Oxygen Reduction In Acidic Media Research Articles

Iron‐Nitrogen‐Carbon Aerogel for Enhanced Oxygen Reduction in Acidic Media: The Influence of Temperature

AbstractThe oxygen reduction reaction (ORR) in an acidic environment is crucial for fuel cell technology. Understanding its complex kinetics and developing advanced catalyst materials have the potential to drive significant improvements in energy efficiency, paving the way for sustainable, green energy solutions. In this work, Iron‐Nitrogen‐Carbon@aerogel (Fe(FcP)x‐N‐C@Aerogel) catalysts are developed by carbonizing polypyrrole (PPy) and ferrocene. The ORR performance of these catalysts is investigated across different annealing temperatures. The catalysts’ shape and structure are validated through scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS), revealing that changes in annealing temperature affect morphology and nitrogen‐containing functional groups of the catalyst. Linear sweep voltammetry (LSV) and rotating disk electrode (RDE) studies demonstrated that Fe(FcP)800‐N‐C@Aerogel catalysts exhibit excellent performance, with a half‐wave potential of 0.687 V and an average electron transfer number of 3.98 under acidic conditions. These findings suggest a near‐four‐electron reaction pathway, highlighting the catalyst's strong ORR activity, high efficiency, and durability, with only 17.4 mV LSV curve decay after 10,000 cycles. In conclusion, Fe(FcP)x‐N‐C@Aerogel advances ORR catalysis in acidic media by delivering exceptional performance and durability, driven by its innovative architecture and precisely engineered active sites, setting a new benchmark for high‐efficiency energy conversion.

Boosting oxygen reduction in acidic media through integration of Pt-Co alloy effect and strong interaction with carbon defects

Boosting oxygen reduction in acidic media through integration of Pt-Co alloy effect and strong interaction with carbon defects

Modification of NiSe2 Nanoparticles by ZIF-8-Derived NC for Boosting H2O2 Production from Electrochemical Oxygen Reduction in Acidic Media

The two-electron oxygen reduction reaction (2e− ORR) has emerged as an attractive alternative for H2O2 production. Developing efficient earth-abundant transition metal electrocatalysts and reaction mechanism exploration for H2O2 production are important but remain challenging. Herein, a nitrogen-doped carbon-coated NiSe2 (NiSe2@NC) electrocatalyst was prepared by successive annealing treatment. Benefiting from the synergistic effect between the NiSe2 nanoparticles and NC, the 2e− ORR activity, selectivity, and stability of NiSe2@NC in 0.1 M HClO4 was greatly enhanced, with the yield of H2O2 being 4.4 times that of the bare NiSe2 nanoparticles. The in situ Raman spectra and density functional theory (DFT) calculation revealed that the presence of NC was beneficial for regulating the electronic state of NiSe2 and optimizing the adsorption free energy of *OOH, which could enhance the adsorption of O2, stabilize the O-O bond, and boost the production of H2O2. This work provides an effective strategy to improve the performance of the transition metal chalcogenide for 2e− ORR to H2O2.

Highly Dispersed Ultrasmall High-Entropy Alloys Nanoparticles as Efficient Electrocatalysts for Oxygen Reduction in Acidic Media.

High-entropy alloys nanoparticles (HEAs NPs) have gained considerable attention due to their extensive compositional tunability and intriguing catalytic properties. However, the synthesis of highly dispersed ultrasmall HEAs NPs remains a formidable challenge due to their inherent thermodynamic instability. In this study, highly dispersed ultrasmall (ca. 2nm) PtCuGaFeCo HEAs NPs are synthesized using a one-pot solution-based method at 160°C and atmospheric pressure. The PtCuGaFeCo NPs exhibit good catalytic activity for the oxygen reduction reaction (ORR). The half-wave potential relative to the reversible hydrogen electrode (RHE) reaches 0.88V, and the mass activity and specific activity are approximately six times and four times higher than that of the commercial Pt/C catalyst. Based on X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses, the surface strain and optimized coordination environments of PtCuGaFeCo have led to high ORR activities in acidic media. Moreover, the ultrasmall size also plays an important role in enhancing catalytic performance. The work presents a facile and viable synthesis strategy for preparing the ultrasmall HEAs NPs, offering great potential in energy and electrocatalysis applications through entropy engineering.

