Water management strategies for PGM-free catalyst layers for polymer electrolyte fuel cells

Platinum group metal–free (PGM-free) catalysts are promising candidates to catalyze the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs). Because of their low activity, larger loadings are used resulting in thicker catalyst layers. Transport, particularly water management, thereby becomes a more prominent performance factor. Currently, very few works attempted to understand water management in PGM-free catalyst layers, mainly because of other challenges that had to be overcome first, such as enhancing their activity and durability. The field has also been active in a hypothesis discussion of micropores flooding that led to the belief that poor stability of the PEFC performance is linked to active sites flooding within the micropores. We present here an overview of recent advances in understanding water management in the PGM-free catalyst layer for oxygen reduction reaction in PEFCs and provide an opinion on design guidance in optimizing catalyst layers to avoid flooding.     

Understanding the Catalytic Sites of Metal–Nitrogen–Carbon Oxygen Reduction Electrocatalysts

The development of low-cost catalysts containing earth-abundant elements as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for the large-scale commercial application of proton exchange membrane fuel cells (PEMFCs). Nonprecious metal–nitrogen–carbon (M-N-C) materials represent the most promising candidates to replace Pt-based catalysts for PEMFCs applications. However, the high-temperature pyrolysis process for the preparation of M-N-C catalysts frequently leads to high structural heterogeneity, that is, the coexistence of various metal-containing sites and N-doped carbon structures. Unfortunately, this impedes the identification of the predominant catalytic active structure, and thus, the further development of highly efficient M-N-C catalysts for the ORR. This Minireview, after a brief introduction to the development of M-N-C ORR catalysts, focuses on the commonly accepted views of predominant catalytic active structures in M-N-C catalysts, including atomically dispersed metal–N_x sites, metal nanoparticles encapsulated with nitrogen-doped carbon structures, synergistic action between metal–N_x sites and encapsulated metal nanoparticles, and metal-free nitrogen-doped carbon structures.     

Single-atom Catalysts for Polymer Electrolyte Membrane Fuel Cells

Searching for high-activity, stability and highly cost-effective electrocatalysts for acid oxygen reaction reduction(ORR) has always been an urgent problem in polymer electrolyte membrane fuel cells(PEMFCs). Nonetheless, the electrochemical properties of various systems have their intrinsic limits and tremendous efforts have been paid out to search for highly efficient electrocatalysts by more rational control over the size, morphology, composition, and structure. In particular, single-atom catalysts(SACs) have attracted extensive interest due to theirs excellent activity, stability, selectivity and the highest metal utilization. In recent years, the number of papers in the field of SACs has increased rapidly, indicating that SACs have made great progress. This review focuses on SACs electrochemical applications in the acid ORR and introduces innovative syntheses, fuel cell performance and long-time durability.     

高活性和耐久性非铂氧还原催化剂的研究进展（英文）

质子交换膜燃料电池(PEMFCs)阴极氧还原反应(ORR)动力学迟缓,需要消耗大量的贵金属催化剂,这限制了其商业化应用。目前,原子级分散的M-N-C (M=Fe,Co,Mn等)催化剂受到人们青睐,有望替代铂催化剂。在过去的几十年里,M-N-C催化剂取得了很大的进步,具有优异的ORR活性,而且燃料电池初始性能有希望接近传统的Pt/C催化剂。然而,这些高活性的Fe-N-C催化剂在燃料电池实际工作条件下的稳定性比较差。这篇综述总结了在高效氧还原M-N-C催化剂方面的最近进展,主要概述了作者课题组在限域策略和自旋调控方面的贡献。此外,我们还总结了几种提高活性的有效方法以及近期的关于揭示M-N-C催化剂的降解机制的认识,如金属浸出、碳腐蚀、质子化和微孔淹没都会造成催化剂降解。为改善M-N-C催化剂的寿命,我们概括了文献中的缓解策略,包括控制催化剂中S1/S2位点、使用非铁基催化剂、增强金属氮键、改善碳载体的耐腐蚀性和使用质子缓冲液等。最后,提出了目前原子级分散的M-N-C催化剂的存在的挑战和可能的解决方案。     

Bioinspired Transition-Metal Complexes as Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the most important reactions in life processes and energy conversion systems. To alleviate global warming and the energy crisis, the development of high-performance electrocatalysts for the ORR for application in energy conversion and storage devices such as metal–air batteries and fuel cells is highly desirable. Inspired by the biological oxygen activation/reduction process associated with heme- and multicopper-containing metalloenzymes, iron and copper-based transition-metal complexes have been extensively explored as ORR electrocatalysts. Herein, an outline into recent progress on non-precious-metal electrocatalysts for the ORR is provided; these electrocatalysts do not require pyrolysis treatment, which is regarded as desirable from the viewpoint of bioinspired molecular catalyst design, focusing on iron/cobalt macrocycles (porphyrins, phthalocyanines, and corroles) and copper complexes in which the ORR activity is tuned by ligand variation/substitution, the method of catalyst immobilization, and the underlying supporting materials. Current challenges and exciting imminent developments in bioinspired ORR electrocatalysts are summarized and proposed.     

