Tungsten‐Doping‐Induced Surface Reconstruction of Porous Ternary Pt‐Based Alloy Electrocatalyst for Oxygen Reduction

Surface engineering has been found to be effective in promoting the catalytic activities of noble‐metal‐based nanocatalysts. In this contribution, by using the PtCu_xNi ternary alloy nanocrystal (NC) as the model catalyst, a surface tungsten(W)‐doping strategy, combining a surface oxidative acid treatment protocol, can effectively boost the electrocatalytic activities of the NCs in oxygen reduction reaction. The W‐doped PtCu_xNi alloy catalysts show obvious enhancement in electrochemical surface area and mass activity and slightly enhanced specific activity compared with the undoped catalyst. Based on the experimental evidence, it is proposed that the W doping involves a surface reconstruction by first removing the surface Pt atoms from the NC and then reducing them back to the surface. The existence of surface Ni atoms may be crucial in promoting the catalytic activities possibly through their electronic interactions to the active sites. The durability of the W‐doped PtCu_xNi catalysts is also enhanced possibly due to the pinning effect of surface W atoms. Therefore, the surface engineering of PtCu_xNi ternary alloy by W atoms can effectively modulate its activity and durability.     

Mesostructured Intermetallic Compounds of Platinum and Non‐Transition Metals for Enhanced Electrocatalysis of Oxygen Reduction Reaction

Alloying techniques show genuine potential to develop more effective catalysts than Pt for oxygen reduction reaction (ORR), which is the key challenge in many important electrochemical energy conversion and storage devices, such as fuel cells and metal‐air batteries. Tremendous efforts have been made to improve ORR activity by designing bimetallic nanocatalysts, which have been limited to only alloys of platinum and transition metals (TMs). The Pt‐TM alloys suffer from critical durability in acid‐media fuel cells. Here a new class of mesostructured Pt–Al catalysts is reported, consisting of atomic‐layer‐thick Pt skin and Pt₃Al or Pt₅Al intermetallic compound skeletons for the enhanced ORR performance. As a result of strong Pt–Al bonds that inhibit the evolution of Pt skin and produce ligand and compressive strain effects, the Pt₃Al and Pt₅Al mesoporous catalysts are exceptionally durable and ≈6.3‐ and ≈5.0‐fold more active than the state‐of‐the‐art Pt/C catalyst at 0.90 V, respectively. The high performance makes them promising candidates as cathode nanocatalysts in next‐generation fuel cells.     

Platinum Porous Nanosheets with High Surface Distortion and Pt Utilization for Enhanced Oxygen Reduction Catalysis

Unlike the well‐established shape/composition control, surface distortion is a newly emerged yet largely unexplored nanosurface engineering for boosting electrocatalysis. Tapping into the novel electrocatalysts for taking full use of the distortion effect is therefore of importance but remains a formidable challenge. Here, an approach to designing highly distorted porous Pt nanosheets (NSs) by electrochemical erosion of ultrathin PtTe₂ NSs is reported. The inherent ultrathin feature and massive leaching of Te have conspired to produce a highly distorted structure. As a result, the generated Pt NSs exhibit a much‐enhanced oxygen reduction reaction (ORR) mass and specific activity of 2.07 A mg_Pt^?1 and 3.1 mA cm^?2 at 0.90 V versus reversible hydrogen electrode, 9.8 and 10.7 times higher than those of commercial Pt/C. The highly distorted Pt NSs can endure 30 000 cycles with negligible activity decay and structure variation. Density functional theory calculations reveal that the electrochemical corrosion induced nanopores, boundaries, and vacancies consist of Pt sites with substantially low coordination numbers deviating from the one of pristine Pt (111) surface. These Pt sites actively act as electron‐depleting centers for highly efficient electron transfer toward the adsorbing O‐species. This study opens a new design for fully using the distortion effect to promote ORR performance and beyond.     

