Oxygen Reduction Reaction In Acidic Media Research Articles

Enhancing Intrinsic Electrocatalytic Activity of Pt/C Nanoparticles for Ox-ygen Reduction Reaction in Acidic Media by Microwave-Assisted Synthesis

Abstract: This study is focused on the enhancement of the intrinsic electrocatalytic activity of Pt nanoparticles supported on C (Pt/C NPs) towards Oxygen Reduction Reaction (ORR) in acidic media. The goal was to investigate the effect of microwave-assisted synthesis on the electrocatalytic perfor-mance of Pt/C NPs towards ORR. Thus, Pt/C NPs were synthesized using a microwave-assisted method and by a conventional heating method; structural and morphological characteristics were an-alyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Electrochemical studies were performed using the rotating disk electrode technique to evaluate the ORR performance. Microwave-assisted synthesis produced Pt/C NPs with a smaller particle size (6.3 ± 0.2 nm) than con-ventionally synthesized nanoparticles (8.6 ± 0.3 nm). Electrochemical analysis showed that the micro-wave-synthesized Pt/C NPs exhibited higher mass activity (4.6 ± 0.8 mA･g-1Pt) for ORR compared to conventionally synthesized nanoparticles (1.9 ± 0.4 mA･mA･g-1Pt). These results demonstrate that microwave-assisted synthesis enhances the intrinsic electrocatalytic activity of Pt/C NPs for ORR in acidic media. These findings have important implications for the development of efficient electrocat-alysts for fuel cell applications. Background: The synthesis and characterization of platinum nanoparticles on C are crucial for ad-vancing electrocatalysis, particularly in the context of potential applications in fuel cells. This study builds on previous research, focusing on two distinct synthesis methods to enhance our understanding of their impact on nanoparticle properties and electrocatalytic performance. Objective: To investigate the synthesis efficiency, structural characteristics, and electrocatalytic activ-ities of platinum nanoparticles on C using microwave-assisted heating and conventional synthesis re-actor heating. The objective is to discern any significant differences in particle size, structure, and electrocatalytic performance between the two synthesis methods. method: The synthesis involved a comparative analysis of platinum nanoparticles using microwave-assisted and conventional heating methods. Chemical composition analysis verified the synthesis efficiency, and structural and morphological characterizations were performed using X-ray Diffraction and Transmission Electron Microscopy. Electrochemical studies employed the rotating disk electrode technique, with activation and evaluation conducted through cyclic voltammetry, and the oxygen reduction reaction studied via linear sweep voltammetry in an acidic media (0.5 mol⋅L-1 H2SO4). Methods: The synthesis involved a comparative analysis of platinum nanoparticles using microwave-assisted and conventional heating methods. Chemical composition analysis verified the synthesis ef-ficiency, and structural and morphological characterizations were performed using X-ray Diffraction and Transmission Electron Microscopy. Electrochemical studies employed the rotating disk electrode technique, with activation and evaluation conducted through cyclic voltammetry, and the oxygen re-duction reaction studied via linear sweep voltammetry in an acidic media (0.5 mol⋅L-1 H2SO4). Results: Well-supported platinum nanoparticles with a face-centered cubic structure were obtained on C using both synthesis methods. However, microwave-synthesized particles (6.3 ± 0.2 nm) exhibited a smaller size compared to conventionally synthesized particles (8.6 ± 0.3 nm). Electrochemical as-sessment revealed superior mass activity for microwave-synthesized material (4.6 ± 0.8 mA･g-1Pt), outperforming commercial Pt nanoparticles (3.0 ± 0.3 mA･g-1Pt) and conventionally synthesized ma-terial (1.9 ± 0.4 mA･mA･g-1Pt). Conclusion: This study concludes that microwave-assisted synthesis yields platinum nanoparticles on C with enhanced electrocatalytic performance, as evidenced by the smaller particle size and superior mass activity compared to conventionally synthesized material and commercial Pt nanoparticles. These findings highlight the potential of microwave-synthesized Pt nanoparticles for applications in fuel cells.

