Rational Design of Hierarchical, Porous, Co-Supported, N-Doped Carbon Architectures as Electrocatalyst for Oxygen Reduction

Electrocatalysis of Co/CoxOy nanofilms supported by synchronously nitrogen-doped Ketjenblack carbon towards oxygen reduction reaction

Developing a highly active and stable non-precious metal catalyst for oxygen reduction reaction (ORR) is of great practical significance for advancing fuel cell technology. In this work, a continuous two-step hydrothermal reaction followed by high temperature pyrolysis were employed to achieve in situ N-doping preferentially into Ketjenblack carbon (KB-N) and composite of KB-N and Co/CoxOy nanofilms (Co/CoxOy-NFs) as Co/CoxOy-NFs@KB-N. The N-doped state strongly affects the ORR activity of catalyst. All prepared Co/CoxOy-NFs@KB-N catalysts exhibit observably improved ORR activity compared with the basal KB-N and N-doped Co/CoxOy-NFs, in which the optimal Co/CoxOy-NFs@KB-N catalyst demonstrate the positive Eonset (0.864 V) and E1/2 (0.788 V) vs. RHE, the low Tafel slope (69.27 mV dec-1), implying quick ORR kinetics. And, the Co/CoxOy-NFs@KB-N catalyst exhibits highly electrochemical durability. The KB-N substrate can purify Co valence in CoO component, promote amorphization of CoO crystalline structure and enhance the interaction between Co/CoxOy-NFs and KB-N in Co/CoxOy-NFs@KB-N catalyst. Thus electronic effect, structural effect and synergistic effect can strengthen O2 adsorption, provide enough adsorbed sites and accelerate electron transfer, resulting in prominent ORR performance of Co/CoxOy-NFs@KB-N catalyst.

Apparent activity and specific activity of lanthanides (La, Ce, Nd) decorated Co-MOF derivatives for electrocatalytic water splitting

Lanthanide (Ln) rare Earth (RE) elements are often used to incorporate and regulate the local coordination environment and electronic configuration of transition metal based electrocatalysts for acquiring improved electrocatalytic performance. But for a given pristine electrode, is a Ln element concentrated more on promoting the apparent activity of original electrode or on enhancing its specific activity? To address this issue, Ln (La, Ce and Nd) decorated ZIF-67 derivative electrodes (Ln/Co/NC) were fabricated following with the detailed experimental testing of apparent activity and specific activity of assembled electrodes. X-ray photoelectron spectroscopy data confirmed that Ce, Nd and La have played their own role in regulating the coordination electronic structure of the surface atoms of the derived Co/NC by forming different types of chemical bonds. Electrochemical (EC) results confirmed that Ce is concentrated more on the apparent activity of derived Co/NC electrode with the smallest overpotential at 50 mA cm^-2(η₅₀), while Nd contributes more to its reaction kinetic property with the smallest value of Tafel slope in alkaline hydrogen evolution reaction process. But for oxygen evolution reaction, all of La, Ce and Nd deteriorate the apparent activity of the pristine Co/NC electrode. Comparatively, La shows a greater ability to modulate the specific activity of Co/NC with a larger electrochemical active surface area normalized current density, while Nd exhibits the best ability to re-establish the properties of reaction centers. This work illustrates the difference influence of La, Ce and Nd on the apparent activity and specific activity of the ZIF-67 derivative Co/NC electrode. It will do some favors in engineering RE elements modified composite electrodes for EC applications.

Combining Multivariate Electrospinning with Surface MOF Functionalization to Construct Tunable Active Sites toward Trifunctional Electrocatalysis

The development of high-performance multifunctional electrocatalysts operating in the same electrolyte is key to reduce the material and process costs of renewable energy conversion and storage devices. Herein, the fabrication of freestanding integral electrodes by combining multivariate electrospinning with surface metal organic framework functionalization to arrest pyrolytic emissions from fiber interior is reported, resulting in the expression of rich active sites with controlled composition, for example, the tunable Co-P coordination. The as-fabricated electrode of CoP@CF-900, when used as both the cathode and anode for overall water splitting, is able to deliver 200 mA cm^-2 at a cell voltage of 1.89 V, significantly outshining the Pt/C‖RuO₂ couple; when used as the air cathode for a zinc-air battery, is able to operate more than 150 h at 10 mA cm^-2 with a nearly constant round-trip energy efficiency of ≈60%, also outperforming the Pt/C+RuO₂ benchmark. The activity and kinetics origin of the superb multi-functionality is further elucidated through extensive electroanalytical, post-mortem, and operando characterizations, which underscore the construction of robust integral electrodes through synergistic structure and composition engineering.

