Enhanced corrosion tolerance and highly durable ORR activity by low Pt electrocatalyst on unique pore structured CNF in PEM fuel cell

Carbon Corrosion Induced by Surface Defects Accelerates Degradation of Platinum/Graphene Catalysts in Oxygen Reduction Reaction

Graphene supported electrocatalysts have demonstrated remarkable catalytic performance for oxygen reduction reaction (ORR). However, their durability and cycling performance are greatly limited by Oswald ripening of platinum (Pt) and graphene support corrosion. Moreover, comprehensive studies on the mechanisms of catalysts degradation under 0.6-1.6 V versus RHE (Reversible Hydrogen Electrode) is still lacking. Herein, degradation mechanisms triggered by different defects on graphene supports are investigated by two cycling protocols. In the start-up/shutdown cycling (1.0-1.6 V vs. RHE), carbon oxidation reaction (COR) leads to shedding or swarm-like aggregation of Pt nanoparticles (NPs). Theoretical simulation results show that the expansion of vacancy defects promotes reaction kinetics of the decisive step in COR, reducing its reaction overpotential. While under the load cycling (0.6-1.0 V vs. RHE), oxygen containing defects lead to an elevated content of Pt in its oxidation state which intensifies Oswald ripening of Pt. The presence of vacancy defects can enhance the transfer of electrons from graphene to the Pt surface, reducing the d-band center of Pt and making it more difficult for the oxidation state of platinum to form in the cycling. This work will provide comprehensive understanding on Pt/Graphene catalysts degradation mechanisms.

Pyridine Functionalized Carbon Nanotubes: Unveiling the Role of External Pyridinic Nitrogen Sites for Oxygen Reduction Reaction

Pyridinic nitrogen has been recognized as the primary active site in nitrogen-doped carbon electrocatalysts for the oxygen reduction reaction (ORR), which is a critical process in many renewable energy devices. However, the preparation of nitrogen-doped carbon catalysts comprised of exclusively pyridinic nitrogen remains challenging, as well as understanding the precise ORR mechanisms on the catalyst. Herein, a novel process is developed using pyridyne reactive intermediates to functionalize carbon nanotubes (CNTs) exclusively with pyridine rings for ORR electrocatalysis. The relationship between the structure and ORR performance of the prepared materials is studied in combination with density functional theory calculations to probe the ORR mechanism on the catalyst. Pyridinic nitrogen can contribute to a more efficient 4-electron reaction pathway, while high level of pyridyne functionalization result in negative structural effects, such as poor electrical conductivity, reduced surface area, and small pore diameters, that suppressed the ORR performance. This study provides insights into pyridine-doped CNTs-functionalized for the first time via pyridyne intermediates-as applied in the ORR and is expected to serve as valuable inspiration in designing high-performance electrocatalysts for energy applications.

Performance Enhancement of Proton Exchange Membrane Fuel Cell through Carbon Nanofibers Grown In Situ on Carbon Paper

We developed an integrated gas diffusion layer (GDL) for proton exchange membrane (PEM) fuel cells by growing carbon nanofibers (CNFs) in situ on carbon paper via the electro-polymerization of polyaniline (PANI) on carbon paper followed by a subsequent carbonization treatment process. The CNF/carbon paper showed a microporous structure and a significantly increased pore volume compared to commercial carbon paper. By utilizing this CNF/carbon paper in a PEM fuel cell, it was found that the cell with CNF/carbon paper had superior performance compared to the commercial GDL at both high and low humidity conditions, and its power density was as high as 1.21 W cm^-2 at 100% relative humidity, which is 26% higher than that of a conventional gas diffusion layer (0.9 W cm^-2). The significant performance enhancement was attributed to a higher pore volume and porosity of the CNF/carbon paper, which improved gas diffusion in the GDL. In addition, the superior performance of the cell with CNF/carbon paper at low relative humidity demonstrated that it had better water retention than the commercial GDL. This study provides a novel and facile method for the surface modification of GDLs to improve the performance of PEM fuel cells. The CNF/carbon paper with a microporous structure has suitable hydrophobicity and lower through-plane resistance, which makes it promising as an advanced substrate for GDLs in fuel cell applications.

Strategies to Improve the Oxygen Reduction Reaction Activity on Pt-Bi Bimetallic Catalysts: A Density Functional Theory Study

Decreasing the level of use of Pt in proton exchange membrane fuel cells is of great research interest both academically and industrially. In this work, we systematically studied the oxygen reduction reaction (ORR) following the four-electron association mechanism at various Pt-Bi surfaces with density functional theory calculations. The results showed that the introduction of Bi changes the potential-determining step of ORR. Moreover, the hydroxy adsorption free energy (G_OH*) can be used as a descriptor of ORR activity, and 0.74 eV is the ideal G_OH* for it to reach its maximum. Notably, we also found that the tensile strain introduced by Bi and electron transfer between Pt and Bi synergize to modulate the d-band of Pt to contract, shift downward, and break the 5d⁹6s¹ valence electron configuration of Pt, and accordingly, PtBi(100), with the lowest d-band center, gives the best ORR activity, which is even slightly higher than that of Pt(111).

