Platinum Group Metal Catalysts Research Articles

Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes.

Metal-catalyzed highly Markovnikov-type selective hydrofunctionalization of terminal alkynes provides a straightforward and atom-economical route to access 1,1-disubstituted alkenes, which have a wide range of applications in organic synthesis. However, the highly Markovnikov-type selective transformations are challenging due to the electronic and steric effects during the addition process. With the development of metal-catalyzed organic synthesis, different metal catalysts have been developed to solve this challenge, especially for platinum group metal catalysts. In this perspective, we review homogeneous metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes according to the classified element types as well as reaction mechanisms. Future avenues for investigation are also presented to help expand this exciting field.

Surprising Route to a Monoazaporphyrin and Full Characterization of Its Complexes with Five Different 3d Metals.

In the search for mild agents for the oxidative cyclization of tetrapyrromethane to the corresponding corrole, we discovered a route that leads to a monoazaporphyrin with three meso-CF3 groups. Optimization studies that allowed access to appreciable amounts of this new macrocycle paved the way for the preparation of its cobalt, copper, nickel, zinc, and iron complexes. All complexes were fully characterized by various spectroscopic methods and X-ray crystallography. Their photophysical and electrochemical properties were determined and compared to those of analogous porphyrins in order to deduce the effect of the peripheral N atom. Considering the global efforts for designing efficient alternatives to platinum group metal (PGM) catalysts, they were also absorbed onto a porous carbon electrode material and studied as electrocatalysts for the oxygen reduction reaction (ORR). The cobalt complex was found to be operative at a quite positive catalytic onset potential and with good selectivity for the desirable 4-electrons/4-protons pathway.

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.

Atomically dispersed Fe-N-C catalyst with densely exposed Fe-N4 active sites for enhanced oxygen reduction reaction

Atomically dispersed Fe-N-C catalysts are a promising alternative to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR). However, attaining efficient ORR activity and superior stability in Fe-N-C catalysts is still crucial yet challenging. Herein, we report a rational SiO2-mediated two-step pyrolysis strategy for stabilizing densely exposed Fe-N4 active sites on hierarchically porous carbon (HP-Fe-N-C/2). Benefiting from the high density of accessible Fe-N4 sites and the high graphitization degree of carbon support, the obtained HP-Fe-N-C/2 catalyst demonstrates outstanding activity and stability for ORR in both alkaline and acidic media. When used as the cathode catalyst, the assembled Zn-air battery shows a high peak power density of 217 mW cm−2 and ultra-long cycling stability for 1342 h without noticeable degradation. As a PGM-free cathode in proton exchange membrane fuel cells, it delivers a maximum output power density of 0.66 W cm−2.

Dual-axial engineering on atomically dispersed catalysts for ultrastable oxygen reduction in acidic and alkaline solutions

Atomically dispersed catalysts are a promising alternative to platinum group metal catalysts for catalyzing the oxygen reduction reaction (ORR), while limited durability during the electrocatalytic process severely restricts their practical application. Here, we report an atomically dispersed Co-doped carbon-nitrogen bilayer catalyst with unique dual-axial Co-C bonds (denoted as Co/DACN) by a smart phenyl-carbon-induced strategy, realizing highly efficient electrocatalytic ORR in both alkaline and acidic media. The corresponding half-wave potential for ORR is up to 0.85 and 0.77 V (vs. reversible hydrogen electrode (RHE)) in 0.5 M H2SO4 and 0.1 M KOH, respectively, representing the best ORR activity among all non-noble metal catalysts reported to date. Impressively, the Zn-air battery (ZAB) equipped with Co/DACN cathode achieves outstanding durability after 1,688 h operation at 10 mA cm-2 with a high current density (154.2 mA cm-2) and a peak power density (210.1 mW cm-2). Density functional theory calculations reveal that the unique dual-axial cross-linking Co-C bonds of Co/DACN significantly enhance the stability during ORR and also facilitate the 4e- ORR pathway by forming a joint electron pool due to the improved interlayer electron mobility. We believe that axial engineering opens a broad avenue to develop high-performance heterogeneous electrocatalysts for advanced energy conversion and storage.

