22 Search Results

To lower the cost of hydrogen produced by anion exchange membrane water electrolysis (AEMWE), it is critical to reduce the use of platinum group metal (PGM) catalysts within the device. While iridium has been successfully replaced with PGM-free catalysts at the anode, platinum-based (Pt) cathode catalysts are still required to meet the activity and durability targets. This study investigates the impact of commercial Pt/C catalyst loading, ionomer type and content, and electrode fabrication method on the cathode catalyst layer properties and AEMWE performance with the aim of determining the feasibility of reduced Pt loadings. While increased Pt loading is foundmore » to improve beginning-of-life performance, the effects are minimal above 0.6 mg/cm². Ink characterization shows that ionomer type and content affect the ink stability, particle size, and percent of unbound ionomer, which further impact the homogeneity of the sprayed catalyst layers. The 5% PiperION cathode exhibited the highest performance, which may be attributed to a balance between the small particle size and the low proportion of unbound ionomer, minimizing kinetic and transport losses. Theoretical calculations show that the ionomers interact differently with the Pt surface, causing different surface charges and water adsorption strength and activating different mechanisms for hydrogen evolution. Pt-PiperION lowered the enthalpy of water-splitting by 0.1 eV compared to Pt alone and allowed for equal site access between adsorbed H* and OH* (both adsorbed at circa −2.2 eV). Although catalyst-coated membrane (CCM) fabrication techniques are desirable for scale-up, no performance enhancement is observed compared with the catalyst-coated substrate approach. Durability, as measured by degradation rates, Pt loss, and catalyst layer restructuring, was found to improve with increased Pt loadings, higher ionomer content, and CCM architectures. These findings provide important insight into the significant role of the cathode in AEMWE and strategies for maintaining the performance with low Pt loading or PGM-free catalysts.« less

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

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

Voltammetric measurements of electrochemical CO ₂ reduction reaction (CO ₂ RR) selectivity on rotating ring disk electrodes (RRDE) are a rapid and sensitive method for quantifying an electrocatalyst’s selectivity, i.e. faradaic efficiency (FE). This method has been applied to polycrystalline Au electrocatalysts where a Au disk electrode catalyzes both the CO ₂ RR and hydrogen evolution reaction while the concentric Au ring electrode selectively senses CO by oxidizing CO back to CO ₂ . Such measurements enabled fundamental mechanistic studies but suffer from poor inter-laboratory reproducibility. This work identifies causes of variability in RRDE selectivity measurements by comparing protocols withmore » different electrochemical methods, reagent purities, and glassware cleaning procedures. We observed FE _CO decrease by 56% during 5 min chronoamperometry measurements, a phenomenon that is not readily apparent in voltammetric scans due to their dynamic nature. Electroplating of electrolyte impurities onto the disk and ring surfaces were identified as a major contributor to Au deactivation. Additionally, the oxygen reduction reaction may lead to higher disk currents in inadequately purged electrolytes, causing an apparent underestimation of FE _CO at low overpotentials. Lastly, we propose operational bounds for CO ₂ RR selectivity measurements on Au using the RRDE method and provide suggestions on steps for improving the accuracy of this technique.« less

Anion exchange membrane water electrolysis (AEMWE) is an emerging technology for the low-cost production of hydrogen. However, the efficiency and durability of AEMWE devices is currently insufficient to compete with other low-temperature electrolysis technologies. The porous transport layer (PTL) is a critical cell component that remains relatively unoptimized for AEMWE. In this study, we demonstrate that device performance is significantly affected by the morphology and composition of the PTL. For Ni fiber-based PTLs with a ~2 μm Co₃O₄ oxygen evolution reaction catalyst layer, decreasing the pore size and porosity resulted in a 20% increase in current density at 2 Vmore » in 1 M KOH supporting electrolyte. Alloy PTLs with even lower porosity had a higher performance; in particular, the stainless steel PTL gave an 80% increase in current density relative to Ni. Without Co₃O₄, the alloy PTLs still demonstrated high activity, indicating that the PTL material was catalytically active. However, characterization of the electrode and electrolyte after testing indicated that the alloy PTLs also underwent restructuring and corrosion processes that may limit long-term stability. This study demonstrates that the design of PTLs with improved morphology and composition is an important area of focus to achieve AEMWE performance targets.« 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

Computationally predicting the performance of catalysts under reaction conditions is a challenging task due to the complexity of catalytic surfaces and their evolution in situ, different reaction paths, and the presence of solid-liquid interfaces in the case of electrochemistry. We demonstrate here how relatively simple machine learning models can be found that enable prediction of experimentally observed onset potentials. Inputs to our model are comprised of data from the oxygen reduction reaction on non-precious transition-metal antimony oxide nanoparticulate catalysts with a combination of experimental conditions and computationally affordable bulk atomic and electronic structural descriptors from density functional theory simulations. Frommore » human-interpretable genetic programming models, we identify key experimental descriptors and key supplemental bulk electronic and atomic structural descriptors that govern trends in onset potentials for these oxides and deduce how these descriptors should be tuned to increase onset potentials. Here, we finally validate these machine learning predictions by experimentally confirming that scandium as a dopant in nickel antimony oxide leads to a desired onset potential increase. Macroscopic experimental factors are found to be crucially important descriptors to be considered for models of catalytic performance, highlighting the important role machine learning can play here even in the presence of small datasets.« 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

In situ techniques are essential to understanding the behavior of electrocatalysts under operating conditions. When employed, in situ synchrotron grazing-incidence X-ray diffraction (GI-XRD) can provide time-resolved structural information of materials formed at the electrode surface. In situ cells, however, often require epoxy resins to secure electrodes, do not enable electrolyte flow, or exhibit limited chemical compatibility, hindering the study of non-aqueous electrochemical systems. Here, a versatile electrochemical cell for air-free in situ synchrotron GI-XRD during non-aqueous Li-mediated electrochemical N ₂ reduction (Li-N ₂ R) has been designed. This cell not only fulfills the stringent material requirements necessary to study thismore » system but is also readily extendable to other electrochemical systems. Under conditions relevant to non-aqueous Li-N ₂ R, the formation of Li metal, LiOH and Li ₂ O as well as a peak consistent with the α-phase of Li ₃ N was observed, thus demonstrating the functionality of this cell toward developing a mechanistic understanding of complicated electrochemical systems.« less