(Digital Presentation) Structure-Activity Relationship of M-N-C Electrocatalysts Synthesized Using a General Solution-Phase Coordination Approach

Metal- and nitrogen-doped carbon (M-N-C) catalysts have great potential in heterogeneous catalysis. They are used to catalyze various crucial electrochemical reactions, such as the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the CO2 reduction reaction (CO2RR).1-4 The nitrogen-coordinated metal sites (MNx) have been considered the main active sites in these M-N-C catalysts. However, the synthesis of MNx moieties often undergoes a high-temperature heat-treatment step, resulting in low site density. On the other side, the coordination environment in the MNx sites is also affected by the metal species which induced the site formation. Therefore, it is challenging to single out the role of central metal in the structure-activity relationships for these MNx sites.In this presentation, we will discuss our recent progress in the development of a solution-phase coordination synthesis approach targeting M-N-C catalysts with high active-site density and well-defined coordination environments.5, 6 A series of M-N-C catalysts are synthesized via this approach by coordinating electroactive target metal ions with the nitrogen-coordinated metal-vacancy (MVNx) sites in N-C templated by sacrificial metals. With a combined experimental and computational approach, we explore the role of sacrificial metals, including s-, p-, 3d-, 4d-, and f-block metals, in forming various MVNx sites with unique coordination configurations. The structure-activity relationship between the coordination environment and the catalytic activity for ORR, HER, OER, and CO2RR is established by comparing the MNx site structures induced by various sacrificial metals. Furthermore, we will present the activity series of metal centers in M-N-C catalysts with the same and well-defined coordination environment for ORR, HER, OER, and CO2RR. These results will guide the future development of M-N-C catalysts. Acknowledgments Financial support from the State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences is greatly appreciated. This study is financially supported by the Shanxi Province grant (Grant No. 20210302123011, 202203021212007, and 202103021224442), and Key Research and Development (R&D) Projects of Shanxi Province (202102070301018). References J. Cui, Q. Chen, X. Li and S. Zhang. Recent advances in non-precious metal electrocatalysts for oxygen reduction in acidic media and PEMFCs: an activity, stability and mechanism study. Green Chemistry, 23, 6898 (2021).H. He, H. H. Wang, J. Liu, X. Liu, W. Li and Y. Wang. Research Progress and Application of Single-Atom Catalysts: A Review. Molecules, 26, 6501 (2021).U. Martinez, S. Komini Babu, E. F. Holby, H. T. Chung, X. Yin and P. Zelenay. Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction. Advanced Materials, 31, 1806545 (2019).Y. Wang, X. Cui, L. Peng, L. Li, J. Qiao, H. Huang and J. Shi. Metal–Nitrogen–Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions. Advanced Materials, 33, e2100997 (2021).M. Huang, R. Ding, J. Yang, W. Shi, S. Shi, L. Chen, S. Liu and X. Yin. Formation of Nitrogen-Coordinated Metal Sites (M = Fe, Co) via Solution-Phase Coordination on Nickel- And Nitrogen-Co-Doped Carbon Templates with Metal Vacancy-Nx Sites. Journal of The Electrochemical Society, 169, 106507 (2022).M. Huang, L. Chen, R. Ding, W. Shi, Q. Qin, J. Yang, S. Liu and X. Yin. Solution-Phase Synthesis of Co-N-C Catalysts Using Alkali Metals-Induced N-C Templates with Metal Vacancy-Nx sites. ChemRxiv, doi: 10.26434/chemrxiv-2022-xnh88 (2022).