A New Catalyst System for the Oxygen Reduction Reaction (ORR) at Fuel Cell Cathodes: P-Block Precious Group Metal-Free Tin and Nitrogen-Doped Carbon (SnNC) Catalyst

The sluggish kinetics of the Oxygen Reduction Reaction (ORR) and the consequently large amount of platinum catalyst required for cathode material in Proton Exchange Membrane Fuel Cell (PEMFC) is today’s primary hurdle for commercializing fuel cell technology in automotive application.1 A class of carbon-doped with metal and nitrogen electrocatalysts (labelled MNC) is promising substitute for platinum-group metals (PGMs) in oxygen reduction reaction (ORR) in PEMFC.2 Particular attention was placed on a specific family of MNC catalysts derived from 3d transition metal precursors with FeNC and CoNC being the most promising for PEMFC applications requiring high power density.3-5 In the present contribution, we report on a new MNC catalyst system, different by its chemical nature from all others catalysts previously reported as ORR-active in acidic medium. The highlights of this work are the successful design and synthesis of SnNC as a non-noble metal catalyst for the first time, and the finding that SnNC hosts Sn-based active sites that can achieve the same turnover frequency as Fe-based active sites in FeNC, and much higher turnover frequency than Co-based active sites in CoNC. FeNC and CoNC materials have, until now, been the state-of-art PGM-free catalysts for ORR in acidic medium. The present SnNC material shows also high and similar selectivity for the four-electron ORR pathway as FeNC, much higher than CoNC materials. The prepared SnNC catalysts meet and exceed the FeNC catalysts in terms of hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a Fenton-inactive character of Sn. Among other techniques, density functional theory and 119Sn Mössbauer spectroscopy were used in combination to investigate SnNC, providing for the first time insights on the structure of its active sites, their rate-determining step for ORR and selectivity for the four-electron reduction pathway. References Holton, O. T.; Stevenson, J. W., The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Metals Review 2013, 57 (4), 259-271.Luo, F.; Wagner, S.; Ju, W.; Primbs, M.; Li, S.; Wang, H.; Kramm, U. I.; Strasser, P., Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors. Journal of the American Chemical Society 2022, 144 (30), 13487-13498.Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8 (1), 957.Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature materials 2015, 14 (9), 937-942.Luo, F.; Roy, A.; Silvioli, L.; Cullen, D. A.; Zitolo, A.; Sougrati, M. T.; Oguz, I. C.; Mineva, T.; Teschner, D.; Wagner, S.; Wen, J.; Dionigi, F.; Kramm, U. I.; Rossmeisl, J.; Jaouen, F.; Strasser, P., P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature materials 2020, 19 (11), 1215-1223.

Catalytic activity of 2D MXenes toward electroreduction processes: Oxygen reduction and hydrogen evolution reactions

In this work, 2D Mo and Ti MXenes (Mo2TiC2 and Ti3C2) were synthesised from the chemical etching of their MAX phases (Mo2TiAlC2 and Ti3AlC2). The physicochemical characterisation demonstrated the removal of Al from precursors obtaining layered structures enriched with metal carbides domains. Focused on potential clean energy applications of these materials, their catalytic activity was assessed in two reactions: the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). The ORR activity of MXenes is quite low: Mo2TiC2 developed a slightly higher activity towards ORR in acidic media than Ti3C2 MXene, with an onset potential for the reaction at 0.4 VRHE. However, 3-fold higher performances were achieved in alkaline media for all the studied materials, suggesting a strong dependence of their catalytic activity with the pH. On the other hand, Mo2TiC2 and Ti3C2 exhibited high catalytic activity toward the HER in acidic media in comparison to their commercial MAX phases and few commercial carbides. This catalytic activity was evidenced from the low onset potentials for HER, as well as low overpotentials at 10 mA cm−2, especially for Mo2TiC2 (0.17 and 0.379 VRHE, respectively). Differential electrochemical mass spectrometry (DEMS) experiments allowed to establish the onset potentials, avoiding the interference of other faradaic processes in the same potential region, and the rate determining steps (rds) for HER at the studied materials. Tafel plots from ionic currents for m/z = 2 indicate that the Heyrovsky step is the rds for HER at Mo2TiC2, whereas Volmer step is the rds at Ti3C2. Long-term stability tests for the MXenes demonstrated that these materials maintain its activity after 10 h under operation conditions. EIS experiments performed at onset and HER evolution potentials corroborated the relevance of the presence of oxide reduction processes competing with the HER, which explains the higher catalytic activity of Mo2TiC2 with respect to Ti3C2.

PtFe Nanoalloys Supported on Fe‐Based Cubic Framework as Efficient Oxygen Reduction Electrocatalysts for Proton Exchange Membrane Fuel Cells

AbstractWhile Pt‐based binary alloys hold promising potential as cathode catalysts in proton exchange membrane fuel cells (PEMFCs) due to their desirable properties compared with monometallic counterparts, their weak interactions with carbon support make them challenging for practical cell implementation. Here, the PtFe alloys are uniformly deposited onto the surface of Fe‐Nx‐enriched nanocubes (FeNC) via surface confinement and internal vapor phase etching. The combined action of PtFe alloy and FeNC support acts as the electronic structure modulator resulting in the optimized intermediate adsorption (revealed by density function theory calculations), thereby boosting intrinsic activity to oxygen reduction reaction (ORR) and simultaneously enhancing their interactions. The resultant Pt1Fe1 @ Fe0.5NC‐900 catalyst with a low Pt loading (0.065 mgPt cm−2) exhibits excellent mass activity and stability toward ORR in acidic media. When incorporated into a PEMFC, it delivers a superior peak power density of 1.05 W cm−2 and satisfactory stability performance.

