Oxygen Reduction In Fuel Cells Research Articles

Spatially Confined Alloying of Pt Accelerates Mass Transport for Fuel Cell Oxygen Reduction.

Pt-based alloy with high mass activity and durability is highly desired for proton exchange membrane fuel cells, yet a great challenge remains due to the high mass transport resistance near catalysts with lowering Pt loading. Herein, an extensible approach employing atomic layer deposition to accurately introduce a gas-phase metal precursor into platinum nanoparticles (NPs) pre-filled mesoporous channels is reported, achieved by controlling both the deposition site and quantity. Following the spatially confined alloying treatment, the prepared PtSn alloy catalyst within mesopores demonstrates a small size and homogeneous distribution (2.10 ± 0.53nm). The membrane electrode assembly with mesoporous carbon-supported PtSn alloy catalyst achieves a high initial mass activity of 0.85 A at 0.9V, which is attributed to the smallest local oxygen transport resistance (3.68 S m-1) ever reported. The mass activity of the catalyst only decreases by 11% after 30000 cycles of accelerated durability test, representing superior full-cell durability among the reported Pt-based alloy catalysts. The enhanced activity and durability are attributed to the decreased adsorption energy of oxygen intermediates on Pt surface and the strong electronic interaction between Pt and Sn inhibiting Pt dissolution.

Engineering a High-Loading Sub-4 nm Intermetallic Platinum-Cobalt Alloy on Atomically Dispersed Cobalt-Nitrogen-Carbon for Efficient Oxygen Reduction in Fuel Cells.

Developing a high-performance membrane electrode assembly (MEA) poses a formidable challenge for fuel cells, which lies in achieving both high metal loading and efficient catalytic activity concurrently for MEA catalysts. Here, we introduce a porous Co@NC carrier to synthesize sub-4 nm PtCo intermetallic nanocrystals, achieving an impressive Pt loading of 27 wt %. The PtCo-CoNC catalyst demonstrates exceptional catalytic activity and remarkable stability for the oxygen reduction reaction. Advanced characterization techniques and theoretical calculations emphasize the synergistic effect between PtCo alloys and single Co atoms, which enhances the desorption of the OH* intermediate. Furthermore, the PtCo-CoNC-based cathode delivers a high power density of 1.22 W cm-2 in the MEA test owing to the enhanced mass transport, which is verified by the simulation results of the O2 distributions and current density inside the catalyst layer. This study lays the groundwork for the design of efficient catalysts with practical applications in fuel cells.

Laccase-Mediated Oxygen Reduction in Liquid Flow Fuel Cells for Efficient Oxidation of Biomass-Derived Aldehydes with Co-Generation of Electricity

Laccase-Mediated Oxygen Reduction in Liquid Flow Fuel Cells for Efficient Oxidation of Biomass-Derived Aldehydes with Co-Generation of Electricity

Pyrazine-linked Iron-coordinated Tetrapyrrole Conjugated Organic Polymer Catalyst with Spatially Proximate Donor-Acceptor Pairs for Oxygen Reduction in Fuel Cells.

Nitrogen-coordinated iron (Fe-N4 ) materials represent the most promising non-noble electrocatalysts for the cathodic oxygen reduction reaction (ORR) of fuel cells. However, molecular-level structure design of Fe-N4 electrocatalyst remains a great challenge. In this study, we develop a novel Fe-N4 conjugated organic polymer (COP) electrocatalyst, which allows for precise design of the Fe-N4 structure, leading to unprecedented ORR performance. At the molecular level, we have successfully organized spatially proximate iron-pyrrole/pyrazine (FePr/Pz) pairs into fully conjugated polymer networks, which in turn endows FePr sites with firmly covalent-bonded matrix, strong d-π electron coupling and highly dense distribution. The resulting pyrazine-linked iron-coordinated tetrapyrrole (Pz-FeTPr) COP electrocatalyst exhibits superior performance compared to most ORR electrocatalysts, with a half-wave potential of 0.933 V and negligible activity decay after 40,000 cycles. When used as the cathode electrocatalyst in a hydroxide exchange membrane fuel cell, the Pz-FeTPr COP achieves a peak power density of ≈210 mW cm-2 . We anticipate the COP based Fe-N4 catalyst design could be an effective strategy to develop high-performance catalyst for facilitating the progress of fuel cells.

