PGM-free Catalysts Research Articles

Asymmetric gas diffusion layers for improved water management in PGM-free electrodes

Proton-exchange-membrane fuel cells (PEMFCs) offer a long-term, carbon-emission free solution to the energy needs of the transportation sector. However, high cost continues to limit PEMFC commercialization. Replacing expensive platinum group metal (PGM) catalysts with PGM-free catalysts could reduce cost, but the low active site density of PGM-free catalysts necessitates the use of thick electrodes that suffer from substantial mass transport losses. In these thick PGM-free electrodes, effective water management and oxygen transport are crucial to achieve high performance. In this work, we investigate the role of anode and cathode gas diffusion layer (GDL) configurations in controlling water management. Asymmetric GDL configurations, in which the anode GDL exhibits higher permeability than the cathode GDL, showed higher performance compared to conventional symmetric configurations. Computational modeling showed that the improved performance is mainly due to improved water management, resulting in lower liquid water saturation and faster oxygen transport in the cathode.

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.

PGM-Free Oxygen Evolution Reaction Catalyst for PEM Water Electrolysis – a Mechanistic Study on Activity and Durability for Practical Applications

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential. Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve add a significant cost barrier to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost.For PEMWE application, a PGM-free anodic catalyst must be highly active to match the performance of Ir. Furthermore, it must be stable against oxidative acid corrosion. At Argonne National Laboratory, we recently developed a new PGM-free OER of a nanofibrous cobalt spinel catalyst co-doped with lanthanum and manganese (LMCF). [1] The catalyst was synthesized from the precursor of cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. The Co-ZIF was mixed with a polymer solution and electrospun into nanofiber before being converted to fibrous cobalt spinel under low temperature oxidation to remove all the organics and carbon. The new PGM-free catalyst demonstrated a low overpotential of 353 mV (RHE) at 10 mA/cm2 and a low OER degradation over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at its anode at ~2.0 mg/cm2 demonstrated a current density of 2000 mA/cm2 at 2.47 volts (Nafion® 115 membrane) or 4000 mA/cm2 at 3.00 volt (Nafion® 212 membrane). The membrane electrode was also subjected to accelerated-stress-test (AST) in both voltage cycling and galvanostatic run at different current densities. Low degradation rates under both ASTs were observed.In addition to catalyst design and synthesis, we also conducted extensive structural characterizations, stability measurement, as well as computational modeling. Our in situ study found that the OER process involves significant lattice oxygen movement, leading to lowering instead of raising the cobalt oxidation state during the electrolysis. The measurement of the stability number, combined with computational Pourbaix diagram, elucidated the importance of the operating potential in reducing catalyst dissolution. These findings offer in-depth understanding and direction of future development for PGM-free OER catalyst in PEM water electrolyzer. Acknowledgement: This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewal Energy, Hydrogen and Fuel Cell Technologies Office, and by Laboratory Directed Research and Development funding of Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DEAC02-06CH11357. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.[1] “La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis” Lina Chong, Guoping Gao, Jianguo Wen, Haixia Li, Haiping Xu, Zach Green, Joshua D. Sugar, A. Jeremy Kropf, Wenqian Xu, Xiao-Min Lin, Hui Xu, Lin-Wang Wang, Di-Jia Liu, Science 380, 609–616 (2023)

Exploring the Role of Coordinating Environments on Durability of Nickel-Based Catalysts for Hydrogen Evolution Reactions

Ni-based platinum-group-metal (PGM)-free catalysts for hydrogen evolution reaction have been considered viable for the commercialization of alkaline exchange membrane water electrolysis (AEMWE) due to their high activity and low cost. Even though several highly active Ni-based catalysts have been developed in recent years, their long-term stability under commercially relevant conditions was not systematically assessed, preventing a direct comparison with the currently used catalysts. While not as active, the current commercial PGM-free catalysts are highly stable, with an operational life measured in decades. However, conducting durability assessments through long-term durability studies can take over 10,000 hours which is impractical. Consequently, accelerated stress tests (ASTs) specifically targeting the catalyst and simulating uncontrolled shutdown and high overpotential scenarios are needed to further improve AEMWE technology.In this presentation, the ASTs of several Ni-based catalysts with different coordination environments (nickel molybdenum, nickel cupferron1, Ni-NiO, and a novel high surface area de-alloyed Ni catalyst) will be explored. The catalysts were subjected to high overpotentials and uncontrolled shutdown cycles in both liquid electrolyte and AEMWE cells. The structure of these catalysts was observed using in situ Raman spectroscopy, XRD, TEM, and BET gas adsorption analysis and correlated to changes in electrochemical performance. The determination of the catalyst structure during these ASTs is paramount in designing durable PGM-free catalysts for improved AEMWE performance.1) Doan, H.; Kendrick, I.; Blanchard, R.; Jia, Q.; Knecht, E.; Freeman, A.; Jankins, T.; Bates, M. K.; Mukerjee, S. Functionalized Embedded Monometallic Nickel Catalysts for Enhanced Hydrogen Evolution: Performance and Stability. J. Electrochem. Soc. 2021, 168 (8), 084501. https://doi.org/10.1149/1945-7111/ac11a1.

