Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

ZIF67@MFC-Derived Co/N-C@CNFs Interconnected Frameworks with Graphitic Carbon-Encapsulated Co Nanoparticles as Highly Stable and Efficient Electrocatalysts for Oxygen Reduction Reactions

Development of nonprecious metal catalysts for oxygen reduction reaction (ORR) to reduce or eliminate Pt-based electrocatalysts is of great importance for fuel cells. Herein, Co/N-codoped carbon with carbon nanofiber (CNF) interconnected three-dimensional (3D) frameworks and graphitic carbon-encapsulated Co nanoparticles were designed and successfully prepared via the in situ growth of zeolitic imidazolate framework-67 (ZIF67) with biomass nano-microfibrillar cellulose (MFC) and then pyrolysis. The catalyst (Co/N-C@CNFs) exhibited outstanding long-term catalytic durability with 92.7% current retention after 70 000 s, which was much higher than that of commercial Pt/C in alkaline media. The support and connection of CNFs to Co/N-C frameworks and the protection of Co nanoparticles by graphite layers contribute to their impressive long-term catalytic stability. Meanwhile, Co/C-N@CNFs displayed excellent ORR catalytic performance (E₀ = 0.952 V vs RHE, E_1/2 = 0.852 V vs RHE, and n: 4.2) in alkaline media. This strategy provides new insights into developing advanced nonprecious metal carbon-based catalysts for ORR.

Oxygen reduction reaction catalyzed by overhanging carboxylic acid strapped iron porphyrins adsorbed on carbon nanotubes

A series of hybrid catalysts for the oxygen reduction reaction (ORR) has been investigated. They are composed of multi-wall carbon nanotubes (MWNTs) coated with iron strapped porphyrins. Two porphyrins have been probed; both are strapped with the same skeleton and differ only by the number of overhung carboxylic acid(s), either one or two. In this structure, the carboxylic acid group can act as a proton relay between the medium and the catalyst or as a polar group surrounding the dioxygen binding cavity. While the number of carboxylic acid group(s) does not exhibit a significant influence on the catalytic properties, the combination of both components–MWNTs and porphyrin–leads to a better catalytic activity than those of the nanotubes or the porphyrins taken separately.

Holistic approach to chemical degradation of Nafion membranes in fuel cells: modelling and predictions

The state of health of polyfluorinated sulfonic-acid ionomer membranes (e.g. Nafion®) in low-temperature proton exchange membrane fuel cells (LT-PEMFCs) is negatively influenced by degradation phenomena occurring during their operation. As a consequence, the performance and durability of the membrane are decreased. In this article, we focus on simulating and predicting chemical membrane degradation phenomena using a holistic zero-dimensional kinetic framework. The knowledge of chemical degradation mechanisms is widely spread. We have collected and evaluated an extensive set of chemical mechanisms to achieve a holistic approach. This yields a set of 23 coupled chemical equations, which provide the whole cause and effect chain of chemical degradation in LT-PEMFCs (based on the Fenton reaction between Fe²⁺ and H₂O₂via the attack of hydroxyl radicals on the membrane, loss of ionomer moieties and emission of fluoride). Our kinetic framework allows the reproduction of experimentally accessible data such as fluoride emission rates and concentrations of ionomer moieties (from both in situ and ex situ tests). We present an approach, which allows estimations of the membrane lifetime based on fluoride emission rates. In addition, we outline the demetallation of Fe-N-C catalysts as a source of additional harmful iron species, which accelerate chemical membrane degradation. To demonstrate the expandability and versatility of the kinetic framework, a set of five chemical equations describing the radical scavenging properties of cerium agents is coupled to the main framework and its influence on membrane degradation is analysed. An automated solving routine for the system of coupled chemical equations on the basis of the chemical kinetic simulation tool COPASI has been developed and is freely accessible online ().

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

The review provides a comprehensive understanding of the atomically dispersed metal–nitrogen–carbon cathode catalysts for proton-exchange membrane fuel cell applications.

