17 Search Results

Despite many recent advances, overpotentials remain high for anion exchange membrane water electrolyzers (AEMWEs). Voltage breakdown analyses (VBA) can help decouple the origins of overpotentials and facilitate design decisions to improve cell performance, but studies investigating how to adapt and apply VBA to AEMWEs are lacking. Specifically, catalyst layer resistances and their contributions to overpotentials are not consistently quantified in water electrolysis and are rarely quantified for AEMWEs. This work presents a systematic methodology for VBA tailored to AEMWEs, including an approach to Tafel analysis in the absence of a reference electrode and under conditions where both the oxygen evolutionmore » reaction and hydrogen evolution reaction exhibit significant overpotentials. Catalyst layer resistance contributions are diagnosed via changes in the catalyst layer thickness, transport layer porosity, ionomer content, and electrolyte concentration. In this study, we explain discrepancies between inherent catalytic kinetics and device level performance and identify catalyst layer design strategies to reduce catalyst layer resistances.« less

Alkanol dehydration offers a pathway to upgrade biomass-derived short-chain oxygenates into alkenes, essential chemical building blocks widely used in industrial applications. Transition metal oxides with Lewis acid-base site pairs are attractive catalysts due to their high reactivity and cost-effectiveness. This work demonstrates a synthetic pathway to manipulate local environments around active Lewis acid-base pairs in anatase TiO2 to enhance their reactivity in alkanol dehydration. Microporous SiO2 layers with an average pore diameter of ~0.6 nm and a controlled thickness of 0.8-33 nm are deposited on anatase TiO2 powders by using a molecular templated SiO2 deposition method. The Lewis acid-base strengthmore » of accessible Ti-O pairs remains unchanged, as shown by temperature-programmed surface reactions of surface-bound formic acid-derived species and temperature-programmed desorption of pyridine. However, measured alkanol dehydration rates on confined Ti-O pairs are much higher (by up to 7-fold) than those on TiO2. The extent of rate enhancements depends on the reactant size and functional group positioning, suggesting that the rate enhancements reflect the interactions between the guest molecules (reactants and transition states) and the surrounding SiO2 micropore environments. By providing a detailed synthetic procedure to tailor the local environments around active sites in bulk oxides, this approach offers an additional avenue for enhancing catalytic performance.« less

The oxygen evolution reaction (OER) is essential to many key electrochemical devices, including H₂O electrolyzers, CO₂ electrolyzers, and metal−air batteries. NiFe oxides have been historically identified as active for the OER, though they have been less studied in their more commercially relevant bulk oxide forms, such as NiFe₂O₄. Past works have demonstrated that the initial starting phase of Ni(Fe) precatalysts can influence their activation to the Ni(Fe)OOH active phase, including the rate and degree of conversion, pointing to the necessity of understanding activation protocols and in situ characteristics of catalyst materials at the device level. In this work, we investigatemore » the characteristics of commercially relevant NiFe bulk oxides (NiFe₂O₄ and a physical mixture of NiO and γ-Fe₂O₃) during multiple activation procedures. Our results demonstrate that significant performance enhancement is observed for these bulk oxides regardless of the Fe incorporation in the initial form (i.e., atomically or macroscopically integrated), leading to significant performance enhancement (up to 30×) over time on stream. We hypothesize that this activation is due to the formation of NiFeOOH active sites on the surface, supported by in situ cyclic voltammetry and Raman spectroscopy results. We further show that not only the starting material but also the method of activation influences the number of Ni(Fe)OOH active sites formed and suggest that these sites can be quantified from the Ni²⁺ to Ni³⁺ redox transition using cyclic voltammetry. Broadly, this work demonstrates the necessity of in situ characterization of catalyst materials for cell-level design and testing.« less