Synthesis and characterization of iron‐ and nitrogen‐functionalized graphene catalysts for oxygen reduction reaction

Iron‐ and nitrogen‐functionalized graphene (Fe‐N‐G), as well as iron‐ and nitrogen‐functionalized oxidized graphene (Fe‐N‐Gox) catalysts were synthesized as non‐noble metal electrocatalysts for oxygen reduction reaction (ORR). The physical properties of the resultant catalysts were characterized using nitrogen adsorption measurements, X‐ray diffraction, Raman and X‐ray photoelectron spectroscopies and transmission electron microscopy. Subsequently, ORR activities of the catalysts were determined electrochemically using a conventional three‐electrode cell via cyclic voltammetry with a rotating disc electrode, the results of which indicated that the synthesized catalysts had a marked electrocatalytic activity towards ORR in acid media. Among the synthesized catalysts, that functionalized using 2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine as nitrogen source had the highest electrocatalytic activity with the highest onset potential (0.98 V/SHE) and limiting current density (5.12 mA cm⁻²). The findings are particularly important to determine a non‐precious metal catalyst for ORR activity in fuel cells.     

Oxygen electroreduction reaction at bidimensional materials

The electrocatalysis of oxygen reduction reaction (ORR) is of paramount importance in energy-converting systems such as fuel cells and metal–air batteries. Unfortunately, the ORR kinetics is sluggish even at prohibitive platinum group metal catalysts. Two-dimensional materials have attracted increasing interest for energy production and storage in the last years, due to their exceptional chemical, physical, optical, and electronic properties. In this review, we briefly report the recent progress of two-dimensional catalysts for the ORR. Particularly, we approach to heteroatom-doped graphenic materials, dichalcogenides, and MXenes offering results that demonstrate outstanding properties of these materials for the construction of competitive ORR electrocatalysts without platinum group metals.     

Self-assembled dopamine nanolayers wrapped carbon nanotubes as carbon-carbon bi-functional nanocatalyst for highly efficient oxygen reduction reaction and antiviral drug monitoring

Oxygen reduction reaction (ORR) catalysts are the heart of eco-friendly energy resources particularly low temperature fuel cells. Although valuable efforts have been devoted to synthesize high performance catalysts for ORR, considerable challenges are extremely desirable in the development of energy technologies. Herein, we report a simple self-polymerization method to build a thin film of dopamine along the tubular nanostructures of multi-walled carbon nanotubes (CNT) in a weak alkaline solution. The dopamine@CNT hybrid (denoted as DA@CNT) reveals an enhanced electrocatalytic activity towards ORR with highly positive onset potential and cathodic current as a result of their outstanding features of longitudinal mesoporous structure, high surface area, and ornamentation of DA layers with nitrogen moieties, which enable fast electron transport and fully exposed electroactive sites. Impressively, the as-obtained hybrid afford remarkable electrochemical durability for prolonged test time of 60,000 s compared to benchmark Pt/C (20 wt%) catalyst. Furthermore, the developed DA@CNT electrode was successfully applied to access the quality of antiviral drug named Valacyclovir (VCR). The DA@CNT electrode shows enhanced sensing performance in terms of large linear range (3–75 nM), low limit of detection (2.55 nM) than CNT based electrode, indicating the effectiveness of the DA coating. Interestingly, the synergetic effect of nanostructured DA and CNT can significantly boost the electronic configuration and exposure level of active species for ORR and biomolecule recognition. Therefore, the existing carbon-based porous electrocatalyst may find numerous translational applications as attractive alternative to noble metals in polymer electrolyte membrane fuel cells and quality control assessment of pharmaceutical and therapeutic drugs.     

A Layer-Structured Metal-Organic Framework-Derived Mesoporous Carbon for Efficient Oxygen Reduction Reaction

Developing highly active and durable electrocatalysts for the oxygen reduction reaction (ORR) is crucial to large-scale commercialization of fuel cells and metal-air batteries. Here we report a facile approach for the synthesis of nitrogen and oxygen dual-doped mesoporous layer-structured carbon electrocatalyst embedded with graphitic carbon coated cobalt nanoparticles by direct pyrolysis of a layer-structured metal-organic framework. The electrocatalyst prepared at 800℃ exhibits comparable ORR performance to Pt/C catalysts but possesses superior stability to Pt/C catalysts. This synthetic approach provides new prospects in developing sustainable carbon-based electrocatalysts for electrochemical energy conversion devices.     

