Platinum Group Metal-free Catalysts Research Articles

Strain engineering of CoSA[sbnd]N[sbnd]C catalyst toward enhancing the oxygen reduction reaction activity

Developing efficient and cost-effective platinum-group metal-free (PGMF) catalysts for the oxygen reduction reaction (ORR) is crucial for energy conversion and storage devices. Among these catalysts, metal-nitrogen-carbon (MNC) materials, particularly cobalt single-atom catalysts (CoSANC), show promise as ORR electrocatalysts. However, their ORR activity is often hindered by strong hydroxyl (OH) adsorption on the Co sites. While the impact of strain engineering on MNC electrocatalysts has been minimally explored, recent studies suggest its potential to enhance catalytic performance and optimize intrinsic activity in traditional bulk catalysts. In this context, we investigate the effect of surface strain on CoSANC for ORR activity and correlate substrate-strain-induced geometric distortions with catalytic activity using experimental and theoretical methods. The findings suggest that the d-band center gap of spin states (Δεd) may be a preferred descriptor for predicting strain-dependent ORR performance in MNC catalysts. Leveraging CoSANC moiety placed on a substrate with an average size of 1.0 μm, we achieve performance comparable to that of commercial Pt/C catalysts when used as a cathode catalyst in zinc-air batteries. This investigation unveils the structure–function relationship of MNC electrocatalysts regarding strain engineering and provides valuable insights for future ORR activity design and enhancement.

Tailoring the Hydrogen Spillover Effect in Ni-Based Heterostructure Catalysts for Boosting the Alkaline Hydrogen Oxidation Reaction.

Improving the catalytic efficiency of platinum group metal-free (PGM-free) catalysts for the sluggish alkaline hydrogen oxidation reaction (HOR) is crucial to the anion exchange membrane fuel cell. Recently, numerous Ni-based heterostructures have been designed based on bifunctional theory to enhance HOR activity by optimizing the binding energy of both H* and OH*; however, their activities are still far inferior to those of PGM catalysts. Indeed, the long transfer pathway for intermediates between different active sites in such heterostructures has rarely been investigated, which could be the reason for the bottleneck. Here, we design a Ni/MoOxHy heterostructure catalyst to promote H* migration from the Ni side to the interface for alkaline HOR via the hydrogen spillover effect. In situ X-ray absorption fine structure, Raman characterizations, H/D kinetic isotope effects, and theoretical calculations have proven facile H* migration from the Ni side to the interface, which further reacts with OH* on the MoOxHy surface. Besides, the hydrogen spillover effect is also beneficial for the preservation of the metallic phase of Ni during the reaction. The catalyst exhibits a high activity with Jk,m of 58.5 mA mgNi-1 and j0,s of 42 μA cmNi-2, which is among the best PGM-free catalysts and is even comparable to some PGM catalysts. It also shows the highest power density (511 mW cm-2) as a PGM-free anode when assembled into fuel cells under moderate back pressure. These findings prove that in addition to optimizing electrophilicity and oxophilicity for active sites, we could also improve the HOR activity from the transfer pathway for intermediates, which provides insight into the design of other efficient HOR catalysts.

Gaseous Nitric Oxide As Probe Molecule for Fe-N-C ORR Electrocatalysts

Iron nitrogen carbon (Fe-N-C) electrocatalysts remain the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Although significant progress has been made in recent years in improving both the ORR activity and stability of the Fe-N-C catalysts, further enhancement in the catalyst durability while maintaining the activity is imperative for the practical applications. One of the major factors that hinders the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites and density of the active sites in the Fe-N-C catalysts due to the complexity of the Fe-N-C catalysts’ composition and structure. Gaseous nitric oxide was proven to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts. In recent years, we have studied the effects of gas-phase adsorption and desorption of nitric oxide on the redox features and ORR activity on Fe-N-C catalysts synthesized by different methods. We have used various characterization tools to quantify the amount of nitric oxide adsorbed and to identify the adsorption sites and geometry, including electrochemical stripping, temperature programmed desorption, and in situ X-ray techniques. The corresponding results obtained will be summarized in this talk. The implication of these results on the nature of the ORR active sites of the Fe-N-C catalysts will be discussed.AcknowledgementsThis work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office. This work was partially authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

Revolution of Electrode Design Toward Oxygen Evolution Reaction for Anion Exchange Membrane Water Electrolysis