Toward Efficient Carbon and Water Cycles: Emerging Opportunities with Single-Site Catalysts Made of 3d Transition Metals

Advances in the chemical and electrochemical transformation of carbon and water are vital for delivering affordable and environmentally friendly energy sources and chemicals. Central to this challenge is the performance of materials. Traditionally, noble metal particles or metal complexes have been used as catalysts for many reactions. Recently, 3d transition-metal single-site catalysts (3dTM-SSCs) have emerged as potentially transformational candidates for the next-generation high-performance noble-metal-free catalysts. Designing catalysts at the molecular level can lead to a more efficient utilization of metal atoms and at the same time enhance catalytic performance under harsh reaction conditions. Despite this promise, several fundamental issues remain, in particular the structural evolution of 3dTM-SSCs during the synthesis, the molecular-level insights into the structure of the active sites, catalytic mechanisms, and the long-term cycling stability. Here, the material chemistries that facilitate the 3dTM-SSCs generation through a controlled pyrolytic synthesis are discussed, with focus on elucidating the underlying performance descriptors that can tune the catalytic properties in various critical reactions in carbon and water cycles. The current challenges and possible solutions for improving these novel catalytic materials are also highlighted.

Theoretical research on the oxidation mechanism of doped carbon based catalysts for oxygen reduction reaction

To understand the essential reasons of poor durability and rapid initial performance loss of heteroatom doped graphene catalysts during the electrochemical oxygen reduction reaction (ORR) process, it is necessary to explore the detailed mechanism of carbon active site oxidation reaction (COR) at different electrode potentials, as it may greatly influence the ORR activity. Herein, density functional theory (DFT) calculation is used to investigate all possible COR mechanisms, including Direct-COR and Indirect-COR, on four typical doped-graphene, and understand the competing relation between COR and ORR from a thermodynamic point of view. Our systematic calculations found that the Direct-COR is affected directly by the structural stability of doped-graphene relative to pure graphite, and the Indirect-COR can be accelerated largely by the ORR process due to the ORR intermediate, such as O and OOH. The competition relation between COR and ORR is mainly influenced by the interaction between the doped-graphene and reaction species, stability of doped-structure, ORR mechanism, and electrode potential. For COR, the partial oxidation of doped-graphene is the dominant oxidation reaction compared to complete oxidation in the ORR potential range. More importantly, both partial and complete oxidation of doped-graphene can remarkably depress the ORR activity. Hence, COR should be one of the major contributors to the rapid initial performance loss of carbon based catalysts in stability testing. Our results provide a comprehensive and deep understanding of the oxidation of carbon active sites on doped-graphene surfaces and can guide the design of more robust doped-carbon based catalysts.

Hierarchically porous carbons as supports for fuel cell electrocatalysts with atomically dispersed Fe-N x moieties

The development of high-performance non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) is still of significance in promoting the commercialization of proton exchange membrane fuel cells (PEMFCs). In this work, a "hierarchically porous carbon (HPC)-supporting" approach was developed to synthesize highly ORR active Fe-phenanthroline (Fe-phen) derived Fe-N x -C catalysts. Compared to commercial carbon black supports, utilizing HPCs as carbon supports can not only prevent the formation of inactive iron nanoparticles during pyrolysis but also optimize the porous morphology of the catalysts, which eventually increases the amount of reactant-accessible and atomically dispersed Fe-N x active sites. The prepared catalyst therefore exhibits a remarkable ORR activity in both half-cells (half-wave potential of 0.80 V in 0.5 M H2SO4) and H2-air PEMFCs (442 mA cm-2 at a working voltage of 0.6 V), making it among the best non-PGM catalysts for PEMFCs.

PGM-Free Cathode Catalysts for PEM Fuel Cells: A Mini-Review on Stability Challenges

In recent years, significant progress has been achieved in the development of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells. At the same time the limited durability of these catalysts remains a great challenge that needs to be addressed. This mini-review summarizes the recent progress in understanding the main causes of instability of PGM-free ORR catalysts in acidic environments, focusing on transition metal/nitrogen codoped systems (M-N-C catalysts, M: Fe, Co, Mn), particularly MN_x moiety active sites. Of several possible degradation mechanisms, demetalation and carbon oxidation are found to be the most likely reasons for M-N-C catalysts/cathodes degradation.