Benchmark and Roadmap for Low Temperature Water Electrolysis for Green H2 Production and Applications

Water electrolysis has attracted world-wide attention due to its ability to produce green hydrogen when coupled with renewable electricity. Green hydrogen is a viable solution to decarbonizing a variety of industries including fertilizer, metal production, petrochemical, and synthetic fuels. There are three major low temperature water electrolysis technologies, alkaline water electrolyzer (AWE), proton exchange membrane water electrolyzer (PEMWE), and anion exchange membrane water electrolyzer (AEMWE). This work will first provide the status of these three technologies in terms of operating conditions, lifetime, and capital cost. Subsequently, the challenges and constraints for these technologies will be discussed in the scenarios of large-scale manufacturing and integration with intermittent renewables. More efforts will be made to illustrate the trajectory of the next-gen water electrolysis technologies, which feature at ultralow PGM or PGM-free catalysts, thin membranes with reduced hydrogen crossover, high-current high-power operation, long lifetime, and remarkable cost reduction. The work will also address the challenges of automated MEA fabrication and stack manufacturing at scale. Finally, a real example of renewable wind turbine integrated with water electrolyzer to produce green hydrogen and subsequently green ammonia will be demonstrated.

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.

Design and Preparation at Scale of Ternary NiFe-M-Ox (M=Co, Mo) Catalysts for OER in Alkaline Media

Due to scarcity of Platinum Group Metals (PGMs) widely used in low temperature water electrolysis as cathodes (Hydrogen Evolution Reaction; HER) and anodes (Oxygen Evolution Reaction; OER) a boosting interest to their substitution to Earth abundant elements is observed. It was successfully demonstrated that elements like iron, nickel, cobalt and other (in their oxide and hydroxide forms) can be effective OER catalysts when operate at high pH ranges from 12 to 14 [1-3].This work is devoted to systematic investigation of influence of third element addition to NiFeOx system on a performance and stability of catalysts in alkaline OER. As a co-catalytic elements Mo and Co were selected, based on our previous experience [4,5]. A systematic study on an effect of NiFe-to-M ratio, concentration of precipitation agent and post-treatment temperature were evaluated in a wide range of parameters. Taking into account that mass loading of PGM-free OER catalysts is substantially higher than iridium/ruthenium once and can be up to several milligrams per cm2 all experiments were designed to be scalable. In a laboratory scale reactor, a fascinating batch size of 25g was achieved, using minimal amount of water-based solvents in order to minimize a waste stream generated. Presentation will discuss characterization of obtained materials by SEM, TEM, XPS, XRD and other methods as well as their performance, stability and durability in alkaline conditions, studying Oxygen Evolution Reaction. Acknowledgements and Statements. The authors acknowledge financial support from H2NEW and ElectroCat consortia. References. [1] X. Lyu, J. Li, J. Yang, A. Serov "Significance of Slight Ambient Temperature Variation on the Electrocatalyst Performance toward Oxygen Evolution Reaction" Journal of Environmental Chemical Engineering (2023).[2] X. lyu, A. Serov “Cutting-edge methods for amplifying the oxygen evolution reaction during seawater electrolysis: a brief synopsis” Ind. Chem. Mater., (2023).[3] X. Lyu, Y. Bai, J. Li, R. Tao, J. Yang, A. Serov "Investigation of oxygen evolution reaction with 316 and 304 stainless-steel mesh electrodes in natural seawater electrolysis" J. of Environmental Chemical Engineering 11 (2023) 109667.[4] A. Serov,N. I. Andersen, A. J. Roy, I. Matanovic, K. Artyushkova, P. Atanassov “CuCo2O4 ORR/OER Bi-Functional Catalyst: Influence of Synthetic Approach on Performance”, J. of The Electrochem. Soc. 162 (4) (2015) F449-F454.[5] N. I. Andersen, A. Serov, P. Atanassov “Metal Oxides/CNT Nano-Composite Catalysts for Oxygen Reduction/Oxygen Evolution in Alkaline Media”, Appl. Catal. B: Environmental 163 (2015) 623-627.