Pyrazine‐linked Iron‐coordinated Tetrapyrrole Conjugated Organic Polymer Catalyst with Spatially Proximate Donor‐Acceptor Pairs for Oxygen Reduction in Fuel Cells

AbstractNitrogen‐coordinated iron (Fe−N4) materials represent the most promising non‐noble electrocatalysts for the cathodic oxygen reduction reaction (ORR) of fuel cells. However, molecular‐level structure design of Fe−N4 electrocatalyst remains a great challenge. In this study, we develop a novel Fe−N4 conjugated organic polymer (COP) electrocatalyst, which allows for precise design of the Fe−N4 structure, leading to unprecedented ORR performance. At the molecular level, we have successfully organized spatially proximate iron‐pyrrole/pyrazine (FePr/Pz) pairs into fully conjugated polymer networks, which in turn endows FePr sites with firmly covalent‐bonded matrix, strong d‐π electron coupling and highly dense distribution. The resulting pyrazine‐linked iron‐coordinated tetrapyrrole (Pz−FeTPr) COP electrocatalyst exhibits superior performance compared to most ORR electrocatalysts, with a half‐wave potential of 0.933 V and negligible activity decay after 40,000 cycles. When used as the cathode electrocatalyst in a hydroxide exchange membrane fuel cell, the Pz−FeTPr COP achieves a peak power density of ≈210 mW cm−2. We anticipate the COP based Fe−N4 catalyst design could be an effective strategy to develop high‐performance catalyst for facilitating the progress of fuel cells.

Engineering Structurally Ordered High-Entropy Intermetallic Nanoparticles with High-Activity Facets for Oxygen Reduction in Practical Fuel Cells.

High-entropy solid-solution alloys have generated significant interest in energy conversion technologies. However, structurally ordered high-entropy intermetallic (HEI) nanoparticles (NPs) have been rarely reported in electrocatalysis applications. Here, we demonstrate structurally ordered PtIrFeCoCu HEI (PIFCC-HEI) NPs with extremely superior performance for both oxygen reduction reaction (ORR) and H2/O2 fuel cells. The PIFCC-HEI NPs show an average diameter of 6 nm. Atomic structural characterizations including atomic-resolution energy-dispersive spectroscopy (EDS) mapping technology confirm the ordered intermetallic structure of PIFCC-HEI NPs. As an electrocatalyst for ORR, the PIFCC-HEI/C achieves an ultrahigh mass activity of 7.14 A mgnoblemetals-1 at 0.85 V and extraordinary durability over 60 000 potential cycles. Moreover, the fuel cell assembled with PIFCC-HEI/C as the cathode delivers an ultrahigh peak power density of 1.73 W cm-2 at a back pressure of 1.0 bar and almost no working voltage decay after 80 h operation, certifying the top-level performance among reported fuel cells. Theoretical calculations combined with experimental results reveal that the superior performance of PIFCC-HEI/C for ORR and fuel cells is attributed to its ultrahigh-activity facets. Especially, the (001) facet affords the lowest activation barriers for the rate-limiting step, the optimal downshift of the d-band center, and more efficient regulation of electron structures for ORR. This work not only opens up a new avenue for the fabrication of high-activity facets in the catalysts but also highlights structurally ordered HEI NPs as sufficiently effective catalysts in practical fuel cells and other potential energy-related applications.

