Titanium Substitution Effects on the Structure, Activity, and Stability of Nanoscale Ruthenium Oxide Oxygen Evolution Electrocatalysts: Experimental and Computational Study

Abstract

Proton-exchange membrane water electrolyzers produce hydrogen from water and electricity and can be powered using renewable energy; however, the high overpotential, high cost, and limited supply of the oxygen evolution reaction (OER) electrocatalyst are key factors that hinder wide-scale adoption. Ruthenium oxide (RuO2) has a lower overpotential, lower cost, and higher global supply compared with iridium oxide (IrO2), but RuO2 is less stable than IrO2. As an approach to improve the catalytic stability, we report the effect of titanium substitution at different concentrations within nanoscale RuO2, Ru1–x Ti x O2 (x = 0–50 at. %), on the structure, OER activity, and stability using combined experiments and theory. Titanium substitution within rutile RuO2 affects the electronic structure, resulting in regions of electron accumulation and electron depletion at the surface, and shifts the d-band and O 2p band centers to higher binding energies. Calculations show that the effects of Ti on the electronic structure are highly dependent on not only the concentration but also the specific dopant location. From electrochemical testing and analysis of the electrolyte and simulations, titanium substitution at low concentrations (12.5 and 20 at. %) improves catalyst stability and lowers Ru dissolution. Experiments of OER activity agree with the theory that Ti substitution results in a higher overpotential when averaging over all adsorption sites. Theoretical analysis shows that specific sites predominately act as catalytic sites for the OER, while metal dissolution occurs at different sites. Specifically, OER has the lowest barriers at penta-coordinated Ru sites, while hexa-coordinated Ru sites have the lowest energetic barriers for dissolution.

Catalytic Activity and Electrochemical Stability of Ru1–xMxO2 (M = Zr, Nb, Ta): Computational and Experimental Study of the Oxygen Evolution Reaction

“…80,81 We adopted the use of the linear response ansatz as proposed by Cococcioni et al The values of the U correction for Zr, Nb, and Ta used in this work are 3.2, 1.1, and 3.5, respectively. The RuO 2 unit cell used to model the Ru 1−x M x O 2 bulk systems and their facets was taken from our previous report, 44 whose lattice parameters are in excellent agreement with experimentally reported values. 82−85 After optimization of the RuO 2 bulk structure, a 2 × 2 supercell was made to evaluate the effect of various M substituents (M = Zr, Nb, Ta) at bulk concentrations of 12.5, 25, and 50%, and theoretical X-ray diffraction patterns were obtained using the software VESTA 86,87 for the calculated structures of each bulk system.…”

“…Hence, the effect of these species on the electronic structure is highly dependent on their concentrations, similar to the effects observed in the geometric structure. On the other hand, Zr follows a similar trend as Ti regarding the d band center value, 44 where the Ru-4d band center shifts progressively toward higher binding energies as the dopant species concentration increases (from −1.24 to −1.32 eV for Zr, respectively). In summary, compared to the pristine oxide, the d band center values suggest overall less reactive Ru sites due to the presence of the substituents.…”

“…We synthesized Ru 1−x M x O 2 , (M = Zr, Nb, Ta) using a hydrothermal method adapted from our previously reported study. 44 The Ru 1−x M x O 2 samples were imaged using scanning electron microscopy (SEM), and representative images are shown in Figure 1d−g. SEM images for the synthesized RuO 2 (RuO 2 -syn) show a distribution of particle sizes ranging from ∼0.25 to 1 μm in diameter.…”

Strategies for the design of ruthenium-based electrocatalysts toward acidic oxygen evolution reaction

“…55 Operando tests coupled with DFT studies confirmed that the introduction of Ni not only significantly stabilized the RuO 2 lattice but also greatly avoided the dissolution of surface Ru and subsurface oxygen for enhanced OER stability. By means of heteroatom doping, such as Na, 49 Ni, 55 Ti, 130 etc. , the d-band center can be kept far away from the Femi level and the lattice oxygen can be suppressed to obey the AEM mechanism, thus improving both catalytic activity and stability.…”

“…In addition to the strategies discussed above, another effective strategy is incorporating foreign metal atoms, such as alkali metals (Li, 45 Na, 49 etc. ), transition metals (Ti, 130 Cr, 131 Mn, 132 Fe, 133 Ni, 55 Co, 134 Cu, 135 Zn, 136 etc. ), main group metals (Mg, 137 Ca, 138 Sr, 139 etc.…”