Anion exchange membrane water electrolyzer (AEMWE) is a promising next-generation technology for producing green hydrogen coupling with cost-effective, platinum group metal-free (PGM-free) catalysts. In AEMWE, oxygen evolution reaction (OER) at the anode plays a more vital role compared with the cathodic reaction of hydrogen evolution reaction (HER). While tremendous attention has been devoted to enhancing the activity of OER catalysts, the long-term stability and cost of OER catalysts, which may play an even more important role in large-scale electrolysis industrialization, have been much less emphasized. Herein, we designed electrodes toward OER from cheap and abundant stainless-steel (SS) foam via a green approach, and the obtained electrode shows excellent activity and stability under an alkaline medium. This work can address all three bottlenecks of activity, stability, and cost, thereby revolutionizing the electrode design for AEMWE.

(Invited) Efficient Ternary Nicuw Hydrogen Oxidation Catalysts for Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) offer prospective approaches to replace proton exchange membrane fuel cells via cost-effective catalysts and membranes. While significant progress has been made in efficient nonnoble electrocatalysts for oxygen reduction reaction in alkaline media, the hydrogen oxidation reaction (HOR) in the alkaline condition still relies on high platinum loadings to compensate for its sluggish kinetics. Therefore, developing efficient non-noble metal electrocatalysts with Pt-like HOR performances in alkaline electrolytes is highly desired but a great challenge.The hydrogen binding energy (HBE) has been identified as the primary descriptor for the HOR. Additionally, an unfavorable hydroxide binding energy (OHBE) is considered the rate-determining step in the Volmer reaction to generate water. Therefore, an ideal catalyst for HOR should achieve a balance between HBE and OHBE. However, despite being a promising non-noble metal, nickel exhibits excessively high HBE and lacks active sites for hydroxide adsorption, which hinders its further application in the real fuel cell.In this work, NiW was successfully deposited onto Cu to form a ternary Ni-Cu-W alloy catalyst by a chemical reduction method. Through XPS spectra, we found the charge transfer from Ni to W to give Ni a higher valence state, contributing to a weakened hydrogen binding energy. Electrochemical measurements observed Ni-Cu-W exhibit an kinetic activity with 117.9 mA/mgNi at η= 50 mV, which is the highest among the reported platinum group metal-free catalysts. Ni-Cu-W alloy showcased robust stability, with no activity degradation during an accelerated long-term durability test between a low overpotential (-0.1 ~ 0.1 V, vs RHE) with 10,000 cyclic voltammetry scans and only a 5.7% activity loss at η= 50 mV when applying more larger overpotential (-0.1 ~ 0.2 V) with the same cycles. When assembled with this catalyst as anode and Pt/C as cathode, it delivered a peak power density of 289 mW/cm2. Both experimental and theoretical studies were conducted to gain insights into the catalyst's enhanced performance. Our findings revealed that the incorporation of Cu into the catalyst significantly weakened the adsorption of hydrogen species, while simultaneously lowering the energy barrier required for the absorption of hydroxide ions. These key observations might shed light on the underlying mechanisms responsible for the catalyst's improved activity and stability.

Simplified Ftacv Model to Quantify the Electrochemically Active Sitedensity in PGM-Free ORR Catalysts

The development of platinum group metal-free catalysts is considered the most prominent path for reducing thecost of low-temperature fuel cells (LTFC). Despite the great advancement in the field, its further progress iscurrently limited by the ability to understand and mitigate the catalysts’ degradation mechanisms, which up torecent years was limited by the lack of activity descriptors. Recent work showed that this could be solved usingFourier-transformed alternating current voltammetry that enables to deconvolute Faradaic currents arising fromthe redox reaction of the active sites from the capacitive currents, and by that accurately measure the electrochemicallyactive site density of these catalysts in situ fuel cells. However, the analysis of the results can becomplex, requiring simulation software for accurate parameter extraction. Herein, a simplified analysis ofFourier-transformed alternating current voltammetry is presented. This was done by mapping the influence ofvarious variables, such as resistivity, capacitance, and thermodynamic and kinetic dispersion, on the accuracy ofelectrochemically active site density measurements. Under specific, yet realistic, operating conditions, a singleequation with the peak current of the 5th harmonic as the only variable can be used to quantify electrochemicallyactive site density with high accuracy. This approach was demonstrated using molecular catalysts, iron phthalocyanine,as a model molecule, and a commercial, heat-treated catalyst.