Self-terminated electrodeposition of Pt group metal: principles, synthetic strategies, and applications

Hydrogen, characterized by its carbon-neutral attributes and high energy density, is gaining momentum as a promising energy source. Platinum group metal (PGM) catalysts have emerged as pivotal components in water electrolysis and fuel cell technologies. However, their constrained availability and high cost impede the advancement of energy conversion systems. To address these challenges, various strategies have been explored within the realm of PGM catalysts. Particularly noteworthy are catalysts that exhibit an overlayer structure, offering exceptional catalyst utilization efficiency, bimetallic synergies, and strain-induced enhancements. Self-terminated electrodeposition (SED) stands out as a technique that enables precise atomic layer electrodeposition within an aqueous electrolyte environment. It allows meticulous control of metal loading quantities and surface coverage while operating at low temperatures and without the need for vacuum conditions. Catalysts with tailored properties achieved through SED exhibit distinct electrochemical reactivity compared to bulk catalysts, showcasing exceptional electrocatalytic activity, particularly in terms of mass and specific activity. This comprehensive review provides insights into the SED phenomenon, elucidates methodologies for fabricating PGM electrocatalysts using SED, and highlights their applications in water electrolysis and fuel cells.

Mixed iridium-nickel oxides supported on antimony-doped tin oxide as highly efficient and stable acidic oxygen evolution catalysts

Proton exchange membrane (PEM) water electrolysis represents a promising technology for green hydrogen production, but its widespread deployment is greatly hindered by the indispensable usage of platinum group metal catalysts, especially iridium (Ir) based materials for the energy-demanding oxygen evolution reaction (OER). Herein, we report a new sequential precipitation approach to the synthesis of mixed Ir-nickel (Ni) oxy-hydroxide supported on antimony-doped tin oxide (ATO) nanoparticles (IrNi y O x /ATO, 20 wt.% (Ir + Ni), y = 0, 1, 2, and 3), aiming to reduce the utilisation of scarce and precious Ir while maintaining its good acidic OER performance. When tested in strongly acidic electrolyte (0.1 M HClO4), the optimised IrNi1O x /ATO shows a mass activity of 1.0 mA µgIr −1 and a large turnover frequency of 123 s−1 at an overpotential of 350 mV, as well as a comparatively small Tafel slope of 50 mV dec−1, better than the IrO x /ATO control, particularly with a markedly reduced Ir loading of only 19.7 µgIr cm−2. Importantly, IrNi1O x /ATO also exhibits substantially better catalytic stability than other reference catalysts, able to continuously catalyse acidic OER at 10 mA cm−2 for 15 h without obvious degradation. Our in-situ synchrotron-based x-ray absorption spectroscopy confirmed that the Ir3+/Ir4+ species are the active sites for the acidic OER. Furthermore, the performance of IrNi1O x /ATO was also preliminarily evaluated in a membrane electrode assembly, which shows better activity and stability than other reference catalysts. The IrNi1O x /ATO reported in this work is a promising alternative to commercial IrO2 based catalysts for PEM electrolysis.

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

Electrochemical Impedance Analysis of Direct Ammonia Fuel Cell Operation and Clamping Effects