Here, we ally steady-state kinetics, kinetic isotope effects, and density functional theory (DFT) calculations to illustrate that Cu-based catalysts remain saturated by H-adatoms (H*) and molecular formic acid (HCOOH**) during CO₂ hydrogenation. High H* coverage under methanol synthesis conditions is evidenced by reverse water-gas shift (RWGS) rates that exhibit positive H₂ reaction orders only at P_H2 ≲ 0.5 bar, above which methanol synthesis and RWGS rates exhibit first and zeroth order dependence on P_H2, respectively. HCOOH** also accumulates on the surface with increasing P_CO2 as informed by the Langmuir-type dependence on P_CO2 (0.25-23 bar) for both methanol synthesis and RWGS.more » As both HCOOH** and H* have one H-atom per site occupied, the two species share the same P_H2 dependence and give rise to CO₂ reaction orders that are independent of P_H2. Surface coverages determined based on kinetic analyses are further corroborated with DFT-derived adsorption energies that show favorable HCOOH** adsorbate-adsorbate interactions as well as repulsive interactions for bidentate formate (HCOO**) on H*-saturated surfaces. Methanol selectivity remains invariant with P_CO2 and P_CO despite CO inhibiting reaction rates, thereby demonstrating methanol synthesis and RWGS occur on the same active site. In contrast, water preferentially inhibits methanol synthesis rates, increases methanol synthesis H₂ reaction order from 1.0 to 1.5, and alters the methanol synthesis H₂/D₂ kinetic isotope effect; the inhibitory effect of H₂O thus cannot be attributed to competitive adsorption alone and instead reflects a change in the rate-determining step for methanol synthesis. The disparate kinetics of methanol synthesis and RWGS evince a branching pathway where methanol is formed from formates and CO is formed from carboxylates. The presented work thus identifies the relevant surface species, underscores the distinct catalytic role of water in branching methanol synthesis and RWGS pathways, and, in doing so, details a mechanistic picture that yields predictable rates and reaction orders for both methanol synthesis and RWGS on Cu-based CO₂ hydrogenation catalysts.« less

Mean-field microkinetic models (MKMs) are appealing for their relatively facile construction, computational tractability, and high-throughput catalyst screening capabilities. As such, they will continue to be a valuable tool for materials design in heterogeneous catalysis even as the field aims to describe more complex systems. Numerous prior reports have provided the groundwork for constructing first-principles-based MKMs, including the analysis of strategies for incorporating lateral interactions into thermodynamic parameters (e.g., adsorption energies). Yet, there remains a need for concerted dialogue on methods for calculating and incorporating coverage-dependent kinetic parameters into MKMs. In this Perspective, we assess strategies for doing so, including themore » corresponding key physical implications and computational challenges. Here, we emphasize that decoupling thermodynamic and kinetic parameters within MKMs can violate thermodynamic consistency and risk unphysical solutions. For some reactions and catalyst materials, scaling relationships can predict coverage-dependent activation energies, but there are several exceptions evident in the literature, indicating that this approach is not universally applicable and that the field could benefit from research aimed at elucidating the limitations. Conducting high-coverage transition state searches is a rigorous but computationally costly strategy, and the effects of various methods for mitigating this cost on resulting energetics have yet to be broadly explored and validated. The goal of this Perspective is to generate discussion on and inspire focused research into the physical relevance of approaches for describing coverage-dependent reaction barriers in MKMs, including the development of computationally tractable methodologies, to advance the applicability of MKMs across diverse reaction chemistries and conditions.« less

While anion exchange membrane water electrolyzers (AEMWEs) have achieved significant performance advances in recent decades, overpotentials remain high relative to their proton exchange membrane water electrolyzer (PEMWE) counterparts, requiring AEMWE-specific catalyst layer design strategies to further advance this technology. In this work, the role of the ionomer in catalyst layer structure and quality, catalyst layer stability, and ion conduction for supporting electrolyte-fed AEMWEs is assessed for catalyst layers composed of NiFe₂O₄ and PiperION TP85 from Versogen at variable ionomer contents (0–30 wt %) for tests up to 200 h. The results reveal that, for supporting electrolyte-fed AEM devices, the ionomermore » is not required for ion conduction through the catalyst layer. Instead, the ionomer is found to play a critical role in catalyst layer structure and stability, where intermediate ionomer contents lead to the lowest overpotentials, highest effective surface areas, and lowest catalyst layer resistances. Catalyst layer stability is found to be a function of both catalyst adhesion and ionomer loss. These results show that an ionomer may be selected which is not of the same chemistry as the anion exchange membrane, mitigating ionomer stability concerns throughout the catalyst layer and offering a pathway towards highly active and stable AEMWEs.« less

Anion exchange membrane water electrolyzers (AEMWEs) are poised to play a key role in reducing capital cost and materials criticality concerns associated with traditional low-temperature electrolysis technologies. To accelerate the development and deployment of this technology, an in-depth understanding of cell materials integration is essential. Notably, the complex chemistries and interactions within the catalyst layer (consisting of the anode/cathode catalyst, anion exchange ionomer, and their interfaces with the transport layers and membrane) collectively influence overall cell performances, lifetimes, and costs. This review outlines recent advances in understanding the catalyst layer in AEMWEs. Specifically, electrode development strategies (including catalyst deposition techniquesmore » and configurations as well as transport layer design strategies) and our current understanding of catalyst–ionomer interactions are discussed. Effects of cell assembly and operational variables (including compression, temperature, pressure, and electrolyte conditions) on cell performance are also discussed. Lastly, we consider cutting-edge in situ and ex situ diagnostic techniques to study the complex chemistries within the catalyst layer as well as discuss degradation mechanisms that arise due to the integration of cell components. Simultaneously, comparisons are made to proton exchange membrane water electrolyzers (PEMWEs) and liquid alkaline water electrolyzers (LAWE) throughout the review to provide context to researchers transitioning into the AEMWE space. We also include recommendations for standard operating procedures, configurations, and metrics for comparing activity and stability.« less

