Recent Advances in Metal-Gas Batteries with Carbon-Based Nonprecious Metal Catalysts.

Metal-gas batteries draw a lot of attention due to their superiorities in high energy density and stable performance. However, the sluggish electrochemical reactions and associated side reactions in metal-gas batteries require suitable catalysts, which possess high catalytic activity and selectivity. Although precious metal catalysts show a higher catalytic activity, high cost of the precious metal catalysts hinders their commercial applications. In contrast, nonprecious metal catalysts complement the weakness of cost, and the gap in activity can be made up by increasing the amount of the nonprecious metal active centers. Herein, recent work on carbon-based nonprecious metal catalysts for metal-gas batteries is summarized. This review starts with introducing the advantages of carbon-based nonprecious metal catalysts, followed by a discussion of the synthetic strategy of carbon-based nonprecious metal catalysts and classification of active sites, and finally a summary of present metal-gas batteries with the carbon-based nonprecious metal catalysts is presented. The challenges and opportunities for carbon-based nonprecious metal catalysts in metal-gas batteries are also explored.

ZIF-8 with Ferrocene Encapsulated: A Promising Precursor to Single-Atom Fe Embedded Nitrogen-Doped Carbon as Highly Efficient Catalyst for Oxygen Electroreduction.

The oxygen reduction reaction (ORR) plays an important role in the fields of energy storage and conversion technologies, including metal-air batteries and fuel cells. The development of nonprecious metal electrocatalysts with both high ORR activity and durability to replace the currently used costly Pt-based catalyst is critical and still a major challenge. Herein, a facile and scalable method is reported to prepare ZIF-8 with single ferrocene molecules trapped within its cavities (Fc@ZIF-8), which is utilized as precursor to porous single-atom Fe embedded nitrogen-doped carbon (Fe-N-C) during high temperature pyrolysis. The catalyst shows a half-wave potential (E1/2 ) of 0.904 V, 67 mV higher than commercial Pt/C catalyst (0.837 V), which is among the best compared with reported results for ORR. Significant electrochemical properties are attributed to the special configuration of Fc@ZIF-8 transforming into a highly dispersed iron-nitrogen coordination moieties embedded carbon matrix.

Sugar Blowing-Induced Porous Cobalt Phosphide/Nitrogen-Doped Carbon Nanostructures with Enhanced Electrochemical Oxidation Performance toward Water and Other Small Molecules.

Rational design of high active and robust nonprecious metal catalysts with excellent catalytic efficiency in oxygen evolution reaction (OER) is extremely vital for making the water splitting process more energy efficient and economical. Among these noble metal-free catalysts, transition-metal-based nanomaterials are considered as one of the most promising OER catalysts due to their relatively low-cost intrinsic activities, high abundance, and diversity in terms of structure and morphology. Herein, a facile sugar-blowing technique and low-temperature phosphorization are reported to generate 3D self-supported metal involved carbon nanostructures, which are termed as Co2 P@Co/nitrogen-doped carbon (Co2 P@Co/N-C). By capitalizing on the 3D porous nanostructures with high surface area, homogeneously dispersed active sites, the intimate interaction between active sites, and 3D N-doped carbon, the resultant Co2 P@Co/N-C exhibits satisfying OER performance superior to CoO@Co/N-C, delivering 10 mA cm-2 at overpotential of 0.32 V. It is worth noting that in contrast to the substantial current density loss of RuO2 , Co2 P@Co/N-C shows much enhanced catalytic activity during the stability test and a 1.8-fold increase in current density is observed after stability test. Furthermore, the obtained Co2 P@Co/N-C can also be served as an excellent nonprecious metal catalyst for methanol and glucose electrooxidation in alkaline media, further extending their potential applications.

Co-N-Doped Mesoporous Carbon Hollow Spheres as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction.

Rational design of cost-effective, nonprecious metal-based catalysts with desirable oxygen reduction reaction (ORR) performance is extremely important for future fuel cell commercialization, etc. Herein, a new type of ORR catalyst of Co-N-doped mesoporous carbon hollow sphere (Co-N-mC) was developed by pyrolysis from elaborately fabricated polystyrene@polydopamine-Co precursors. The obtained catalysts with active Co sites distributed in highly graphitized mesoporous N-doped carbon hollow spheres exhibited outstanding ORR activity with an onset potential of 0.940 V, a half-wave potential of 0.851 V, and a small Tafel slope of 45 mV decade-1 in 0.1 m KOH solution, which was comparable to that of the Pt/C catalyst (20%, Alfa). More importantly, they showed superior durability with little current decline (less than 4%) in the chronoamperometric evaluation over 60 000 s. These features make the Co-N-mC one of the best nonprecious-metal catalysts to date for ORR in alkaline condition.