Fe,Cu‐Coordinated ZIF‐Derived Carbon Framework for Efficient Oxygen Reduction Reaction and Zinc–Air Batteries

Zeolitic imidazole frameworks (ZIFs) offer rich platforms for rational design and construction of high‐performance nonprecious‐metal oxygen reduction reaction (ORR) catalysts owing to their flexibility, hierarchical porous structures, and high surface area. Herein, an Fe, Cu‐coordinated ZIF‐derived carbon framework (Cu@Fe‐N‐C) with a well‐defined morphology of truncated rhombic dodecahedron is facilely prepared by introducing Fe²⁺ and Cu²⁺ during the growth of ZIF‐8, followed by pyrolysis. The obtained Cu@Fe‐N‐C, with bimetallic active sites, large surface area, high nitrogen doping level, and conductive carbon frameworks, exhibits excellent ORR performance. It displays 50 mV higher half‐wave potential (0.892 V) than that of Pt catalysts in an alkaline medium and comparable performance to Pt catalysts in an acidic medium. In addition, it also has excellent durability and methanol resistance ability in both acidic and alkaline solutions, which makes it one of the best Pt‐free catalysts reported to date for ORR. Impressively, when being employed as a cathode catalyst in zinc–air batteries, Cu@Fe‐N‐C presents a higher peak power density of 92 mW cm^?2 than that of Pt/C (74 mW cm^?2) as well as excellent durability.     

The Quasi‐Pt‐Allotrope Catalyst: Hollow PtCo@single‐Atom Pt1 on Nitrogen‐Doped Carbon toward Superior Oxygen Reduction

Single‐atom Pt and bimetallic Pt₃Co are considered the most promising oxygen reduction reaction (ORR) catalysts, with a much lower price than pure Pt. The combination of single‐atom Pt and bimetallic Pt₃Co in a highly active nanomaterial, however, is challenging and vulnerable to agglomeration under realistic reaction conditions, leading to a rapid fall in the ORR. Here, a sustainable quasi‐Pt‐allotrope catalyst, composed of hollow Pt₃Co (H‐PtCo) alloy cores and N‐doped carbon anchoring single atom Pt shells (Pt₁N‐C), is constructed. This unique nanoarchitecture enables the inner and exterior spaces to be easily accessible, exposing an extra‐high active surface area and active sites for the penetration of both aqueous and organic electrolytes. Moreover, the novel Pt₁N‐C shells not only effectively protect the H‐PtCo cores from agglomeration but also increase the efficiency of the ORR in virtue of the isolated Pt atoms. Thus, the H‐PtCo@Pt₁N‐C catalyst exhibits stable ORR without any fade over a prolonged 10 000 cycle test at 0.9 V in HClO₄ solution. Furthermore, this material can offer efficient and stable ORR activities in various organic electrolytes, indicating its great potential for next‐generation lithium–air batteries as well.     

Tuning Bifunctional Oxygen Electrocatalysts by Changing the A‐Site Rare‐Earth Element in Perovskite Nickelates

Perovskite‐structured (ABO₃) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare‐earth nickelates (RNiO₃) thin films is investigated with controlled A‐site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three‐electrode system in O₂‐saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A‐site element ionic radius which lowers the conductivity of RNiO₃ (R = La, La_0.5Nd_0.5, La_0.2Nd_0.8, Nd, Nd_0.5Sm_0.5, Sm, and Gd) films, with LaNiO₃ being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La_0.2Nd_0.8NiO₃. Moreover, the OER activity remains comparable within error through Sm‐doped NdNiO₃. Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni³⁺ to Ni²⁺ as a result of oxygen vacancies, which increases the average occupancy of the e_g antibonding orbital to more than one. The work highlights the importance of tuning A‐site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts.     

Evolution of Nanoporous Pt–Fe Alloy Nanowires by Dealloying and their Catalytic Property for Oxygen Reduction Reaction

The short life and high cost of carbon‐supported Pt nanoparticle catalysts (Pt/C) are two main problems with proton exchange membrane fuel cells. Porous Pt alloy nanowires have more durability and catalytic activity than Pt/C. Dealloying is a facile way to make nanoporous Pt. However, the process of porosity formation is difficult to control. In this paper, electrospinning and chemical dealloying techniques are used to make long, thin and yet nanoporous Pt–Fe alloy nanowires. The evolution of nanoporosity is observed and studied. It is found that non‐uniform composition in the precursor PtFe₅ alloy nanowires helps the formation of nanoporous structure. The overall wire diameter is about 10–20 nm and the ligament diameter only 2–3 nm. These porous long nanowires interweave to form a self‐supporting network with a high specific activity, 2.3 times that of conventional Pt/C catalysts, and also have better durability.     