NaNO3-mediated synthesis of Pt-doped titanate perovskites for oxygen-reduction reactions in acidic media

NaNO3-mediated synthesis of Pt-doped titanate perovskites for oxygen-reduction reactions in acidic media

Monolayer supertubes of Carbon-Armored platinum nanocrystals enabling robust oxygen reduction electrocatalysis

Self-assembled superstructures composed of nanocrystals (NCs) have shown immense potential for enhancing the performance in electrocatalytic applications. However, there has been limited research on the self-assembly of platinum (Pt) into low-dimensional superstructures as efficient electrocatalysts for oxygen reduction reaction (ORR). In this study, we designed a unique tubular superstructure composed of monolayer or sub-monolayer carbon-armored platinum nanocrystals (Pt NCs) using a template-assisted epitaxial assembly approach. The organic ligands on the surface of Pt NCs were in situ carbonized, resulting in few-layer graphitic carbon shells that encapsulate Pt NCs. Due to their monolayer assembly and tubular geometry, the Pt utilization of the supertubes was 1.5 times higher than that of conventional carbon-supported Pt NCs. As a result, such Pt supertubes exhibit remarkable electrocatalytic performance for the ORR in acidic media, with a high half-wave potential of 0.918 V and a high mass activity of 181 A g-1Pt at 0.9 V, which are comparable to commercial carbon-supported Pt (Pt/C) catalysts. Furthermore, the Pt supertubes demonstrate robust catalytic stability, as confirmed by long-term accelerated durability tests and identical-location transmission electron microscopy. This study presents a new approach to designing Pt superstructures for highly efficient and stable electrocatalysis.

Metal-support interaction promoted multifunctional electrocatalysis on PtCo/NC with ultralow Pt loading for oxygen reduction reaction and zinc-air battery

Tri-functional PtCo/NC nanocatalysts were assembled via in-situ integration with ultralow Pt loading supported on ZIF-67 derived porous carbon material, and used as electrocatalysts for oxygen reduction reaction (ORR), ethanol oxidation reaction (EOR), hydrogen evolution reaction (HER). Detailedly, PtCo-3/NC with ultralow Pt loading (1 wt%) exhibits the halfwave potential of 0.877 V for ORR in acidic media and considerable current density for both ORR and EOR at 0.85 V, and low overpotential (η10 =10 mV) for HER. The as-assembled zinc-air batteries (ZABs) based on PtCo/NC have higher power density than Pt/C based ZABs. DFT calculation reveals the strong metal-support interactions (MSI) in PtCo/NC, which leads to the change of electronic state, redistribution of charge density and optimization of free energy, allowing it to present greater performances than Pt/C. This work highlights the possibility of using it as highly effective electrode catalysts with ultralow Pt loading in fuel cells and metal-air battery.

Carbon-Based Electrocatalysts for Acidic Oxygen Reduction Reaction.

Oxygen reduction reaction (ORR) is vital for clean and renewable energy technologies, which require no fossil fuel but catalysts. Platinum (Pt) is the best-known catalyst for ORR. However, its high cost and scarcity have severely hindered renewable energy devices (e.g., fuel cells) for large-scale applications. Recent breakthroughs in carbon-based metal-free electrochemical catalysts (C-MFECs) show great potential for earth-abundant carbon materials as low-cost metal-free electrocatalysts towards ORR in acidic media. This article provides a focused, but critical review on C-MFECs for ORR in acidic media with an emphasis on advances in the structure design and synthesis, fundamental understanding of the structure-property relationship and electrocatalytic mechanisms, and their applications in proton exchange membrane fuel cells. Current challenges and future perspectives in this emerging field are also discussed.