Self-Assembled Fe-N-Doped Carbon Nanotube Aerogels with Single-Atom Catalyst Feature as High-Efficiency Oxygen Reduction Electrocatalysts.

Self-assembled M-N-doped carbon nanotube aerogels with single-atom catalyst feature are for the first time reported through one-step hydrothermal route and subsequent facile annealing treatment. By taking advantage of the porous nanostructures, 1D nanotubes as well as single-atom catalyst feature, the resultant Fe-N-doped carbon nanotube aerogels exhibit excellent oxygen reduction reaction electrocatalytic performance even better than commercial Pt/C in alkaline solution.

Hydrothermal Synthesis of Metal-Polyphenol Coordination Crystals and Their Derived Metal/N-doped Carbon Composites for Oxygen Electrocatalysis.

Cobalt (or iron)-polyphenol coordination polymers with crystalline frameworks are synthesized for the first time. The crystalline framework is formed by the assembly of metal ions and polyphenol followed by oxidative self-polymerization of the organic ligands (polyphenol) during hydrothermal treatment in alkaline condition. As a result, such coordination crystals are even partly stable in strong acid (such as 2 m HCl). The metal (Co or Fe)-natural abundant polyphenol (tannin) coordination crystals are a renewable source for the fabrication of metal/carbon composites as a nonprecious-metal catalyst, which show high catalytic performance for both oxygen reduction reaction and oxygen evolution reaction. Such excellent performance makes metal-polyphenol coordination crystals an efficient precursor to fabricate low-cost catalysts for the large-scale application of fuel cells and metal-air batteries.

Electrodeposited cobalt-phosphorous-derived films as competent bifunctional catalysts for overall water splitting.

One of the challenges to realize large-scale water splitting is the lack of active and low-cost electrocatalysts for its two half reactions: H2 and O2 evolution reactions (HER and OER). Herein, we report that cobalt-phosphorous-derived films (Co-P) can act as bifunctional catalysts for overall water splitting. The as-prepared Co-P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mA cm(-2) at overpotentials of -94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100% Faradaic efficiency, rivalling the integrated performance of Pt and IrO2. The major composition of the as-prepared and post-HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER.

Single-shell carbon-encapsulated iron nanoparticles: synthesis and high electrocatalytic activity for hydrogen evolution reaction.

Efficient hydrogen evolution reaction (HER) through effective and inexpensive electrocatalysts is a valuable approach for clean and renewable energy systems. Here, single-shell carbon-encapsulated iron nanoparticles (SCEINs) decorated on single-walled carbon nanotubes (SWNTs) are introduced as a novel highly active and durable non-noble-metal catalyst for the HER. This catalyst exhibits catalytic properties superior to previously studied nonprecious materials and comparable to those of platinum. The SCEIN/SWNT is synthesized by a novel fast and low-cost aerosol chemical vapor deposition method in a one-step synthesis. In SCEINs the single carbon layer does not prevent desired access of the reactants to the vicinity of the iron nanoparticles but protects the active metallic core from oxidation. This finding opens new avenues for utilizing active transition metals such as iron in a wide range of applications.

Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts.

Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low-temperature fuel cells. A novel type of catalysts prepared by high-pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe3 C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide-based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts.

First-Row Transition Metal Antimonates for the Oxygen Reduction Reaction.

The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical-experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates─MSb2O6, where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb2O6 shows the highest theoretical ORR activity based on the 4e- ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb2O6 is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.

Hydrogen Passivation of M-N-C (M = Fe, Co) Catalysts for Storage Stability and ORR Activity Improvements.

M-N-C (M = Fe, Co) are highly active nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) and other applications. Although their operation stability has been extensively studied in proton-exchange-membrane fuel cells, the storage stability that determines the performance maintenance before use has not yet been understood. Here, it is found that long-term exposure of M-N-C catalysts in air would cause surface oxidation and hydroxylation, resulting in significant decrease of ORR activity and fuel-cell performances. Hydrogen passivation is demonstrated to be an effective strategy to protect the atomic M-N4 active sites and improve the storage stability of the catalysts. In addition, the hydrogen-termination can also reduce the ORR energy barrier and increase the utilization of active sites, leading to the improvements of fuel-cell activity and power density. Notably, these findings help to understand the storage-associated degradation and protection of M-N-C catalysts.

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons.

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

Atomically Isolated Iron Atom Anchored on Carbon Nanotubes for Oxygen Reduction Reaction.