Degradation Analysis for Platinum Group Metal-Free Fuel Cell Cathodes

Recent advancements in platinum-group metal-free (PGM-free) catalysts have overwhelmingly prioritized improving initial electrocatalytic activity rather than focusing on catalyst stability. Unfortunately, this emphasis has led to less significant progress in addressing catalyst degradation, ultimately falling well below the level required for commercialization. Shifting the focus towards understanding and mitigating degradation unlocks the true potential of PGM-free catalysts and paves the way for their successful implementation in commercial applications. This work explores methods to mitigate degradation and expand models to predict PGM-free cathode fuel cell degradation due to voltage cycling, specifically current density loss over time. The tested PGM-free cathode fuel cells, using iron (Fe)-based catalyst, were electrochemically characterized before and after the accelerated stress tests (AST) using H2/N2 electrochemical impedance spectroscopy, cyclic voltammetry, and air polarization curves. This study reports the functions of each acceleration factor due to changes in temperature, cycling upper potential limits, and relative humidity are reported. The data indicates that degradation rates increase as temperature, upper potential limit, and relative humidity increase in Fe-based cathode proton exchange membrane fuel cells (PEMFCs). This finding is essential to more accurately quantify losses in active site density with respect to temperature, varying upper potential cycling limit, and relative humidity. More generally, these results can aid in predicting Fe-based cathode fuel cell degradation due to changes in operational conditions.

Unraveling Stoichiometry Effect in Nickel-Tungsten Alloys for Efficient Hydrogen Oxidation Catalysis in Alkaline Electrolytes.

Anion-exchange membrane fuel cells provide the possibility to use platinum group metal-free catalysts, but the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics and its source is still debated. Here, over nickel-tungsten (Ni-W) alloy catalysts, we show that the Ni : W ratio greatly governs the HOR performance in alkaline electrolyte. Experimental and theoretical studies unravel that alloying with W can tune the unpaired electrons in Ni, tailoring the potential of zero charge and the catalytic surface to favor hydroxyl adsorption (OHad). The OHad species coordinately interact with potassium (K+) ions, which break the K+ solvation sheath to leave free water molecules, yielding an improved connectivity of hydrogen-bond networks. Consequently, the optimal Ni17W3 alloy exhibits alkaline HOR activity superior to the state-of-the-art platinum on carbon (Pt/C) catalyst and operates steadily with negligible decay after 10,000 cycles. Our findings offer new understandings of alloyed HOR catalysts and will guide rational design of next-generation catalysts for fuel cells.

Pyrolysis-Free Covalent Organic Polymers Directly for Oxygen Electrocatalysis.

ConspectusOxygen electrode catalysis is crucial for the efficient operation of clean energy devices, such as proton exchange membrane fuel cells (PEMFCs) and Zn-air batteries (ZABs). However, sluggish oxygen electrocatalysis kinetics in these infrastructures put forward impending requirements toward seeking efficient oxygen-electrode catalytic materials with a clear active-site configuration and geometrical morphology to study in depth the structure-property relationship of materials. Although transition-metal-nitrogen-carbon (M-N-C) electrocatalysts have shown great prospects currently and potential in oxygen electrocatalysis as promising platinum group metal-free catalysts, the universal pyrolysis operation in the preparation process often inevitably brings about randomness and diversity of active sites, for which it is difficult to determine the structure-activity relationship, understand the catalytic mechanism, and further improve facilities performance.Covalent organic polymers (COPs) are a class of molecular geometric constructs linked by irreversible kinetic covalent bonds through reticular chemistry. Unique structural tailorability, diverse design principles, and inherent well-defined construction in pristine COPs naturally provide a great platform to study the structure-property relationship of active sites and exhibit unique features for application. In this Account, we afford an overview of our recent attempts toward the utilization of COP materials as free-pyrolysis oxygen electrode catalysts, enabling accurate construction of oxygen electrodes with clear active site and geometrical morphology characteristics in PEMFC and ZAB devices yet without enduring any high-temperature pyrolysis treatments. Starting from the needs of modern electrocatalysis, we discussed the unique properties for the design and development of pyrolysis-free pristine COPs as high-performance oxygen electrode catalytic materials in terms of intrinsic electronic structure properties and membrane-electrode-assembly (MEA) application distinguished from pyrolysis M-N-C catalysts. First, the pyrolysis-free COP catalysts provide a viable molecular model catalyst platform, which is conducive to mechanism comprehension for the relationship between catalyst activity and structure. Second, the simple and low-energy consumption synthesis process for pyrolysis-free catalysts lays the foundation for the large-scale production of catalysts, oxygen electrodes, and even the entire stack assembly without considering numerous complicated factors as traditional pyrolytic catalysts. Besides, most traditional COPs are difficult to dissolve and solution process due to their cross-linked skeleton. Our newly developed COP materials with solution processability bring about new opportunities to the process and assemble oxygen electrodes into device. These properties are unparalleled and have not been systematically reviewed and analyzed by any research reports so far. Here, we have clarified the specific advantage and potential of pyrolysis-free COP materials as oxygen electrodes applied in PEMFC and ZAB devices in response to the latest progress and requirements of current electrocatalytic research.

