Understanding ion-transfer reactions in silver electrodissolution and electrodeposition from first-principles calculations and experiments.

The electrified aqueous/metal interface is critical in controlling the performance of energy conversion and storage devices, but an atomistic understanding of even basic interfacial electrochemical reactions challenges both experiment and computation. We report a combined simulation and experimental study of (reversible) ion-transfer reactions involved in anodic Ag corrosion/deposition, a model system for interfacial electrochemical processes generating or consuming ions. With the explicit modeling of the electrode potential and a hybrid implicit-explicit solvation model, the density functional theory calculations produce free energy curves predicting thermodynamics, kinetics, partial charge profiles, and reaction trajectories. The calculated (equilibrium) free energy barriers (0.2 eV), and their asymmetries, agree with experimental activation energies (0.4 eV) and transfer coefficients, which were extracted from temperature-dependent voltage-step experiments on Au-supported, Ag-nanocluster substrates. The use of Ag nanoclusters eliminates the convolution of the kinetics of Ag+(aq.) generation and transfer with those of nucleation or etch-pit formation. The results indicate that the barrier is controlled by the bias-dependent competition between partial solvation of the incipient ion, metal-metal bonding, and electrostatic stabilization by image charge, with the latter two factors weakened by stronger positive biases. We also report simulations of the bias-dependence of defect generation relevant to nucleating corrosion by removing an atom from a perfect Ag(100) surface, which is predicted to occur via a vacancy-adatom intermediate. Together, these experiments and calculations provide the first validated, accurate, molecular model of the central steps that govern the rates of important dissolution/deposition reactions broadly relevant across the energy sciences.

Solvent-controlled ion-coupled charge transport in microporous metal chalcogenides.

Interactions between ions and itinerant charges govern electronic processes ranging from the redox chemistry of molecules to the conductivity of organic semiconductors, but remain an open frontier in the study of microporous materials. These interactions may strongly influence the electronic behavior of microporous materials that confine ions and charges to length scales comparable to proton-coupled electron transfer. Yet despite mounting evidence that both solvent and electrolyte influence charge transport through ion-charge interactions in metal-organic frameworks, fundamental microscopic insights are only just beginning to emerge. Here, through electrochemical analysis of two open-framework chalcogenides TMA2FeGe4S10 and TMA2ZnGe4S10, we outline the key signatures of ion-coupled charge transport in band-type and hopping-type microporous conductors. Pressed-pellet direct-current and impedance techniques reveal that solvent enhances the conductivity of both materials, but for distinct mechanistic reasons. This analysis required the development of a fitting method that provides a novel quantitative metric of concerted ion-charge motion. Taken together, these results provide chemical parameters for a general understanding of electrochemistry in nanoconfined spaces and for designing microporous conductors and electrochemical methods used to evaluate them.

Reinvigorating electrochemistry education.

Electrochemistry is an established discipline with modern frontiers spanning energy conversion and storage, neuroscience, and organic synthesis. In spite of the expanding opportunities for academic and industrial electrochemists, particularly in the growing energy-storage sector, rigorous training of electrochemists is generally lacking at academic institutions in the United States. In this perspective, we highlight the core concepts of electrochemistry and discuss ways in which it has been historically taught. We identify challenges faced when teaching inherently interdisciplinary electrochemical concepts and discuss how technology provides new tools for teaching, such as inexpensive electronics and open-source software, to help address these challenges. Finally, we outline example programs and discuss how new tools and approaches can be brought together to prepare scientists and engineers for careers in electrochemical technology where they can accelerate the research, development, and deployment of the clean energy technology essential to combat climate change in the coming decades.

Spontaneous Formation of >90% Optically Transmissive, Electrochemically Active CoP Films for Photoelectrochemical Hydrogen Evolution.

Earth-abundant catalysts for the hydrogen-evolution reaction require increased mass loadings, relative to Pt films, to achieve comparable activity and stability in acidic electrolytes. We report herein that spontaneous nanostructuring of opaque, electrodeposited CoP films, 40-120 nm in thickness, leads to transparent electrocatalyst films that exhibit up to 90% optical transmission in the visible spectrum. The photocurrent density under simulated sunlight at a representative n+p-Si(100)/CoP photocathode increases by 200% after exposure to 0.50 M H2SO4(aq) and remains stable for 12 h of continuous operation. Atomic force microscopy and scanning electron microscopy of the film before and after exposure to 0.50 M H2SO4(aq) validate an optical model for transparent CoP films as probed with spectroscopic ellipsometry.

High Broadband Light Transmission for Solar Fuels Production Using Dielectric Optical Waveguides in TiO2 Nanocone Arrays.

We describe the fabrication and use of arrays of TiO2 nanocones to yield high optical transmission into semiconductor photoelectrodes covered with high surface loadings of light-absorbing electrocatalysts. Covering over 50% of the surface of a light absorber with an array of high-refractive-index TiO2 nanocones imparted antireflective behavior (<5% reflectance) to the surface and allowed >85% transmission of broadband light to the underlying Si, even when thick metal contacts or opaque catalyst coatings were deposited on areas of the light-facing surface that were not directly beneath a nanocone. Three-dimensional full-field electromagnetic simulations for the 400-1100 nm spectral range showed that incident broadband illumination couples to multiple waveguide modes in the TiO2 nanocones, reducing interactions of the light with the metal layer. A proof-of-concept experimental demonstration of light-driven water oxidation was performed using a p+n-Si photoanode decorated with an array of TiO2 nanocones additionally having a Ni catalyst layer electrodeposited onto the areas of the p+n-Si surface left uncovered by the TiO2 nanocones. This photoanode produced a light-limited photocurrent density of ∼28 mA cm-2 under 100 mW cm-2 of simulated air mass 1.5 illumination, equivalent to the photocurrent density expected for a bare planar Si surface even though 54% of the front surface of the Si was covered by an ∼70 nm thick Ni metal layer.

Surface-Initiated Ring-Opening Metathesis Polymerization of Dicyclopentadiene from the Vapor Phase.

Surface-attached polydicyclopentadiene (pDCPD) films were prepared on gold and silicon substrates via surface-initiated ring-opening metathesis polymerization (SI-ROMP) of dicyclopentadiene (DCPD). The films were grown utilizing monomer in both the vapor phase and the solution phase with the former process exhibiting rapid kinetics, producing ∼400-nm-thick pDCPD films in less than 1 min of polymerization. No significant differences in thickness were observed for films grown from monomer in the vapor phase with the different isomers (exo and endo) of DCPD. Decane was used as an inert additive to control the concentration of DCPD monomer in the vapor phase enabling the preparation of pDCPD films with thickness ranging from tens of nanometers to hundreds of nanometers. The thickness of pDCPD films polymerized using monomer in the vapor phase was enhanced by the presence of a rinse solvent on the surface of the ROMP-active gold substrates. The choice of ROMP catalyst was found to be an important consideration when SI-ROMP was conducted on different substrates. Electrochemical impedance spectroscopy was used to reveal that the films provide effective barriers to the diffusion of aqueous ions in excess of 1 × 106 Ω·cm2. The mechanical properties of the surface-tethered pDCPD films were quantified with AFM PeakForce quantitative nanomechanical mapping (QNM) with a measured reduced Young's modulus (Er) of 15 GPa. The measured Er was greater than that of a non-cross-linked surface-tethered polymer, pNB, indicating that the pDCPD films are stiffer.