Ternary PtZrNi nanorods for efficient multifunctional electrocatalysis towards oxygen reduction and alcohol oxidation

Pt-based alloys with precise structure and composition design have been considered to be effective and robust novel electrocatalysts for fuel cells. Whereas, the sluggish kinetics of oxygen reduction reaction (ORR) and low intrinsic activity of Pt limited their real application on a large scale. Herein, a novel ternary PtZrNi nanorods (PtZrNi NRs) was synthesized via a facile wet-chemical method to achieve high electrocatalytic performance for both ORR and alcohol oxidation reaction owing to the synergism of chosen three elements and prominent one-dimensional morphology. Specifically, the PtZrNi NRs show enhanced mass and specific activities of 0.755 A mgPt-1 and of 0.97 mA/cm2 at 0.9 VRHE towards ORR in acidic media, which are 4.7 and 4.4 times of those of commercial Pt/C, respectively. Additionally, in alkaline media, the PtZrNi NRs also exhibit superior ORR mass and specific activities of 3.216 A mgPt-1and 4.13 mA/cm2, enhanced by 34.6 and 31.3 times compared with those of commercial Pt/C, respectively. The PtZrNi NRs retain the nanorod shape well without agglomeration after an accelerated durability test (20000 cycles). This work may offer a new perspective for engineering high-performance Pt-based electrocatalysts for commercial fuel cells.

Deactivation Mechanism of Pyridinic Nitrogen-Doped Carbon for ORR in Acidic Media

Nitrogen doped (N-doped) carbons are attracting attention as Pt-free catalysts for oxygen reduction reaction (ORR) in fuel cells. N-doped carbon catalysts exhibit high activity comparable to that of platinum in basic media whereas the activity is significantly reduced in acidic media. To solve the deactivation mechanism based on kinetics, we have investigated the reaction pathway of N-doped carbon catalysts. The active site of pyridinic nitrogen (pyri-N) is protonated to be pyridinium (pyri-NH+) in acidic media due to acid-base equilibrium ofpyri-N + H+ ⇄ pyriNH+ (1)Following to Eq. (1), we recently exhibited thatpyriNH+ + e- + O2⇄ O2a + pyri-NH(2)proceeds using model catalysts with homogenous active sites. In the step, the electrochemical reduction from pyri-NH+ to pyri-NH and thermal O2 adsorption simultaneously occurs wherein the adsorption of O2 promotes the electron transfer into π* orbital of pyri-NH. We proposed that the hydration of pyri-NH+ lowers the redox potential of Eq. (2) decreasing the ORR activity in acidic media. In the present study, we further considered the acid-base equilibrium of pyri-N in Eq. (1) to comprehensively answer why the ORR activity of N-doped carbon catalyst is high in alkaline media and is decreased in acidic media.To investigate the relationship between acid-base equilibrium and the activity, the electrochemical measurements were performed in electrolyte solutions of various pH. Figure 1 shows the onset potential (voltage at -100 μA/cm2) as a function of pH measure with a model catalyst of 1,10-phenanthroline/(carbon black). In the range from pH 5 to 13, the onset potential decreased with a slope of ~59 mV/pH, while at below pH 5, the onset potentials were almost constant. This indicates that the reaction mechanism changes due to the shift of the acid-base equilibrium. In low pH, the reactions determining the onset potential is assumed to be Eq. (2). On the other hand, at high pH, the reaction should start from pyri-N but not from pyri-NH+.We have figured out that pyri-N + H2O +e- + O2⇄ O2a + pyri-NH + OH- (3) takes place in alkaline madia. Figure 2 shows the N1s peak of the model catalyst applied voltage at pH 13 in N2-(left) and O2-(right)saturated electrolyte. A peak at 400.5 eV appeared at higher applied voltage in O2-saturated electrolyte. This means that pyri-N (399.2 eV) is reduced at a higher potential in the presence of O2 to form pyri-NH. In other words, oxygen adsorption and reduction of pyri-N simultaneously occurred in the presence of O2.The redox potentials of Eq. (2) and (3) explain the pH dependences of the onset potentials. The Nernst equations of Eq. (2) and Eq. (3) show pH-independent redox potential vs. RHE and pH-dependent redox potential vs. RHE with the slope of 59 mV/pH, respectively. This also means that the activity depends on the acid-base equilibrium of pyri-N.We also calculated the redox potentials based on DFT as shown in Figure 3. The additional important point is that the redox potentials of Eq. (2) is strongly influenced by the hydration whereas the redox potentials of Eq. (3) is not influenced by the hydration because the stability of pyri-N is not changed by the hydration. Therefore, the ORR activity in alkaline media is high because the redox potential of Eq. (2) vs. RHE is higher for the higher pH and is not influenced by the hydration. On the other hand, the activity in acidic media is lowered by the hydration. This is that the suppression of the hydration by the introduction of the hydrophobicity at the vicinity of the active site can significantly promote the activity in acidic media. Figure 1

Surfactant-Assisted Electrodeposition of Stable Porous Platinum Structures for the Oxygen Reduction Reaction