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.

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.

Investigating the Effects of Anode Catalyst Conductivity and Loading on Performance for Anion Exchange Membrane Water Electrolysis

Water electrolysis (WE) is a promising route for carbon-free production of green hydrogen, which is increasingly considered to be an important energy carrier. Among the low-temperature WE technologies, anion exchange membrane water electrolysis (AEMWE) has the potential advantages of high operating current densities and low cost due to the use of zero-gap assembly and platinum group metal (PGM)-free components, respectively [1]. However, significant challenges remain for this technology, including low efficiency, insufficient durability, and poor performance in pure water operation. Recent efforts have yielded improvements in the individual components, particularly the oxygen evolution reaction (OER) catalysts and anion exchange polymers; however, to further improve the performance of the AEMWE, it is also necessary to understand and improve the integration of these materials into the membrane electrode assembly (MEA). Within the MEA, the anode and cathode catalyst layers (CLs) incorporate catalyst and ion-conducting and/or binding polymers, which are deposited onto either the membrane or the porous transport layer. The material properties of each component, as well as the loadings of the catalyst and ionomer and methods of dispersion and deposition, have significant impact on the catalyst layer morphology and uniformity [2]. These CL properties affect the transport of electrons, hydroxide ions, and evolved gases, ultimately controlling the accessibility and utilization of catalyst active sites, with significant consequences for both the performance and durability of AEMWEs.Most of the effort in catalyst development for AEMWE has focused on the OER, aiming to improve sluggish kinetics and improve stability at the highly oxidizing operating potentials. A number of PGM-free catalysts have shown promising activity in rotating disk electrode (RDE) tests, particularly oxides with varying ratios of Ni, Fe, and Co. The activity improvements with tuning the composition are often attributed to the increase in surface area and shift in redox potentials for the in-situ conversion to higher valence sites with higher intrinsic OER activity. However, these activity trends do not always translate into device-level performance because of differences in the operating environment, e.g. hydroxide concentration, reaction rates, etc. In the AEMWE, where typically an order of magnitude higher loadings and current densities are required, the challenges of conductivity and mass transport to allow for full site access are exacerbated. Electrochemical impedance spectroscopy (EIS) is a valuable tool for understanding the origin of voltage losses in the AEMWE, typically broken down into ohmic, kinetic, and mass transport losses. Recently, a transmission line model has been applied to non-Faradaic EIS to isolate another contribution to voltage loss: the catalyst layer resistance (CLR), which corresponds to in-plane and through-plane electronic and ionic resistances in the catalyst layer [3]. Using this approach, the roles of intrinsic catalyst activity and catalyst layer properties can be decoupled, helping to guide materials integration efforts.In this study, we investigate two types of OER catalyst materials, NiFe- and Co-based oxides. Within each composition, the catalysts have similar active sites but vary in electronic conductivity, crystal structure, and surface area. By comparing the AEMWE performance at different catalyst loadings, we are able to determine the role of catalyst properties in determining active site utilization and electronic resistance within the catalyst layer. Figure 1 shows the trends in performance and ohmic and catalyst layer resistance as a function of loading for high-conductivity (Co@Co3O4) and low-conductivity (Co3O4) catalysts. Despite nominally having the same surface chemistry and similar particle morphology, they display both different performance and different trends with loading. Using in-situ EIS diagnostics and ex-situ characterization of the catalyst layer morphology, the relationship between catalyst utilization, CLR, and the physical properties of the catalyst and catalyst layer is elucidated. This work provides insight into the catalyst properties that most strongly affect AEMWE performance, as well as how materials integration strategies must be tailored based on these properties.

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.

Photo-Thermal Dry Reforming of Methane with PGM-Free and PGM-Based Catalysts: A Review.

