Pourbaix Machine Learning Framework Identifies Acidic Water Oxidation Catalysts Exhibiting Suppressed Ruthenium Dissolution.

The demand for green hydrogen has raised concerns over the availability of iridium used in oxygen evolution reaction catalysts. We identify catalysts with the aid of a machine learning-aided computational pipeline trained on more than 36,000 mixed metal oxides. The pipeline accurately predicts Pourbaix decomposition energy (Gpbx) from unrelaxed structures with a mean absolute error of 77 meV per atom, enabling us to screen 2070 new metallic oxides with respect to their prospective stability under acidic conditions. The search identifies Ru0.6Cr0.2Ti0.2O2 as a candidate having the promise of increased durability: experimentally, we find that it provides an overpotential of 267 mV at 100 mA cm-2 and that it operates at this current density for over 200 h and exhibits a rate of overpotential increase of 25 µV h-1. Surface density functional theory calculations reveal that Ti increases metal-oxygen covalency, a potential route to increased stability, while Cr lowers the energy barrier of the HOO* formation rate-determining step, increasing activity compared to RuO2 and reducing overpotential by 40 mV at 100 mA cm-2 while maintaining stability. In situ X-ray absorption spectroscopy and ex situ ptychography-scanning transmission X-ray microscopy show the evolution of a metastable structure during the reaction, slowing Ru mass dissolution by 20× and suppressing lattice oxygen participation by >60% compared to RuO2.

An ultra-stable reference electrode for scaled all-vanadium redox flow batteries.

Redox flow batteries (RFBs) have been investigated as a promising energy storage system (ESS) for grid applications over the past several decades due to their unique features, which include the separation of energy and power output, high safety, and long cycle life. It is therefore vital but still in severe deficiency to understand the reliability of RFBs, and the mechanisms that cause degradation with time. One of the primary challenges involves the unseparated contributions from individual electrodes due to the absence of a stable reference electrode (RE), particularly for long-term cycle testing in a scaled cell. Herein, we first develop an ultra-stable RE for scaled all-vanadium RFBs. The newly developed RE, based on a dynamic hydrogen electrode (DHE) with a novel design on the area (size) and surface roughness of platinum electrodes, demonstrates high accuracy and long-term stability that enables in situ monitoring of individual electrode potentials throughout 500 cycles. By introducing the RE approach to decouple the cathode and anode in conjunction with the measurement of voltage profiles, overpotentials and polarization curves, the reliability and degradation mechanism of a scaled all-vanadium RFB are further explored, revealing the diverse behaviors of individual electrodes. This exploratory work will benefit the future design and development of a stable RE for a scaled ESS, as well as the fundamental understanding of the RFB's reliability and degradation mechanism.