Pyridinic‐Nitrogen‐Dominated Graphene Aerogels with Fe–N–C Coordination for Highly Efficient Oxygen Reduction Reaction

Here, pyridinic nitrogen dominated graphene aerogels with/without iron incorporation (Fe‐NG and NG) are prepared via a facile and effective process including freeze‐drying of chemically reduced graphene oxide with/without iron precursor and thermal treatment in NH₃. A high doping level of nitrogen has been achieved (up to 12.2 at% for NG and 11.3 at% for Fe‐NG) with striking enrichment of pyridinic nitrogen (up to 90.4% of the total nitrogen content for NG, and 82.4% for Fe‐NG). It is found that the Fe‐NG catalysts display a more positive onset potential, higher current density, and better four‐electron selectivity for ORR than their counterpart without iron incorporation. The most active Fe‐NG exhibits outstanding ORR catalytic activity, high durability, and methanol tolerance ability that are comparable to or even superior to those of the commercial Pt/C catalyst at the same catalyst loading in alkaline environment. The excellent ORR performance can be ascribed to the synergistic effect of pyridinic N and Fe‐N _x sites (where iron probably coordinates with pyridinic N) that serve as active centers for ORR. Our Fe‐NG can be developed into cost‐effective and durable catalysts as viable replacements of the expensive Pt‐based catalysts in practical fuel cell applications.     

Shape Control of Mn3O4 Nanoparticles on Nitrogen‐Doped Graphene for Enhanced Oxygen Reduction Activity

Three kinds of Mn₃O₄ nanoparticles with different shapes (spheres, cubes, and ellipsoids) are selectively grown on nitrogen‐doped graphene sheets through a two‐step liquid‐phase procedure. These non‐precious hybrid materials display an excellent ORR activity and good durability. The mesoporous microstructure, nitrogen doping, and strong bonding between metal species and doped graphene are found to facilitate the ORR catalytic process. Among these three kinds of Mn₃O₄ particles, the ellipsoidal particles on nitrogen‐doped graphene exhibit the highest ORR activity with a more positive onset‐potential of –0.13 V (close to that of Pt/C, –0.09 V) and a higher kinetic limiting current density (J_K) of 11.69 mA cm^–2 at –0.60 V. It is found that the ORR performance of hybrid materials can be correlated to the shape of Mn₃O₄ nanocrystals, and specifically to the exposed crystalline facets associated with a given shape. The shape dependence of Mn₃O₄ nanoparticles integrated with nitrogen‐doped graphene on the ORR performance, reported here for the first time, may advance the development of fuel cells and metal‐air batteries.     

Iron–Nitrogen‐Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction

A highly active iron–nitrogen‐doped carbon nanotube catalyst for the oxygen reduction reaction (ORR) is produced by employing vertically aligned carbon nanotubes (VA‐CNT) with a high specific surface area and iron(II) phthalocyanine (FePc) molecules. Pyrolyzing the composite easily transforms the adsorbed FePc molecules into a large number of iron coordinated nitrogen functionalized nanographene (Fe–N–C) structures, which serve as ORR active sites on the individual VA‐CNT surfaces. The catalyst exhibits a high ORR activity, with onset and half‐wave potentials of 0.97 and 0.79 V, respectively, versus reversible hydrogen electrode, a high selectivity of above 3.92 electron transfer number, and a high electrochemical durability, with a 17 mV negative shift of E _1/2 after 10 000 cycles in an oxygen‐saturated 0.5 m H₂SO₄ solution. The catalyst demonstrates one of the highest ORR performances in previously reported any‐nanotube‐based catalysts in acid media. The excellent ORR performance can be attributed to the formation of a greater number of catalytically active Fe–N–C centers and their dense immobilization on individual tubes, in addition to more efficient mass transport due to the mesoporous nature of the VA‐CNTs.     

Phase and Composition Tuning of 1D Platinum‐Nickel Nanostructures for Highly Efficient Electrocatalysis

Among various platinum (Pt)‐based nanostructures, porous or hollow ones are of great importance because they exhibit fantastic oxygen reduction reaction (ORR) enhancements and maximize atomic utilization by exposing both exterior and interior surfaces. Here, a new class of porous Pt₃Ni nanowires (NWs) with 1D architecture, an ultrathin Pt‐rich shell, high index facets, and a highly open structure is designed via a selective etching strategy by using the phase and composition segregated Pt‐Ni NWs as the starting material. The porous feature of Pt₃Ni NWs can be readily fulfilled by changing the Pt/Ni atomic ratio of the starting Pt‐Ni NWs. Such porous Pt₃Ni NWs show extraordinary activity and stability enhancements toward methanol oxidation reaction and ORR. The porous Pt₃Ni NWs can deliver ORR mass activity of 5.60 A mg^?1, which is 37.3‐fold higher than that of the Pt/C. They also show outstanding stability with negligible activity loss after 20 000 cycles. This study offers a unique approach for the design of complex nanostructures as efficient catalysts through precisely tailoring.     

