Green hydrogen production from water is one attractive route to non-fossil fuel and a potential source of clean energy. Hydrogen is not only a zero-carbon energy source but can also be utilized as an efficient storage of electrical energy generated through various other sources, such as wind and solar. Cost-effective and environmentally benign direct hydrogen production through neutral water (similar to pH 7) reduction is particularly challenging due to the low concentration of protons. There is currently a major need for easy-to-prepare, robust, as well as active electrode materials. Herein we report three new molecular electrodes that were prepared by anchoring commercially available, and environmentally benign cobalt-containing electrocatalysts with three different ligand frameworks (porphyrin, phthalocyanine, and corrin) on a structurally modified graphite foil surface. Under the studied reaction conditions (over 7 h at 22 degrees C), the electrode with Co-porphyrin is the most efficient for the water reduction with starting similar to 740 mV onset potential (OP) (vs. RHE, current density 2.5 mA/cm(2)) and a Tafel slope (TS) of 103 mV/dec. It is followed by the molecular electrodes having Co-phthalocyanine [825 mV (OP), 138 mV/dec (TS)] and Vitamin-B-12 (Co-corrin moiety) [830 mV (OP), 194 mv/dec (TS)]. A clear time-dependent improvement (>200 mV over 3 h) in the H-2 production overpotential with the Co-porphyrin-containing cathode was observed. This is attributed to the activation due to water coordination to the Co-center. A long-term chronopotentiometric stability test shows a steady production of hydrogen from all three cathode surfaces throughout seven hours, confirmed using an H(2 )needle sensor. At a current density of 10 mA/cm(2), the Co-porphyrin-containing electrode showed a TOF value of 0.45 s(-1) at 870 mV vs. RHE, whereas the Co-phthalocyanine and Vitamin-B-12-containing electrodes showed 0.37 and 0.4 s(-1) at 1.22 V and 1.15 V (vs. RHE), respectively.

Green hydrogen production via water splitting is vital for decarbonization of hard-to-abate industries. Its integration with renewable energy sources remains to be a challenge, due to the susceptibility to hazardous gas mixture during electrolysis. Here, we report a hybrid membrane-free cell based on earth-abundant materials for decoupled hydrogen production in either acidic or alkaline medium. The design combines the electrocatalytic reactions of an electrolyzer with a capacitive storage mechanism, leading to spatial/temporal separation of hydrogen and oxygen gases. An energy efficiency of 69% lower heating value (48 kWh/kg) at 10 mA/cm2 (5 cm-by-5 cm cell) was achieved using cobalt-iron phosphide bifunctional catalyst with 99% faradaic efficiency at 100 mA/cm2. Stable operation over 20 hours in alkaline medium shows no apparent electrode degradation. Moreover, the cell voltage breakdown reveals that substantial improvements can be achieved by tunning the activity of the bifunctional catalyst and improving the electrodes conductivity. The cell design offers increased flexibility and robustness for hydrogen production.

The instability of molecular electrodes under oxidative/reductive conditions and insufficient understanding of the metal oxide-based systems have slowed down the progress of H2-based fuels. Efficient regeneration of the electrode's performance after prolonged use is another bottleneck of this research. This work represents the first example of a bifunctional and electrochemically regenerable molecular electrode which can be used for the unperturbed production of H2 from water. Pyridyl linkers with flexible arms (-CH2-CH2-) on modified fluorine-doped carbon cloth (FCC) were used to anchor a highly active ruthenium electrocatalyst [RuII(mcbp)(H2O)2] (1) [mcbp2− = 2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine]. The pyridine unit of the linker replaces one of the water molecules of 1, which resulted in RuPFCC (ruthenium electrocatalyst anchored on -CH2-CH2-pyridine modified FCC), a high-performing electrode for oxygen evolution reaction [OER, overpotential of ∼215 mV] as well as hydrogen evolution reaction (HER, overpotential of ∼330 mV) at pH 7. A current density of ∼8 mA cm−2 at 2.06 V (vs. RHE) and ∼−6 mA cm−2 at −0.84 V (vs. RHE) with only 0.04 wt% loading of ruthenium was obtained. OER turnover of >7.4 × 103 at 1.81 V in 48 h and HER turnover of >3.6 × 103 at −0.79 V in 3 h were calculated. The activity of the OER anode after 48 h use could be electrochemically regenerated to ∼98% of its original activity while it serves as a HE cathode (evolving hydrogen) for 8 h. This electrode design can also be used for developing ultra-stable molecular electrodes with exciting electrochemical regeneration features, for other proton-dependent electrochemical processes.

