Fe, Cu-codoped metal-nitrogen-carbon catalysts with high selectivity and stability for the oxygen reduction reaction

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Architecting FeNx on High Graphitization Carbon for High-Performance Oxygen Reduction by Regulating d-Band Center

Fe single atoms and N co-doped carbon nanomaterials (Fe-N-C) are the most promising oxygen reduction reaction (ORR) catalysts to replace platinum group metals. However, high-activity Fe single-atom catalysts suffer from poor stability owing to the low graphitization degree. Here, an effective phase-transition strategy is reported to enhance the stability of Fe-N-C catalysts by inducing increased degree of graphitization and incorporation of Fe nanoparticles encapsulated by graphitic carbon layer without sacrificing activity. Remarkably, the resulted Fe@Fe-N-C catalysts achieved excellent ORR activity (E_1/2  = 0.829 V) and stability (19 mV loss after 30K cycles) in acid media. Density functional theory (DFT) calculations agree with experimental phenomena that additional Fe nanoparticles not only favor to the activation of O₂ by tailoring d-band center position but also inhibit the demetallization of Fe active center from FeN₄ sites. This work provides a new insight into the rational design of highly efficient and durable Fe-N-C catalysts for ORR.

Nanosized Proton Conductor Array with High Specific Surface Area Improves Fuel Cell Performance at Low Pt Loading

The use of ordered catalyst layers, based on micro-/nanostructured arrays such as the ordered Nafion array, has demonstrated great potential in reducing catalyst loading and improving fuel cell performance. However, the size (diameter) of the basic unit of the most existing ordered Nafion arrays, such as Nafion pillar or cone, is typically limited to micron or submicron sizes. Such small sizes only provide a limited number of proton transfer channels and a small specific area for catalyst loading. In this work, the ordered Nafion array with a pillar diameter of only 40 nm (D40) was successfully prepared through optimization of the Nafion solvent, thermal annealing temperature, and stripping mode from the anode alumina oxide (AAO) template. The density of D40 is 2.7 × 10¹⁰ pillars/cm², providing an abundance of proton transfer channels. Additionally, D40 has a specific area of up to 51.5 cm²/cm², which offers a large area for catalyst loading. This, in turn, results in the interface between the catalyst layer and gas diffusion layer becoming closer. Consequently, the peak power densities of the fuel cells are 1.47 (array as anode) and 1.29 W/cm² (array as cathode), which are 3.3 and 2.9 times of that without array, respectively. The catalyst loading is significantly reduced to 17.6 (array as anode) and 61.0 μg/cm² (array as cathode). Thus, the nanosized Nafion array has been proven to have high fuel cell performance with low Pt catalyst loading. Moreover, this study also provides guidance for the design of a catalyst layer for water electrolysis and electrosynthesis.

Si Doping Enables Activity and Stability Enhancement on Atomically Dispersed Fe-Nx /C Electrocatalysts for Oxygen Reduction in Acid

Fe-N-C represents the most promising non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) in fuel cells, but often suffers from poor stability in acid due to the dissolution of metal sites and the poor oxidation resistance of carbon substrates. In this work, silicon-doped iron-nitrogen-carbon (Si/Fe-N-C) catalysts were developed by in situ silicon doping and metal-polymer coordination. It was found that Si doping could not only promote the density of Fe-N_x /C active sites but also elevated the content of graphitic carbon through catalytic graphitization. The best-performing Si/Fe-N-C exhibited a half-wave potential of 0.817 V vs. reversible hydrogen electrode in 0.5 m H₂ SO₄ , outperforming that of undoped Fe-N-C and most of the reported Fe-N-C catalysts. It also exhibited significantly enhanced stability at elevated temperature (≥60 °C). This work provides a new way to develop non-precious metal ORR catalysts with improved activity and stability in acidic media.

Non-Noble Metal Catalysts in Cathodic Oxygen Reduction Reaction of Proton Exchange Membrane Fuel Cells: Recent Advances

The oxygen reduction reaction (ORR) is one of the crucial energy conversion reactions in proton exchange membrane fuel cells (PEMFCs). Low price and remarkable catalyst performance are very important for the cathode ORR of PEMFCs. Among the various explored ORR catalysts, non-noble metals (transition metal: Fe, Co, Mn, etc.) and N co-doped C (M-N-C) ORR catalysts have drawn increasing attention due to the abundance of these resources and their low price. In this paper, the recent advances of single-atom catalysts (SACs) and double-atom catalysts (DACs) in the cathode ORR of PEMFCs is reviewed systematically, with emphasis on the synthesis methods and ORR performance of the catalysts. Finally, challenges and prospects are provided for further advancing non-noble metal catalysts in PEMFCs.

Fe-ZIF8 Coating Cu Foil Derived Carbon as A pH-universal Electrocatalyst for Efficient Oxygen Reduction Reaction

It is a great challenge to fabricate highly efficient pH-universal electrocatalysts for oxygen reduction reaction (ORR). Herein, a facile strategy, which includes coating the Fe modified ZIF8 on Cu foil and in-situ pyrolysis to evaporate and dope Cu into the MOF derived carbon, is developed to fabricate Fe/Cu-N co-doped carbon material (Cu/Fe-NC). Profiting from the modulated electron distribution and textual properties, well-designed Cu/Fe-NC exhibits superior half-wave potential (E 1/2 ) of 0.923 V in alkaline, 0.757 V in neutral and comparable 0.801 V in acid electrolytes, respectively. Furthermore, the ultralow peroxides yield of ORR demonstrates the high selectivity of Cu/Fe-NC in full pH scale electrolytes. As expected, the self-made alkaline and neutral zinc-air batteries equipped with Cu/Fe-NC cathode display excellent discharge voltages, outstanding peak power densities and remarkable stability. This work opens a new way to fabricate highly efficient and pH-universal electrocatalysts for ORR through strategy of Fe/Cu-N co-doping, Cu foil evaporation and carbon defects capture.