Electrochemical On-line ICP-MS in Electrocatalysis Research

Electrocatalyst degradation due to dissolution is one of the major challenges in electrochemical energy conversion technologies such as fuel cells and electrolysers. While tendencies towards dissolution can be grasped considering available thermodynamic data, the kinetics of material's stability in real conditions is still difficult to predict and have to be measured experimentally, ideally in-situ and/or on-line. On-line inductively coupled plasma mass spectrometry (ICP-MS) is a technique developed recently to address exactly this issue. It allows time- and potential-resolved analysis of dissolution products in the electrolyte during the reaction under dynamic conditions. In this work, applications of on-line ICP-MS techniques in studies embracing dissolution of catalysts for oxygen reduction (ORR) and evolution (OER) as well as hydrogen oxidation (HOR) and evolution (HER) reactions are reviewed.

Investigation of Earth-Abundant Oxygen Reduction Electrocatalysts for the Cathode of Passive Air-Breathing Direct Formate Fuel Cells

The development of direct formate fuel cells encounters important obstacles related to the sluggish oxygen reduction reaction (ORR) and low tolerance to formate ions in Pt-based cathodes. In this study, electrocatalysts formed by earth-abundant elements were synthesized, and their activity and selectivity for the ORR were tested in alkaline electrolyte. The results showed that carbon-encapsulated iron-cobalt alloy nanoparticles and carbon-supported metal nitrides, characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), do not present significant activity for the ORR, showing the same half-wave potential of Vulcan carbon. Contrarily, nitrogen-doped carbon, synthesized using imidazole as the nitrogen source, showed an increase in the half-wave potential, evidencing an influential role of nitrogen in the ORR electrocatalysis. The synthesis with the combination of Vulcan, imidazole, and iron or cobalt precursors resulted in the formation of nitrogen-coordinated iron (or cobalt) moieties, inserted in a carbon matrix, as revealed by X-ray absorption spectroscopy (XAS). Steady-state polarization curves for the ORR evidenced a synergistic effect between Fe and Co when these two metals were included in the synthesis (FeCo-N-C material), showing higher activity and higher limiting current density than the materials prepared only with Fe or Co. The FeCo-N-C material presented not only the highest activity for the ORR (approaching that of the state-of-the-art Pt/C) but also high tolerance to the presence of formate ions in the electrolyte. In addition, measurements with FeCo-N-C in the cathode of an passive air-breathing direct formate fuel cells, (natural diffusion of formate), showed peak power densities of 15.5 and 10.5 mW cm−2 using hydroxide and carbonate-based electrolytes, respectively, and high stability over 120 h of operation.

Dicyandiamide and iron-tannin framework derived nitrogen-doped carbon nanosheets with encapsulated iron carbide nanoparticles as advanced pH-universal oxygen reduction catalysts

The development of an efficient and cost-effective electrocatalyst toward the oxygen reduction reaction (ORR) is of critical importance for diverse renewable electrical energy techniques. Herein, a dicyandiamide and iron-tannin framework-derived nitrogen-doped carbon nanosheet with encapsulated iron carbide nanoparticle (Fe₃C/N-CNS) is developed. Particularly, dicyandiamide is the key to achieve this two-dimensional nitrogen-doped lamellar carbon nanosheet. Owing to the synergistic characteristics including composition and structure, the optimal catalyst exhibits the comparable or even better catalytic activity, as well as superior methanol tolerance and stability compared with platinum/carbon catalyst over the whole pH range. More notably, the current approach can be potentially extended to synthesize additional two-dimensional structured transition-metal/carbon composites for various energy conversion and storage technologies.

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

Abstract

Recent progresses in proton exchange membrane fuel cell electrocatalysts are reviewed in this article in terms of cathodic and anodic reactions with a focus on rational design. These designs are based around gaining active sites using model surface studies and include high-index faceted Pt and Pt-alloy nanocrystals for anodic electrooxidation reactions as well as Pt-based alloy/core–shell structures and carbon-based non-precious metal catalysts for cathodic oxygen reduction reactions (ORR). High-index nanocrystals, alloy nanoparticles, and support effects are highlighted for anodic catalysts, and current developments in ORR electrocatalysts with novel structures and different compositions are emphasized for cathodic catalysts. Active site structures, catalytic performances, and stability in fuel cells are also reviewed for carbon-based non-precious metal catalysts. In addition, further developmental perspectives and the current status of advanced fuel cell electrocatalysts are provided.