Sphere-like PdNi Alloy: Unveiling the Twin Functional Properties toward Oxygen Reduction and Temperature-Dependent Methanol Oxidation for Alkaline Direct Methanol Fuel Cells

The development of high-performance bifunctional palladium-based (Pd) alloys for the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) is a great challenge in direct methanol fuel cells. The incorporation of an oxophilic atom into the Pd atom is an effective strategy to enhance the electrocatalytic activity of the catalysts. Herein a sphere-like PdNi alloy (sPdNiA) was developed via a simple hydrazine-assisted reduction method to overcome the kinetic drawbacks of palladium and delivers superior ORR/MOR performance. The successful alloy formation, the induced strain effect in the electronic structure, and structural characteristics of the sPdNiA were confirmed through XRD, XPS, and HR-TEM studies. As a result, for ORR, the sPdNiA exhibits a positive halfwave potential (E1/2) of ∼0.854 V (vs RHE) compared to the pure Pd (0.833 V vs RHE) nanoparticles. For MOR, the sPdNiA presents the low onset potential and high mass activity of 518 mA mgPd–1 in 1.0 M KOH electrolyte. Furthermore, the temperature-dependent MOR and 3D Bode plot investigations reveal a five-fold higher mass activity at elevated temperature (60 °C) with a lower activation energy (Ea). For the first time, the direct methanol fuel cell is fabricated utilizing sPdNiA as a bifunctional electrode, exhibited a higher peak power density (Pmax) of 34 mW cm–2, whereas 22 mW cm–2 was obtained for Pd Np-based fuel cells. Incorporating oxophilic atom (Ni) not only improves the performance of Pd but significantly reduces the cost of the catalyst material by lowering the Pd content.

Nitrogen doped titania stabilized Pt/C catalyst via selective atomic layer deposition for fuel cell oxygen reduction

The electrochemical dissolution of platinum (Pt) based nanoparticles and the consequent Pt active sites losses and particle aggregation have become a critical stability issue for the commercial proton exchange membrane fuel cell. Herein, nitrogen doped titania (N-TiO2) stabilized Pt low-coordinated sites on a commercial Pt/C catalyst is achieved by simply coupling selective atomic layer deposition of TiO2 and following nitrogen doping process. Selectively deposited N-TiO2 on Pt nanoparticles keeps the exposure of Pt sites on (111) facets and increases the reduction states of Pt, which results in 1.7 times improvement of mass activity than commercial Pt/C. Besides, N-TiO2 stabilized commercial Pt/C catalyst exhibits outstanding durability enhancement, showing only a 14.0% loss of mass activity after 30,000 potential cycles of rotating disk electrode tests and 91.7% retention after durability tests in humid fuel cell condition. The shielding role of N-TiO2 could effectively inhibit the degradation of low-coordinated sites on Pt nanoparticles and maintain Pt size distribution, which is a promising strategy to prolong the lifetime of commercial Pt/C catalysts.

Complexes of Sodium Pectate with Nickel for Hydrogen Oxidation and Oxygen Reduction in Proton-Exchange Membrane Fuel Cells.

A number of nickel complexes of sodium pectate with varied Ni2+ content have been synthesized and characterized. The presence of the proton conductivity, the possibility of the formation of a dense spatial network of transition metals in these coordination biopolymers, and the immobilization of transition ions in the catalytic sites of this class of compounds make them promising for proton-exchange membrane fuel cells. It has been established that the catalytic system composed of a coordination biopolymer with 20% substitution of sodium ions for divalent nickel ions, Ni (20%)-NaPG, is the leading catalyst in the series of 5, 15, 20, 25, 35% substituted pectates. Among the possible reasons for the improvement in performance the larger specific surface area of this sample compared to the other studied materials and the narrowest distribution of the vertical size of metal arrays were registered. The highest activity during CV and proximity to four-electron transfer during the catalytic cycle have also been observed for this compound.

Heterointerface Effect in Accelerating the Cathodic Oxygen Reduction for Intermediate-Temperature Solid Oxide Fuel Cells.

