(Invited) First Principles and Thermodynamics Modeling of Battery Materials and Electrocatalysts in Aqueous Settings: The Search for Sustainable and Functional Nanomaterials

Electrochemical modeling of processes at the solid-aqueous solution interface poses inherent challenges owing to structural complexity, heterogeneity, and redox chemistry. The first-principles methods that can reliably predict materials properties are not compatible with the macroscopic electrochemical driving forces of concentration, pH, and applied potential. The ability to partition overall reactions into elementary steps enables a modeling strategy that uses electronic structure calculations combined with information from thermodynamics, statistical mechanics, and electrochemical principles. The "DFT + Thermodynamics" approach is discussed and demonstrated for electrochemical systems. First, we discuss a body of work comprised of modeling, synthesis, characterization, analytical measurements, and biological studies of the Li(Ni x Mn y Co 1-x-y )O 2 ("NMC") family of materials. NMC is a technologically relevant complex metal oxide used in rechargeable Li-ion battery technology, with projected increased use and demand. A lagging consequence of the growing demand of NMC is the (slower growing yet increasing) amount of end-of-life battery materials entering permanent waste disposal. Environmental exposure results in leaching of toxic metals, in particular nickel and cobalt. It was shown experimentally that equistoichiometric NMC undergoes incongruent metal release in water, resulting in Mn-enriched materials after long periods of exposure. DFT + Thermodynamics modeling captures the incongruent trend, and provides an interpretation based on the interplay of metal oxidation states, coordination environment, and thermodynamically preferred aqueous speciation. The results are extended to Mn- and Ni-rich variations of NMC in compositional tuning studies. In subsequent work, it is shown that variations in the aqueous media can go on to further govern metal release, again with strong correlation between theory and experiment. In separate work, we model transition metal phosphides as promising electrocatalysts for HER. Here, the ability of DFT + Thermodynamics to predict how the surface structure evolves as a function of pH and U is essential to understanding the reactive sites and HER mechanisms.