Graphical Abstract

Critical advancements in achieving high power and stable nonprecious metal catalyst-based MEAs for real-world proton exchange membrane fuel cell applications

Despite great progress in the development of nonprecious metal catalysts (NPMCs) over the past several decades, the performance and stability of these promising catalysts have not yet achieved commercial readiness for proton exchange membrane fuel cells (PEMFCs). Through rational design of the cathode catalyst layer (CCL), we demonstrate the highest reported performance for an NPMC-based membrane electrode assembly (MEA), achieving a peak power of 570 mW/cm² under air. This record performance is achieved using a precommercial catalyst for which nearly all pores are <3 nm in diameter, challenging previous beliefs regarding the need for larger catalyst pores to achieve high current densities. This advance is achieved at industrially relevant scales (50 cm² MEA) using a precommercial NPMC. In situ electrochemical analysis of the CCLs is also used to help gain insight into the degradation mechanism observed during galvanostatic testing. Overall, the performance of this NPMC-based MEA has achieved commercial readiness and will be introduced into an NPMC-based product for portable power applications.

A Novel Metal–Organic Framework Route to Embed Co Nanoparticles into Multi-Walled Carbon Nanotubes for Effective Oxygen Reduction in Alkaline Media

Metal–organic framework (MOF) materials can be used as precursors to prepare non-precious metal catalysts (NPMCs) for oxygen reduction reaction (ORR). Herein, we prepared a novel MOF material (denoted as Co-bpdc) and then combined it with multi-walled carbon nanotubes (MWCNTs) to form Co-bpdc/MWCNTs composites. After calcination, the cobalt ions from Co-bpdc were converted into Co nanoparticles, which were distributed in the graphite carbon layers and MWCNTs to form Co-bpdc/MWCNTs. The prepared catalysts were characterized by TEM (Transmission electron microscopy), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), BET (Brunauer–Emmett–Teller), and Raman spectroscopy. The electrocatalytic activity was measured by using rotating disk electrode (RDE) voltammetry. The catalysts showed higher ORR catalytic activity than the commercial Pt/C catalyst in alkaline solution. Co-bpdc/MWCNTs-100 showed the highest ORR catalytic activity, with an initial reduction potential and half-wave potential reaching 0.99 V and 0.92 V, respectively. The prepared catalysts also showed superior stability and followed the 4-electron pathway ORR process in alkaline solution.

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts

Fe-N-C catalysts with high O2 reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H2 O2 reduction, a key intermediate during indirect O2 reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeNx Cy moieties and Fe particles encapsulated in N-doped carbon layers (0-100 %) show that both types of sites are active, although moderately, toward H2 O2 reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeNx Cy moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.

"Wiring" Fe-Nx -Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media

This study presents a novel metal-organic-framework-engaged synthesis route based on porous tellurium nanotubes as a sacrificial template for hierarchically porous 1D carbon nanotubes. Furthermore, an ultrathin Fe-ion-containing polydopamine layer has been introduced to generate highly effective FeN_x C active sites into the carbon framework and to induce a high degree of graphitization. The synergistic effects between the hierarchically porous 1D carbon structure and the embedded FeN_x C active sites in the carbon framework manifest in superior catalytic activity toward oxygen reduction reaction (ORR) compared to Pt/C catalyst in both alkaline and acidic media. A rechargeable zinc-air battery assembled in a decoupled configuration with the nonprecious pCNT@Fe@GL/CNF ORR electrode and Ni-Fe LDH/NiF oxygen evolution reaction (OER) electrode exhibits charge-discharge overpotentials similar to the counterparts of Pt/C ORR electrode and IrO₂ OER electrode.

The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction

Fe-N-C series catalysts are always attractive for their high catalytic activity towards the oxygen reduction reaction (ORR). However, they usually consist of various components such as iron nitrides, metallic iron, iron carbides, N-doped carbon and Fe-N₄ moieties, leading to controversial contributions of these components to the catalysis of the ORR, especially iron nitrides. In this work, to investigate the function of iron nitrides, Fe_xN nanoparticles (NPs) embedded in mesoporous N-doped carbon without Fe-N₄ moieties are designed and constructed by a simple histidine-assisted method. Herein, the use of histidine can increase the N and Fe contents in the product. The obtained catalyst exhibits excellent ORR catalytic activity which is very close to that of the commercial Pt/C catalyst in alkaline electrolytes. Combining the catalytic activity, structural characterization (especially from Mössbauer spectroscopy), and the results of DFT calculations for adsorption energies of oxygen on the main surfaces of Fe₂N including ε-Fe₂N and ζ-Fe₂N, it can be deduced that Fe₂N NPs as active species make a contribution to the ORR catalysis, of which ε-Fe_xN (x ≤ 2.1) is more active than ζ-Fe₂N. In addition, we find that there exists an obvious synergistic effect between Fe₂N NPs and N-doped carbon, leading to the greatly enhanced ORR catalytic activity.