Simultaneously Realizing Rapid Electron Transfer and Mass Transport in Jellyfish‐Like Mott–Schottky Nanoreactors for Oxygen Reduction Reaction

Fundamental understanding of constructing elevated catalysts to realize fast electron transfer and rapid mass transport in oxygen reduction reaction (ORR) chemistry by interface regulation and structure design is important but still ambiguous. Herein, a novel jellyfish‐like Mott–Schottky‐type electrocatalyst is developed to realize fast electron transfer and decipher the structure–mass transport connection during ORR process. Both spectroscopy techniques and density functional theory calculation demonstrate electrons spontaneously transfer from Fe to N‐doped graphited carbon at the heterojunction interface, thus accelerating electron transfer from electrode to reactant. Dynamic analysis indicates unique structure can significantly improve mass transport of oxygen‐species due to two factors: one is electrolyte streaming effect caused by tentacle‐like carbon nanotubes; the other is effective collision probability in the semi‐closed cavity. Therefore, this Mott–Schottky‐type catalyst delievers superior ORR performance with high onset potential, positive half wave potential, and large current density. It also exhibits low overpotential when serving as an air cathode in Zn–air batteries. This work deepens understanding of the two key factors—electron transfer and mass transport—on determining the kinetic reaction of ORR process and offers a new avenue in constructing efficient Mott–Schottky electrocatalysts.     

Structurally Ordered Low‐Pt Intermetallic Electrocatalysts toward Durably High Oxygen Reduction Reaction Activity

Carbon‐supported low‐Pt ordered intermetallic nanoparticulate catalysts (PtM₃, M = Fe, Co, and Ni) are explored in order to enhance the oxygen reduction reaction (ORR) activity while achieving a high stability compared to previously reported Pt‐richer ordered intermetallics (Pt₃M and PtM) and low‐Pt disordered alloy catalysts. Upon high‐temperature thermal annealing, ordered PtCo₃ intermetallic nanoparticles are successfully prepared with minimum particle sintering. In contrast, the PtFe₃ catalyst, despite the formation of ordered structure, suffers from obvious particle sintering and detrimental metal–support interaction, while the PtNi₃ catalyst shows no structural ordering transition at all but significant particle sintering. The ordered PtCo₃ catalyst exhibits durably thin Pt shells with a uniform thickness below 0.6 nm (corresponding to 2–3 Pt atomic layers) and a high Co content inside the nanoparticles after 10 000 potential cycling, leading to a durably compressive Pt surface and thereby both high activity (fivefold vs a commercial Pt catalyst and 1.7‐fold vs an ordered PtCo intermetallic catalyst) and high durability (5 mV loss in half‐wave potential and 9% drop in mass activity). These results provide a new strategy toward highly active and durable ORR electrocatalysts by rational development of low‐Pt ordered intermetallics.     

Recent Advances in Non‐Noble Bifunctional Oxygen Electrocatalysts toward Large‐Scale Production

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial reactions in energy conversion and storage systems including fuel cells, metal–air batteries, and electrolyzers. Developing low‐cost, high‐efficiency, and durable non‐noble bifunctional oxygen electrocatalysts is the key to the commercialization of these devices. Here, based on an in‐depth understanding of ORR/OER reaction mechanisms, recent advances in the development of non‐noble electrocatalysts for ORR/OER are reviewed. In particular, rational design for enhancing the activity and stability and scalable synthesis toward the large‐scale production of bifunctional electrocatalysts are highlighted. Prospects and future challenges in the field of oxygen electrocatalysis are presented.     

Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and Application for Efficient Oxygen Reduction

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m² g^?1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm^?2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐N_x/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐N_x/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.     