Electrospun Carbon Nanofibre‐Based Catalysts Prepared with Co and Fe Phthalocyanine for Oxygen Reduction in Acidic Medium

AbstractA Pt‐free cathode catalyst is necessary for proton‐exchange membrane fuel cell (PEMFC) to enable the widespread use of these environmentally friendly energy conversion devices at affordable price. Herein, a pyrolyzed electrospun carbon nanofibre (CNF) catalyst is prepared embedded with cobalt(II) phthalocyanine and iron(II) phthalocyanine compounds to provide the transition metal N4‐macrocyclic complex‐derived sites (MNX) possessing better electrocatalytic oxygen reduction reaction (ORR) activity. The physical characterisation showed the nanofibrous structure of catalyst with rough surface texture and considerable amount of N, Fe, and Co. The D−MN4−CNF−IL−A catalyst prepared using ionic liquid as a porogen displayed the best electrocatalytic activity for O2 electroreduction proceeding via 4e− pathway in 0.5 M H2SO4 electrolyte solution with the ORR onset and half‐wave potential of 0.83 and 0.71 V vs reversible hydrogen electrode (RHE), respectively.

MOF-Conductive Polymer Composite Film as Electrocatalyst for Oxygen Reduction in Acidic Media

A metal-organic framework (MOF)-conductive polymer composite film was constructed from PCN-222(Fe) nanoparticles and PEDOT:PSS solution by simple drop-casting approach. The composite film was tested as an electrocatalytic device for oxygen reduction reaction (ORR). The combination of PCN-222(Fe) MOF particles and conductive PEDOT matrix facilitates electron transfer in the composite material and improves the ORR performance of PCN-222(Fe). Levich plot and H2O2 quantification experiment show that PCN-222(Fe)/PEDOT:PSS film mainly catalyzes two-electron oxygen reduction and produces H2O2.

Bifunctional Zinc-Molybdate or Zinc molybdenum Oxide/Metal-Nitrogen-Carbon catalytic layers with improved four-electron selectivity for oxygen reduction in acidic medium

Platinum-group-metal-free metal-nitrogen-carbon catalysts for the oxygen reduction reaction have demonstrated high initial activity and power performance in proton exchange membrane fuel cells. The main challenge facing this class of catalysts is their poor durability in operating acidic fuel cells. Their key operando degradation mechanisms involve oxidative attacks triggered by the catalysis of the oxygen reduction on atomically-dispersed 3d transition metals. Reactive oxygen species such as H2O2 or radical species are particularly aggressive to the carbon matrix. Minimizing H2O2 by-product formation during oxygen reduction or actively scavenging any formed H2O2 are therefore promising routes to improve their durability.Here, we report that Zn-molybdate and Zn molybdenum oxide have low activity towards H2O2 decomposition and electro-reduction in acid, but result in a strong synergistic effect with various M-N-C catalysts. The addition of α-ZnMoO4 or Zn3(OH)2(MoO4)2 in cathode layers resulted in improved four-electron ORR selectivity of M-N-C catalysts (M = Fe, Co, Cr) as well as moderately improved durability in accelerated stress tests in O2-saturated pH 1 electrolyte. The improvement in selectivity could be rationalized by hydrogen peroxide electro-reduction measurements and catalase enzyme-like activity tests. Both types of measurements revealed higher reactivity towards H2O2 scavenging when Metal-N-C catalysts are physically mixed (in a catalytic layer) or together in a suspension in solution with Zn-molybdate or Zn molybdenum oxide, revealing a synergy effect. Control experiments with ZnCl2 and Na2MoO4 salts however suggest that leached ions from ZnMoO4 in acidic solution are likely at the root of the synergy effect.

MOF-derived Fe–N–C electrocatalyst via a dual ligand strategy for efficient oxygen reduction in acidic media

A metal–organic-framework based dual ligand strategy is proposed to develop high-performance acid ORR Fe–N–C electrocatalyst via the synergistic effect between in situ Zn evaporation and ligand-manipulated pyrolysis.