The demands on the alternative energy sector necessitates high quality research and development to produce economical, reliable and environment friendly energy sources. Proton exchange fuel cells (PEFCs) offer a versatile and dependable alternative to conventional energies both in the transportation and stationary power sectors. Their widespread use is, however, still restricted due to their high cost and the limited availability of platinum (Pt) resources. The high cost of PEFCs is attributed in part to the higher loading of Pt based catalyst at the cathode. The reduction of Pt loadings at the cathode is generally accompanied by significant performance losses due to sluggish oxygen reduction reaction (ORR) kinetics. This issue can be resolved by maximizing the Pt utilization without sacrificing the performance. Hence, on-going research and development work is seeking to better utilize Pt with lower loadings along with optimized performance. Compared to their solid or bulk counterparts, porous structures exhibit a high electrochemical active surface area (A ecsa) and, thus, can enhance the Pt utilization. These porous nanostructures can restructure during fuel cell cycling, and structural collapse can lead to an eventual decrease in Pt utilization.1 Stabilizing agents such as surfactants are usually employed during the Pt nanoparticle (NP) synthesis to prevent NP aggregation. These surfactants can also assist in the formation of the mesoporous structure.2 This study describes the electrochemical deposition of a stable porous Pt structure in the presence of surfactants.A site-directed electrodeposition offers advantages to the wet chemical synthesis of Pt nanocatalysts as it ensures that the Pt is deposited onto specific regions of a support that have a sufficient ionic and electrical conductivity. In this study, we demonstrated the use of anionic, cationic, and non-ionic surfactants to produce porous Pt using electrodeposition and the influence of these surfactants in stabilizing the porous structure during the ORR in acidic medium. The conditions for electrodeposition, such as concentration of surfactants and potential for the nucleation and growth stages were each optimized through a series of experiments. These surfactants included cetyl trimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS), and polyethylene glycol octadecyl ether (Brij 78). We utilized scanning electron microscopy techniques to evaluate the porosity of the deposited NPs and their surface coverage. In these experiments, an initial applied pulse was used to induce nucleation of the Pt followed by the growth of these materials with further electrodeposition at a lower potential. These parameters were evaluated for their influence on the final product, such as the porosity, A ecsa, and the surface coverage these materials.The porosity, composition, and crystallinity of these mesoporous particles were confirmed using transmission electron microscopy (TEM) and selected area electron diffraction techniques. These surfactant systems each resulted in the formation of porous Pt. The porous Pt was also subjected to durability testing by cycling the applied potential over a range of oxidizing potentials. Further analysis of these materials by TEM indicated that the mesoporous structure was maintained after the durability tests with a negligible change in their half-wave potential towards the ORR. The durability testing conducted after the removal of surfactants using Soxhlet extractor confirmed the role of the surfactants in helping to stabilize the porous Pt structure.