In addition to being an essential chemical for fertilizers, ammonia can also be used as a hydrogen carrier or directly as a fuel. Compared to hydrogen, the main advantages are that ammonia can be liquefied in mild conditions and the technology for ammonia shipping and pipelines already exists. Compared to carbon-based synthetic fuels, no carbon dioxide is produced by the oxidation, only water and nitrogen. The theoretical voltage of a direct ammonia fuel cell (DAFC) is 1.17 V, nearly equal to the 1.23 V of hydrogen fuel cells (HFC) but a central drawback is the slow kinetics of the ammonia oxidation reaction (AOR). The AOR is even slower than the oxygen reduction reaction (ORR) at the cathode, which in turn is the main voltage loss component in HFCs. Consequently, due to the AOR kinetics, DAFCs typically require significantly higher platinum group metal (PGM) catalyst loadings than HFCs for the same power output.To improve the performance of our aqueous-feed DAFC, especially using non-PGM catalysts, we carried out comparisons of catalysts and anion exchange membranes (AEM), and supplemented the comparison of power, voltage, and current parameters with electrochemical impedance spectroscopy (EIS). Additionally, we also compared measurements in two different test cells to try to identify the cause of measurement repeatability issues that we had when using one of the cells.Initially, we assumed that the reproducibility of catalyst deposition (or lack of it) and AEM or catalyst degradation in the ammonia-containing electrolyte had caused our repeatability problems. However, following a preliminary analysis of measurement statistics, we consider problems with the cell itself a more likely cause. The maximum power of the cell depended almost inversely on the series resistance (R S, i.e., the high-frequency resistance/real-axis intercept of the EIS spectrum), and the current at given voltage varied more than the differences in the resistance alone could explain when most measurement parameters were not varied (e.g., ammonia and hydroxide concentrations, liquid and gas flow rates). The general inverse dependence on R S remained also when measuring the same electrodes in different cells, and the performance did not necessarily degrade over time so, clearly, degradation and differences between electrodes could not explain everything. When testing different screw torques and gaskets in our AEM electrolyser, we had noticed that the voltage differences were often high compared to the R S differences, and it is known that clamping torque can affect the operation of fuel cells and electrolysers significantly. Nevertheless, more detailed analysis is ongoing to determine possible other contributing factors, to optimize the use of the cell(s) in measurements, and to guide future work.This research was done under the TELEGRAM project. This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under grant agreement No 101006941. The project started on the 1st of November 2020 with a duration of 42 months. The authors acknowledge support from the Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A). Figure 1

(Invited) Strategies to Improve the Stability of Fe/N/C Catalysts in PEM Fuel Cells

Proton-exchange membrane (PEM) fuel cells hold promising applications in transportation and stationary, however, their widespread commercialization is greatly hindered by the high cost. Platinum (Pt) represents one of the largest cost components of a fuel cell, therefore, my research interests have been focusing on strategies that will increase the activity and utilization of platinum group metal (PGM) catalysts, and improve the performance and stability, as well as prolong the lifetime of the PGM-free catalysts. In this talk, I will first briefly present our work on developing low-PGM catalysts (various unique nanostructured Pt nanowires, nanotubes, and single atoms) to significantly increase the activity and stability of the Pt-based catalysts for ORR in fuel cells. Then, I will mainly focus on our work on PGM-free catalysts. Based on the major breakthroughs on Fe/N/C catalyst achieved by the Dodelet team at INRS (with the MEA activity and performance approaching that of Pt catalyst) [1,2], in collaboration with Ballard Power Systems and Toyota, we have made important progress on improving the stability of PGM-free catalyst [3-17]. Specifically in the following aspects: (i) identifying the active sites, (ii) understanding the fuel cell degradation mechanisms experimentally and theoretically, (iii) developing approaches to improving the stability of the Fe/N/C catalyst, such as pore size control, and fluorination, and (iv) catalyst layer and electrode optimization, such as catalyst hydrophobicity adjustment, Fe/N/C and ultra-low loading Pt/C hybrid catalyst.

Unveiling the Impact of Carbon Corrosion on the Degradation of Fe-N-C Catalyst Layers in Alkaline Media

One of the most promising candidates to replace platinum group metal catalysts for oxygen reduction reaction (ORR) in fuel cells (FCs) is the sub-class of iron-nitrogen-carbon (Fe-N-C) catalysts. However, Fe-N-C materials considerably suffer from varied degradation mechanisms. [1-2] Compared to FC operating conditions, start/stop events where the cathodes experience anodic potential may be more damaging due to the carbon corrosion phenomenon. [3] The correlation between carbon corrosion and Fe dissolution rates has been reported in aqueous model systems in acidic media. [3] However, different carbon corrosion mechanisms are reported for acidic and alkaline media. [4] While anion-exchange membrane FCs (AEMFCs) have gained increased attention, studies on the effects of carbon corrosion on realistic AEMFC Fe-N-C catalyst layers are still missing.In this work, using a gas diffusion electrode (GDE) half-cell coupled with inductively coupled plasma mass spectrometry (ICP-MS), [5] we observed that the rate of Fe loss significantly accelerates with rising potential (E > 1.0 VRHE), commonly experienced during the start/stop events. Increased temperature intensifies the rate of Fe leaching during carbon corrosion (see Figure 1), while the gas atmosphere (Ar or O2) shows a negligible influence. On the contrary, the subsequent Fe deposition and the drop of ORR activity depend on the presence of O2 and the varied temperature. Combining in situ and post-mortem analyses, we report how carbon corrosion in alkaline media degrades Fe-N-C catalyst layers in various atmospheres and at different temperatures. These insights can contribute to rational designs of AEMFCs' start/stop protocol and more robust Fe-N-C materials. Figure 1. Fe-N-C demetallation during anodic potential holds (1.0 - 1.5 VRHE) in O2-saturated alkaline (0.1 M NaOH) environment at 22 ± 2 and 62 ± 2 ℃. (A) Potential profile. (B) The Fe dissolution rate normalized to catalyst loading.