A solid-state mixing method was adopted to prepare a new Pr0.8Sr0.2Fe0.7Ni0.3O3−δ-Pr1.2Sr0.8Fe0.4Ni0.6O4+δ (PSFN113-214) composite cathode oxide for the solid oxide fuel cells (SOFCs). Herein, heterointerface engineering was investigated for the performance enhancement. It was found that the oxygen vacancy content could be increased by mixing the PSFN214 with PSFN113, which gave rise to the formation of a heterostructure, and resulted in the promotion of oxygen ion transport as well as the specific surface area. The optimum mixing ratio 5:5 resulted in the highest oxygen vacancy content and the largest specific surface area, indicating the strongest interface effect. Polarization resistance of PSFN113-214 (5:5) was 0.029 Ω cm2 at 800°C, which was merely 24% of PSFN113 and 39% of PSFN214. The corresponding maximum power density was 0.699 W cm−2, which was nearly 1.44 times of PSFN113 and 1.24 times of PSFN214. Furthermore, the voltage attenuation rate after 100 h was merely 0.0352% h−1. Therefore, the new PSFN113-214 composite could be a prospective cathode oxide for SOFCs.

Ultrasmall, Coating-Free, Pyramidal Platinum Nanoparticles for High Stability Fuel Cell Oxygen Reduction.

Ultrasmall (<5 nm diameter) noble metal nanoparticles with a high fraction of {111} surface domains are of fundamental and practical interest as electrocatalysts, especially in fuel cells; the nanomaterial surface structure dictates its catalytic properties, including kinetics and stability. However, the synthesis of size-controlled, pure Pt-shaped nanocatalysts has remained a formidable chemical challenge. There is an urgent need for an industrially scalable method for their production. Here, a one-step approach is presented for the preparation of single-crystal pyramidal nanocatalysts with a high fraction of {111} surface domains and a diameter below 4 nm. This is achieved by harnessing the shape-directing effect of citrate molecules, together with the strict control of oxidative etching while avoiding polymers, surfactants, and organic solvents. These catalysts exhibit significantly enhanced durability while, providing equivalent current and power densities to highly optimized commercial Pt/C catalysts at the beginning of life (BOL). This is even the case when they are tested in full polymer electrolyte membrane fuel cells (PEMFCs), as opposed to rotating disk experiments that artificially enhance electrode kinetics and minimize degradation. This demonstrates that the {111} surface domains in pyramidal Pt nanoparticles (as opposed to spherical Pt nanoparticles) can improve aggregation/corrosion resistance in realistic fuel cell conditions, leading to a significant improvement in membrane electrode assembly (MEA) stability and lifetime.

Structural Engineering of Cobalt-Free Perovskite Enables Efficient and Durable Oxygen Reduction in Solid Oxide Fuel Cells.

Developing low-cost, efficient, and durable cobalt-free perovskite oxides for oxygen reduction reaction at intermediate-to-low temperatures is crucial to enhance the viability of solid oxide fuel cells (SOFCs), a promising ingredient for establishing a more sustainable future. Herein, a highly active and robust cobalt-free perovskite Ba0.75 Sr0.25 Fe0.95 P0.05 O3-δ (BSFP) oxygen electrode via a facile co-doping strategy for intermediate-to-low temperature SOFCs (ILT-SOFCs) is reported by a combined experimental and theoretical approach. Attributed to stable and oxygen defect-rich structure, and remarkable intrinsic oxygen transport kinetics, the BSFP cathode shows exceptional catalytic performance, including record-level power output among iron-based perovskite cathodes (1464mW cm-2 at 600 °C), low area-specific resistance (≈0.1 Ω cm2 at 600 °C), robust stability both in symmetrical and single cell configurations, and outstanding CO2 tolerance/reversibility. The first-principle calculations validate the role of co-doping of strontium and phosphorus for the high activity and durability. Central to this work is the combined experiment-calculation approach to point to an effective strategy in the development of highly active and stable perovskite-type cathodes for ILT-SOFCs and related applications.

N-Doped Carbon Xerogels from Urea-Resorcinol-Formaldehyde as Carbon Matrix for Fe-N-C Catalysts for Oxygen Reduction in Fuel Cells

N-Doped Carbon Xerogels from Urea-Resorcinol-Formaldehyde as Carbon Matrix for Fe-N-C Catalysts for Oxygen Reduction in Fuel Cells

Minimizing Liquid/Solid Interfacial Energy Boosts Fe−N Doping Inside Hollow Carbon Sphere for Oxygen Reduction in Membrane-Less Direct Formate Fuel Cells

Minimizing Liquid/Solid Interfacial Energy Boosts Fe−N Doping Inside Hollow Carbon Sphere for Oxygen Reduction in Membrane-Less Direct Formate Fuel Cells

Stability challenges of carbon-supported Pt-nanoalloys as fuel cell oxygen reduction reaction electrocatalysts.

