the CO2 conversion efficiency is over three times higher, while the electrolysis energy consumption is decreased by over 40%.