Different crystal facets with different surface atomic configurations and physical/chemical properties will have distinct electrochemical performances during their surface/near-surface redox reactions, and it is important to realize the controllable synthesis of high active surfaces for electrode materials. Herein, using first-principles calculations, the electrochemical performances of different surfaces of β-MnO2 were investigated. Higher surface adsorption pseudocapacitance and lower ion diffusion barrier from the surface to the near surface make the {001} surface of β-MnO2 superior to other surfaces when acting as an electrode material. Moreover, β-MnO2 with a large percentage of the {001} surface was predicted to be obtained through surface F-termination. F-termination decreases the surface energy of the {001} surface while suppressing the growth of {110} surface, which demonstrated as the surface with a much lower electrochemical performance. This work might provide a feasible strategy to synthesize anticipated surfaces with a high electrochemical performance for transition metal oxides.Keywords: adsorption; DFT; diffusion; F-termination; surface energy; β-MnO2;

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. In this research, a series of TMDs (MX2, M = Co, Cr, Fe, Mn, Mo, Nb, Ni, Re, Sc, Tc, Ti, V, W, Zr, and X = S, Se, Te) in 2H- and 1T-phases were investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔGH) and electronic structure using a simple descriptor. The results showed a positive linear relation between ΔGH and the work required of the H electron to fill the unoccupied electronic states of the TMDs. Based on such linear relationships, the various defects (B-, C-, N-, O-, F-, P-, Se-doping and S-vacancy) were used to activate the inert basal planes of the 2H-phase molybdenum disulfide, which can introduce impurity states in the lower energy level to effectively accommodate the H electron. Furthermore, HER activity can be further optimized with the increasing concentration of the defects. These findings provide a practicable map of the HER performances, as well as indicating an appropriate direction for optimizing HER activity.

•Design a two action sites strategy to break interdependence restriction.•Synergistic effect of two actions sites can beyond the top of the volcano.•Plasma-engraved CoOOH exhibit outstanding OER performance.•Decreased Tafel slope confirm the change of calculative rate-limiting step.Developing effective ways to promote the sluggish oxygen evolution reaction (OER) still remains a great challenge due to the interdependence multiple steps procedure. Herein, we design a two action sites strategy to break interdependence restriction to reduce the calculative overpotential. Density functional theory demonstrated that the introduced oxygen vacancies could accelerate the oxidation of H2O by induced an extra reaction step, corresponding deprotonation of H2O* decomposed into two separated reaction steps: H2O* ↔ (HO + H)* and (HO + H)*↔ HO* + H+ + e-. Meanwhile, experimental observations confirm that two action sites promote the Vo-CoOOH OER performance including a lower onset potential, a lower Tafel slope and an incremental Turnover frequency (TOF) from 0.01 to 0.04 s−1. Through this work, a viable design principle for, but not limited to, CoOOH OER catalyst is proposed: injected hole can activate the synergistic catalytic effect of two actions sites to accelerate the water oxidation.This study can be described as following figure.Download high-res image (271KB)Download full-size image

•A novel three-dimensional MoS2 framework coupled with Ni(OH)2 nanoparticlesis firstly synthesized.•This facile Ni(OH)2/MoS2 heterostructure delivers an excellent alkaline HER activity.•The interfacial synergistic effect plays a key role in reducing the energy barrier of the initial water dissociation step.•This work demonstrates a potentially interface engineering strategy for accelerating sluggish alkaline HER kinetics.Earth-abundant, noble-metal-free catalysts with outstanding electrochemical hydrogen evolution reaction catalytic activity in alkaline media play a key role in sustainable production of H2 fuel. Herein, the novel three-dimensional Ni(OH)2/MoS2 hybrid catalyst with synergistic effect has been synthesized by a facile approach for efficient alkaline hydrogen evolution reaction. Benefiting from abundant active interfaces, this hybrid catalyst shows high hydrogen evolution catalytic activity in 1 M KOH aqueous solution with an onset overpotential of 20 mV, an overpotential of 80 mV at 10 mA cm−2 and a Tafel slope of 60 mV dec−1. Further theoretical calculations offers a deeper insight of the synergistic effect of Ni(OH)2/MoS2 interface: Ni(OH)2 provides the active sites for hydroxyl adsorption, and MoS2 facilitates adsorption of hydrogen intermediates and H2 generation. This interfacial cooperation leads to a favorable hydrogen and hydroxyl species energetics and reduce the energy barrier of the initial water dissociation step, which is the rate-limiting step of MoS2 catalyst in alkaline media. The combination of experimental and theoretical investigations demonstrates that the sluggish alkaline hydrogen evolution process can be circumvented by rational catalysts interface engineering.Download high-res image (178KB)Download full-size image