Journey of Fe-N4 Sites from Birth in Furnace till Death in Cell

One central topic in the Fe-N-C electrocatalysis for the oxygen reduction reaction (ORR) is the nature of the active site(s) responsible for the superior ORR activity of Fe-N-C electrocatalysts in acidic media. Significant progress has been made in this regard in the past decade, thanks to the employment of advanced in situ/operando characterization techniques including Mossbauer and X-ray absorption spectroscopy (XAS). The key findings are that the local structure of at least one type of Fe-N4 sites changes dramatically from produced in the furnace, exposed to air, and immersed in electrochemical media in aqueous solution or proton exchange membrane fuel cells (PEMFCs) with applied potentials. Despite these new findings, what the local structure of Fe-N4 sites is when they are catalyzing the ORR in acidic media, assuming Fe-N4 sites as the primary ORR-active centers of Fe-N-C electrocatalysts, remains unclear. Another mystery is the cause(s) of the rapid degradation of Fe-N4 sites during PEMFC operation.Herein, we walk through a possible journey of Fe-N4 sites from being produced by pyrolysis till decomposition in PEMFCs. In particular, we propose a new local structure of the exposed Fe-N4 site at lower potentials in acidic media: (H2O)2-Fe(II)-N4 with an octahedral structure subject to Jahn-Teller distortions. Specifically, in-plane Fe(II)-N4 sites are formed upon high-temperature (> 600 ℃) pyrolysis. Those Fe(II)-N4 sites accessible by air get spontaneously oxidized forming (O)2-Fe(III)-N4 ((O)2 represents the two axial oxygen ligands rather than one oxygen molecule) when exposed to air. The (O)2-Fe(III)-N4 structure is most likely also the form of the exposed Fe-N4 sites in acidic media at elevated potentials ( > 0.8 V versus reversible hydrogen electrode (RHE). As the potential scans cathodically across the Fe(III)/Fe(II) redox potential, the (O)2-Fe(III)-N4 structure transforms to the (H2O)2-Fe(II)-N4 structure with the axial ligands of (O)2 replaced by water molecules with elongated Fe-OH2 bonds due to the Jahn-Teller effect on high-spin Fe(II) octahedral complexes. This high-spin (O)2-Fe(III)-N4 structure was inferred from the combination of the presence of two Fourier-Transform of extended X-ray absorption fine structure (FT-EXAFS) peaks of several Fe-N-C catalysts1,2 at lower potentials; the low intensity of the pre-edge peak in the X-ray absorption near edge structure (XANES) spectra; with in situ Mossbauer results.3 In contrast, the Jahn-Teller effect is absent on high-spin Fe(III) octahedral complexes such as (O)2-Fe(III)-N4, correspondingly only one Fe-N/O FT-EXAFS peak was observed at elevated potentials or ex situ. The H2O molecule(s) in the (H2O)2-Fe(II)-N4 is replaceable by O2 molecules during the ORR. The exposed Fe-N4 sites transform to inorganic iron species such as iron oxides during PEMFC operation according to the recent operando Mossbauer results,4 which accounts partly for the degradation of Fe-N-C catalysts during the ORR. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416. We acknowledge the support from the DOE, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis Consortium (ElectroCat) and the DOE programme and technology managers, D. Papageorgopoulos, D. Peterson and N. Garland. This research used beamline 7-BM and 8-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. In-temperature XAS experiments were performed at the MRCAT in Advanced Photon Source (APS), a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. Jiao, L. et al. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Mater. 20, 1385-1391 (2021).Deng, Y. et al. g-C3N4 promoted MOF derived hollow carbon nanopolyhedra doped with high density/fraction of single Fe atoms as an ultra-high performance non-precious catalyst towards acidic ORR and PEM fuel cells. Journal of Materials Chemistry A 7, 5020-5030 (2019).Zelenay, P. ElectroCat (Electrocatalysis Consortium). Report No. LA-UR-20-24045 United States 10.2172/1631569 LANL English, Medium: ED; Size: 67 p. (; Los Alamos National Lab. (LANL), Los Alamos, NM (United States), 2020)Li, J. et al. Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells. Catal. 4, 10-19 (2021). Figure 1

Recent progress in carbon nanotubes support materials for Pt-based cathode catalysts in PEM fuel cells

Support materials have a significant impact on catalytic activity, stability, and performance of catalysts toward the oxygen reduction reaction (ORR). The properties of carbon-based materials have made them an excellent alternative for use as support for nanosized catalysts. Recently, carbon nanotubes (CNTs) have been explored as catalyst support materials, and their properties make them a promissory alternative. Furthermore, catalysts supported on CNTs exhibit higher resistance to electrochemical oxidation, better catalytic performance, and higher durability than catalysts supported on carbon black. In recent years, CNTs have acquired great relevance as catalysts support materials for ORR in acid media. This review addresses the most relevant studies on CNTs modification using methods such as functionalization, doping, and hybrid supports (CNTs-metal oxide) used as supports for Pt-based cathode catalysts in proton exchange membrane fuel cells.

Fe3O4 Templated Pyrolyzed Fe−N−C Catalysts. Understanding the role of N‐Functions and Fe3C on the ORR Activity and Mechanism

AbstractPyrolyzed non‐precious metal catalysts have been proposed as an alternative to substitute the expensive and scarce noble metal catalysts in several conversion energy reactions. For the oxygen reduction reaction (ORR), the pyrolyzed catalyst M−N−C (M: Fe or Co) presents remarkable catalytic activity in acid and alkaline media. These pyrolyzed materials show a high heterogeneity of active sites being the most active in the MNx moieties. The activity and stability of these catalysts are also conditioned by other structural parameters such as the area, the N‐doping, and by the presence of metal particles. In this study, we explore the use of Fe3O4 nanoparticles as templates and as iron sources to synthesize Fe−N−C. The best performance for the ORR in acidic media was reached with the catalysts using nanoparticles covered by PANI and iron salts as the precursor, with an onset potential of 0.85 vs. RHE and a direct 4‐electrons mechanism. We corroborated the use of the catalysts’ redox potential as reactivity descriptors and discussed the detrimental role of the presence of Fe3C metallic particles in the mechanism. Based on the experimental results, we performed DFT calculations to explore the influence of N‐doped species on the electronic density of the iron centers of FeN4 active sites, and we propose a theoretical model for increasing the activity based on the distance and ratio of N‐doping to iron center.

