Electrowinning for Room-Temperature Ironmaking: Mapping the Electrochemical Aqueous Iron Interface.

A promising route toward room-temperature ironmaking is electrowinning, where iron ore dissolution is coupled with cation electrodeposition to grow pure iron. However, poor faradaic efficiencies against the hydrogen evolution reaction (HER) is a major bottleneck. To develop a mechanistic picture of this technology, we conduct a first-principles thermodynamic analysis of the Fe110 aqueous electrochemical interface. Constructing a surface Pourbaix diagram, we predict that the iron surface will always drive toward adsorbate coverage. We calculate theoretical overpotentials for terrace and step sites and predict that growth at the step sites are likely to dominate. Investigating the hydrogen surface phases, we model several hydrogen absorption mechanisms, all of which are predicted to be endothermic. Additionally, for HER we identify step sites as being more reactive than on the terrace and with competitive limiting potentials to iron plating. The results presented here further motivate electrolyte design toward HER suppression.

Phosphorene as a Catalyst for Highly Efficient Nonaqueous Li-Air Batteries.

Li-air batteries can yield exceptionally high predicted energy densities. However, for this technology to become realizable, round-trip efficiency issues and slow kinetics at the cathode require implementation of a catalyst. With design parameters not well understood and limitations on material selection, choosing an ideal catalyst is complex. In Li-air batteries, energy storage is achieved by reactions between Li and O (oxygen reduction reaction for discharge and oxygen evolution reaction for charge). Here, phosphorene is proposed as a solution through simulations of its catalytic behavior toward discharge initiated via either O2 dissociation or Li adsorption. After obtaining intermediate geometries for both nucleation paths leading to either Li2O2 or 2(Li2O), free-energy diagrams are generated to predict the promoted discharge product of Li2O2. Furthermore, considering a final product of Li2O2, the overpotentials are predicted to be 1.44 V for discharge and 2.63 V for charge. Activation barriers for the catalytic decomposition of Li2O2 (during charge) are found to be 1.01 eV for phosphorene versus 2.06 eV for graphene. This leads to a major difference in reaction rate up to 1017 times in favor of phosphorene. These results, complemented by electronic analysis, establish phosphorene as a promising catalyst for Li-air batteries.

Adsorption and diffusion of lithium polysulfides over blue phosphorene for Li-S batteries.

Lithium-sulphur (Li-S) batteries suffer from capacity loss due to the dissolution of lithium polysulfides (LiPSs). Although finding cathodes that can trap LiPSs strongly is a possible solution to suppress the "shuttle" effect, fast diffusion of lithium and LiPSs is also pivotal to prevent agglomeration. We report that monolayer blue phosphorene (BP), a recently synthesized two-dimensional material, possesses these characteristics as a cathode in Li-S batteries. Density functional theory calculations showed that while the adsorption energies (Eb) of various LiPSs over pristine BP are reasonably strong (from -0.86 eV to -2.45 eV), defect engineering of the lattice by introducing a single vacancy (SV) increased the binding strength significantly, with Eb in the range of -1.41 eV to -4.34 eV. Ab-initio molecular dynamics simulations carried out at 300 K showed that the single vacancies trap the Li atoms in the LiPSs compared to pristine BP. Projected density of states revealed that the creation of an SV induces metallicity in the cathode. Furthermore, an increase in the adsorption strength did not cause significant structural deformation, implying that the soluble large LiPSs did not decompose, which is essential to suppress capacity fading. The energy barriers for LiPSs' migration over pristine BP are minimal to ensure ultrafast diffusion, with the lowest diffusion energy barriers being 0.23 eV, 0.13 eV and 0.18 eV for Li2S4, Li2S6 and Li2S8, respectively. Furthermore, the energy barrier associated with the catalytic oxidation of Li2S over pristine and defective BP was found to be greater than three times smaller compared to graphene, which suggests that charging processes could be faster by orders of magnitude. Therefore, BP with a suitable combination of defects would be an excellent cathode material in Li-S batteries.

Ultrahigh Storage and Fast Diffusion of Na and K in Blue Phosphorene Anodes.

In the wake of blue phosphorene's (BP) computational discovery and experimental realization, it has emerged as a versatile material with interesting optical, electrical, and mechanical properties. In this study, using first principles density functional theory calculations, we have investigated the adsorption and diffusion of Na and K over monolayer BP to assess its suitability as Na-ion and K-ion battery anodes. The optimized adsorption energies were found to be -0.96 eV for Na and -1.54 eV for K, which are sufficiently large to ensure stability and safety during operation. In addition, BP could adsorb Na and K atoms up to a stoichiometric ratio of 1:1 which yields a high storage capacity of 865 mA h/g for both adatom species. Through examination of the electronic structure and projected density of states of BP as a function of Na/K concentration, we predict that the band gap of the system increasingly shrinks, and in the case of maximum K adsorption, the band gap diminishes completely. Additionally, the diffusion of Na and K over BP is observed to be ultrafast, especially for K, and anisotropic with modest energy barriers of 0.11 and 0.093 eV for Na and K, respectively. Building upon these findings, we employed vibrational analysis techniques with transition state theory to incorporate kinetic effects and predicted a diffusivity of 7.2 × 10-5 cm2/s for Na and 8.58 × 10-5 cm2/s for K on BP. Given these advantages, that is, ultrahigh capacity, electrical conductivity, and high Na/K diffusivity, we conclude that BP can be considered as an excellent candidate for anodes in Na- and K-ion batteries.