•Unravel contrastive pseudocapacitance and similar EDLC in T and H phases of MoS2.•Possible pseudocapacitance introduced by Mo-edge and vacancy of H-MoS2.•EDLC of MoS2 is comparable with graphene, which agrees with previous experiments.Two-dimensional MoS2 is a promising candidate for high performance electrochemical capacitors. However, the understanding of different capacitive performances of MoS2 with different phases is still limited. Herein, the pseudocapacitive and electrical double layer characters in T and H phases of MoS2 monolayer are systematically studied, by using first-principles calculations and molecular dynamic simulations. The electronic levels referenced to the vacuum level are utilized to discuss the intrinsic pseudocapacitive characters. The large density of states of T-MoS2 located at 0.25 V versus the standard hydrogen electrode indicates possible redox pseudocapacitance. Contrastively, the large band gap of H-MoS2 limits its pseudocapacitive behavior. Furthermore, the reaction with the Li atom is employed to shed more light on the possible redox behavior. The different shifts of Fermi levels in H and T phase confirm their different capacitive properties. Moreover, the electrical double layer capacitances character of MoS2 in aqueous solution is also investigated. The H-MoS2 exhibits similar double layer character with T-MoS2. The calculated double layer capacitances of MoS2 are comparable with graphene, and can reach 9.07 μF/cm2 and 7.42 μF/cm2 for anode and cathode, respectively. The combined calculations shed more light to explore other two-dimensional transition-metal dichalcogenides as supercapacitors electrode materials.

•Ionic insertion and exchange and ionic diffusion inside the MXene (Ti3C2(OH)2) slit nanopore during galvanostatic cycling process is simulated.•Charge storage takes place by a fast counter-ion insertion and counter-ion/co-ion exchange during charge/discharge process.•Spatial distribution of ions and orientation of cation dipoles vary cyclically to screen the external electric potential.•A complement to the restriction of the in situ measurements in understanding the underlying charge storage mechanism.While most theoretical studies on slit nanopore electrodes are limited in their static properties, understanding the dynamic charge storage is crucial for promoting their electrochemical performance in the field of energy storage systems. In this article, the charging/discharging dynamics in MXene (Ti3C2(OH)2) electrode featuring intrinsic slit nanopore with room temperature ionic liquid are investigated by molecular dynamic simulation. The galvanostatic cycling is modeled to help to understand the dynamic charge storage mechanism. The simulation results show that the electrolyte ions spontaneously wet the Ti3C2(OH)2 slit nanopore in the absence of external potential, even at 0.7 nm layer distance pore case. An intriguing electroneutrality breakdown and faster diffusion property is observed inside the nanopore. During charge/discharge process, the charge storage takes place by a fast counter-ion insertion and counter-ion/co-ion exchange. Besides, the spatial distribution of ions and the orientation of the cation dipoles inside the pore vary cyclically alone with the charge/discharge process, which helps to screen the external electric potential. Having elucidated the charge mechanism, we investigate the layer distance factor and show that the ions exhibit more significantly acceleration and more ordered dipole orientation inside the narrower pore. This will help Ti3C2(OH)2 electrode with narrower pore exhibit better power performance.

Pseudocapacitors, which can store more energy at high charge/discharge rates, have attracted considerable attention. The performance of a pseudocapacitive material mainly depends on the interaction between electrode materials and the electrolyte ions. However, the understanding of the interaction is still limited. Here, the performance of Ti2CT2 (T = O, F, and OH) nanosheets as pseudocapacitor electrode materials has been investigated through a novel first-principles approach. The results suggest that O-terminated Ti2C nanosheets are shown to be pseudocapacitive cathode materials. The pseudocapacitance is attributed to the large intrinsic capacitance of Ti2CO2 nanosheets and contact adsorbed cations. The former mainly decides the capacity of charges and the latter reduces the change of potential in the electrode. The integral capacitance and Na-ion capacity of Ti2CO2 nanosheets are simulated to be 291.5 F g−1 with a broad potential window range from 0 to 2.80 V (versus Na/Na+). Low diffusion energy barriers on Ti2CO2 and Ti2CF2 nanosheets indicate fast transportation and high charge and discharge rate for Na-ions. Our results provide insight into the origin of pseudocapacitance on stacked two-dimensional materials.