High-performance oxygen reduction electrocatalysts derived from bimetal-organic framework and sulfur-doped precursors for use in microbial fuel cells

Development of platinum -free cathode electrocatalysts with high oxygen reduction reaction (ORR) activity for use in microbial fuel cells (MFCs) is challenging. The current study reports a cathode electrocatalyst with high oxygen reduction reaction produced by pyrolysis of sulfur-doped Co–Zn Bi-MOF precursors. SEM, TEM, FTIR and XPS findings showed that sulfur heteroatoms were successfully coated on the Co-ZIF-8, and a carbon material with a core-shell structure (Co–S shell) was synthesized. Presence of initial Zn creates spatial isolation of Co that prevents it from sintering during pyrolysis, and Zn evaporation increases the surface area of the resulting catalysts. Atomically Co site distributed catalysts with ideal chemical and structural properties exhibit high activity and stability for the ORR in acidic media (E 1/2 = 0.75 V vs. RHE) and alkaline media (E 1/2 = 0.8 V vs. RHE) as shown by analysis of the effect of S doping contents. MFCs devices coupled with Co–N/S–C-3.5 cathode showed high open-circuit potentials (E OCV = 0.61 V) and high power densities (1190 ± 10 mW m −2 ). • A new Co–N/S–C-3.5 catalyst derived from ZIF-67 is used in MFC. • Sulfur atoms are distributed on the surface of the polyhedron to form a core-shell structure. • The doping of sulfur is beneficial to the activity of the catalyst. • The catalyst exhibits good durability in all pH mediums. • The Co–N/S–C-3.5 exhibited much higher practicability than that of Pt/C.

PGM-Free Oxygen-Reduction Catalyst Development for Proton-Exchange Membrane Fuel Cells: Challenges, Solutions, and Promises

ConspectusProton-exchange membrane fuel cells (PEMFCs) are efficient and clean hydrogen energy technologies for transportation and stationary applications. Highly active and durable low-cost cathode catalysts for the oxygen-reduction reaction (ORR) under challenging acidic environments are desperately needed to address the cost and durability issues of PEMFCs. The most promising platinum group metal (PGM)-free catalysts for the ORR in acidic media are atomically dispersed and nitrogen-coordinated metal site catalysts denoted as M–N–C, M = Fe, Co, or Mn. Due to significant efforts in the past few decades, these catalysts have demonstrated much-improved ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the insufficient long-term stability (up to 5000 h) under PEMFC operation represents a primary technical barrier to making current PGM-free catalysts less viable yet in PEMFCs. In this Account, we highlight recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing effective strategies to improve mass and intrinsic activity and the possible degradation mechanisms. In particular, a chemical doping method based on the zeolitic imidazolate framework (ZIF)-8 represents the key to developing efficient M–N–C catalysts containing atomically dispersed and nitrogen-coordinated single metal active sites (i.e., MN4). The newly acquired understanding of the formation mechanism of MN4 active sites during the thermal activation and its correlation to catalytic properties guide the rational catalyst design rather than relying on current trial-and-error approaches. Considerable efforts have further been invested in increasing the active site density and enhancing intrinsic activity by regulating carbon-phase structures and the local coordination environment. These highly active catalysts usually suffer from significant activity loss during the ORR. Therefore, breaking the activity–stability trade-off is the key to simultaneously achieving activity and stability in one catalyst, which is discussed on the basis of our recent successes in regulating local carbon structures surrounding active single metal sites. Significant research efforts toward understanding the degradation mechanisms and improving the lifetime of PGM-free catalysts are still crucial for viable applications in the future. Novel electrode designing strategies are needed to translate the PGM-free catalysts’ ORR activity to solid-state electrolyte-based membrane electrode assemblies (MEAs) with robust three-phase (i.e., gas–liquid–solid) interfaces for efficient charge and mass transports for performance improvement. On the basis of our effort at the University at Buffalo supported by ElectroCat Consortium associated with U.S. DOE’s Hydrogen and Fuel Cell Technologies Office, we provide a perspective on PGM-free cathode catalysts concerning remaining bottlenecks and future opportunities, aiming to inspire the community in both mechanistic understanding and technological development.

OH scavenging pyridinic N for facile oxygen reduction reaction on free Pt surface.