Air Breathing Cathodes for Microbial Fuel Cell using Mn-, Fe-, Co- and Ni-containing Platinum Group Metal-free Catalysts

The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in "clean" environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ± 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ± 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe-AAPyr also had the highest output of 251 ± 2.3 μWcm^-2, followed by Co-AAPyr with 196 ± 1.5 μWcm^-2, Ni-AAPyr with 171 ± 3.6 μWcm^-2, Mn-AAPyr with 160 ± 2.8 μWcm^-2 and AC 129 ± 4.2 μWcm^-2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm^-1 to 63.1 mScm^-1. A maximum power density of 482 ± 5 μWcm^-2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

Engineering Favorable Morphology and Structure of Fe-N-C Oxygen-Reduction Catalysts through Tuning of Nitrogen/Carbon Precursors

Structures and morphologies of Fe-N-C catalysts are believed to be crucial because of the number of active sites and local bonding structures governing the overall catalyst performance for the oxygen reduction reaction (ORR). However, the knowledge how to rationally design catalysts is still lacking. By combining different nitrogen/carbon precursors, including polyaniline (PANI), dicyandiamide (DCDA), and melamine (MLMN), we aim to tune catalyst morphology and structure to facilitate the ORR. Instead of the commonly studied single precursors, multiple precursors were used during the synthesis; this provides a new opportunity to promote catalyst activity and stability through a likely synergistic effect. The best-performing Fe-N-C catalyst derived from PANI+DCDA is superior to the individual PANI or DCDA-derived ones. In particular, when compared to the extensively explored PANI-derived catalysts, the binary precursors have an increased half-wave potential of 0.83 V and an enhanced electrochemical stability in challenging acidic media, indicating a significantly increased number of active sites and strengthened local bonding structures. Multiple key factors associated with the observed promotion are elucidated, including the optimal pore size distribution, highest electrochemically active surface area, presence of dominant amorphous carbon, and thick graphitic carbon layers with more pyridinic nitrogen edge sites likely bonded with active atomic iron.

Janus structured Pt–FeNC nanoparticles as a catalyst for the oxygen reduction reaction

Stable Janus structured Pt–FeNC nanoparticles as an ORR catalyst.

Ionically dispersed Fe(ii)–N and Zn(ii)–N in porous carbon for acidic oxygen reduction reactions

A bimetallic Fe, Zn/N/C catalyst with high metal loading, demonstrating high activity for ORR processes in acidic electrolytes.

Co-generation of hydrogen and power/current pulses from supercapacitive MFCs using novel HER iron-based catalysts

In this work, four different supercapacitive microbial fuel cells (SC-MFCs) with carbon brush as the anode and an air-breathing cathode with Fe-Aminoantipyrine (Fe-AAPyr) as the catalyst have been investigated using galvanostatic discharges. The maximum power (P_max) obtained was in the range from 1.7 mW to 1.9 mW for each SC-MFC. This in-series connection of four SC-MFCs almost quadrupled P_max to an operating voltage of 3025 mV and a P_max of 8.1 mW, one of the highest power outputs reported in the literature. An additional electrode (Ad_HER) connected to the anode of the first SC-MFC and placed in the fourth SC-MFC evolved hydrogen. The hydrogen evolution reaction (HER) taking place at the electrode was studied on Pt and two novel platinum group metal-free (PGM-free) catalysts: Fe-Aminoantipyrine (Fe-AAPyr) and Fe-Mebendazole (Fe-MBZ). The amount of H₂ produced was estimated using the Faraday law as 0.86 mMd^-1cm^-2 (0.132 L day^-1) for Pt, 0.83 mMd^-1cm^-2 (0.127 L day^-1) for Fe-AAPyr and 0.8 mMd^-1cm^-2 (0.123 L day^-1) for Fe-MBZ. Hydrogen evolution was also detected using gas chromatography. While HER was taking place, galvanostatic discharges were also performed showing simultaneous H₂ production and pulsed power generation with no need of external power sources.