The urgent demands for sustainable and renewable energy resources facilitate the researches of energy storage devices, and the electrode materials are important for the performance of these devices. Transition metal oxides as the promising candidates are impeded by their limited electronic conductivity and low intrinsic activity. In this work, we propose a strategy of lower valence-state doping for ramsdellite-MnO2 to facilitate the formation of oxygen vacancies, which are effective to improve the conductivity and activity of ramsdellite-MnO2. Using DFT + U calculations, we find out that the formation energies of oxygen vacancies both in the bulk and on the surface of ramsdellite-MnO2 are decreased apparently after Zn doping. Notably, the surface oxygen vacancies could form spontaneously without any other impetus. In addition, the bulk Zn dopants will provide the enhanced electrons diffusion to the surface, and the positive surface oxygen vacancies will draw the electrons to the reaction sites. In the reaction sites, the oxygen vacancies and reduced Mn ions will improve the activity of the electrode reactions. This may be anticipated to improve the electrochemical performance of the similar binary metal oxides.

•We have investigated the electrochemical double layer capacity of polar reduced graphene oxide electrode with aqueous solution by using molecular dynamic simulation, which is rare in previous studies currently.•The dipole orientation of water molecules near the polar reduced graphene oxide electrode will strongly depend on the polarity of functional group on reduced graphene oxide surface. The polarization interaction also restricts the rotation response of water molecules and weakens their mobility near the charged electrode, resulting in the decrease of integral capacitance value of reduced graphene oxide with increasing of functional groups concentration.•To obtain the accurate partial charge distribution of decorated functional group and corresponding surface polarization of reduced graphene oxide electrode, Bader charge analysis based on density functional theory calculations is used.Reduced graphene oxide (rGO) has emerged as an attractive choice for electrochemical double layer capacitors. Based on the accurate atomic partial charge distribution determined by density functional theory calculations, the electrochemical double layer structural and capacitive properties at the rGO/NaCl aqueous electrolyte interface are studied using the molecular dynamic simulations. Due to the charge redistribution mainly around oxygen functional groups, rGO will form a strong polar surface. It will significantly change the arrangement of surrounding water molecules through a hydrogen bond like interaction. The change of dipole orientation of water will induce corresponding net charge redistribution, leading to a change of potential of zero charge of rGO electrode. The polarization interaction also restricts the orientation of electrolyte molecules and weakens their mobility, resulting in the decrease of integral capacitance value of rGO with increasing of functional groups concentration. This work about electrochemical double layer near polar rGO electrode could offer a reasonable guidance for future performance improvements of electrochemical double layer capacitors.

The band gap engineering of nanostructures is the key point in the application of nanoelectronic devices. By using first-principles calculations, the band structures of graphyne nanoribbons with armchair (a-GNRs) and zigzag (z-GNRs) edges under various strains are investigated. A controllable band gap of a strained narrow a-GNR (1.36–2.85 eV) could be modulated almost linearly under an increasing strain in the range of −5% to 16%. In contrast, the band gap of a strained narrow z-GNR (2.68–2.91 eV) is relatively insensitive to −16% to 16% strain. This contrastive band gap engineering of narrow GNRs is attributed to different structure deformation of the specific graphyne structure including two kinds of carbon atoms different from those of graphene. For wider strained GNRs, the band gap (depending on its width and edge morphology) generally decreases as tensile strain increases, similar to 2D graphyne sheet. The charge density distributions of key states around Fermi level are presented to investigate the reason for band gap variation.

The adsorption of several acidic gases (CO2, NO2 and SO2) on light metal (Li, Al) decorated graphene oxide (GO) is theoretically studied, based on the first-principles calculations. Configuration relaxation, binding energy and charge transfer are carried out to discuss the acidic gas adsorption ability of light metal decorated GO. It is found out that Li, Al could be anchored stably by hydroxyl and epoxy groups on GO, and then a strong adsorption of CO2, NO2 and SO2 will occur above these light metals. In contrast to Ti, Li decorated GO exhibits a comparable adsorption ability of acidic gases, but a much smaller interaction with O2 about 2.85–3.98 eV lower in binding energy; and Al decorated GO displays much higher binding energy of all acidic gases with an enhancement of about 0.59–2.29 eV. The results of enhanced acidic gas adsorption ability and a reduced interference by O2 imply that Li, Al decorated GO may be useful and promising for collection and filtration of exhaust gases.

The mechanism of electric field induced orientation-selective unzipping of carbon nanotubes into carbon nanoribbons upon oxidation is studied based on first-principles calculations. Under a controlled external electric field, the initial sparse O atoms are ordered to attack the transverse C–C bonds along a certain side of the carbon nanotube instead of a random site. In addition, the next O atoms tend to follow each initial O atom and stand on the adjacent transverse C–C bonds on this side, sequentially and automatically leading to a linear epoxy chain. Besides, it is noteworthy that an effortless O diffusion with reduced energy barriers allows the regular arrangement of an epoxy chain due to electron doping under a suitable electric field. An easier process for the unzipping of carbon nanotubes and longer graphene nanoribbons with smoother edges is expected under an external electric field.