The corrosion behavior of a new Nb-doped stainless steel (SDSS-Nb) designed by the CALPHAD method, produced using an open atmosphere process based on recycled materials, was investigated to improve the circular economy. Three heat-treatment conditions were evaluated to assess the sensitization effects of the precipitates and inclusions. XRD and SEM-EDS were used for phase identification, and sensitization was analyzed by cyclic polarization and Scanning Kelvin Probe Force Microscopy (SKPFM). The thermodynamic stability predicted by Thermo-Calc agrees with that observed by SEM-EDS. It was observed by cyclic polarization that the corrosion sensitization was mainly provided by the σ phase, which was deduced from the results obtained by SEM-EDS, XRD, and Thermo-Calc simulations. Furthermore, it was obtained that the sensitization due to Cr2N precipitates and nonmetallic inclusions was low, and the mechanical response is comparable to commercial UNS32750 super duplex stainless steel, which allows a good performance in severe environments and an efficient industrial application. Additionally, it has been obtained by SKPFM that the shear potential between the σ phase and the austenite is between 210 mV and 241 mV and that its value depends on the stability and equilibrium reached by the σ phase during thermal cycling.

Capacitive deionization (CDI) is an emerging technique for purifying water by removing ions. Recent experimental studies have reported that the anion/cation adsorption can be naturally imbalanced, even for a solution with just sodium and chloride, and suggested a link between imbalance and Faradaic leakages. However, these effects have been missing from conventional models. In this work, we developed a new circuit model to better understand the connection between Faradaic leakages and adsorption imbalance. The theory demonstrates that the effect emerges in a model that includes leakages, considers leakages on both electrodes separately, and considers different leakage resistance on the two electrodes. Having the model, it is possible to analyze and quantify the influence of the leakage resistance and other material properties on the adsorption imbalance. Leveraging these results, we further present a multi-electrode (ME) device design. The setup adds a third electrode to the spacer channel and can tune or eliminate the adsorption imbalance based on appropriately distributing the voltage across the electrodes. In conclusion, we describe a charge leakage mechanism responsible for the imbalance of ion adsorption and a flexible device design to tune the anion/cation removal.

Sodium hypochlorite (NaOCl) is widely used as a disinfectant agent for water treatment and surface cleaning. A straightforward way to produce NaOCl is by the electrolysis of an aqueous sodium chloride (NaCl) solution. This process presents several side reactions decreasing its efficiency with hypochlorite reduction on the cathode surface being one of the main detrimental reactions. In this work, we have studied carbon-based electrodes modified with cerium oxide (CeO2), fluorine, and platinum nanoparticles as cathodes for hypochlorite production. Fluorination was carried out electrochemically; the polyol method was used to synthesize platinum nanoparticles; and the hydrothermal process was applied to form a CeO2 layer. Scanning electron microscopy, FTIR, and inductively coupled plasma (ICP) indicated the presence of cerium oxide as a film, fluorine groups on the substrate, and a load of 3.2 mg/cm2 of platinum nanoparticles and 2.7 mg/cm2 of CeO2. From electrochemical impedance spectroscopy, it was possible to demonstrate that incorporating platinum and fluorine decreases the charge transfer resistance by 16% and 28%, respectively. Linear sweep voltammetry showed a significant decrease in hypochlorite reduction when the substrate was doped with fluorine from -16.6 mA/cm2 at -0.6 V to -9.64 mA/cm2 that further reduced to -8.78 mA/cm2 with cerium oxide covered fluorinated electrodes. The performance of the cathode materials during hypochlorite production improved by 80% compared with pristine activated carbon cloth (ACC) electrodes. The improvement toward hindering NaOCl reduction is probably caused by the incorporation of a partial negative charge upon doping with fluorine.