包覆型非贵金属氧还原催化剂的研究进展

燃料电池中广泛使用的铂基催化剂价格昂贵、储量低、容易失活,因此亟待开发廉价、高效非铂催化剂. 过渡金属(Fe、Co、Ni等)/杂原子共掺杂催化剂、杂原子掺杂（N、P、S、F等）碳材料以及碳材料包覆过渡金属复合物是目前发现的几类性能优异的非贵金属氧还原催化剂. 其中碳材料包覆过渡金属催化剂作为一类新型的高性能催化剂,对其研究还有待深入. 本文主要阐述了国内外在包覆型非贵金属氧还原催化剂方面的研究进展,从合成,性能,机理等方面对该类催化剂进行了总结,力求助益于该类催化剂的发展.     

Theoretical Modelling and Facile Synthesis of a Highly Active Boron‐Doped Palladium Catalyst for the Oxygen Reduction Reaction

A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode‐electrode reaction of fuel cells, is sought for higher fuel‐cell performance. Our theoretical modelling reveals that B‐doped Pd (Pd‐B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O₂ dissociation, suggesting Pd‐B should be highly active for ORR. In fact, Pd‐B, facile synthesized by an electroless deposition process, exhibits 2.2 times and 8.8 times higher specific activity and 14 times and 35 times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti‐bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity.     

Pomegranate‐Inspired Design of Highly Active and Durable Bifunctional Electrocatalysts for Rechargeable Metal–Air Batteries

Rational design of highly active and durable electrocatalysts for oxygen reactions is critical for rechargeable metal–air batteries. Herein, we report the design and development of composite electrocatalysts based on transition metal oxide nanocrystals embedded in a nitrogen‐doped, partially graphitized carbon framework. Benefiting from the unique pomegranate‐like architecture, the composite catalysts possess abundant active sites, strong synergetic coupling, enhanced electron transfer, and high efficiencies in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The Co₃O₄‐based composite electrocatalyst exhibited a high half‐wave potential of 0.842 V for ORR, and a low overpotential of only 450 mV at the current density of 10 mA cm^?2 for OER. A single‐cell zinc–air battery was also fabricated with superior durability, holding great promise in the practical implementation of rechargeable metal–air batteries.     

Oxygen Reduction Reaction in a Droplet on Graphite: Direct Evidence that the Edge Is More Active than the Basal Plane

Carbon‐based metal‐free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium have been extensively investigated with the aim of replacing the commercially available, but precious platinum‐based catalysts. For the proper design of carbon‐based metal‐free electrocatalysts for the ORR, it would be interesting to identify the active sites of the electrocatalyst. The ORR was now studied with an air‐saturated electrolyte solution droplet (diameter ca. 15 μm), which was deposited at a specified position either on the edge or on the basal plane of highly oriented pyrolytic graphite. Electrochemical measurements suggest that the edge carbon atoms are more active than the basal‐plane ones for the ORR. This provides a direct way to identify the active sites of carbon materials for the ORR. Ball‐milled graphite and carbon nanotubes with more exposed edges were also prepared and showed significantly enhanced ORR activity. DFT calculations elucidated the mechanism by which the charged edge carbon atoms result in the higher ORR activity.     

The influence of the type of N dopping on the performance of bifunctional N-doped ordered mesoporous carbon electrocatalysts in oxygen reduction and evolution reaction

To develop more ideal bifunctional heteroatom-doped carbon electrocatalysts toward the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) for regenerative fuel cells and rechargeable metal–air batteries, herein, tobacco-derived N-containing ordered mesoporous carbon(N-OMC) electrocatalysts with different N species distributions are designed. Results indicate that the as-prepared N-OMC with more pyrrolic and pyridinic Ns exhibits much higher activities for the ORR and OER than N-OMC with more graphitic N in both acidic and alkaline media, suggesting that the increase of pyrrolic and pyridinic Ns favors the improvement of ORR and OER activities of the N-containing carbon catalysts, and showing a great potential for the designing of more effective, lower-cost ORR and OER bifunctional electrocatalysts for future regenerative fuel cells and rechargeable metal–air batteries.     

Rational design of platinum-group-metal-free electrocatalysts for oxygen reduction reaction

Platinum-group-metal (PGM)-free materials have been promised as potential replacement for Pt as the cathodic catalyst in proton exchange membrane fuel cells. Critical design criteria of the PGM-free catalyst reside on the high active site density to compensate its generally lower turn-over frequency and improved mass-charge transfers during the electrocatalysis. This short review summarizes the research activities in recent years from our team at Argonne National Laboratory in preparing highly active oxygen reduction reaction (ORR) catalysts using rationally designed porous organic precursors, as reported in the First Telluride Science Research Center (TSRC) Workshop on PGM-free Electrocatalysis in 2019. More recent studies by others are also discussed.     