Recently, electrocatalysts based on anchored dispersive/isolated single metal atoms on conductive carbon supports have demonstrated great promise to substitute costly Pt for the oxygen reduction reaction (ORR) in the field of fuel cells or metal-air batteries. However, developments of cost-efficient single-atom Fe catalysts with high activities are still facing various hardships. Here, we developed a facile way to synthesize isolated iron atoms anchored on the carbon nanotube (CNT) involving a one-pot pyrrole polymerization on a self-degraded organic template and a subsequent pyrolysis. The as-obtained electrocatalyst possessed unique characteristics of abundant nanopores in the wall of conductive CNTs to host the abundant atomic Fe-Nx active sites, showing ultrahigh ORR activity (half-wave potential: 0.93 V, kinetic current density: 59.8 mA/cm2 at 0.8 V), better than that of commercial Pt/C (half-wave potential: 0.91 V; kinetic current density: 38.0 mA/cm2 at 0.8 V) in an alkaline electrolyte. Furthermore, good ORR activity has been proven in acidic solution with a half-wave-potential of 0.73 V.

Insight into the Rapid Degradation Behavior of Nonprecious Metal Fe-N-C Electrocatalyst-Based Proton Exchange Membrane Fuel Cells.

In the past few years, great progress has been made in nonprecious metal catalysts, which hold the potential as alternative materials to replace platinum in proton exchange membrane fuel cells. One type of nonprecious metal catalyst, Fe-N-C, has displayed similar catalytic activity as platinum in rotating disk electrode tests; however, rapid degradation of Fe-N-C catalyst-based fuel cells is always observed, which limits its practical application. Although considerable research has been devoted to study the degradation of the catalyst itself, rather less attention has been paid to the membrane electrode assembly that makes the mechanism of fuel cell degradation remain unclear. In this work, a high-performance Fe-N-C catalyst-based membrane electrolyte assembly is prepared and used to study its degradation mechanism. The fuel cell performs with an initial peak power density as high as 1.1 W cm-2 but suffers a current loss of 52% at 0.4 V over 20 h only. The experimental and DFT calculation results indicate that Fe at active sites of catalysts is attacked by hydroxyl free radicals decomposed from H2O2, which is further leached out, causing an increase in activity loss. The ionomer of the catalyst layer and the membrane is further contaminated by the leached Fe ions, which results in an enlarged membrane resistance and cathode catalyst layer proton conduction resistance, greatly influencing the cell performance. In addition, it has been assumed in previous studies that the quick performance loss of Fe-N-C-based fuel cells is caused by water flooding within the catalyst layer, which is proved to be incorrect in our study through a dry-out experiment.

Iron Single Atoms on Graphene as Nonprecious Metal Catalysts for High-Temperature Polymer Electrolyte Membrane Fuel Cells.

Iron single atom catalysts (Fe SACs) are the best-known nonprecious metal (NPM) catalysts for the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), but their practical application has been constrained by the low Fe SACs loading (<2 wt%). Here, a one-pot pyrolysis method is reported for the synthesis of iron single atoms on graphene (FeSA-G) with a high Fe SAC loading of ≈7.7 ± 1.3 wt%. The as-synthesized FeSA-G shows an onset potential of 0.950 V and a half-wave potential of 0.804 V in acid electrolyte for the ORR, similar to that of Pt/C catalysts but with a much higher stability and higher phosphate anion tolerance. High temperature SiO2 nanoparticle-doped phosphoric acid/polybenzimidazole (PA/PBI/SiO2) composite membrane cells utilizing a FeSA-G cathode with Fe SAC loading of 0.3 mg cm-2 delivers a peak power density of 325 mW cm-2 at 230 °C, better than 313 mW cm-2 obtained on the cell with a Pt/C cathode at a Pt loading of 1 mg cm-2. The cell with FeSA-G cathode exhibits superior stability at 230 °C, as compared to that with Pt/C cathode. Our results provide a new approach to developing practical NPM catalysts to replace Pt-based catalysts for fuel cells.

Precious Metal-Free Nickel Nitride Catalyst for the Oxygen Reduction Reaction.

With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells. Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method. In rotating disk electrode testing in a 0.1 M HClO4 electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR. It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively. A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis. Design strategies for activity and stability improvement through alloying and nanostructuring are discussed.

CuO/Co(OH)2 Nanosheets: A Novel Kind of Electrocatalyst for Highly Efficient Electrochemical Oxidation of Methanol.

