Molecular Analysis of the Unusual Stability of an IrNbO_x Catalyst for the Electrochemical Water Oxidation to Molecular Oxygen (OER)

Abstract

Adoption of proton exchange membrane (PEM) water electrolysis technology on a global level will demand a significant reduction of today’s iridium loadings in the anode catalyst layers of PEM electrolyzers. However, new catalyst and electrode designs with reduced Ir content have been suffering from limited stability caused by (electro)­chemical degradation. This has remained a serious impediment to a wider commercialization of larger-scale PEM electrolysis technology. In this combined DFT computational and experimental study, we investigate a novel family of iridium–niobium mixed metal oxide thin-film catalysts for the oxygen evolution reaction (OER), some of which exhibit greatly enhanced stability, such as minimized voltage degradation and reduced Ir dissolution with respect to the industry benchmark IrO x catalyst. More specifically, we report an unusually durable IrNbO x electrocatalyst with improved catalytic performance compared to an IrOx benchmark catalyst prepared in-house and a commercial benchmark catalyst (Umicore Elyst Ir75 0480) at significantly reduced Ir catalyst cost. Catalyst stability was assessed by conventional and newly developed accelerated degradation tests, and the mechanistic origins were analyzed and are discussed. To achieve this, the IrNbO x mixed metal oxide catalyst and its water splitting kinetics were investigated by a host of techniques such as synchrotron-based NEXAFS analysis and XPS, electrochemistry, and ab initio DFT calculations as well as STEM-EDX cross-sectional analysis. These analyses highlight a number of important structural differences to other recently reported bimetallic OER catalysts in the literature. On the methodological side, we introduce, validate, and utilize a new, nondestructive XRF-based catalyst stability monitoring technique that will benefit future catalyst development. Furthermore, the present study identifies new specific catalysts and experimental strategies for stepwise reducing the Ir demand of PEM water electrolyzers on their long way toward adoption at a larger scale.

Cation Dissolution-Induced Structural Transformation and Enhanced Stability for Ni–Ir Bimetallic Electrocatalysts during the Oxygen Evolution Reaction in Acid

“…15 In another case of the NiNbO x mixed oxide catalyst, the specific niobium (Nb) substitutions at bridge sites of the IrO 2 surface can stabilize the μ 2 -O ligands which changes the potential-determining step of the OER. 16 However, the exact degradation processes involved on the catalyst−electrolyte interfaces of bimetallic catalysts could be even more complicated, which have been less investigated. Herein, in this work, two layered oxides Li 2 Ir 1−x Ni x O 3 with tuned compositions of x = 0.25 and 0.75, denoted as LINO-0.25 and LINO-0.75, respectively, were synthesized as model catalysts.…”

“…For instance, the addition of Ni to Ir-based catalyst materials was found to achieve unusually high catalytic reactivity. ,− It was suggested that nickel leaching generates lattice vacancies, which in turn creates abundant d-band holes in the iridium oxide shell and energetically facilitates the acid–base O–O bond formation . In another case of the NiNbO x mixed oxide catalyst, the specific niobium (Nb) substitutions at bridge sites of the IrO 2 surface can stabilize the μ 2 -O ligands which changes the potential-determining step of the OER . However, the exact degradation processes involved on the catalyst–electrolyte interfaces of bimetallic catalysts could be even more complicated, which have been less investigated.…”

Characterization of a Dynamic Y2Ir2O7 Catalyst during the Oxygen Evolution Reaction in Acid

“…Surface IrO x layers on AIr x O y after OER in acidic electrolytes have been observed with ex situ characterization methods. For example, SrIrO 3 and IrNbO x thin films and BaIrO x nanoparticles have shown the presence of highly disordered IrO x surface layers (depleted of Sr, Nb, and Ba) and surface roughening after OER testing. − It is still unclear, however, if the IrO x surface layer is universal to all AIr x O y compositions or if the initial composition can impact the formation and stability of this surface layer. In situ characterization methods to monitor local structural and electronic environment of Ir in these materials can help build fundamental understanding of such dynamic catalysts.…”

Iridium Stabilizes Ceramic Titanium Oxynitride Support for Oxygen Evolution Reaction

“…Presently, the crucial bottleneck is the anode side of the electrolyzers, where the sluggish oxygen evolution reaction (OER) dictates the employment of expensive and scarce iridium to an unsustainable extent . Therefore, there is a strong incentive to minimize the amount of currently still irreplaceable iridium in the catalyst layer and enhance its activity and durability. , Following the main concepts of fuel cells and platinum-based catalysts, most approaches for decreasing the loading of iridium are based on synthesizing catalyst core–shell morphologies with minimal Ir content − by alloying iridium with other metals ,− or by mixing iridium nanoparticles with less expensive oxides of earth-abundant elements . Especially effective is dispersing iridium nanoparticles on a high-surface-area support. ,− However, supported OER catalysts represent a multidimensional platform encompassing many unresolved phenomena such as support electroconductivity, metal–support interactions, support morphology, and stability, , to name a few.…”

“… 2 Therefore, there is a strong incentive to minimize the amount of currently still irreplaceable iridium in the catalyst layer and enhance its activity and durability. 3 , 4 Following the main concepts of fuel cells and platinum-based catalysts, most approaches for decreasing the loading of iridium are based on synthesizing catalyst core–shell morphologies with minimal Ir content 5 − 8 by alloying iridium with other metals 7 , 9 − 13 or by mixing iridium nanoparticles with less expensive oxides of earth-abundant elements. 14 Especially effective is dispersing iridium nanoparticles on a high-surface-area support.…”