Co@Co3O4 Encapsulated in Carbon Nanotube‐Grafted Nitrogen‐Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode

Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal–air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core–shell Co@Co₃O₄ nanoparticles embedded in CNT‐grafted N‐doped carbon‐polyhedra obtained by the pyrolysis of cobalt metal–organic framework (ZIF‐67) in a reductive H₂ atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO₂, and RuO₂ and thus ranking them among one of the best non‐precious‐metal electrocatalysts for reversible oxygen electrodes.     

Highly Dispersed and Nano-sized Pt-based Electrocatalysts for Low-Temperature Fuel Cells

Among metals, Pt is so far the best material to be used as anode and cathode in low-temperature fuel cells. However, Pt has the drawback of being expensive and easily CO-poisoned. Thus, to produce useful electrocatalysts, significant efforts have been made worldwide on developing Pt-based catalysts with low Pt contents as well as searching for alternative materials with high catalytic activity for anodic and cathodic reactions in low-temperature fuel cells. This article presents the development of highly dispersed and nano-sized Pt-based electrocatalysts synthesized by several new methods based on our experimental results. In the case of anode materials, our proposed new method consists of the synthesis of Pt-based nanoparticles in order to maximize their surface availability, combined with the use of secondary metals that promote the oxidations of methanol and CO. On the other hand, for the cathode materials, the use of the Pt catalysts mixed with metal oxides enhances their oxygen reduction reaction (ORR) activity. We anticipate that the highly dispersed Pt-based nanoparticles introduced in this article will improve the performance of anode and cathode for low-temperature fuel cells.     

Bottom‐Up Fabrication of a Sandwich‐Like Carbon/Graphene Heterostructure with Built‐In FeNC Dopants as Non‐Noble Electrocatalyst for Oxygen Reduction Reaction

High‐performance non‐noble electrocatalysts for oxygen reduction reaction (ORR) are the prerequisite for large‐scale utilization of fuel cells. Herein, a type of sandwiched‐like non‐noble electrocatalyst with highly dispersed FeN_x active sites embedded in a hierarchically porous carbon/graphene heterostructure was fabricated using a bottom‐up strategy. The in situ ion substitution of Fe³⁺ in a nitrogen‐containing MOF (ZIF‐8) allows the Fe‐heteroatoms to be uniformly distributed in the MOF precursor, and the assembly of Fe‐doped ZIF‐8 nano‐crystals with graphene‐oxide and in situ reduction of graphene‐oxide afford a sandwiched‐like Fe‐doped ZIF‐8/graphene heterostructure. This type of heterostructure enables simultaneous optimization of FeN_x active sites, architecture and interface properties for obtaining an electron‐catalyst after a one‐step carbonization. The synergistic effect of these factors render the resulting catalysts with excellent ORR activities. The half‐wave potential of 0.88 V vs. RHE outperforms most of the none‐noble metal catalyst and is comparable with the commercial Pt/C (20 wt %) catalyst. Apart from the high activity, this catalyst exhibits excellent durability and good methanol‐tolerance. Detailed investigations demonstrate that a moderate content of Fe dopants can effectively increase the intrinsic activities, and the hybridization of graphene can enhance the reaction kinetics of ORR. The strategy proposed in this work gives an inspiration towards developing efficient noble‐metal‐free electrocatalysts for ORR.     

Titanium Dioxide‐Grafted Copper Complexes: High‐Performance Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathodes of fuel cells significantly hampers fuel cell performance. Therefore, the development of high‐performance, non‐precious‐metal catalysts as alternatives to noble metal Pt‐based ORR electrocatalysts is highly desirable for the large‐scale commercialization of fuel cells. TiO₂‐grafted copper complexes deposited on multiwalled carbon nanotubes (CNTs) form stable and efficient electrocatalysts for the ORR. The optimized catalyst composite CNTs@TiO₂–ZA–[Cu(phen)(BTC)] shows surprisingly high selectivity for the 4 e^? reduction of O₂ to water (approximately 97 %) in alkaline solution with an onset potential of 0.988 V vs. RHE, and demonstrates superior stability and excellent tolerance for the methanol crossover effect in comparison to a commercial Pt/C catalyst. The copper complexes were grafted onto the surface of TiO₂ through coordination of an imidazole‐containing ligand, zoledronic acid (ZA), which binds to TiO₂ through its bis‐phosphoric acid anchoring group. Rational optimization of the copper catalyst’s ORR performance was achieved by using an electron‐deficient ligand, 5‐nitro‐1,10‐phenanthroline (phen), and bridging benzene‐1,3,5‐tricarboxylate (BTC). This facile approach to the assembly of copper catalysts on TiO₂ with rationally tuned ORR activity will have significant implications for the development of high‐performance, non‐precious‐metal ORR catalysts.