Regeneration of single-atom catalysts deactivated under acid oxygen reduction reaction conditions

Single-atom catalysts serve as a promising candidate to realize noble-metal-free electrocatalytic oxygen reduction in acid media. However, their poor stability under working conditions strictly restrains their practical applications. Therefore, regeneration of their electrocatalytic activity is of great significance. Herein, the regeneration of a Fe–N–C single-atom catalyst is demonstrated to be feasible by a facile annealing regeneration strategy. The activity after regeneration recovers to that of the pristine electrocatalyst and surpasses the deactivated electrocatalyst. The regeneration mechanism is identified to be self-etching of the surface carbon layer and consequent exposure of the previously buried single-atom sites. Furthermore, the regeneration strategy is applicable to other single-atom catalysts. This work demonstrates the feasibility of regenerating oxygen reduction electrocatalysts and affords a pioneering approach to deal with rapid deactivation under working conditions.

Surface ion isolated platinum-thiocyanate catalysts for hydrogen peroxide production via 2-electron oxygen reduction in acidic media

Electrosynthesis of hydrogen peroxide (H2O2) through oxygen reduction reaction (ORR) is an attractive alternative to the traditional anthraquinone process. It offers decentralized and on-site generation of H2O2 with a cost-effective system. Noble metals electrocatalysts through alloying or in a single-atomic structure and a few non-precious metals have been demonstrated with high selectivity but the stability remains challenging. Here, we propose a surface ion isolated platinum-thiocyanate (PtSCNx) catalyst as a promising electrocatalyst for the H2O2 electrosynthesis in acidic media. The SCN– binding remarkably weakens the absorption of crucial *OOH intermediate on the Pt surface, leading to the change of ORR electron transfer pathway. The carbon supported PtSCNx exhibits an onset potential of 0.68 V (versus RHE) and ∼ 90% selectivity. Significantly, the PtSCNx/C shows a continual and stable electrolysis over 130 h, which outperforms the state-of-the-art 2e– ORR catalysts. Moreover, an on-site electrochemical rapid removal of organic pollutant is achieved through an electro-Fenton process.

Highly dispersed PtNi nanoparticles modified carbon black as high-performanced electrocatalyst for oxygen reduction in acidic medium

• Rational optimization of Ni/C precursor for Pt galvanic replacement. • Regulation on the electronic structure of active site stemmed from PtNi alloys. • PtNi/C shows excellent and stable ORR activity via 4e - pathway in acidic medium. • The mass activity of PtNi/C at 0.8 V is 9.4 times that of commercial Pt/C. Due to the excellent intrinsic activity, platinum-based materials are widely used to produce highly efficient electrocatalysts towards oxygen reduction reactions (ORR), but their ultrahigh price and poor stability severely hinder the mass production and further wide application. Alloying Pt with 3d transition metals through galvanic replacement is an effective way to reduce Pt loading based on the premise of outstanding ORR activity. In this work, to fabricate the optimal precursor for subsequent galvanic replacement, a series of carbon-supported Ni samples were firstly prepared by tuning the Ni loading amount and pyrolysis temperature. Then, the highly dispersed PtNi nanoparticles modified carbon nanohybrid (PtNi/C) with low Pt content of 2.29 wt% can be obtained, which exhibits remarkable and durable ORR activity with E 1/2 of 0.841 V vs. RHE via efficient 4e - ORR process in acidic media. Moreover, it should be emphasized that the as-synthesized PtNi/C behaves the impressively high mass activity of 0.57 A/mg Pt at 0.8 V, which is 9.4 times that of commercial Pt/C. It is confident that our work can offer constructive suggestions for the design and synthesis of high-performanced electrocatalysts fabricated through galvanic replacement method.

Design of Fe‐Nx/Tungsten Carbide for Efficient Electrocatalyst Oxygen Reduction in Acidic Media

AbstractImproving the activity and stability of Fe/N/C catalyst in oxygen reduction reaction (ORR) is a huge challenge in the commercial application of polymer electrolyte membrane fuel cells (PEMFCs). In the past decade, there have been significant break‐throughs in the performance of transition metal catalysts, but little progress has been made in their stability. Herein, a zinc‐based zeolite imidazole framework (ZIF‐8) and tungsten carbide engaged strategy was reported to prepare Fe/N/C catalyst. Particularly, physical vapor deposition (PVD) was used to trap tungsten carbide nanoparticles with particle size of less than 3 nm limited into the FeNC catalytic micropores to synthesis composite catalyst (WC@FeNC). Compared with original Fe/N/C cata‐lysts, confined WC nanoparticles in Fe/N/C porous has improved the ORR activity (2.7 mA mg−1 vs. 2.2 mA mg−1 at 0.85 V vs. RHE) as well as stability (decay 18.7 mV vs. 21.6 mV after 10 h charged) in 0.1 M H2SO4. This work puts forward some unique insights for improving the stability of transition metal oxygen reduction catalysts.