N-Doped and Sulfonated Reduced Graphene Oxide Supported PtNi Nanoparticles as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction

N-doping and sulfonation is prepared on the reduced graphene oxide (rGO) support for PtNi nanoparticles (PtNi/S-(N)rGO) by a simple method of hydrothermal synthesis and thermal decomposition. The specific surface area increases from 180.7 m2/g of PtNi/rGo to 293.5 m2/g of PtNi/S-(N)rGO. The surface morphology shows wrinkles sites, which are separated by the sulfonated groups. The catalytic stability and efficiency are improved by the anchoring effect of sulfonated groups and evenly distribution of nanoparticles, respectively. The synergistic effect of N-doping and sulfonation can be in favor of catalytic efficiency by the increase of number of electron transfer. The half-wave potential of the PtNi/S-(N)rGO catalyst is up to 0.632 V, a small positive shift compared to the Pt/C catalyst. The durability of the PtNi/S-(N)rGO is 2.6 times higher than of the Pt/C catalyst after 5000 repeated cycles. The peak power of the PtNi/S-(N)rGO catalyst increased 37.5% compared to the Pt/C catalyst. Therefore, the stability and catalytic efficiency are improved by the PtNi/S-(N)rGO catalyst applied in proton exchange membrane fuel cell (PEMFC) compared to the commercial Pt/C catalyst.

Carbon-Supported Noble-Metal Nanoparticles for Catalytic Applications—A Review

Noble-metal nanoparticles (NMNPs), with their outstanding properties, have been arousing the interest of scientists for centuries. Although our knowledge of them is much more significant today, and we can obtain NMNPs in various sizes, shapes, and compositions, our interest in them has not waned. When talking about noble metals, gold, silver, and platinum come to mind first. Still, we cannot forget about elements belonging to the so-called platinum group, such as ruthenium, rhodium, palladium, osmium, and iridium, whose physical and chemical properties are very similar to those of platinum. It makes them highly demanded and widely used in various applications. This review presents current knowledge on the preparation of all noble metals in the form of nanoparticles and their assembling with carbon supports. We focused on the catalytic applications of these materials in the fuel-cell field. Furthermore, the influence of supporting materials on the electrocatalytic activity, stability, and selectivity of noble-metal-based catalysts is discussed.

Anchoring Highly Dispersed Pt Electrocatalysts on TiOx with Strong Metal-Support Interactions via an Oxygen Vacancy-Assisted Strategy as Durable Catalysts for the Oxygen Reduction Reaction

Pt electrocatalysts with high activity and durability have still crucial issues for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). In this study, a novel catalyst consisting of Pt nanoparticles (NPs) on TiO_x/C composites (TiO_x-V_o-H/C) with abundant oxygen vacancies (V_o) is proposed, which is abbreviated as PTO-V_o-H/C. The introduction of V_o helps anchor highly dispersed Pt NPs with low loading and strengthen the strong metal-support interaction (SMSI), which benefits to the enhanced ORR catalytic activity. Moreover, the accelerated durability test (ADT) demonstrates the higher retention of ORR activity for PTO-V_o-H/C. Experimental and theoretical analyses reveal that electronic interactions between Pt NPs and TiO_x/C composite support give rise to an electron-rich Pt NPs and strong SMSI effect, which is favorable for the electron transfer and stabilization of Pt NPs. More importantly, the assembled PEMFC with PTO-V_o-H/C shows only 6.9% of decay on maximum power density after 3000 ADT cycles while the performance of Pt/C sharply decreased. This work provides a new insight into the unique vacancy regulation of dispersive Pt on metal oxides for superior ORR performance.

A High-Durability Graphitic Black Pearl Supported Pt Catalyst for a Proton Exchange Membrane Fuel Cell Stack

Graphitized black pearl (GBP) 2000 supported Pt nanoparticle catalysts is synthesized by a formic acid reduction method. The results of a half-cell accelerated degradation test (ADT) of two protocols and a single-cell ADT show that, Pt/GBP catalyst has excellent stability and durability compared with commercial Pt/C. Especially, the survival time of Pt/GBP-membrane electrode assembly (MEA) reaches 205 min, indicating that it has better reversal tolerance. After the 1003-hour durability test, the proton exchange membrane fuel cell (PEMFC) stack with Pt/GBP presents a slow voltage degradation rate of 5.19% and 36 μV h⁻¹ at 1000 mA cm⁻². The durability of the stack is improved because of the durability and stability of the catalyst. In addition, the post morphology characterizations indicate that the structure and particle size of the Pt/GBP catalyst remain unchanged during the dynamic testing protocol, implying its better stability under dynamic load cycles. Therefore, Pt/GBP is a valuable and promising catalyst for PEMFC, and considered as an alternative to classical Pt/C.