High-Level Oxygen Reduction Catalysts Derived from the Compounds of High-Specific-Surface-Area Pine Peel Activated Carbon and Phthalocyanine Cobalt

Non-platinum carbon-based catalysts have attracted much more attention in recent years because of their low cost and outstanding performance, and are regarded as one of the most promising alternatives to precious metal catalysts. Activated carbon (AC), which has a large specific surface area (SSA), can be used as a carrier or carbon source at the same time. In this work, stable pine peel bio-based materials were used to prepare large-surface-area activated carbon and then compound with cobalt phthalocyanine (CoPc) to obtain a high-performance cobalt/nitrogen/carbon (Co-N-C) catalyst. High catalytic activity is related to increasing the number of Co particles on the large-specific-area activated carbon, which are related with the immersing effect of CoPc into the AC and the rational decomposed temperature of the CoPc ring. The synergy with N promoting the exposure of CoNx active sites is also important. The Eonset of the catalyst treated with a composite proportion of AC and CoPc of 1 to 2 at 800 °C (AC@CoPc-800-1-2) is 1.006 V, higher than the Pt/C (20 wt%) catalyst. Apart from this, compared with other AC/CoPc series catalysts and Pt/C (20 wt%) catalyst, the stability of AC/CoPc-800-1-2 is 87.8% in 0.1 M KOH after 20,000 s testing. Considering the performance and price of the catalyst in a practical application, these composite catalysts combining biomass carbon materials with phthalocyanine series could be widely used in the area of catalysts and energy storage.

Engineering Active Sites of Gold-Cuprous Oxide Catalysts for Electrocatalytic Oxygen Reduction Reaction

Understanding how the catalyst morphology influences surface sites is crucial for designing active and stable catalysts and electrocatalysts. We here report a new approach to this understanding by decorating gold (Au) nanoparticles on the surface of cuprous oxides (Cu₂O) with three different shape morphologies (spheres, cubes, and petals). The Au-Cu₂O particles are dispersed onto carbon nanotube (CNT) matrix with high surface area, stability, and conductivity for oxygen reduction reaction. A clear morphology-dependent enhancement of the electrocatalytic activity is revealed. Oxygenated gold species (AuO^-) are found to coexist with Au⁰ on the cube and petal catalysts, whereas only Au⁰ species are present on the sphere catalyst. The AuO^- species function effectively as active sites, resulting in the improved catalytic performance by changing the reaction mechanism. The enhanced catalytic performance of the petal-shaped catalyst in terms of onset potential, half-wave potential, diffusion-limited current density, and stability is closely associated with the presence of the most abundant AuO^- species on its surface. Highly active AuO^- species are identified on the surface of the catalysts as a result of the unique structural characteristics, which is attributed to the structural origin of high activity and stability. This insight constitutes the basis for assessing the detailed correlation between the morphology and the electrocatalytic properties of the nanocomposite catalysts, which has implications for the design of surface-active sites on metal/metal oxide electrocatalysts.

A mild reduction of Co-doped MnO2 to create abundant oxygen vacancies and active sites for enhanced oxygen evolution reaction

Efficient and non-precious-metal-based catalysts (e.g., manganese-based oxides) for the oxygen evolution reaction (OER) remain a substantial challenge. Creation of oxygen vacancies of manganese-based oxides with the aim to enhance their intrinsic activities is rarely reported, and there is a critical requirement for a mild and facile synthesis strategy to create abundant oxygen vacancies on manganese-based oxides. Herein, Co-doped MnO₂ nanowires were reduced by NaBH₄ solution at room temperature; then, MnCo₂O_4.5 nanosheets with abundant oxygen vacancies and active sites were formed on the surface of Co-doped MnO₂ nanowires. Benefiting from the reduction strategy, the fabricated hierarchical Co-doped-MnO₂@MnCo₂O_4.5 nanowire/nanosheet nanocomposites exhibit higher catalytic activity (an overpotential of 250 mV at a current density of 10 mA cm^-2 in 1.0 M KOH solution) than pristine Co-doped MnO₂ nanowires. The calculated TOF of Co-doped-MnO₂@MnCo₂O_4.5 is 0.034 s^-1 at the overpotential of 300 mV, which is 136-fold higher than that of Co-doped-MnO₂. The excellent OER performance was attributed to the synergistic advantages of abundant oxygen vacancies and active sites over the hierarchical nanowire-nanosheet architectures.

