Tracking oxygen atoms in electrochemical CO oxidation - Part II: Lattice oxygen reactivity in oxides of Pt and Ir

The low overpotential regime of acidic water oxidation part II: trends in metal and oxygen stability numbers

The operating conditions of low pH and high potential at the anodes of polymer electrolyte membrane electrolysers restrict the choice of catalysts for the oxygen evolution reaction (OER) to oxides based on the rare metals iridium or ruthenium. In this work, we investigate the stability of both the metal atoms and, by quantitative and highly sensitive 18O isotope labelling experiments, the oxygen atoms in a series of RuO x and IrO x electrocatalysts during the OER in the mechanistically interesting low overpotential regime. We show that materials based on RuO x have a higher dissolution rate than the rate of incorporation of labelled oxygen from the catalyst into the O2 evolved ("labelled OER"), while for IrO x -based catalysts the two rates are comparable. On amorphous RuO x , metal dissolution and labelled OER are found to have distinct Tafel slopes. These observations together lead us to a full mechanistic picture in which dissolution and labelled OER are side processes to the main electrocatalytic cycle. We emphasize the importance of quantitative analysis and point out that since less than 0.2% of evolved oxygen contains an oxygen atom originating from the catalyst itself, lattice oxygen evolution is at most a negligible contribution to overall OER activity for RuO x and IrO x in acidic electrolyte.

The low overpotential regime of acidic water oxidation part I: the importance of O2 detection

The high overpotential required for the oxygen evolution reaction (OER) represents a significant barrier for the production of closed-cycle renewable fuels and chemicals. Ruthenium dioxide is among the most active catalysts for OER in acid, but the activity at low overpotentials can be difficult to measure due to high capacitance. In this work, we use electrochemistry - mass spectrometry to obtain accurate OER activity measurements spanning six orders of magnitude on a model series of ruthenium-based catalysts in acidic electrolyte, quantifying electrocatalytic O₂ production at potential as low as 1.30 V_RHE. We show that the potential-dependent O₂ production rate, i.e., the Tafel slope, exhibits three regimes, revealing a previously unobserved Tafel slope of 25 mV decade^-1 below 1.4 V_RHE. We fit the expanded activity data to a microkinetic model based on potential-dependent coverage of the surface intermediates from which the rate-determining step takes place. Our results demonstrate how the familiar quantities "onset potential" and "exchange current density" are influenced by the sensitivity of the detection method.