Investigating Oxygen Reduction Performance of Fe-N-C Catalysts for Proton Exchange Membrane Fuel Cells

Non-precious Fe-N-C catalysts have reached high activity levels for acidic oxygen reduction reaction (ORR) in rotating ring disk electrode (RRDE) setup approaching to that of the platinum group metals (PGM) when compared at high catalyst loadings of 0.6-1.0 mgFe-N-C cm-2. The improvements in the ORR activity of Fe-N-Cs can be attributed to new synthetic routes developed in recent years that have led to increased Fe-N4 active site density and utilization and/or better intrinsic activity/turnover frequency (TOF) of those sites.1-3 Despite impressive activities in half-cells, Fe-N-Cs still require quite high loadings in the range of 4.0-5.0 mg cm-2 in proton exchange membrane fuel cells (PEMFCs), which makes the Fe-N-C cathodes 4-5x thicker than Pt-based cathodes (~100 µm for Fe-N-C layer vs ~10-20 µm for Pt layer). Thick Fe-N-C cathodes suffer from quick performance drop in medium to high current density region due to mass-transport related losses. Reducing the Fe-N-C loading levels in RRDE and in PEMFC without comprising the ORR activity would need further improvements in active site density, site utilization and TOF. In this talk, I will present our work on how morphological tailoring of carbon framework in Fe-N-Cs affects the utilization of active sites and water adsorption-desorption behaviour with the aim of improving the ORR performance and mass-transport properties. Active-site imprinted carbon frameworks are used to prepare Fe-N-Cs that consist of purely Fe-N4 sites without any other iron species.1,4,5 Besides activity, performance stability against extensive voltage cycling in RRDE and long-term durability in PEMFC single-cells is another important criterion that Fe-N-Cs need to fulfil for becoming a viable substitute to PGM catalysts. Very recently, encouraging results on the improvement of Fe-N-C durability in PEMFC tests have been reported using radical scavengers and by coating a thin carbon layer on the surface of Fe-N-C.6,7 However, significantly more efforts are required in that direction for reaching the durability targets of thousands of hours. Second part of the talk will discuss the performance stability of Fe-N-Cs.

Improving Oxygen Reduction Reaction Activity through Defect Engineering of Atomically Dispersed Iron Electrocatalysts for Proton Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen-coordinated iron catalysts(Fe-N-Cs) have the potential as an alternative to platinum-group metal catalysts for the oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-N-Cs remain unsatisfactory. To address this issue, a defect engineering strategy has been developed to prepare high-performance PEMFC MEAs using atomically dispersed Fe-N-C catalysts. This strategy involves the use of a zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation to create atomically dispersed iron sites with a controlled number of defects. By adjusting the extent of defect formation in the carbon plane using CO2 activation, it is anticipated that changes in the oxidation state and spin state of the Fe center will modify the electronic structure of the Fe-N4 active sites. The Fe-N-C species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2SO4. Fine-tuning the number of defects can be optimized the ORR activity by adjusting the contribution of the Fe d-orbitals to the reaction intermediate binding energies. The resulting MEA based on the defect-engineered Fe-NC catalyst exhibits remarkable peak power densities in both H2/O2 and H2/air fuel cells, making it one of the most active atomically dispersed catalyst materials at the MEA level.