Heterogeneous catalysis is driven by the interaction of reactant molecules and the catalyst surface. The locus of this interaction as well as the surrounding ensemble of atoms is referred to as the catalyst active site. Active site characterization attempts to distinguish active catalytic sites from inactive surface sites, to elucidate the structural and chemical nature of active sites, and to quantify active site concentration. Numerous techniques have been demonstrated to provide compositional and structural information about the active sites within a catalyst. However, each technique has its own limitations and experimental pitfalls that can lead to data misinterpretation or irreproduciblemore » results. Further, this work aims to provide an overview of the types of data that can be collected, to outline common experimental challenges and how to avoid them, and to assemble relevant references for the most used active site characterization techniques. More broadly, we aim to outline best practices for researchers to collect, interpret, and report active site characterization data in a way that provides the most benefit to the broader catalysis community. Increasing the rigor and reproducibility of active site characterization offers a strategy to better link properties with catalytic performance and to enable the community to develop consensus concerning these relationships.« less

The activities and stabilities of non-platinum group metals (PGMs) in the forms of monometallic (Mn₂O₃, Fe₂O₃, Co₃O₄, NiO) and bimetallic (NiFe₂O₄, CoNiO₂) oxides were assessed for the oxygen evolution reaction (OER) in alkaline media and compared with IrO₂. Both half-cell, rotating disc electrode (RDE) apparatus and single-cell, membrane electrode assemblies (MEA) were used to study kinetic and device-level performance in parallel and to provide insights into the use of these materials in anion exchange membrane (AEM) electrolyzers. Normalization of RDE results by geometric and physical surface areas, double layer capacitance, and metal content probed differences in physically vs electrochemically accessiblemore » surface areas and ensured reported trends were independent of the normalization method. The results showed that: (i) Ni- and Co- containing materials met or exceeded IrO₂ performance in both RDE and MEA testing, (ii) Co₃O₄ deactivated over time-on-stream (1.8 V for 13.5 h) due to oxide and, relatedly, particle growth, (iii) NiFe₂O₄ increased in activity over time-on-stream due to dissolution of Fe and an increased Ni/Fe ratio, and (iv) reduction of catalyst layer resistance is an avenue to further increase device-level performance. These results demonstrated the clear viability for non-PGMs to be used as anode catalysts in AEM devices.« less

The direct synthesis of H₂O₂ from O₂ and H₂ provides a green pathway to produce H₂O₂, a popular industrial oxidant. Here, in this study, we theoretically investigate the effects of Pd oxidation states, coordination environments, and particle sizes on primary H₂O₂ selectivities, assessed by calculating the ratio of rate constants for the formation of H₂O₂ (via OOH* reduction; k_O–H) and the decomposition of OOH* (via O–O cleavage; k_O–O). For Pd metals, the k_O–H/k_O–O ratio decreased from 10^-4 for Pd(111) to 10^-10 for the Pd₁₃ cluster at 300 K, indicating poorer H₂O₂ selectivity as Pd particle size decreases and low primarymore » selectivities for H₂O₂ overall. As the oxygen chemical potential increases and metals form surface and bulk oxides, the perturbation of Pd–Pd ensemble sites by lattice O atoms results in selectivities that become dramatically higher than unity. For instance, at 300 K, the k_O–H/k_O–O ratio increases significantly from 10^-4 to 10⁹ to 10¹⁶ as Pd(111) oxidizes to Pd₅O₄/Pd(111) and to PdO(100), respectively. In contrast, such selectivity enhancements are not observed for surface and bulk oxides that persistently contain rows of more metallic, undercoordinated Pd–Pd ensemble sites, such as PdO(101)/Pd(100) and PdO(101). These Pd–Pd ensembles are also absent when smaller Pd nanoparticles fully oxidize, indicating that smaller PdO clusters can be more selective for H₂O₂ synthesis. These trends for primary H₂O₂ selectivities were found to inversely correlate with trends for H₂O₂ decomposition rates via O–O bond cleavage, demonstrating that catalysts with high primary H₂O₂ selectivity can also hinder H₂O₂ decomposition. Ab initio thermodynamic calculations are used to estimate the thermodynamically favored phase among Pd, PdO/Pd and PdO in O₂, H₂O₂/H₂O, and O₂/H₂ environments. These results are combined to show that smaller Pd nanoparticles are more prone to be oxidized at lower oxygen chemical potentials, upon which they become more selective than larger Pd particles for H₂O₂ synthesis.« less