Dry reforming of methane (DRM) is considered one of the most promising technologies for efficient greenhouse gas management thanks to the fact that through this reaction, it is possible to reduce CO2 and CH4 to obtain syngas, a mixture of H2 and CO, with a suitable ratio for the Fischer-Tropsch production of long-chain hydrocarbons. Two other main processes can yield H2 from CH4, i.e., Steam Reforming of Methane (SRM) and Partial Oxidation of Methane (POM), even though, not having CO2 as a reagent, they are considered less green. Recently, scientists' challenge is to overcome the many drawbacks of DRM reactions, i.e., the use of precious metal-based catalysts, the high temperatures of the process, metal particle sintering and carbon deposition on the catalysts' surfaces. To overcome these issues, one proposed solution is to implement photo-thermal dry reforming of methane in which irradiation with light is used in combination with heating to improve the efficiency of the process. In this paper, we review the work of several groups aiming to investigate the pivotal promoting role of light radiation in DRM. Focus is also placed on the catalysts' design and the progress needed for bringing DRM to an industrial scale.

Rational design of membrane electrode assembly for anion exchange membrane water electrolysis systems

Anion exchange membrane water electrolysis (AEMWE) is a promising and potentially low-cost technology for producing green hydrogen, but a novel manufacturing technique with rational design of the electrodes is essential to improve the performance and stability. In this work, we investigate the effect of electrode structure on activity and the stability of AEMWEs by fabricating membrane electrode assemblies (MEAs). For the first time, the decal transfer method with platinum-group-metal-free (PGM-free) catalyst was successfully used in AEMWEs. With this method, deposition of a compact catalyst layer (CL) on the membrane was achieved without damaging neither the CL nor the membrane. The MEAs were designed for AEMWE using 1 M KOH as the electrolyte and the ionomer content was optimized for both cathode and anode. In the anode, a low ionomer loading improved activity and ionic conductivity, however, a higher ionomer content was beneficial for the cathode. Furthermore, the type of ionomer on the anode side has shown to be the major reason of loss of performance over time. An ionomer with low (1.4–1.7 meq g−1) Ion Exchange Capacity (IEC) and Nafion™ ionomer greatly improved the stability.

Enhancing Electrode Efficiency in Proton Exchange Membrane Fuel Cells with PGM-Free Catalysts: A Mini Review

Enhancing Electrode Efficiency in Proton Exchange Membrane Fuel Cells with PGM-Free Catalysts: A Mini Review

The Surface/Interface Modulation of Platinum Group Metal (PGM)-Free Catalysts for VOCs and CO Catalytic Oxidation.

Effective management of volatile organic compounds (VOCs) and carbon monoxide (CO) is critical to human health and the ecological environment. Catalytic oxidation is one of the most promising technologies for achieving efficient VOCs and CO emission control. Platinum group metal (PGM)-free catalysts are recently receiving sustainable attention in catalyzing VOCs and CO removal due to their low cost, superior catalytic activity, and excellent stability, but PGM-free catalysts face challenges in low-temperature catalytic efficiency. In this mini-review, starting with discussing the catalytic mechanism of VOCs and CO oxidation, we summarize the surface/interface modulation strategies of PGM-free catalysts to promote oxygen and VOCs/CO molecule activation for enhanced low-temperature oxidation activity, including oxygen vacancy engineering, heteroatom doping, surface acidity modification, and active interface construction. We highlight the currently remaining challenges and prospects of advanced PGM-free catalyst development for highly efficient VOCs and CO emission control in practical applications.

Understanding the Effects of Anode Catalyst Conductivity and Loading on Catalyst Layer Utilization and Performance for Anion Exchange Membrane Water Electrolysis.

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

Nanostructure Engineering of Cathode Layers in Proton Exchange Membrane Fuel Cells: From Catalysts to Membrane Electrode Assembly.

The membrane electrode assembly (MEA) is the core component of proton exchange membrane fuel cells (PEMFCs), which is the place where the reaction occurrence, the multiphase material transfer and the energy conversion, and the development of MEA with high activity and long stability are crucial for the practical application of PEMFCs. Currently, efforts are devoted to developing the regulation of MEA nanostructure engineering, which is believed to have advantages in improving catalyst utilization, maximizing three-phase boundaries, enhancing mass transport, and improving operational stability. This work reviews recent research progress on platinum group metal (PGM) and PGM-free catalysts with multidimensional nanostructures, catalyst layers (CLs), and nano-MEAs for PEMFCs, emphasizing the importance of structure-function relationships, aiming to guide the further development of the performance for PEMFCs. Then the design strategy of the MEA interface is summarized systematically. In addition, the application of in situ and operational characterization techniques to adequately identify current density distributions, hot spots, and water management visualization of MEAs is also discussed. Finally, the limitations of nanostructured MEA research are discussed and future promising research directions are proposed. This paper aims to provide valuable insights into the fundamental science and technical engineering of efficient MEA interfaces for PEMFCs.