Compared to hydrogen oxidation reaction, oxygen reduction reaction (ORR) is a sluggish reaction in the proton exchange membrane fuel cell. Many studies have focused on the development of complex synthesis methods for new catalysts. In this study, we introduce a simple catalyst layer preparation method using an additive based on physical mixing for facile ORR in acid media. N-doped carbon containing mainly pyridinic N was used as the additive in the Pt or PtCo catalyst layer. The adjacent pyridinic N near the Pt catalyst works as a Lewis base and removes hydroxyl ions from Pt, thus leading to the simultaneous suppression of Pt oxidation. Based on electrochemical and spectroscopic analyses, we found that pyridinic N reacts with hydroxyl ions and is oxidized. Consequently, Pt catalyst without Pt oxide species exhibited higher ORR activity than pristine Pt or PtCo catalyst.

Pt-Free Metal Nanocatalysts for the Oxygen Reduction Reaction Combining Experiment and Theory: An Overview.

The design and manufacture of highly efficient nanocatalysts for the oxygen reduction reaction (ORR) is key to achieve the massive use of proton exchange membrane fuel cells. Up to date, Pt nanocatalysts are widely used for the ORR, but they have various disadvantages such as high cost, limited activity and partial stability. Therefore, different strategies have been implemented to eliminate or reduce the use of Pt in the nanocatalysts for the ORR. Among these, Pt-free metal nanocatalysts have received considerable relevance due to their good catalytic activity and slightly lower cost with respect to Pt. Consequently, nowadays, there are outstanding advances in the design of novel Pt-free metal nanocatalysts for the ORR. In this direction, combining experimental findings and theoretical insights is a low-cost methodology—in terms of both computational cost and laboratory resources—for the design of Pt-free metal nanocatalysts for the ORR in acid media. Therefore, coupled experimental and theoretical investigations are revised and discussed in detail in this review article.

Boron modulating electronic structure of FeN4C to initiate high-efficiency oxygen reduction reaction and high-performance zinc-air battery

The biggest challenge is to develop a low cost and readily available catalyst to replace expensive commercial Pt/C for efficient electrochemical oxygen reduction reaction (ORR). In this research, closo-[B12H12]2− and 1,10-phenanthroline-iron complexes were introduced into the porous metal-organic framework by impregnation method, and further annealing treatment achieved the successful anchoring of single-atom-Fe in B-doped CN Matrix (FeN4CB). The ORR activity of FeN4CB is comparable to the widely used commercial 20 wt% Pt/C. Where the half-wave potential (E1/2) in alkaline medium up to 0.84 V, and even in the face of challenging ORR in acidic medium, the E1/2 of ORR driven by FeN4CB is still as high as 0.81 V. When FeN4CB was used as air cathode, the open circuit voltage of Zn-air battery reaches 1.435 V, and the power density and specific capacity are as high as 177 mW cm−2 and 800 mAh gZn−1 (theoretical value: 820 mAh gZn−1), respectively. The dazzling point of FeN4CB also appears in the high ORR stability, whether in alkaline or acidic media, E1/2 and limiting current density are still close to the initial value after 5000 times cycles. After continuously running the charge-discharge test for 220 h, the charge voltage and discharge voltage of the rechargeable zinc-air battery with FeN4CB as the air cathode maintained the initial state. Density functional theory calculations reveals that introducing B atom to Fe–N4–C can adjust the electronic structure to easily break O = O bond and significantly reduce the energy barrier of the rate-determining step resulting in an improved ORR activity.

Recent Progress in Carbon-based Materials of Non-Noble Metal Catalysts for ORR in Acidic Environment

Proton exchange membrane fuel cell (PEMFC) has important implications for the success of clean transportation in the future. One of the key factors affecting the cost and performance of PEMFC is the cathode electrocatalyst for the oxygen reduction reaction (ORR) to overcome sluggish kinetics and instability in an acidic environment. As an essential component of the electrocatalyst, the support material largely determines the activity, mass transfer, charge transfer, and durability of the electrocatalyst. Thereby, the support material plays a critical role in the overall performance of the electrocatalyst. Carbon-based materials are widely used as electrocatalyst supports because of their high porosity, conductivity, chemical stability, and tunable morphology. Recently, some new carbon-based materials with excellent structure have been introduced, such as carbon nanotubes, carbon nanowires, graphene, metal–organic framework (MOF)-derived carbon, and biomass-derived carbon materials. Combined with a variety of strategies, such as controllable construction of porous structures and surface defects, proper doping heteroatoms, the ingenious design of model electrocatalysts, and predictive theoretical calculation, a new reliable path was provided for further improving the performance of electrocatalysts and exploring the catalytic mechanism. Based on the topic of carbon-based materials for ORR in acidic medium, this review summarizes the up-to-date progress and breakthroughs, highlights the factors affecting the catalytic activity and stability of ORR electrocatalysts in acids, and discusses their future application and development.