Carbon-supported Pt-based nanoalloys (CSPtNs) as the oxygen reduction reaction (ORR) electrocatalysts are considered state-of-the-art electrocatalysts for use in proton exchange membrane fuel cells (PEMFCs). Although their ORR activity performance is already adequate to allow lowering of the Pt loading and thus commercialisation of the fuel cell technology, their stability remains an open challenge. In this Feature Article, the recent achievements and acquired knowledge on the degradation behaviour of these electrocatalysts are overviewed and discussed.

Coupling the Atomically Dispersed Fe‐N3 Sites with Sub‐5 nm Pd Nanocrystals Confined in N‐Doped Carbon Nanobelts to Boost the Oxygen Reduction for Microbial Fuel Cells

AbstractBoth the monodispersed Pd/C (2–5 nm) and Fe‐NC single atoms (SAs) are promising non‐Pt catalysts for oxygen reduction reaction (ORR), which belongs to precious metal and nonprecious metal camps, respectively. However, the poles apart of sub‐5 nm Pd/C and Fe‐NC SAs in synthesis and thermostability leave the challenge to integrate them together in one system. Herein, a 1‐naphthylamine protected pyrolysis mechanism is devised to couple the atomically dispersed Fe sites with sub‐5 nm Pd nanocrystals embedded in N‐doped carbon nanobelts (FeN3‐Pd@NC NBs). The FeN3 SAs represent the minimal surface blockage to tune the electronic structure of Pd, while the carbon frameworks are born with ultrathin, porous, and N‐doped feature's. As inspired, the FeN3‐Pd@NC NBs exhibit outstanding activity (E1/2 = 0.926 V) and durability (2 mV decay in E1/2 after 2000 cycles) for ORR, as well as achieving a maximum power density of 831.2 mW cm−2 in a microbial fuel cell operated for over 100 d. Density functional theory calculation reveals that the FeN3 SAs can shift the density of states of Pd toward the Fermi level, and their coupling can decrease the limiting reaction barrier with a value of −0.62 eV, thus greatly accelerating the ORR kinetics.

Advanced Atomically Dispersed Metal–Nitrogen–Carbon Catalysts Toward Cathodic Oxygen Reduction in PEM Fuel Cells

AbstractProton exchange membrane fuel cells (PEMFCs) are a highly efficient hydrogen energy conversion technology, which shows great potential in mitigating carbon emissions and the energy crisis. Currently, to accelerate the kinetics of the oxygen reduction reaction (ORR) required for PEMFCs, extensive utilization of expensive and rare platinum‐based catalysts are required at the cathodic side, impeding their large‐scale commercialization. In response to this issue, atomically dispersed metal–nitrogen–carbon (M–N–C) catalysts with cost‐effectiveness, encouraging activity, and unique advantages (e.g., homogeneous activity sites, high atom efficiency, and intrinsic activity) have been widely investigated. Considerable progress in this domain has been witnessed in the past decade. Herein, a comprehensive summary of recent development in atomically dispersed M–N–C catalysts for the ORR under acidic conditions and of their application in the membrane electrode assembly (MEA) of PEM fuel cells, are presented. The ORR mechanisms, composition, and operating principles of PEMFCs are introduced. Thereafter, atomically dispersed M–N–C catalysts towards improved acidic ORR and MEA performance is summarized in detail, and improvement strategies for MEA performance and stability are systematically analyzed. Finally, remaining challenges and significant research directions for design and development of high‐performance atomically dispersed M–N–C catalysts and MEA are discussed.