All Platinum Group Metal-Free and Durable Catalysts for Direct Borohydride Fuel Cells

All Platinum Group Metal-Free and Durable Catalysts for Direct Borohydride Fuel Cells

(Invited) Progress in Development of Platinum Group Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

Platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells have emerged as a promising alternative to PGM-based catalysts thanks to their abundance in the Earth crust and, consequently, low cost. Metal-nitrogen-carbon (M-N-C) catalysts possibly represent the most attractive PGM-free ORR materials. However, despite the significant progress accomplished in M-N-C catalyst development over the past two decades, this class of materials continues to face challenges that need to be overcome to narrow the performance gap to PGM-based catalysts.[1] Our team has made significant efforts to improve the activity and stability of M-N-C catalysts by developing novel synthesis approaches, such as the ‘dual-zone’ synthesis, targeting specifically improvements in catalyst stability.[2, 3] We have also established standardized protocols for the activity and stability evaluation of PGM-free catalysts under relevant operating conditions of the fuel cell.[4] Additionally, our modeling of PGM-free M-N-C ORR electrocatalysts has provided much needed insight into the materials and helped with their optimization. We have utilized machine learning methods coupled with high-throughput synthesis and characterization to optimize the conditions for a variety of synthesis approaches, which has led to substantial increase in experimentally realized activity.[5, 6] Our prior fundamental studies of ORR activity [1, 2, 7, 8] and materials stability [9] at the density functional theory (DFT) level are actively being linked to achieve simultaneously high activity and long-term stability through a Pareto optimization. Coupling machine learning and DFT efforts is presently underway.In this talk, we will summarize our recent progress in the development of PGM-free M-N-C catalysts for ORR, focusing on improvements in catalyst activity and stability from both experimental and theoretical perspectives. We will highlight various synthesis strategies for improving catalyst activity and stability, and key design principles for M-N-C catalysts, learned from DFT calculations and machine learning-driven, high-throughput catalyst synthesis. We will also discuss the challenges and opportunities in the development of PGM-free catalysts for ORR. Overall, PGM-free catalysts represent a promising pathway for the advancement of low-cost and efficient fuel cell technologies. References Martinez, U. et al., J. Electrochem. Soc., 2019. 166, F3136.Chung, H.T. et al., Science, 2017. 357, 479.Zelenay, P. and D. Myers, ElectroCat (Electrocatalysis Consortium); U.S. Department of Energy Hydrogen and Fuel Cell Program Annual Merit Review 2019. https://www.hydrogen.energy.gov/pdfs/review19/fc160_myers_zelenay_2019_o.pdf.Zhang, H. et al., Nat. Catal., 2022. 5 (5): 455.Kort-Kamp, W.J.M. et al. . J. Power Sources, 2023. 559: 232583.Karim, M.R. et al., ACS Appl. Energy Mater.. 2020. 3 (9): 9083.Holby, E.F., Curr. Opin. Electrochem., 2021. 25, 100631.Anderson, A.B. and E. F. Holby, J. Phys. Chem. C, 2019. 123 (30): 18398.Holby, E.F., G. Wang, and P. Zelenay, ACS Catal., 2020. 10 (24): 14527.