The low stability of the electrocatalysts at water oxidation (WO) conditions and the use of expensive noble metals have obstructed large-scale H2 production from water. Herein, we report the electrocatalytic WO activity of a cobalt-containing, water-soluble molecular WO electrocatalyst [CoII(mcbp)(OH2)] (1) [mcbp2−=2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine] in homogeneous conditions (overpotential of 510 mV at pH 7 phosphate buffer) and after anchoring it on pyridine-modified fluorine-doped carbon cloth (PFCC). The formation of cobalt phosphate was identified only after 4 h continuous oxygen evolution in homogeneous conditions. Interestingly, a significant enhancement of the stability and WO activity (current density of 5.4 mA/cm2 at 1.75 V) was observed for 1 after anchoring onto PFCC, resulting in a turnover (TO) of >3.6×103 and average TOF of 0.05 s−1 at 1.55 V (pH 7) over 20 h. A total TO of >21×103 over 8 days was calculated. The electrode allowed regeneration of∼ 85 % of the WO activity electrochemically after 36 h of continuous oxygen evolution. 

Multiple and hierarchical manganese (Mn)-based Prussian blue analogues obtained on different substrates are successfully prepared using a universal, facile, and simple strategy. Different functional groups and surface charge distributions on carbon cloth have significant effects on the morphologies and nanostructures of Mn-based Prussian blue analogues, thereby indirectly affecting their physicochemical properties. Combined with the advantages of the modified carbon cloth and the nanostructured Mn-based Prussian blue analogues, the composite with negative surface charge formed by the electronegativity differences shows good electrochemical properties, leading to improvement in charge efficiency during capacitive desalination. An asymmetric device fabricated with Mn-based Prussian blue analogue-modified F-doped carbon cloth as the cathode and acid-treated carbon cloth as the anode presents the highest salt adsorption capacity of 10.92 mg g-1 with a charge efficiency of 82.28% and the lowest energy consumption of 0.45 kW h m-3 at 1 V due to the main influencing factor from the negative surface charge leading to co-ion expulsion boosting the capacitive deionization performance. We provide insights for further exploration of the relationship between second-phase materials and carbon cloth, while offering some guidance for the design and preparation of electrodes for desalination and beyond. 

Prussian blue (PB) and its analogues (PBAs) are drawing attention as promising materials for sodium-ion batteries and other applications, such as desalination of water. Because of the possibilities to explore many analogous materials with engineered, defect-rich environments, computational optimization of ion-transport mechanisms that are key to the device performance could facilitate real-world applications. In this work, we have applied a multiscale approach involving quantum chemistry, self-consistent mean-field theory, and finite-element modeling to investigate ion transport in PBAs. We identify a cyanide-mediated ladder mechanism as the primary process of ion transport. Defects are found to be impermissible to diffusion, and a random distribution model accurately predicts the impact of defect concentrations. Notably, the inclusion of intermediary local minima in the models is key for predicting a realistic diffusion constant. Furthermore, the intermediary landscape is found to be an essential difference between both the intercalating species and the type of cation doping in PBAs. We also show that the ladder mechanism, when employed in multiscale computations, properly predicts the macroscopic charging performance based on atomistic results. In conclusion, the findings in this work may suggest the guiding principles for the design of new and effective PBAs for different applications.

Clean water and affordable energy are critical worldwide challenges for which electrolytic capacitors are increasingly considered as viable alternatives. The upcoming technology of capacitive deionization (CDI) uses similar electrolytic capacitors for the desalination of water. The current work presents a new method that leverages existing support for supercapacitors in the form of current-distribution models, which enables detailed and separated descriptions of the rate-limiting resistances. Crucially, the new model blends this basis with a novel formulation centered on the adsorption of chemical species in CDI. Put together, it is adaptable to solving a wide range of problems related to chemical species in electrochemical cells. The resulting electrolytic-capacitor (ELC) model has enhanced stability and ease-of-implementation for simulations in 2D. The results demonstrate that the model accurately simulates dynamics CDI performance under a variety of operational conditions. The enhanced stability together with the adaptability further allows tractable simulations of leakage reactions and even handling multi-ion deionization in 2D. Moreover, the model naturally blends with existing interfaces in COMSOL Multiphysics, which automatically generalizes, stabilizes, and simplifies the implementation. In conclusion, the ELC model is user-friendly and tractable for standard simulations while also being especially powerful when simulating complex structures, leakage reactions, and multi-ion solutions.