Recent Developments in Carbon-Based Nanocomposites for Fuel Cell Applications: A Review

Carbon-based nanocomposites have developed as the most promising and emerging materials in nanoscience and technology during the last several years. They are microscopic materials that range in size from 1 to 100 nanometers. They may be distinguished from bulk materials by their size, shape, increased surface-to-volume ratio, and unique physical and chemical characteristics. Carbon nanocomposite matrixes are often created by combining more than two distinct solid phase types. The nanocomposites that were constructed exhibit unique properties, such as significantly enhanced toughness, mechanical strength, and thermal/electrochemical conductivity. As a result of these advantages, nanocomposites have been used in a variety of applications, including catalysts, electrochemical sensors, biosensors, and energy storage devices, among others. This study focuses on the usage of several forms of carbon nanomaterials, such as carbon aerogels, carbon nanofibers, graphene, carbon nanotubes, and fullerenes, in the development of hydrogen fuel cells. These fuel cells have been successfully employed in numerous commercial sectors in recent years, notably in the car industry, due to their cost-effectiveness, eco-friendliness, and long-cyclic durability. Further; we discuss the principles, reaction mechanisms, and cyclic stability of the fuel cells and also new strategies and future challenges related to the development of viable fuel cells.

Electrospun One-Dimensional Electrocatalysts for Oxygen Reduction Reaction: Insights into Structure-Activity Relationship

Oxygen reduction reaction (ORR) is an efficiency-determining process at the cathode in several energy storage and conversion devices, typically such as metal-air batteries and fuel cells. To date, a considerable amount of ORR electrocatalysts have been purposely exploited to address the key issues of high overpotentials and sluggish electrochemical kinetics. Electrospinning is a popular additive manufacturing technology, enabling the production of one-dimensional (1D) electrocatalysts with outstanding chemical stability and structural diversity. However, compared with the well-studied composite/structural design as well as performance advancement, insights into structure-activity relationship are yet to be settled. To clarify this key issue, herein, a dedicated review on the structure-activity relationship between the 1D architectures of electrospun electrocatalysts and their catalytic ORR property is presented. First, the development and principles of electrospinning technique, the composition regulation- and structure design-oriented fundamentals are summarized by imputing the perspectives of mechanistic understanding. Then, the typical examples of nanofiber-shaped and nanofiber-supported electrocatalysts with different compositions and structures for ORR are implemented to establish different structure-activity relationship by comparative studies. Finally, we also identify some ongoing challenges and present future perspectives to direct the precise manipulation of structure-activity relationship for further activation and optimization of electrospun 1D electrocatalysts.

Anchored Pt-Co Nanoparticles on Honeycombed Graphene as Highly Durable Catalysts for the Oxygen Reduction Reaction

Durability is an important factor in evaluating the performance of a catalyst. In this work, the spatial protection of the carrier to nanoparticles was considered to improve the durability of the catalyst. It is found that a honeycombed graphene with a three-dimensional (3D)-hierarchical porous structure (3D HPG) can help to reduce the shedding of Pt-Co nanoparticles (Pt-Co NPs) because 3D HPG can form a protective layer to reduce the direct erosion of Pt-Co NPs on the interface by an electrolyte. Then, appropriate oxygen groups were introduced on the 3D reduced hierarchical porous graphene oxide (3D rHPGO) to improve the dispersion of Pt-Co NPs on the surface of the carrier. It was found that the Pt d-band of the catalyst was anchored by π sites of carbonyl of an oxygen group. After optimization, the catalyst (referred to as Pt-Co/3D rHPGO) achieved a 2-fold enhancement in mass activity than that of a commercial Pt/C catalyst. More importantly, after the accelerated durability test (ADT) of 20 000 cycles, the Pt-Co/3D rHPGO catalyst can almost sustain this level of performance, whereas other catalysts showed a comparatively large loss of activity. According to the results, the high durability of Pt-Co/3D rHPGO was attributed to spatial protection of Pt-Co NPs and the defects on the surface allowed the electrolyte to enter. In addition, oxygen groups provided an anchoring effect on nanoparticles. Thus, the Pt-Co/3D rHPGO electrocatalyst exhibited splendid durability, holding a potential to be applied in PEMFC for long-term work.