Scalable Solid-State Synthesis of Carbon-Supported Ir Electrocatalysts for Acidic Oxygen Evolution Reaction: Exploring the Structure-Activity Relationship.

Enhancing iridium (Ir)-based electrocatalysts to achieve high activity and robust durability for the oxygen evolution reaction (OER) in acidic environments has been an ongoing mission in the commercialization of proton exchange membrane (PEM) electrolyzers. In this study, we present the synthesis of carbon-supported Ir nanoparticles (NPs) using a modified impregnation method followed by solid-state reduction, with Ir loadings of 20 and 40 wt % on carbon. Among the catalysts, the sample with an Ir loading of 20 wt % synthesized at 1000 °C with a heating rate of 300 °C/h demonstrated the highest mass-normalized OER performance of 1209 A gIr-1 and an OER current retention of 80% after 1000 cycles of cyclic voltammetry (CV). High-resolution STEM images confirmed the uniform dispersion of NPs, with diameters of 1.6 ± 0.4 nm across the support. XPS analysis revealed that the C-O and C═O peaks shifted slightly toward higher binding energies for the best-performing catalyst. In comparison, the metallic Ir state shifted toward lower binding energies compared to other samples. This suggests electron transfer from the carbon support to the Ir NPs, indicating a potential interaction between the catalyst and the support. This work underscores the strong potential of the solid-state method for the scalable synthesis of supported Ir catalysts.

Tailoring iridium-palladium nanoparticles with Ir-rich skin: a highly durable anode electrocatalyst for acidic water electrolysis via a facile microwave-assisted chemical reduction method.

Electrochemical water splitting under acidic conditions is a clean way towards producing hydrogen fuels. The slow kinetics of the oxygen evolution reaction (OER) at the anode is currently a bottleneck for commercial acceptance of this technology. Therefore, arriving at more efficient and sustainable OER electrocatalysts is highly desirable. We here demonstrate the synthesis of iridium-palladium (IrPd) alloy nanoparticles (2-5 nm) with variable average composition (Ir : Pd = 1 : 0, 1 : 1, 1 : 3, 1 : 6, 1 : 9 and 0 : 1) using a facile one-pot microwave-assisted chemical reduction method. The IrPd nanoparticles show structure- and composition-dependent OER performance in acidic media. Utilizing different reduction strengths and precursor ratios, successful alloy catalysts were prepared with Ir-rich skin and sublayers of different Pd compositions. Their structures were revealed using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen underpotential deposition (Hupd) studies. It turned out that (1) the alloy OER catalyst also has a high electrochemically active surface area for hydrogen adsorption/desorption, (2) the OER performance is strongly dependent on the surface Ir contribution and (3) the intact Ir skin is essential for electrocatalyst stability.

Multiple Bubble Removal Strategies to Promote Oxygen Evolution Reaction: Mechanistic Understandings from Orientation, Rotation, and Sonication Perspectives.

Bubble coverage of catalytically active sites is one of the well-known bottlenecks to the kinetics of the oxygen evolution reaction (OER). Herein, various bubble removal approaches (electrode orientation, rotating, and sonication) were considered for the OER performance evaluation of a state-of-the-art Ir-based electrocatalyst. Key parameters, such as catalyst mass loss, activity, overpotential, and charge- and mass-transfer mechanisms, were analyzed. First, it was suggested that a suitable orientation of the working electrode facilitates coalescence and sliding bubble effects on the catalyst surface, leading to better electrochemical performance than those of the traditional rotating disk electrode (RDE) configuration. Then, the convection and secondary Bjerknes force were explained as the responsible phenomena in improving the OER activity in the RDE and sonication methods. Finally, simultaneous implementation of the methods enhanced the catalyst mass activity up to 164% and provided fast charge-transfer kinetics and low double-layer capacitance, which eventually led to a 22% reduction in overpotential, while the catalyst loss slightly increased from 1.93 to 3.88%.

Recovery, Regeneration, and Reapplication of PFSA Polymer from End-of-Life PEMFC MEAs.

We have developed a recovery, regeneration, and reapplication process for Nafion, a perfluorinated sulfonic acid (PFSA) ionomer, from end-of-life (EoL) low-temperature proton-exchange membrane (PEM) fuel cells (FCs). Samples of PFSA PEM recovered from EoL membrane-electrode assemblies (MEAs) with a history of close to 19,000 h of operation were recycled by dissolving the polymeric material in ethanol and applying the dissolved PFSA ionomer for producing the ionomer phase of the catalyst layer of new PEMFC cathodes. Structural characterizations show a marginally lower abundance of sulfonic groups for the EoL PEM compared to a fresh sample. Sulfonation of the former was employed to regenerate sulfonic groups to compensate for the lost ones. New gas-diffusion electrodes (GDEs) were prepared with the recycled PFSA ionomer both with and without sulfonation, and MEAs with these GDEs as cathodes were assembled through a state-of-the-art procedure. Electrochemical characterizations of the GDEs and single-cell studies of the MEAs showed that the electrochemical performances of catalyst layers containing recycled PFSA ionomer were at least similar to those containing fresh. Durability studies of the GDEs and MEAs, performed through a three-electrode liquid cell and a single cell, respectively, show the highest durability for the GDE/MEA with PFSA ionomer recycled without applying the sulfonation step. However, the GDE with PFSA ionomer obtained from recycling a re-sulfonated PEM shows a durability comparable to that of the GDE with fresh PFSA ionomer. Hence, PFSA material aged during PEMFC operation may be employed to produce highly functional and durable regenerated PFSA ionomer for PEMFC catalyst layers. The studied process of PFSA ionomer recycling is highly attractive for industrial adoption.