Recent Advances on MOF Derivatives for Non-Noble Metal Oxygen Electrocatalysts in Zinc-Air Batteries

Oxygen electrocatalysts are of great importance for the air electrode in zinc-air batteries (ZABs). Owing to the high specific surface area, controllable pore size and unsaturated metal active sites, metal-organic frameworks (MOFs) derivatives have been widely studied as oxygen electrocatalysts in ZABs. To date, many strategies have been developed to generate efficient oxygen electrocatalysts from MOFs for improving the performance of ZABs. In this review, the latest progress of the MOF-derived non-noble metal-oxygen electrocatalysts in ZABs is reviewed. The performance of these MOF-derived catalysts toward oxygen reduction, and oxygen evolution reactions is discussed based on the categories of metal-free carbon materials, single-atom catalysts, metal cluster/carbon composites and metal compound/carbon composites. Moreover, we provide a comprehensive overview on the design strategies of various MOF-derived non-noble metal-oxygen electrocatalysts and their structure-performance relationship. Finally, the challenges and perspectives are provided for further advancing the MOF-derived oxygen electrocatalysts in ZABs.

Microporous Organic Polymers: A Synthetic Platform for Engineering Heterogeneous Carbocatalysts

The conceptual, bottom-up design of functional carbon materials from microporous organic polymers was investigated. Owing to their structural rigidity and synthetic flexibility, the porous polymers streamlined the thermal carbonization process while excluding the need for exogenous additives or extra synthesis procedures and allowed for simultaneous elemental engineering of the resultant carbonaceous materials. As designed, heteroatoms such as nitrogen and sulfur could be uniformly incorporated into the carbon matrices from the microporous polymers during thermal carbonization with a concomitant change in the macroscopic properties of the materials. In particular, doping with sulfur atoms could provide reactive sites, thereby conferring superior catalytic performance to the carbon materials. This study demonstrates expansion of the capability of microporous polymers as a functional carbon source and advances the synthetic concept for carbonaceous materials.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Investigation of ZIF-derived Co, N co-doped porous carbon-supported Au nanoparticles as an effective catalyst for borohydride electrooxidation

An Au/CoNPC nanoparticle composite electrocatalyst with lower cost and higher catalytic activity would be a promising anode for direct borohydride fuel cells.

Synthesis and Performance of MOF-Based Non-Noble Metal Catalysts for the Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells: A Review

Hydrogen is widely regarded as a prime energy carrier for bridging the gap between renewable energy supply and demand. As the energy-generating component of the hydrogen cycle, affordable and reliable fuel cells are of key importance to the growth of the hydrogen economy. However, the use of scarce and costly Pt as an electrocatalyst for the oxygen reduction reaction (ORR) remains an issue to be addressed, and in this regard, metal-organic frameworks (MOFs) are viewed as promising non-noble alternatives because of their self-assembly capability and tunable properties. Herein, recent (2018-2020) works on MOF-based electrocatalysts containing N-doped C, Mn, Fe, Co, multiple metals, and multiple sites are reviewed and summarized with a focus on ORR activity, and the principal physicochemical properties and electrochemical performance of these catalysts realized using rotating electrodes are compared.

Optimization of Sulfurization Process of Cobalt Sulfide and Nitrogen Doped Carbon Material for Boosting the Oxygen Reduction Reaction Catalytic Activity in Alkaline Medium

In order to reduce fuel cell material cost and promote its application, it is urgent to develop non-noble metal materials to replace platinum as the cathode catalysts in fuel cells. The cobalt sulfide and nitrogen co-doped carbon (S-Co-N/C) materials with metal-organic frameworks as precursors have shown good oxygen reduction reaction (ORR) catalytic activity. Benefiting from the protection of catalytic active sites by sulfur atoms, the stability and alcohol-tolerance of the S-Co-N/C catalyst can be significantly improved. In order to fully understand the effect of the sulfurization process on the properties of the material, zeolite imidazole frameworks (ZIF)-8, and ZIF-67 are used as precursors to prepare a novel material of S-Co-N/C by using a sulfurization-pyrolysis method. Another S-Co-N/C material by using a pyrolysis- sulfurization method is prepared for comparison. The effects of the sulfurization process in the preparation on the morphology, chemical structure, specific surface area, and ORR catalytic activity of the final material are investigated. The experimental results show that the surface of the S-Co-N/C material tends to be rough due to the sulfurization reaction of the metal elements. The porosity of the material is reduced to some extent due to the remaining Zn elements in the final product. Interestingly, some carbon nanotubes are found to be generated on the surface of the S-Co-N/C material because of the synergistic effect of Zn and Co on the carbon material during the pyrolysis process, which is beneficial to accelerate the adsorption of oxygen on the S-Co-N/C surface and the electron transportation during the oxygen reduction reaction. In addition, the generated CoS during the sulfurization process can further protect the Co elements from agglomeration, which can effectively increase the ORR catalytic active sites in the final material. The S-Co-N/C material prepared by the sulfurization-pyrolysis method performs a superior ORR catalytic activity to the one synthesized by the pyrolysis-sulfurization method.