Efforts to Measure Reduction Potentials for Hydrogen Insertion in Transition Metal Oxides

Hydrogen is a common industrial reactant that is almost exclusively sourced from nonrenewable resources. Electrochemical hydrogen insertion into transition metal oxides to form metal oxide hydrogen bronzes (H_xMO_y where M is a transition metal like W, Mo, or Ti) offers a potential alternative to conventional hydrogen generation. After electrochemically inserting hydrogen atoms from an abundant source like water into a transition metal oxide, hydrogen atoms can be stored inside the metal oxide lattice until oxidizing potentials are applied and the hydrogen atoms are de-inserted to make H₂(g) or react with industrial feedstocks directly.

Ongoing work in our lab to understand the basic thermodynamics of hydrogen insertion/de-insertion processes depends on the ability to precisely measure the equilibrium reduction potentials (E⁰') for these reactions. My research project aimed to develop a standardized procedure to make these measurements across a wide range of metal oxide hydrogen bronze formers.

The procedure I developed involves depositing inks composed of powdered metal oxide, polyvinylidene fluoride (PVDF) as a binder, and variable amounts of acetylene black for enhanced conductivity onto glassy carbon working electrodes. Using a three-electrode cell setup and aqueous electrolytes at various pH values, I ran chronopotentiometry, cyclic voltammetry, and galvanostatic intermittent titration technique (GITT) experiments to measure the potentials at which hydrogen insertion/de-insertion occurs. Despite successive efforts to increase the reversibility of the reactions—adjusting the proportion of acetylene black; decreasing the amount of metal oxide on the working electrode; passing smaller amounts of current; and purging the electrolyte with inert gas—I was unable to identify a consistent set of conditions that approached microscopic reversibility to the extent required for precise measurements. We speculate that the reversibility is limited in part by side reactions with atmospheric oxygen that we have not yet been able to adequately exclude from the cell; we hope to improve upon measurements in the future by implementing a design to minimize oxygen exposure during experimentation. We further conjecture that it may be impossible to reversibly de-insert all hydrogen from most metal oxides due to inherent changes in conductivity as a metal oxide incorporates hydrogen into its crystal structure. If such a limitation exists, it may be appropriate to pursue non-electrochemical approaches to these measurements of equilibrium dynamics in future studies.