Oscar Diaz-Morales

Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

Electrochemical water splitting by gold: evidence for an oxide decomposition mechanism

In this paper we study through a multiplicity of experimental and theoretical techniques the electrochemical evolution of oxygen on gold, the metal on which water splitting was initially discovered more than two centuries ago. The evidence obtained with a combination of in situ surface-enhanced Raman spectroscopy, online electrochemical mass spectrometry and density functional theory calculations suggests the existence of several mechanisms for the evolution of O2 on Au electrodes, depending on the electrode potential. Significantly, at approximately 2.0 V vs. RHE the first O2 that is evolved consists of two oxygens from the surface oxide, suggesting an oxide decomposition or oxide disproportionation step. At somewhat higher potentials, O2 is formed by a combination of oxygen from the oxide lattice and oxygen provided by water. The oxide decomposition step implies a more three-dimensional mechanism for oxygen evolution than suggested in previous mechanisms, which involve only surface-adsorbed intermediates.

Electrochemical and Spectroelectrochemical Characterization of an Iridium-Based Molecular Catalyst for Water Splitting: Turnover Frequencies, Stability, and Electrolyte Effects

We present a systematic electrochemical and spectroelectrochemical study of the catalytic activity for water oxidation of an iridium-N-dimethylimidazolin-2-ylidene (Ir-NHC-Me2) complex adsorbed on a polycrystalline gold electrode. The work aims to understand the effect of the electrolyte properties (anions and acidity) on the activity of the molecular catalyst and check its stability toward decomposition. Our results show that the iridium complex displays a very strong dependence on the electrolyte properties such that large enhancements in catalytic activity may be obtained by adequately choosing pH and anions in the electrolyte. The stability of the adsorbed compound was investigated in situ by Surface Enhanced Raman Spectroscopy and Online Electrochemical Mass Spectrometry showing that the catalyst exhibits good stability under anodic conditions, with no observable evidence for the decomposition to iridium oxide.

Iridium-based double perovskites for efficient water oxidation in acid media

The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.

Current transient study of the kinetics of nucleation and diffusion-controlled growth of bimetallic phases

A model to describe the kinetics of nucleation and diffusion-controlled growth of bimetallic phases has been developed, and analytical expressions have been obtained for the elucidation of nucleation kinetics through determination of the number density N0 of active sites and their nucleation frequency A, from experimental current transients obtained under potentiostatic control. The validity of the model has been demonstrated with the electrodeposition of Ag–Hg phases at two distinct compositions.

The stability number as a metric for electrocatalyst stability benchmarking

Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium. The so-called stability number is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favour dissolution. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure crystalline iridium oxide and finding solutions to stabilize amorphous iridium oxides.The proper verification of the stability of metal oxide catalysts for water electrolysis in acid electrolyte remains unresolved. Here, the ‘stability number’ is introduced to evaluate the dissolution mechanisms of various iridium-based oxides and to facilitate benchmarking of catalysts independent of loading, surface area or involved active sites.