Recent advances of carbon nanotubes synthesis by the electric arc technique using atomized platinum-group metal catalysts

The traditional application of platinum-group metals (PGMs: Ru, Rh, Pd, Os, Ir, Pt) as powder catalysts in the electric arc synthesis of carbon nanotubes (CNTs) is limited due to the inclusion of metal particles dispersed under electric arc conditions and the spoiling regularity of synthesized carbon nanotubes. In the current work, the authors applied the atomized platinum metal catalysts formed from the ionic compounds of PGMs under electric arc conditions, to avoid the occurrence of multiatomic metal particles in the CNT mass. Such an approach permits a synthesis of defectless CNTs, using PGMs as catalysts. All the six PGMs are good catalysts for CNT synthesis and CNTs do not form from graphite without a catalyst under electric arc conditions. The aim of this work is to comparatively characterize the six PGMs as atomized catalysts in the formation of carbon nanotubes, to enable a wider use of these highly catalytic metals.

The effect of additional nitrogen source on iron phthalocyanine-based nanocarbon catalysts for oxygen reduction reaction in acidic media

The focus in the development of catalysts doped with transition metals aims to replace platinum group metal catalysts in fuel cells. However, these non-precious metal catalysts exhibit limited performance in acidic environment for the oxygen reduction reaction (ORR) due to issues such as metal agglomeration and the subsequent loss of active sites. Herein, we synthesised catalysts doped with iron and nitrogen on a composite material consisting of carbide-derived carbon (CDC) and graphene (G), employing an additional nitrogen source dicyandiamide (DCDA), denoted as FeN-CDC/G/DCDA. Our physico-chemical analysis unveiled that the inclusion of DCDA was effective in mitigating metal agglomeration during the synthesis process and increasing the presence of Fe-Nx sites in the catalysts. Notably, the FeN-CDC/G/DCDA catalyst exhibited enhanced ORR activity in acid media with half-wave potential (E1/2) of 0.76 V, surpassing the performance of the FeN-CDC/G catalyst, which had an E1/2 value of 0.70 V. Furthermore, the rotating ring-disk electrode results indicated a reduced formation of hydrogen peroxide when employing the FeN-CDC/G/DCDA catalyst. The findings from this study represent a significant step towards the development of efficient catalysts for fuel cells, underscoring the pivotal role of additional nitrogen doping and its positive impact on the ORR performance.

Activating doped graphene surface by cobalt-rich sulfide encapsulation toward oxygen reduction electrocatalysis

Similar to proton exchange membrane fuel cell, anion-exchange membrane fuel cell is also a significant energy conversion device for achieving the utilization of clean hydrogen energy. However, the cathodic alkaline oxygen reduction reaction (ORR) is kinetically not favored and usually requires platinum-group metal (PGM) catalysts such as Pt/C to reduce the overpotential. The major challenge in using PGM-free catalysts for ORR is their low efficiency and poor stability, which urgently demands new concepts and strategies to address this issue. Herein, we controllably manufactured a N, S-co doped graphene encapsulating uniform cobalt-rich sulfides (Co8FeS8@NSG) by a universal synthesis strategy. After encapsulation, electron transfer from the encapsulated cobalt-rich sulfides to the doped graphene was greatly promoted, which effectively optimizes the electronic structure of the doped graphene, thereby enhancing the ORR activity of the doped graphene surface. Consequently, the Co8FeS8@NSG exhibits enhanced ORR activity with a higher half-wave potential of 0.868 V (versus reversible hydrogen electrode, vs. RHE) when compared with pure NSG (0.765 V vs. RHE). Density functional theory calculations further confirm that the construction of interface for NSG encapsulating cobalt-rich sulfides could conspicuously elevate the ORR activity through slightly positively-charged C active site and thus simultaneously enhancing electronic conductivity.

Insight into Defect Engineering of Atomically Dispersed Iron Electrocatalysts for High-Performance Proton Exchange Membrane Fuel Cell.