Materials Integration and Catalyst Interfaces in Anion Exchange Membrane, Low Temperature Electrolysis

As an energy carrier, hydrogen has unique advantages due to its high energy density, ability for long-term storage, and the ability to convert between chemical bonds and electricity. [1] Although hydrogen currently has a small role in energy pathways, low-cost renewable power sources can allow for significant growth. Anion exchange membrane (AEM) electrolyzers leverage several advantages, including a zero-gap approach that typically improves upon the efficiency of traditional alkaline systems. [2] Compared to proton exchange membrane (PEM) electrolyzers, the high pH also allows for platinum group metal-free (PGM-free) catalysts and component coatings (transport layers, separators) that can reduce system cost and improve long-term durability. In recent years, membrane advancements have further enabled higher AEM performance, particularly in supporting electrolytes. [3]This presentation includes an overview of efforts in AEM electrolysis and focuses on materials choices and their integration. While components are readily available that can achieve high performance, individual properties and their interactions create integration challenges. [4] In catalyst and transport layers, large particle sizes and lower catalyst conductivity coupled with the high distance between transport layer fibers can lead to lower catalyst utilization and high catalyst layer resistances. The ionomer, while critical for ionic transport in water, is primarily needed in supporting electrolytes for structural integrity, and changes to the ionomer content, type, and its distribution throughout the catalyst layer have been explored for its effect on performance and durability. These experiments demonstrate the complications of developing a single set of materials and test protocols for component evaluations.

Platinum Group Metal-Free Catalyst Loading Optimization for Dry Cathode Operation in Anion Exchange Membrane Water Electrolysis

With an increasing interest for green hydrogen as an important fuel for the global energy transition and as a raw material for green chemistry, extensive research efforts are currently devoted to increase the performance of water electrolysis technologies. In this frame work, Anion Exchange Water Electrolysis (AEMWE) has a strategic role promising to boost electrolytic hydrogen production without relying on scarce resources as AEMWE does not need Iridium or other Platinum group metals (PGM) catalysts, unlike Proton Exchange Membrane Water Electrolysis (PEMWE), to reach high power density [1].In this work, the focus is on electrode optimization for dry cathode operation. Operating without a liquid feed on the cathode reduces the extent of the drying of hydrogen gas and simplifies the balance of the plant. However, water must be transported from the anode to the cathode where it is consumed by the hydrogen evolution reaction (HER) [2]. Also, ionic connection from the membrane to catalyst surface must be provided by the ionomer without a supporting electrolyte which is generally employed in AEMWE to improve performance [1]. Under these premises, it is clear that using high catalyst loading to compensate for the intrinsic low activity of Platinum group metals-free (PGM-free) catalysts will result in lower utilization of the catalyst layer as a function of the electrode thickness [3].To study how transport limitations hinder cathode performance, we manufactured catalyst coated membranes (CCMs) via decal transfer method and varied the cathode catalyst loading thereby adjusting catalyst layer (CL) thickness. RaneyTM Nickel served as the cathode catalyst and Super PTM Carbon as the conductive additive. It was found that increasing the catalyst loading between 0.65 mg/cm2 and 1.65 mg/cm2 greatly improved cell performance (Figure 1). However, cell voltage proved to be less sensitive to cathode loading for higher loadings. In fact, to reasonably improve the performance further, a loading of 4.55 mg/cm2 was necessary. Noticeably, the high frequency resistance (HFR) was found to be independent of catalyst loading (i.e. CL thickness) due to the high electronic conductivity of the cathode CL with carbon additive.This works sheds light on how AEMWE performance are related to cathode catalyst loading when the cathode is dry operated. It is found that performance improvement is limited at high loading (above 1.65 mg/cm2). This shows how the ionic conductivity of the electrode should be improved to mitigate the low mass activity of PGM-free catalyst.[1] Ayers, Katherine, Nemanja Danilovic, Ryan Ouimet, Marcelo Carmo, Bryan Pivovar, and Marius Bornstein. “Perspectives on Low-Temperature Electrolysis and Potential for Renewable Hydrogen at Scale.” Annual Review of Chemical and Biomolecular Engineering 10, no. 1 (June 7, 2019): 219–39.[2] Koch, Susanne, Joey Disch, Sophia K. Kilian, Yiyong Han, Lukas Metzler, Alessandro Tengattini, Lukas Helfen, Michael Schulz, Matthias Breitwieser, and Severin Vierrath. “Water Management in Anion-Exchange Membrane Water Electrolyzers under Dry Cathode Operation.” RSC Advances 12, no. 32 (2022): 20778–84.[3] Liu, Jiangjin, Zhenye Kang, Dongguo Li, Magnolia Pak, Shaun M. Alia, Cy Fujimoto, Guido Bender, Yu Seung Kim, and Adam Z. Weber. “Elucidating the Role of Hydroxide Electrolyte on Anion-Exchange-Membrane Water Electrolyzer Performance.” Journal of The Electrochemical Society 168, no. 5 (May 1, 2021): 054522.Figure 1 Polarization curve of CCMs with different cathode catalyst loading. The cathode contains 65 % catalyst (RaneyTM Nickel), 10% ionomer and 25% Super PTM Carbon. NiFe nanoparticles are used in the anode with 5% ionomer (AP-1-HNN5-00-X). The measurements are conducted at 60 °C feeding 1 M KOH to anode. Figure 1