Insights into Degradation of the Membrane-Electrode Assembly Performance in Low-Temperature PEMFC: the Catalyst, the Ionomer, or the Interface?

Here, we report a study on the structural characteristics of membrane electrode assembly (MEA) samples obtained from a low-temperature (LT) polymer electrolyte membrane (PEM) fuel cell (FC) stack subjected to long-term durability testing for ∼18,500 h of nominal operation along with ∼900 on/off cycles accumulated over the operation time, with the total power production being 3.39 kW h/cm2 of MEA and the overall degradation being 87% based on performance loss. The chemical and physical states of the degraded MEAs were investigated through structural characterizations aiming to probe their different components, namely the cathode and anode electrocatalysts, the Nafion ionomer in the catalyst layers (CLs), the gas diffusion layers (GDLs), and the PEM. Surprisingly, X-ray diffraction and electron microscopy studies suggested no significant degradation of the electrocatalysts. Similarly, the cathode and anode GDLs exhibited no significant change in porosity and structure as indicated by BET analysis and helium ion microscopy. Nevertheless, X-ray fluorescence spectroscopy, elemental analysis through a CHNS analyzer, and comprehensive investigations by X-ray photoelectron spectroscopy suggested significant degradation of the Nafion, especially in terms of sulfur content, that is, the abundance of the -SO3- groups responsible for H+ conduction. Hence, the degradation of the Nafion, in both of the CLs and in the PEM, was found to be the principal mechanism for performance degradation, while the Pt/C catalyst degradation in terms of particle size enlargement or mass loss was minimal. The study suggests that under real-life operating conditions, ionomer degradation plays a more significant role than electrocatalyst degradation in LT-PEMFCs, in contrast to many scientific studies under artificial stress conditions. Mitigation of the ionomer degradation must be emphasized as a strategy to improve the PEMFC's durability.

Platinum recycling through electroless dissolution under mild conditions using a surface activation assisted Pt-complexing approach.

High industrial demand and limited global abundance of precious metals (PMs) make their recycling essential for industrial and societal sustainability. Owing to their high surface-to-volume ratio, recycling of nanoparticulate precious metals through dissolution in dilute acids at room temperature is quite relevant. However, their dissolution by approaches such as the cyclic oxidation-reduction of metal surfaces through surface potential manipulation may not be suitable for large-scale production. Here, we demonstrate fast dissolution of Pt-nanoparticles under mild conditions (normal temperature and pressure) in Cl- containing dilute acidic/neutral baths without using cyclic oxidation-reduction. We demonstrate that the dissolution of Pt nanoparticles through [PtClx]2- complexing is hindered by blockage of the Pt surface due to adsorption of non-oxide species (impurities), a phenomenon termed herein as non-oxide passivation (NOP). The nanoparticles can be kept active for the [PtClx]2- complexing through removal of the adsorbed species by surface activation, a process to remove the NOP layer by application of cyclic/continuous perturbation. As an example, average % dissolution rate (calculated on initial Pt loading) increases from ∼10% per h (∼30% dissolution in 3 h) for dissolution without NOP removal to ∼19% per h (∼55% dissolution in 3 h) for dissolution through cyclic activation of the Pt surface by HCl-water cycling. The approach may be implemented with a range of cost-efficient and non-toxic reagents for industrial-scale and environmentally friendly recycling of Pt.

Particle Size-Controlled Growth of Carbon-Supported Platinum Nanoparticles (Pt/C) through Water-Assisted Polyol Synthesis.