Atomically dispersed and nitrogen coordinated iron catalysts (Fe-NCs) demonstrate potential as alternatives to platinum-group metal (PGM) catalysts in oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-NCs remain unsatisfactory. Herein, improved MEA performance is achieved by tuning the local environment of the Fe-NC catalysts through defect engineering. Zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation is employed to construct atomically dispersed iron sites with a controlled defect number. The Fe-NC species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83V in 0.5M H2 SO4 . Variation in the number of defects allows for fine-tuning of the reaction intermediate binding energies by changing the contribution of the Fe d-orbitals, thereby optimizing the ORR activity. The MEA based on a defect-engineered Fe-NC catalyst is found to exhibit a remarkable peak power density of 1.1W cm-2 in an H2 /O2 fuel cell, and 0.67W cm-2 in an H2 /air fuel cell, rendering it one of the most active atomically dispersed catalyst materials at the MEA level. This article is protected by copyright. All rights reserved.

(Invited) MEA Technologies for PEM Electrolyzers: Overview and Perspectives

Hydrogen has gained increasing attention recently as an alternative energy carrier leading towards a sustainable hydrogen economy [1]. Green hydrogen has less pollution (water as the only byproduct) and higher energy density than its counterpart fossil fuels. Additionally, it has much higher refueling speed than lithium-ion batteries, which makes it’s more suitable for mobility applications.Proton exchange membrane (PEM) water electrolysis is one of the most popular approaches for green hydrogen generation due to its high efficiency and high differential pressure operation capability [2]. The membrane electrode assembly (MEA) is the core of a PEM electrolyzer in which water molecules are spilt into hydrogen and oxygen molecules. In this overview, the challenges and paths regarding the MEA materials, operation/diagnosis/maintenance, and final recycling will be discussed.An electrolyzer MEA contains an anode and cathode made of platinum group metal (PGM) catalysts, a perfluorosulfonic acid (PFSA) membrane, and other gasketing materials. PGM catalysts are a major source of the cost of the PEM electrolyzer stack. Ir black based catalysts have been used for decades in the PEM electrolyzer industry. Due to the high cost and scarcity of Ir, efforts have been dedicated to reducing the catalyst loading and developing highly active catalysts [3]. The membrane is another major component of a PEM electrolyzer. 5 mil and 7 mil thick membranes have been used in electrolyzer industries. However, the ionic resistive losses of these relatively thick PEMs significantly reduce the efficiency of the MEA. Hence, reducing membrane thickness has been a central topic for PEM electrolyzer membrane development. With the progressive thinning of the PEM, the hydrogen gas crossover will become significant, which will compromise the safe operation of a PEM electrolyzer system. An effective gas recombination layer in the PEM is therefore urgently needed to mitigate the resulting higher hydrogen crossover. Additionally, the mechanical stresses imposed due to operation at high differential pressure operation (up to 40 bar) increases the attractiveness of a reinforcement layer.The unique properties of the membrane and catalysts in an MEA also require ultra-pure feed water. However, this cannot always be guaranteed. Metal ions will leach out from system plumbing and the DI system can’t always effectively remove all ions from tap water. The contamination will also poison the catalysts in an MEA. It is therefore critical to establish a universal method to efficiently identify the issue and recover the MEA performance.It is expected that the MEA can last 60k to 90k hours [4]. Though functionality may be compromised at the this point, the value in the PGM catalysts, and to a lesser extent membrane, and other carbon-based materials [5] remains. MEA recycling thus needs to be considered at the very early stage of the large-scale PEM electrolyzer application. Tradition calcination approaches for catalyst refinery is not optimized for PEM MEAs due to the presence of the PFSA membrane. The burning of MEAs will not only waste costly PFSA material but also poses a threat to the environment due to carbon and fluoride emissions. Current research efforts have focused on catalyst recycling and reuse/repurpose of PFSA based materials.[1] Oliveira, A. M., Beswick, R. R. & Yan, Y. A green hydrogen economy for a renewable energy society. Current Opinion in Chemical Engineering 33, 100701 (2021).[2] Ayers, K. et al. Perspectives on low-temperature electrolysis and potential for renewable hydrogen at scale. Annual review of chemical and biomolecular engineering 10 (2019).[3] Mittelsteadt, C. (Invited) Ir Strangelove: Or How I Learned to Stop Worrying and Embrace the PEM. ECS Meeting s MA2022-01, 1335-1335 (2022).[4] Schmidt, O., Future cost and performance of water electrolysis: an expert elicitation study. International journal of hydrogen energy 42 (2017), 30470-30492.[5] Sverdrup, H. U. & Ragnarsdottir, K. V. A system dynamics model for platinum group metal supply, market price, depletion of extractable amounts, ore grade, recycling and stocks-in-use. Resources, Conservation and Recycling 114, 130-152 (2016)