The urgent demands for sustainable and renewable energy resources facilitate the researches of energy storage devices, and the electrode materials are important for the performance of these devices. Transition metal oxides as the promising candidates are impeded by their limited electronic conductivity and low intrinsic activity. In this work, we propose a strategy of lower valence-state doping for ramsdellite-MnO2 to facilitate the formation of oxygen vacancies, which are effective to improve the conductivity and activity of ramsdellite-MnO2. Using DFT + U calculations, we find out that the formation energies of oxygen vacancies both in the bulk and on the surface of ramsdellite-MnO2 are decreased apparently after Zn doping. Notably, the surface oxygen vacancies could form spontaneously without any other impetus. In addition, the bulk Zn dopants will provide the enhanced electrons diffusion to the surface, and the positive surface oxygen vacancies will draw the electrons to the reaction sites. In the reaction sites, the oxygen vacancies and reduced Mn ions will improve the activity of the electrode reactions. This may be anticipated to improve the electrochemical performance of the similar binary metal oxides.

The mechanism of electric field induced orientation-selective unzipping of carbon nanotubes into carbon nanoribbons upon oxidation is studied based on first-principles calculations. Under a controlled external electric field, the initial sparse O atoms are ordered to attack the transverse C–C bonds along a certain side of the carbon nanotube instead of a random site. In addition, the next O atoms tend to follow each initial O atom and stand on the adjacent transverse C–C bonds on this side, sequentially and automatically leading to a linear epoxy chain. Besides, it is noteworthy that an effortless O diffusion with reduced energy barriers allows the regular arrangement of an epoxy chain due to electron doping under a suitable electric field. An easier process for the unzipping of carbon nanotubes and longer graphene nanoribbons with smoother edges is expected under an external electric field.

The adsorption of several acidic gases (CO2, NO2 and SO2) on light metal (Li, Al) decorated graphene oxide (GO) is theoretically studied, based on the first-principles calculations. Configuration relaxation, binding energy and charge transfer are carried out to discuss the acidic gas adsorption ability of light metal decorated GO. It is found out that Li, Al could be anchored stably by hydroxyl and epoxy groups on GO, and then a strong adsorption of CO2, NO2 and SO2 will occur above these light metals. In contrast to Ti, Li decorated GO exhibits a comparable adsorption ability of acidic gases, but a much smaller interaction with O2 about 2.85–3.98 eV lower in binding energy; and Al decorated GO displays much higher binding energy of all acidic gases with an enhancement of about 0.59–2.29 eV. The results of enhanced acidic gas adsorption ability and a reduced interference by O2 imply that Li, Al decorated GO may be useful and promising for collection and filtration of exhaust gases.

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. In this research, a series of TMDs (MX2, M = Co, Cr, Fe, Mn, Mo, Nb, Ni, Re, Sc, Tc, Ti, V, W, Zr, and X = S, Se, Te) in 2H- and 1T-phases were investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔGH) and electronic structure using a simple descriptor. The results showed a positive linear relation between ΔGH and the work required of the H electron to fill the unoccupied electronic states of the TMDs. Based on such linear relationships, the various defects (B-, C-, N-, O-, F-, P-, Se-doping and S-vacancy) were used to activate the inert basal planes of the 2H-phase molybdenum disulfide, which can introduce impurity states in the lower energy level to effectively accommodate the H electron. Furthermore, HER activity can be further optimized with the increasing concentration of the defects. These findings provide a practicable map of the HER performances, as well as indicating an appropriate direction for optimizing HER activity.

Pseudocapacitors, which can store more energy at high charge/discharge rates, have attracted considerable attention. The performance of a pseudocapacitive material mainly depends on the interaction between electrode materials and the electrolyte ions. However, the understanding of the interaction is still limited. Here, the performance of Ti2CT2 (T = O, F, and OH) nanosheets as pseudocapacitor electrode materials has been investigated through a novel first-principles approach. The results suggest that O-terminated Ti2C nanosheets are shown to be pseudocapacitive cathode materials. The pseudocapacitance is attributed to the large intrinsic capacitance of Ti2CO2 nanosheets and contact adsorbed cations. The former mainly decides the capacity of charges and the latter reduces the change of potential in the electrode. The integral capacitance and Na-ion capacity of Ti2CO2 nanosheets are simulated to be 291.5 F g−1 with a broad potential window range from 0 to 2.80 V (versus Na/Na+). Low diffusion energy barriers on Ti2CO2 and Ti2CF2 nanosheets indicate fast transportation and high charge and discharge rate for Na-ions. Our results provide insight into the origin of pseudocapacitance on stacked two-dimensional materials.