Chemical Vapor Deposition for N/S-Doped Single Fe Site Catalysts for the Oxygen Reduction in Direct Methanol Fuel Cells

Single metal site catalysts are the most promising candidates to replace platinum-group-metal (PGM) catalysts for the oxygen reduction reaction (ORR), yet insufficient performance and scalable preparation approaches remain great challenges. Here, we report a nitrogen (N)/sulfur (S) codoped single Fe site catalyst (Fe–N/S–C) through a chemical vapor deposition (CVD) strategy. Using the cyclopentadiene-shielded Fe atom ferrocene (Fc) as the precursor, atomically dispersed single Fe sites were successfully embedded into the N, S codoped 2D carbon nanosheets. The superior catalytic activity for the ORR in alkaline media is stemmed from the N, S codoping, tuning the optimal charge distribution of Fe sites. In addition, the CVD approach could surpress the formation of iron-carbide-containing iron clusters (“FexC/Fe”), thereby leading to high surface areas and porosity. Furthermore, the Fe–N/S–C catalyst was further studied as a cathode catalyst in direct methanol fuel cells showing encouraging performance.

Single Atomic Cerium Sites with a High Coordination Number for Efficient Oxygen Reduction in Proton-Exchange Membrane Fuel Cells

Fe–N–C electrocatalysts, as a representative of platinum group metal-free (PGM-free) catalysts, exhibit a comparable oxygen reduction reaction (ORR) activity but insufficient stability to that of commercial Pt/C in proton-exchange membrane fuel cells (PEMFCs), due to the unavoidable Fenton’s reactions. Herein, we report a hard-template approach to synthesize the rare-earth single-cerium-atom-doped metal–organic frameworks with a hierarchically macro–meso–microporous structure. Spherical aberration correction electron microscopy confirms the atomic dispersion of Ce sites. Additionally, X-ray absorption spectroscopy (XAS) was employed to further verify the coordination environment of Ce sites, which were stabilized by four-coordinated nitrogen atoms and six-oxygen atoms (Ce–N4/O6). The Ce sites were embedded in a hierarchically macro–meso–microporous N-doped carbon (Ce SAS/HPNC) catalyst, which exhibits a half-wave potential of 0.862 V in ORR and the highest power density of 0.525 W cm–2 under 2.0 bar H2/O2 in the fuel cell test.

Resolving the Dilemma of Nanoparticles’ Structure-Property Relationships at the Atomic Level: Case Study of Pt-Based PEM Fuel Cell Oxygen Reduction Electrocatalysts

Achieving highly active and stable oxygen reduction reaction (ORR) performance at low platinum-group-metal (PGM) loadings remains one of the grand challenges in the proton-exchange membrane fuel cells (PEMFCs) community. Currently, state-of-the-art electrocatalysts are high-surface-area-carbon-supported nanoalloys of platinum (Pt) with different transition metals M (M = Cu, Ni, Fe, Co, etc.). Nevertheless, despite years of focused research, the established structure-property relationships are not able to explain and predict the electrochemical performance and behaviour of the real nanoparticulate systems. Unlike the perfect core-shell systems, usually used to model the observed behaviour of the electrocatalysts by the community, nanoparticles in real nanocatalysts exhibit much greater local atomic-scale structural complexity. Structure-property relationships addressing averaged material properties, such as the actual ORR activity and metal degradation (e.g. dissolution) behaviour, are at risk of oversimplifying the underlying phenomena when they rely on top-down characterization techniques and attribute averaged affects to idealized nanoparticle structures. In this work, we reveal the complexity of commercially available Pt/C and Pt-M/C electrocatalysts and discuss the relevance of understanding their stability behaviour in fuel cell systems. We expose the limitations of the established top-down characterization methodologies for studying degradation and provide a conceptual approach that offers a new dimension and transcends the limits of conventional structure-stability studies. We outline a bottom-up approach where atomically resolved properties, structural changes and strain analysis are recorded as well as analysed in great detail on an individual nanoparticle. While fundamental, this approach addresses the complexity of practical electrocatalysts and enables studying their stability when exposed to realistic electrochemical conditions (i.e. high current density). This methodology, in our opinion, offers the new level of understanding structure-stability relationships of practically viable nanoparticulate systems.