In Situ Growth of Pt-Co Nanocrystals on Different Types of Carbon Supports and Their Electrochemical Performance toward Oxygen Reduction.

Carbon-supported Pt-M (M = Co, Ni, and Fe) alloy nanocrystals are widely used as catalysts toward oxygen reduction, a reaction key to the operation of proton-exchange membrane fuel cells. Here we report a colloidal method for the in situ growth of Pt-Co nanocrystals on various commercial carbon supports. The use of different carbon supports resulted in not only variations in size and composition for the nanocrystals but also their catalytic activity and durability toward oxygen reduction in acidic media. Among the nanocrystals, those grown on Vulcan XC72 and Ketjenblack EC300J showed the highest specific and mass activities in the 0.1 M HClO4 and 0.05 M H2SO4 electrolytes, respectively. Additionally, the catalysts also showed different durability depending on the strength of the interaction between the nanocrystals and the carbon support. Our analysis demonstrated that the difference in catalytic performance could be ascribed to the distinct effects of carbon support on both the synthetic and catalytic processes.

Synthesis and Characterization of Cobalt and Nitrogen Co-Doped Peat-Derived Carbon Catalysts for Oxygen Reduction in Acidic Media

In this study, several peat-derived carbons (PDC) were synthesized using various carbonization protocols. It was found that depending on the carbonization method, carbons with very different surface morphologies, elemental compositions, porosities, and oxygen reduction reaction (ORR) activities were obtained. Five carbons were used as carbon supports to synthesize Co-N/PDC catalysts, and five different ORR catalysts were acquired. The surface analysis revealed that a higher nitrogen content, number of surface oxide defects, and higher specific surface area lead to higher ORR activity of the Co-N/PDC catalysts in acidic solution. The catalyst Co-N/C-2(ZnCl2), which was synthesized from ZnCl2-activated and pyrolyzed peat, showed the highest ORR activity in both rotating disk electrode and polymer electrolyte membrane fuel cell tests. A maximum power density value of 210 mW cm−2 has been obtained. The results of this study indicate that PDCs are promising candidates for the synthesis of active non-platinum group metal type catalysts.

Atomically Dispersed Single Metal Site Catalysts for Oxygen Reduction in Acidic Media

Proton exchange membrane fuel cells (PEMFCs) are leading candidates in utilizing clean energy resources for application in transportation, stationary, and portable devices. In PEMFCs, cathode catalysts are crucial for overall performance and durability due to kinetically slow oxygen reduction reactions (ORR). Because platinum (Pt), a state-of-the-art ORR catalyst, is rare and expensive, the development of high-performance platinum metal group (PGM)-free catalysts is highly desirable for future fuel cell technologies. Among various PGM-free catalyst formulations, metal and nitrogen co-doped carbon (M-N-C, M: Fe, Co, or Mn) catalysts have exhibited encouraging activity and stability in acidic media for the ORR, which possess great potential to replace Pt in the future. Therefore, based on our extensive experience in the field of ORR catalysis, the talk will comprehensively summarize the basic principles in the design and synthesis of M-N-C catalysts for durable, inexpensive, and high-performance PEMFCs with an emphasis on Co- and Mn-N-C catalysts to avoid Fenton reactions between Fe2+ and H2O2, which can generate free radicals and lead to the degradation of catalysts, ionomers, and membranes in PEMFCs. Furthermore, template-free 3D hydrocarbon frameworks as attractive precursors to advanced M-N-C catalysts will be discussed to enhance intrinsic ORR activities in acidic media significantly. Also, the long-term performance durability of M-N-C cathodes will be discussed extensively to provide potential solutions to enhance catalyst stability in PEMFCs. Finally, this talk will provide an overall perspective on the progress, challenges, and solutions of PGM-free catalysts for future PEMFC technologies.

