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Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last ten years, the fastestgrowing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals.Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most upto-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Carbon-based, metal-free catalysts showed excellent activity, durability and potential to replace Pt in acidic fuel cells.

It is still a grand challenge to develop a highly efficient nonprecious-metal electrocatalyst to replace the Pt-based catalysts for oxygen reduction reaction (ORR). Here, we propose a surfactant-assisted method to synthesize single-atom iron catalysts (SA-Fe/NG). The half-wave potential of SA-Fe/NG is only 30 mV less than 20% Pt/C in acidic medium, while it is 30 mV superior to 20% Pt/C in alkaline medium. Moreover, SA-Fe/NG shows extremely high stability with only 12 mV and 15 mV negative shifts after 5,000 cycles in acidic and alkaline media, respectively. Impressively, the SA-Fe/NG-based acidic proton exchange membrane fuel cell (PEMFC) exhibits a high power density of 823 mW cm Combining experimental results and density-functional theory (DFT) calculations, we further reveal that the origin of high-ORR activity of SA-Fe/NG is from the Fe-pyrrolic-N species, because such molecular incorporation is the key, leading to the active site increase in an order of magnitude which successfully clarifies the bottleneck puzzle of why a small amount of iron in the SA-Fe catalysts can exhibit extremely superior ORR activity.

Sulfur-stabilized intermetallic nanoparticles Nanoparticles of intermetallic alloys of platinum could have enhanced electronic properties that improve their catalytic activity, but the high temperatures needed to ensure complete atomic diffusion often lead to the growth of larger nanoparticles—sintering—with low surface area and hence low overall activity. Yang et al . show that sulfur-doped carbon supports create strong platinum-sulfur bonds that stabilize small platinum alloy nanoparticles (<5 nanometers in diameter) to temperatures up to 1000ºC. They screened libraries of platinum alloys and identified ones with high mass activity for the oxygen reduction reaction in hydrogen fuel cells. —PDS

Nonprecious metal catalysts (NPMCs) FeNC are promising alternatives to noble metal Pt as the oxygen reduction reaction (ORR) catalysts for proton‐exchange‐membrane fuel cells. Herein, a new modulation strategy is reported to the active moiety FeN4 via a precise “single‐atom to single‐atom” grafting of a Pt atom onto the Fe center through a bridging oxygen molecule, creating a new active moiety of Pt1O2Fe1N4. The modulated FeNC exhibits remarkably improved ORR stabilities in acidic media. Moreover, it shows unexpectedly high catalytic activities toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), with overpotentials of 310 mV for OER in alkaline solution and 60 mV for HER in acidic media at a current density of 10 mA cm−2, outperforming the benchmark RuO2 and comparable with Pt/C(20%), respectively. The enhanced multifunctional electrocatalytic properties are associated with the newly constructed active moiety Pt1O2Fe1N4, which protects Fe sites from harmful species. Density functional theory calculations reveal the synergy in the new active moiety, which promotes the proton adsorption and reduction kinetics. In addition, the grafted Pt1O2 dangling bonds may boost the OER activity. This study paves a new way to improve and extend NPMCs electrocatalytic properties through a precisely single‐atom to single‐atom grafting strategy.

Fuel cell vehicles, the only all-electric technology with a demonstrated >300 miles per fill travel range, use Pt as the electrode catalyst. The high price of Pt creates a major cost barrier for large-scale implementation of polymer electrolyte membrane fuel cells. Nonprecious metal catalysts (NPMCs) represent attractive low-cost alternatives. However, a significantly lower turnover frequency at the individual catalytic site renders the traditional carbon-supported NPMCs inadequate in reaching the desired performance afforded by Pt. Unconventional catalyst design aiming at maximizing the active site density at much improved mass and charge transports is essential for the next-generation NPMC. We report here a method of preparing highly efficient, nanofibrous NPMC for cathodic oxygen reduction reaction by electrospinning a polymer solution containing ferrous organometallics and zeolitic imidazolate framework followed by thermal activation. The catalyst offers a carbon nanonetwork architecture made of microporous nanofibers decorated by uniformly distributed high-density active sites. In a single-cell test, the membrane electrode containing such a catalyst delivered unprecedented volumetric activities of 3.3 A·cm −3 at 0.9 V or 450 A·cm −3 extrapolated at 0.8 V, representing the highest reported value in the literature. Improved fuel cell durability was also observed.nanofibrous | nonprecious metal catalyst | metal-organic framework | fuel cell | oxygen reduction P olymer electrolyte membrane fuel cells (PEMFCs) electrochemically convert the chemical energy of hydrogen and oxygen to electricity while producing water as a byproduct. They have significantly higher power and energy densities than the competing electrochemical devices, such as Li-ion batteries and supercapacitors, and represent the only all-electric technology with a demonstrated cruising range of over 300 miles between refueling (1). Current PEMFCs use platinum as a catalyst to promote an oxygen reduction reaction (ORR) at the cathode and a hydrogen oxidation reaction at the anode. The Pt use at the cathode is typically three to four times more than that at the anode to overcome the kinetically more sluggish ORR. Because platinum is expensive and there are limited worldwide reserves, technologies that could substantially reduce or replace its use have to be realized before widespread PEMFC commercialization. Nonprecious metal catalysts (NPMCs) represent one such technology.* Among NPMCs, transition metal (TM) and N-doped carbonaceous composites (TM/N/Cs) have demonstrated promising ORR catalytic activities in both acidic and alkaline media, whereas TM-free composites (N/Cs) showed activities primarily in an alkaline medium (2-17). The initial discovery of ORR catalytic activity by N-ligated cobalt was reported half a century ago (18). However, it was not until recently that breakthrough performances were achieved (19-23). New surface property and synthesis strategies for continuously improving catalytic activity were also identified. For example, Lef...

Multifunctionalization is the future development direction for microwave absorbing materials, but has not yet been explored. The effective integration of multiple functions into one material remains a huge challenge. Herein, an aerogel‐type microwave absorber assembled with multidimensional organic and inorganic components is synthesized. Polyacrylonitrile fibers and polybenzoxazine membranes work as the skeleton and crosslinker, respectively, forming a 3D framework, in which carbon nanotubes are interconnected into an electrically conductive network, and Fe3O4 nanoparticles are uniformly dispersed throughout the aerogel. Remarkably, the microwave absorption performances of the aerogel achieve ultralight, ultrathin (1.5 mm), and strong absorption (reflection loss of −59.85 dB) features. In particular, its specific reflection loss values considerably outperform the current magnetic–dielectric hybrids with similar components. Moreover, the aerogel possesses strong hydrophobicity and good thermal insulation, endowing it attractive functions of self‐cleaning, infrared stealth, and heat insulation that is even comparable to commercial products. The excellent multifunction benefits from the cellular structure of aerogel, the assembly of multidimensional nanomaterials, and the synergistic effect of organic–inorganic components. This study paves the way for designing next‐generation microwave absorbing materials with great potential for multifunctional applications.