Metallic Glass Nanofoam Anode Catalysts for Anion-Exchange Membrane Water Electrolyzers

Alkaline anion-exchange membrane water electrolyzers (AEMWEs) for hydrogen production are now receiving intensive attention due to their feasibility to use sustainable, low-cost platinum group metal (PGM-free) catalysts. Although a variety of highly efficient PGM-free catalysts for the oxygen evolution reaction (OER) have been explored, few of them demonstrated satisfied performance in real AEMWEs due to the insufficient electrical conductivity and unfavorable interfaces with ionomers in 3D porous electrodes. Herein, we report a series of highly porous ternary NiFeM (M: Cu, Co, and Mn) metallic glassy catalysts featured with nanofoam network morphologies, which are composed of amorphous OER active metal oxide shells and highly electrically conductive metallic glass alloy cores. Due to these unique properties, these NiFeM nanofoam catalysts demonstrated promising OER activities and stabilities in the half-cell with aqueous alkaline electrolytes, especially at high potentials. We also examined their magnetic properties and found no direct correlation with measured OER activity. These ternary NiFeM catalysts are further integrated with unique ionomers and AEMs to fabricate AEMWEs, showing superior performance to binary NiFe and commercial IrO2 catalysts when utilizing diluted KOH electrolytes. A different trend was identified when directly using desirable but challenging pure water, and the NiFeCu catalyst performed the best comparable to IrO2 especially at high current densities. Although deep understanding on limiting factor of pure water AEMWEs is still required, these NiFeM catalysts with favorable catalytic and morphological properties representing a new class of highly efficient PGM-free anode catalysts for viable AEMWEs toward clean hydrogen generation.

Regulating Catalytic Properties and Thermal Stability of Pt and PtCo Intermetallic Fuel-Cell Catalysts via Strong Coupling Effects between Single-Metal Site-Rich Carbon and Pt.

Developing low platinum-group-metal (PGM) catalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles (HDVs) remains a great challenge due to the highly demanded power density and long-term durability. This work explores the possible synergistic effect between single Mn site-rich carbon (MnSA-NC) and Pt nanoparticles, aiming to improve intrinsic activity and stability of PGM catalysts. Density functional theory (DFT) calculations predicted a strong coupling effect between Pt and MnN4 sites in the carbon support, strengthening their interactions to immobilize Pt nanoparticles during the ORR. The adjacent MnN4 sites weaken oxygen adsorption at Pt to enhance intrinsic activity. Well-dispersed Pt (2.1 nm) and ordered L12-Pt3Co nanoparticles (3.3 nm) were retained on the MnSA-NC support after indispensable high-temperature annealing up to 800 °C, suggesting enhanced thermal stability. Both PGM catalysts were thoroughly studied in membrane electrode assemblies (MEAs), showing compelling performance and durability. The Pt@MnSA-NC catalyst achieved a mass activity (MA) of 0.63 A mgPt-1 at 0.9 ViR-free and maintained 78% of its initial performance after a 30,000-cycle accelerated stress test (AST). The L12-Pt3Co@MnSA-NC catalyst accomplished a much higher MA of 0.91 A mgPt-1 and a current density of 1.63 A cm-2 at 0.7 V under traditional light-duty vehicle (LDV) H2-air conditions (150 kPaabs and 0.10 mgPt cm-2). Furthermore, the same catalyst in an HDV MEA (250 kPaabs and 0.20 mgPt cm-2) delivered 1.75 A cm-2 at 0.7 V, only losing 18% performance after 90,000 cycles of the AST, demonstrating great potential to meet the DOE targets.