Highly Accessible Atomically Dispersed Fe-N x Sites Electrocatalyst for Proton-Exchange Membrane Fuel Cell.

Atomically dispersed transition metal‐Nx sites have emerged as a frontier for electrocatalysis because of the maximized atom utilization. However, there is still the problem that the reactant is difficult to reach active sites inside the catalytic layer in the practical proton exchange membrane fuel cell (PEMFC) testing, resulting in the ineffective utilization of the deeply hided active sites. In the device manner, the favorite structure of electrocatalysts for good mass transfer is vital for PEMFC. Herein, a facile one‐step approach to synthesize atomically dispersed Fe‐Nx species on hierarchically porous carbon nanostructures as a high‐efficient and stable atomically dispersed catalyst for oxygen reduction in acidic media is reported, which is achieved by a predesigned hierarchical covalent organic polymer (COP) with iron anchored. COP materials with well‐defined building blocks can stabilize the dopants and provide efficient mass transport. The appropriate hierarchical pore structure is proved to facilitate the mass transport of reactants to the active sites, ensuring the utilization of active sites in devices. Particularly, the structurally optimized HSAC/Fe‐3 displays a maximum power density of up to 824 mW cm−2, higher than other samples with fewer mesopores. Accordingly, this work will offer inspirations for designing efficient atomically dispersed electrocatalyst in PEMFC device.

Recent advances in non-precious metal electrocatalysts for oxygen reduction in acidic media and PEMFCs: an activity, stability and mechanism study

The state-of-the-art PGM-free ORR catalysts for PEMFCs, including their active sites, ORR and deactivation mechanisms are reviewed.

Coordination Number Regulation of Molybdenum Single-Atom Nanozyme Peroxidase-like Specificity

Summary Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. Overcoming this and controlling the targeted enzyme-like performance of traditional nanozymes is extremely challenging due to the intrinsic structural complexity of these systems. We report theoretical design and experimental realization of a series of heterogeneous molybdenum single-atom nanozymes (named MoSA–Nx–C), wherein we find that the peroxidase-like specificity is well regulated by the coordination numbers of single Mo sites. The resulting MoSA–N3–C catalyst shows exclusive peroxidase-like behavior. It achieves this behavior via a homolytic pathway, whereas MoSA–N2–C and MoSA–N4–C catalysts have a different heterolytic pathway. The mechanism of this coordination-number-dependent enzymatic specificity is attributed to geometrical structure differences and orientation relationships of the frontier molecular orbitals toward these MoSA–Nx–C peroxidase mimics. This study demonstrates the rational design of peroxidase-specific nanozymes and precise regulation of their enzymatic properties.

Trifunctional Single‐Atomic Ru Sites Enable Efficient Overall Water Splitting and Oxygen Reduction in Acidic Media

Development of cost-effective, active trifunctional catalysts for acidic oxygen reduction (ORR) as well as hydrogen and oxygen evolution reactions (HER and OER, respectively) is highly desirable, albeit challenging. Herein, single-atomic Ru sites anchored onto Ti3 C2 Tx MXene nanosheets are first reported to serve as trifunctional electrocatalysts for simultaneously catalyzing acidic HER, OER, and ORR. A half-wave potential of 0.80 V for ORR and small overpotentials of 290 and 70 mV for OER and HER, respectively, at 10 mA cm-2 are achieved. Hence, a low cell voltage of 1.56 V is required for the acidic overall water splitting. The maximum power density of an H2 -O2 fuel cell using the as-prepared catalyst can reach as high as 941 mW cm-2 . Theoretical calculations reveal that isolated Ru-O2 sites can effectively optimize the adsorption of reactants/intermediates and lower the energy barriers for the potential-determining steps, thereby accelerating the HER, ORR, and OER kinetics.