A water-assisted control of Pt nanoparticle size during a surfactant-free, microwave-assisted polyol synthesis of the carbon-supported platinum nanoparticles (Pt/C) in a mixture of ethylene glycol and water using (NH4)2PtCl6 as the Pt precursor is demonstrated. The particle size was tuned between ∼2 and ∼6 nm by varying either the H2O volume percent or the Pt precursor concentration during synthesis. The electrochemical surface area (ECSA) and the oxygen-reduction reaction activity obtained for the Pt/C electrocatalyst show a catalytic performance competitive to that of the state-of-the-art commercial Pt/C electrocatalysts used for polymer electrolyte membrane fuel cell electrodes (ECSA: ∼70 m2/g; half-wave potential for oxygen reduction reaction: 0.83 V vs reversible hydrogen electrode). The synthesized Pt/C electrocatalysts show durability equivalent to or better than that of the commercial Pt/C. The durability was found to improve with increasing particle size, with the ECSA loss values being ∼70 and ∼55% for the particle sizes of 2.1 and 4.3 nm, respectively. The study may be used as a route to synthesize Pt/C electrocatalysts from a convenient and economic Pt precursor (NH4)2PtCl6 and avoiding the use of alkaline media.

Accurate Determination of Catalyst Loading on Glassy Carbon Disk and Its Impact on Thin Film Rotating Disk Electrode for Oxygen Reduction Reaction.

Thin film-rotating disc electrode (TF-RDE) experiment provides a fast research platform for screening of newly developed electrocatalysts for oxygen reduction reaction (ORR) activity; however, precise estimation of their performance parameters is necessary to avoid wastage of resources in the testing of otherwise unpromising electrocatalyst in actual fuel cells. Here we show the importance of the accurate amount of catalyst (e.g., Pt) on glassy carbon (GC) disk of RDE in TF-RDE experiment by characterizing the commercial catalysts for their electrocatalysis performance (electrochemical surface area and ORR activity) values. The Pt loadings used to calculate these performance values were obtained using two schemes, namely, using the literature based (conventional) scheme and an X-ray fluorescence (XRF) based scheme. A parameter called "catalyst-density-of-the-ink" is used to correlate the variations observed in performance values and the amount of Pt on GC disk of RDE obtained using both schemes. The investigation suggests that the actual Pt loading on the GC disk of RDE varies with the ink-conditions, which is considered constant in the conventional scheme and might be one of the reasons of irreproducibility of the data obtained by TF-RDE experiments. The XRF-based scheme, which is simple and direct, can have the potential to replace conventional scheme for accurate catalyst loading estimation, improve experimental reproducibility, and open many other possibilities (e.g., post-mortem analysis of catalyst) in electrocatalysis studies.

Zoom in Catalyst/Ionomer Interface in Polymer Electrolyte Membrane Fuel Cell Electrodes: Impact of Catalyst/Ionomer Dispersion Media/Solvent.

Large-scale applications of polymer electrolyte membrane fuel cells (PEMFCs) are throttled primarily by high initial cost and durability issues of the electrodes, which essentially consist of the nanoparticulate catalysts (e.g., Pt) having accessibility to electrons (e-), protons (H+), and fuel/oxidant through catalyst support, polymer electrolyte ionomer, and porous gas diffusion layer, respectively. Hence, to achieve high electrode performance in terms of activity and/or durability, understanding and optimization of the catalyst/support and catalyst/ionomer interfaces are of significant importance. Present study demonstrates an alternative route to inspect the catalyst/ionomer interface through an accelerated stress test combined with electrochemical impedance spectroscopy. Various interfaces are created through catalyst inks prepared using commercial Pt/C catalyst powder dispersed in different solvents. Electrode degradation pattern turns out to be a very useful tool to interpret a catalyst/ionomer interface structure. Variations of interfacial impedance, electrochemical surface area (ECSA), and double layer capacitance with the number of potential cycles suggested significant impact of catalyst/ionomer interface on the catalyst performance. A quantification of the degradation mechanisms responsible for ECSA loss during AST was employed to further understand the correlations between the electrochemical performance of the electrodes and their catalyst/ionomer interface structures. The knowledge may be implied to further optimize the electrode structure and hence to advance the PEMFC technology.

Environmentally and Industrially Friendly Recycling of Platinum Nanoparticles Through Electrochemical Dissolution-Electrodeposition in Acid-Free/Dilute Acidic Electrolytes.

A process to recycle platinum from industrial waste, for example, spent catalysts from polymer fuel cells and electrolyzers, through potentiodynamic dissolution along with potentiostatic electrodeposition in dilute acidic/acid-free baths has been explored. During potentiodynamic dissolution, owing to Ostwald ripening, redeposition of the dissolved Pt species on source nanoparticles becomes significant, leading to lower overall dissolution efficiency. Alternatively, high concentrations of Pt-complexing agents (e.g., Cl- ) are required to stabilize dissolved species through complex formation. The present process overcomes those limitations by removing the dissolved Pt species continuously through electrodepositing them in the form of Pt0 on another electrode. Such a process significantly promotes the overall reaction kinetics, and an increase in dissolution rate by a factor of two or more has been observed in non-complexing electrolytes. The process may be implemented for environmentally and industrially friendly recycling of Pt in dilute acidic/acid-free baths, thus eliminating the additional steps such as electrolyte upconcentration and post-dissolution reduction of dissolved Pt species.