With the booming of non-noble-metal electrocatalysts for efficient oxygen reduction reaction under alkaline conditions, corresponding anodic catalysts for methanol oxidation are urgently needed especially for direct methanol fuel cells with alkaline membranes. Here, we report the facile synthesis of a CuO/Co(OH)2-nanosheet composite as a novel kind of high-performance electrochemical methanol oxidation reaction (MOR) catalyst. The obtained material with an optimized Cu/Co ratio shows much enhanced mass activity and area-specific activity, as well as excellent stability. The electronic structure interaction between Cu and Co, which results in the Co ion binding-energy elevation, is considered to be the origin of high MOR performance. This work promises the great potential of cobalt hydroxide as a novel kind of MOR catalyst and may arouse much interest in exploring more hydroxides as efficient nonprecious-metal electrocatalysts for MOR.

Magnetron Sputtering Deposition Cu@Onion-like N-C as High-Performance Electrocatalysts for Oxygen Reduction Reaction.

The idea of a core-shell structure can promote the utilization of nonprecious metallic catalysts by enhancing their activity and stability for the oxygen reduction reaction (ORR). Developing a low-cost, high-efficiency, and high-reproducibility method for synthesizing core-shell-structured materials represents an urgent challenge. Here, we fabricate encapsulated Cu nanoparticles with nitrogen-doped onion-like graphite nanoshells (Cu@onion-like N-C) as an efficient ORR catalyst by magnetron sputtering, in which the graphite shells grow by an in situ self-assembly process activated by the surface-catalyzed behavior with Cu nanoparticles. The results show that the CuCN-650 °C catalyst achieves the optimized Cu@onion-like N-C structure with small-sized Cu nanoparticles and a few-layer nanoshells and exhibits excellent ORR electrocatalytic properties, including a half-wave potential and onset potential similar to those of commercial Pt/C, coupled with better stability and higher methanol tolerance than for commercial Pt/C in alkaline electrolytes. The internal Cu nanoparticles in the core-shell structure not only promote the formation of a high content of pyridinic N but also donate the electronic charges to outer N-doped C shells, and thus the synergistic effect between the encapsulated Cu nanoparticles and N-doped C shells is responsible for the excellent electrocatalytic activity for the ORR.

Two-Dimensional N,S-Codoped Carbon/Co9S8 Catalysts Derived from Co(OH)2 Nanosheets for Oxygen Reduction Reaction.

The development of highly active and cost-efficient electrocatalysts for the oxygen reduction reaction (ORR) is of great importance in a wide range of clean energy devices, including fuel cells and metal-air batteries. Herein, the simultaneous formation of Co9S8 and N,S-codoped carbon with high ORR catalytic activity was achieved in an efficient strategy with a dual templates system. First, Co(OH)2 nanosheets and tetraethyl orthosilicate were utilized to direct the formation of two-dimensional carbon precursors, which were then dispersed into thiourea solution. After subsequent pyrolysis and template removal, N,S-codoped porous carbon-sheet-confined Co9S8 catalysts (Co9S8/NSC) were obtained. Owing to the morphological and compositional advantages as well as the synergistic effects, the resultant Co9S8/NSC catalysts with a modified doping level and pyrolysis degree exhibit superior ORR catalytic activity and long-term stability compared with the state-of-the-art Pt/C catalysts in alkaline media. Remarkably, the as-prepared carbon composites also reveal exceptional tolerance of methanol, indicating their potential applications in fuel cells.

High-Performance Direct Methanol Fuel Cells with Precious-Metal-Free Cathode.

Direct methanol fuel cells (DMFCs) hold great promise for applications ranging from portable power for electronics to transportation. However, apart from the high costs, current Pt-based cathodes in DMFCs suffer significantly from performance loss due to severe methanol crossover from anode to cathode. The migrated methanol in cathodes tends to contaminate Pt active sites through yielding a mixed potential region resulting from oxygen reduction reaction and methanol oxidation reaction. Therefore, highly methanol-tolerant cathodes must be developed before DMFC technologies become viable. The newly developed reduced graphene oxide (rGO)-based Fe-N-C cathode exhibits high methanol tolerance and exceeds the performance of current Pt cathodes, as evidenced by both rotating disk electrode and DMFC tests. While the morphology of 2D rGO is largely preserved, the resulting Fe-N-rGO catalyst provides a more unique porous structure. DMFC tests with various methanol concentrations are systematically studied using the best performing Fe-N-rGO catalyst. At feed concentrations greater than 2.0 m, the obtained DMFC performance from the Fe-N-rGO cathode is found to start exceeding that of a Pt/C cathode. This work will open a new avenue to use nonprecious metal cathode for advanced DMFC technologies with increased performance and at significantly reduced cost.