Nickel-molybdenum hydrogen evolution and oxidation reaction electrocatalyst obtained by electrospinning

To accelerate the deployment of anion exchange membrane (AEM) technologies, and break free from our reliance on expensive and rare platinum group metals (PGM), this study explores a novel family of non-PGM electrocatalysts capable of both hydrogen evolution and oxidation. By thermal reduction, nanoparticles of nickel-molybdenum, 9:1 atomic ratio, were grown on fibrous carbon supports obtained by electrospinning and doped with carbon nanotubes (CNT). Complimentary characterization techniques confirm the integration of the CNT with the carbon fibers and a high loading of active alloy material on the support. In particular, XANES indicates a strong interaction between NiMo and the CNT. When supported on undoped carbon fibers, NiMo shows superior activity towards the hydrogen oxidation reaction (HOR) compared to the doped carbon supports. However, heat treating the catalysts in 10 % NH3/ 90 % N2 upsets this trend, while also improving the HOR performance of all the catalysts based on several performance indicators. In the case of NiMo on carbon fibers containing 7 wt% CNT, denoted as NiMo/CF(7 %), the ammonia heat treatment produced a five-fold increase in the kinetic current. The catalysts NiMo/CF(7 %)-NH3 and NiMo/CF(24 %)-NH3, also capable of hydrogen evolution, match other state-of-the-art PGM-free HOR catalysts for their remarkable specific activity.

Real-time investigation of reactive oxygen species and radicals evolved from operating Fe-N-C electrocatalysts during the ORR: potential dependence, impact on degradation, and structural comparisons.

Improving the stability of platinum-group-metal-free (PGM-free) catalysts is a critical roadblock to the development of economically feasible energy storage and conversion technologies. Fe-N-C catalysts, the most promising class of PGM-free catalysts, suffer from rapid degradation. The generation of reactive oxygen species (ROS) during the oxygen reduction reaction (ORR) has been proposed as a central cause of this loss of activity. However, there is insufficient understanding of the generation and dynamics of ROS under catalytic conditions due to the difficulty of detecting and quantifying short-lived ROS such as the hydroxyl radical, OH˙. To accomplish this, we use operando scanning electrochemical microscopy (SECM) to probe the production of radicals by a commercial pyrolyzed Fe-N-C catalyst in real-time using a redox-active spin trap methodology. SECM showed the monotonic production of OH˙ which followed the ORR activity. Our results were thoroughly backed using electron spin resonance confirmation to show that the hydroxyl radical is the dominant radical species produced. Furthermore, OH˙ and H2O2 production followed distinct trends. ROS studied as a function of catalyst degradation also showed a decreased production, suggesting its relation to the catalytic activity of the sample. The structural origins of ROS production were also probed using model systems such as iron phthalocyanine (FePc) and Fe3O4 nanoparticles, both of which showed significant generation of OH˙ during the ORR. These results provide a comprehensive insight into the critical, yet under-studied, aspects of the production and effects of ROS on electrocatalytic systems and open the door for further mechanistic and kinetic investigation using SECM.

Tuning the performance of Fe-porphyrin aerogel-based PGM-free oxygen reduction reaction catalysts in proton exchange membrane fuel cells.

Fe-N-C catalysts are currently the leading candidates to replace Pt-based catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells. To maximize their activity, it is necessary to optimize their structure to allow high active site density on one hand, and hierarchical porous structure that will allow good mass transport of reactants and products to and from the active sites on the other hand. Hence, the hierarchical structure of the catalyst plays an important role in the balance between the electrochemical active site density and the mass transport resistance. Aerogels were synthesized in this work to study the interplay between these two parameters. Aerogels are covalent organic frameworks with ultra-low density, high porosity, and large surface area. The relative ease of tuning the composition and pore structure of aerogels make them prominent candidates for catalysis. Herein, we report on a tunable Fe-N-C catalyst based on an Fe porphyrin aerogel, which shows high electrocatalytic oxygen reduction reaction activity with tunable hierarchical pore structure and studied the influence of the porous structure on the overall performance in proton exchange membrane fuel cells.