A One‐Pot Route to Faceted FePt‐Fe3O4 Dumbbells: Probing Morphology–Catalytic Activity Effects in O2 Reduction Catalysis

The design and synthesis of faceted nanoparticles with a controlled composition is of enormous importance to modern catalyst engineering. Faceted FePt‐Fe₃O₄ dumbbell nanoparticles are prepared by a simple, one‐pot technique that avoids the need for expensive additives or preformed seeds. The faceted product consists of an FePt octopod and a cubic Fe₃O₄ lobe, of mean diameter 13.6 and 14.9 nm, respectively. The mass normalized activity for electrocatalytic oxygen reduction shows that this new structure types outperforms related catalysts in alkaline media. This work illustrates the power of morphology control and tailoring crystal facet abundance at the nanoparticle surface for enhancing catalytic performance.     

Sulfur‐Enriched Conjugated Polymer Nanosheet Derived Sulfur and Nitrogen co‐Doped Porous Carbon Nanosheets as Electrocatalysts for Oxygen Reduction Reaction and Zinc–Air Battery

Among the rising 2D soft materials, conjugated polymer nanosheets are one of the most promising and new classes of polymeric materials, which are rarely developed because of the challenge in controlling the dimensionality and lack of synthetic strategies. In this study, one kind of sulfur‐enriched conjugated polymer nanosheet (2DP‐S) with a high aspect ratio of up to ≈400 is successfully synthesized. On the basis of structural characterization, as‐prepared 2DP‐S possesses the chemical identity of cruciform‐fused polymeric backbone consisting of quinoidal polythiophene and poly(p‐phenylenevinylene) along horizontal and vertical directions, respectively, by sharing two alternating single–double carbon–carbon bonds in each repeat unit. The unique structural conformation of 2DP‐S renders carrier mobilities of up to 0.1 ± 0.05 cm² V^?1 s^?1, a figure inferred from Terahertz time domain spectroscopy. Moreover, upon thermal treatment, 2DP‐S is readily converted into N/S dual‐doped porous carbon nanosheets (2DPCs) under ammonia atmosphere, whose N/S ratio can be rationally controlled by adjusting the activation time. The catalytic performance of the oxygen reduction reaction of as‐prepared 2DPCs is well tunable by the rationally controlled N/S contents. These results offer a new pathway for exploring heteroatom‐doped porous carbons applicable for energy conversion and storage.     

Hybrid of Iron Nitride and Nitrogen‐Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction

It is extremely desirable but challenging to create highly active, stable, and low‐cost catalysts towards oxygen reduction reaction to replace Pt‐based catalysts in order to perform the commercialization of fuel cells. Here, a novel iron nitride/nitrogen doped‐graphene aerogel hybrid, synthesized by a facile two‐step hydrothermal process, in which iron phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π–π stacking and oxygen‐containing functional groups, is reported. As a result, there exist strong interactions between Fe _x N nanoparticles and graphene substrates, leading to a synergistic effect towards oxygen reduction reaction. It is worth noting that the onset potential and current density of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure nitrogen‐doped graphene aerogel, free Fe _x N and their physical mixtures. The hybrid also exhibits comparable catalytic activity as commercial Pt/C at the same catalyst loading, while its stability and resistance to methanol crossover are superior. Interestingly, it is found that, apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high density of Fe–N–C sties and small size of Fe _x N particles boost charge transfer rate.     

Enhanced Electrocatalytic Oxygen Evolution Activity by Tuning Both the Oxygen Vacancy and Orbital Occupancy of B‐Site Metal Cation in NdNiO3

Perovskite oxides have been explored as promising electrocatalysts for the oxygen evolution reaction (OER), while a lack of understanding of key factors impacting the catalytic activity restricts their further design and development. Here, for the first time, the contributions of oxygen vacancy (V_O) and orbital occupancy of B‐site cations to the catalytic activity of NdNiO₃ films are systematically investigated. It is found that OER activity follows a typical volcano‐shaped dependence on the oxygen pressure. In the range of 0.2–10 Pa, proper concentration of V_O can provide a moderate bonding strength with intermediate hydroxyl OH* and the increased ratio of Ni³⁺/Ni²⁺ provides a more favorable occupancy of e_g orbital for the catalytic activity; while in the range of 10–60 Pa, insufficient concentration of V_O leads to an enhanced strength of hybridization between Ni 3d and O 2p band and thus deteriorated catalytic activity. The superior OER catalytic performance can be only achieved with both appropriate concentration of V_O and the ratio of B‐site metal cations with different valences.