In-Situ Analysis of Electrodes Using Fine-Tuned Poly(Terphenyl Piperidinium) Ionomers in Anion Exchange Membrane Fuel Cells

In the field of low temperature polymer electrolyte fuel cells, anion exchange membrane fuel cells (AEMFCs) have been considered a promising alternative by circumventing the obstacles to commercialization faced by the established PEMFC technology. The application of platinum group metal-free catalysts for the oxygen reduction reaction, the potential to utilize cheaper and more sustainable polymers than the industry benchmark Nafion®, and the generally less corrosive alkaline operation conditions, have all been identified as inherent advantages of the AEMFC technology, potentially enabling more cost-effective and environmental-friendly mass production of AEMFC system.The first major challenge in developing AEMFC systems was the lack of stable and conductive anion exchange polymers to be used as membrane and ionomer materials. This challenge was at least partly overcome by the successful development of a variety of next-generation polymers. Among those, poly(arylene piperidinium) show great potential through the combination an ether-free aromatic backbone for increased mechanical stability with a piperidinium-based cationic groups displaying excellent resistance to hydroxide attack in highly alkaline environment [1]. Furthermore, the crucial parameters ion exchange capacity (IEC), water uptake and dimensional swelling can be finely controlled via copolymerization and partial substitution of the cationic functional group in the repeating unit [2].In a previous study using a series of poly(terphenyl piperidinium) membranes with different IECs, we established the importance of these three parameters, in particular for the ionomer used in the catalyst layer of the electrode [3]. Furthermore, it was established that water balance of AEMFC systems proved to be one of the greatest challenges in reaching and maintaining high cell performance. The asymmetrical nature of the simultaneous electrochemical production and consumption of water at the anode and cathode, respectively, necessitates fine-tuning the ionomer properties, as well as selecting operational parameters for each electrode individually in order to strike an optimal water balance [4].In this study, we expand on our previous work by analyzing a matrix of electrode combination based on three poly(terphenyl piperidinium)ionomers with different IEC values. Furthermore, we complemented our analysis by evaluating the effects of substituting a fraction of regular dissolvable ionomer with insoluble non-conformal particles of cross-linked poly(terphenyl piperidinium). The beneficial effects of insoluble non-conformal ionomer particles in the catalyst layer has been previously shown by Varcoe and coworkers utilizing radion-grafted anion exchange membranes [5], and by Holdcroft and coworkers utilizing phenylated poly(phenylene) ionomers for PEMFCs [6].Different electrode combinations were evaluated while varying operational parameters such as gas flow rates, gas relative humidities and back pressure. The recording of observed cell performances were complemented by in-situ analysis of in-let/out-let gas relative humidities, H2/O2 electrochemical impedance spectroscopy for kinetic analysis and information on ohmic losses, H2/Ar electrochemical impedance spectroscopy to measure ionic conductivity, cyclic voltammetry to measure electrochemically active surface area as well as ex-situ characterization of the electrodes before/after the test run.The study was carried out with the aim to establish a correlation between the properties of individually tuned ionomers and operational parameters, and to investigate reasons for performance losses due to mass transport limitations, drop in ionomer conductivity, loss of active area, and decreased kinetics caused by phenyl adsorption.Finally, we demonstrate how the application of individually optimized electrodes using fine-tuned poly(terphenyl piperidinium) based ionomers in conjunction with suitable operating conditions will lead to a drastic increase in fuel cell performance compared to symmetrical non-optimized electrodes.A selection of experimental data is shown in Figure 1.[1] J. S. Olsson, T. H. Pham, P. Jannasch, Adv. Funct. Mater. 2017, 28, 1702758.[2] J. S. Olsson, T. H. Pham, P. Jannasch, Tuning poly(arylene piperidinium) anion-exchange membranes by copolymerization, partial quaternization and crosslinking, J. Membrane Sci., 2019, 578, 183-195.[3] T. Novalin, D. Pan, G. Lindbergh, C. Lagergren, P. Jannasch, R.W. Lindström, Electrochemical performance of poly(arylene piperidinium) membranes and ionomers in anion exchange membrane fuel cells, J. Power Sources, 2021, 507, 230287. [4] D. P. Leonard, S. Maurya , E. J. Park , S. Noh , C. Bae , E. D. Baca , C. Fujimoto and Y. S. Kim , J. Mater. Chem. A, 2020, 8 , 14135-14144.[5] L. Q. Wang , J. J. Brink , Y. Liu , A. M. Herring , J. Ponce-Gonzalez , D. K. Whelligan and J. R. Varcoe, Energy Environ. Sci., 2017, 10 , 2154-2167.[6] E. Balogun, S. Cassegrain, P. Mardle, M. Adamski, T. Saatkamp, and S. Holdcroft, ACS Energy Lett. 2022, 7, 2070-2078. Figure 1

Enhancement of Mn-N-C single atom catalysts via sulfur and/or oxygen co-doping for oxygen reduction in acidic conditions: Unveiling the catalyst durability in fuel cells

Carbon-supported single Mn sites coordinated with nitrogen (Mn-N-C) catalysts are amongst the most favorable platinum group metal-free (PGM-free) catalysts for proton exchange membrane fuel cells (PEMFCs). However, the high overpotential of these catalysts, limits their application for oxygen reduction reaction (ORR). Experiments showed that O and S heteroatom co-doping increases the catalytic activity of Mn-N-C catalysts for electrochemical gas conversion. This prompted us to perform a systematic investigation of the formed co-doped configurations at the atomic scale and to study the corresponding reaction mechanisms for oxygen reduction in acidic environment. All probable configurations for Mn-NxOySz/C10 complexes are considered and the most stable and durable structures are selected as ORR active catalysts. Our results confirm the strong stabilization of the Mn sites over N4- and N3-doped carbonaceous support and consequently their stability against oxidation in contrast to other O and/or S co-doped heterostructures.

Rapid and facile preparation of freestanding dense polymer membranes from high-boiling-point solvents for anion exchange membrane water electrolyzers

Transparent, dense, and freestanding polymer membranes are typically prepared by solution casting (SC). However, SC is a time-consuming (>24 h) multi-step process that requires high-temperature vacuum treatment. Here, we report a rapid and facile method—nonsolvent-induced film formation (NIFF)—for preparing dense, freestanding membranes by immersing a polymer solution in a nonsolvent, using two types of poly(arylene piperidinium) (PAP) and their quaternized versions. Thermodynamic, kinetic, and molecular dynamics analyses of polymer/solvent/nonsolvent ternary systems reveal that the PAP polymers have high affinities for the nonsolvent (water), yielding a gel between homogeneous and heterogeneous regions. The protective gel layer at the polymer/solvent–nonsolvent interface averts penetration of the nonsolvent and permits withdrawal of the solvent from the polymer solution, forming a nonporous dense membrane. The physicochemical, mechanical, morphological, and electrochemical characteristics of NIFF- and conventional-SC-derived membranes are comparable. Moreover, the alkaline water electrolysis cell performance is evaluated using the membranes and platinum group metal-free catalyst. Both NIFF- and SC-derived membrane-based cells achieve an equivalent current density of 910 mA cm−2 at 2.0 V, which is 20 % higher than commercial FAA-3-50 membrane-based cells. This study establishes an innovative strategy to facilely fabricate freestanding polymer films with a time- and energy-saving process for ion-conducting membranes.

Kinetic Analysis of Carbon Corrosion-Induced Degradation of Platinum Group Metal-Free Catalysts in Acidic Media

Kinetic Analysis of Carbon Corrosion-Induced Degradation of Platinum Group Metal-Free Catalysts in Acidic Media

Effect of Fe–N–Cs as Catalytic Active Support for Platinum towards ORR in Acidic Environment

Metal-nitrogen-carbon (M–N–C) compounds such as Fe–N–Cs are currently the most promising platinum group metal free catalysts for oxygen reduction in acidic environment. Regarding the overriding goal of reducing PEMFC production costs by reducing the platinum content, the use of Fe–N–Cs as catalytic active support for low Pt amounts is investigated in this study. Activity and stability of Pt in different contents on a commercial Fe–N–C is compared to Pt on a typical carbon black. Pt nanoparticles are well-distributed on both support substrate classes. Although the electrochemical surface and mass activity of Pt is lower on Fe–N–C compared to carbon black, the Fe–N–C has a contribution to total ORR activity depending on the Pt/Fe–N–C ratio, which is quantified. In the low Pt content case of 1 wt%, the ORR activity is increased by factor of two in presence of Fe–N–C. This boosting effect on ORR activity is important for future strategies to lower the Pt content in PEMFCs.

Highly durable platinum group metal-free catalyst fiber cathode MEAs for proton exchange membrane fuel cells

Fe-based platinum group metal (PGM)-free catalysts were incorporated into electrospun fiber mat or powder cathode membrane-electrode-assemblies (MEAs) with a Nafion 211 membrane and a Pt/C powder anode. Fabrication and characterization tests were performed on MEAs with: (1) a conventional powder cathode with a neat Nafion binder, (2) a fiber mat cathode with a Nafion/polyethylene oxide (PEO) binder, where PEO was extracted before MEA testing, (3) a powder cathode with a blended binder of Nafion and polyvinylidene fluoride (PVDF), and (4) a series of fiber mat cathodes with different Nafion/PVDF binder weight ratios. Cathode degradation occurred, with a loss in power output, in MEAs with a neat Nafion powder cathode or with a Nafion fiber cathode. In contrast, little or no power loss was observed for powder or fiber cathodes when the binder was a blend of Nafion and PVDF. The presence of hydrophobic PVDF drove water and electrogenerated peroxide out of the cathode, away from catalyst particles, which improved cathode durability, but PVDF also decreased the binder conductivity and slowed oxygen reduction kinetics, resulting in lower power densities. A 75:25 w:w Nafion:PVDF fiber cathode MEA was the best compromise for maximizing power and minimizing catalyst degradation. For such a cathode, with a PGM-free cathode catalyst loading of 3.0 mg cm−2, a power density of 88 mW cm−2 at 0.5 V, 80 °C, and 200 kPaabs pressure was maintained for 80 h of continuous operation.

Advanced Analytical Electron Microscopy Techniques for Probing Individual PGM-Free Catalyst Sites

Hydrogen fuel cells currently utilize expensive platinum group metal nanoparticles as catalysts [1]. To reduce the cost of catalyst materials, large efforts to develop viable platinum group metal-free (PGM-free) catalyst materials, which often consist of single transition metal atoms embedded in nitrogen-doped graphitic carbon structures, are being undertaken [2]. Given the atomic-scale nature of the metal sites in these materials, their performance and stability are highly dependent on the local coordination environment and consequently electronic structure of the sites [3]. A detailed understanding of the local elemental composition and lattice structure of individual sites, and how these properties vary between sites, is therefore needed to further develop improved PGM-free catalyst materials. While many bulk techniques are available that can provide average properties of these sites, such as Mössbauer and X-ray absorption spectroscopies, techniques with high spatial resolution are needed to measure the properties of individual sites. As a result, scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) has proven an invaluable tool for characterization of these materials due to its ability to probe structural and electronic properties at the atomic scale [4]. However, combined with the instability of many metal sites under the electron probe, the quantity and type of information accessible by conventional STEM instrumentation and acquisition methods is limited. To better understand the local coordination environment and electronic structure of individual sites, as well as how these properties vary site to site, new techniques and hardware are therefore needed.Here, we demonstrate advancements in STEM-EELS and -EDS that allow the quality and quantity of spectroscopic information obtained from individual sites to be significantly improved over conventional method. First, we demonstrate direct real-space mapping of metal sites by using a direct electron detector in the EELS spectrometer. This increases the data acquisition rate and signal-to-noise ratio sufficiently to map metal atom positions at the atomic scale. In addition, we show how automated metal atom position identification and subsequent probe positioning allow the rate at which metal sites are probed to be increased by another order of magnitude while simultaneously increasing the quantity of data obtained from each site. Using EELS and EDS signals acquired in parallel, this automated acquisition method enables a wide range of metal site elements to be identified, as well as the local coordination environment around the sites to be determined. Combined with electronic structure measurements of the metal atoms themselves, this technique provides significantly more information about individual sites than conventional methods. The techniques presented here are therefore poised to significantly improve our understanding of the local coordination environment and electronic structure of individual PGM-free metal sites, as well how these properties vary site to site, accelerating development of these low-cost hydrogen fuel cell catalyst materials [5].