Developing highly efficient, low-cost oxygen reduction catalysts, especially in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinumbased catalytic materials, further improvement requires precise control of the Fe-N x structure at the molecular level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. In this report, we present a microporous metal−organic-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe 3+ -N 4 configuration is revealed by the 57 Fe Mossbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe 2+ -N 4 at low potential. The in situ reduced Fe 2+ -N 4 moiety from high-spin O x -Fe 3+ -N 4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ca. 1.71 e s −1 sites −1 .

Electrocatalytic reduction of carbon dioxide (CO2ER) in rechargeable Zn–CO2 battery still remains a great challenge. Herein, a highly efficient CO2ER electrocatalyst composed of coordinatively unsaturated single‐atom copper coordinated with nitrogen sites anchored into graphene matrix (Cu–N2/GN) is reported. Benefitting from the unsaturated coordination environment and atomic dispersion, the ultrathin Cu–N2/GN nanosheets exhibit a high CO2ER activity and selectivity for CO production with an onset potential of −0.33 V and the maximum Faradaic efficiency of 81% at a low potential of −0.50 V, superior to the previously reported atomically dispersed Cu–N anchored on carbon materials. Experimental results manifest the highly exposed and atomically dispersed Cu–N2 active sites in graphene framework where the Cu species are coordinated by two N atoms. Theoretical calculations demonstrate that the optimized reaction free energy for Cu–N2 sites to capture CO2 promote the adsorption of CO2 molecules on Cu–N2 sites; meanwhile, the short bond lengths of Cu–N2 sites accelerate the electron transfer from Cu–N2 sites to *CO2, thus efficiently boosting the *COOH generation and CO2ER performance. A designed rechargeable Zn–CO2 battery with Cu–N2/GN nanosheets deliver a peak power density of 0.6 mW cm−2, and the charge process of battery can be driven by natural solar energy.

Emerging as a new frontier in heterogeneous catalysis, single-atom site catalysts (SSCs) have sparked enormous attention and bring about new opportunities to oxygen reduction electrocatalysis. Despite considerable progress achieved recently, most of the reported SSCs suffer from either insufficient activity or unsatisfactory stability, which severely retards their practical application. Here, we demonstrate a novel Ru-SSC with appropriate adsorption free energy of OH* (ΔG OH*) to confer excellent activity and low Fenton reactivity to maintain long-term stability. The as-developed Ru-SSC exhibits encouraging oxygen reduction reaction turnover frequency of 4.99 e– s–1 sites–1, far exceeding the state-of-the-art Fe-SSC counterpart (0.816 e– s–1 sites–1), as a result of Ru energy level regulation via spontaneous OH binding. Furthermore, Ru-SSC exhibits greatly suppressed Fenton reactivity, with restrained generation of reactive oxygen species directly observed, thus endowing the Ru-SSC with much more superior stability (only 17 mV negative shift after 20 000 cycles) than the Fe-SSC counterpart (31 mV). The practical application of Ru-SSC is further validated by its excellent activity and stability in a real fuel cell device.

A new nanocomposite catalyst consisting of high-loading cobalt oxide (CoO) on nitrogen-doped reduced graphene oxide (rGO) for oxygen reduction reaction (ORR) was prepared in this work. Its high activity for the ORR in alkaline electrolyte was determined using the rotating disk electrode technique, and further confirmed in real alkaline membrane fuel cells. A combination of physicochemical characterization (e.g., X-ray absorption and X-ray photoelectron spectra) and density functional theory (DFT) calculation suggests that cobalt(II) cations in the composite catalyst may coordinate with the pyridinic nitrogen atoms doped into graphene planes, most likely the active species for the ORR. Especially, the DFT calculations indicate that a stable rGO(N)–Co(II)–O–Co(II)–rGO(N) structure can be formed in the nitrogen-doped graphene catalyst. Kinetic parameter analysis shows a high selectivity of four-electron reduction on the composite catalyst during the ORR with an average electron transfer number of 3.75. A synergistic effect between the rGO(N) and CoO may exist, yielding a much higher catalytic activity on the CoO/rGO(N) catalyst, compared to either rGO(N) or CoO controls. The novel synthesis procedure utilizing rGO(N) to further couple Co(II) yields a high loading of Co species (24.7 wt %). Thus, a relatively thinner cathode in fuel cell can accommodate more active Co species and facilitate O2 transfer. Due to the high intrinsic activity and efficient mass transport, the CoO–rGO(N) ORR catalyst achieved approaching performance to state-of-the-art Pt/C cathodes in anion-exchange-membrane alkaline fuel cells.

The detrimental effects of phosphate anion adsorption on the oxygen reduction reactions (ORR) on low index Pt single crystal electrodes were studied in 0.1 M perchloric acid by using a hanging meniscus rotating disk electrode in the presence of varied concentrations of H(3)PO(4). The kinetic current for ORR decreased dramatically on Pt(100), Pt(110), Pt(111), and PtSn(111) even with the addition of a small amount (1 mM) of H(3)PO(4) into the perchloric acid solution, most probably due to the adsorption of phosphate anions onto the Pt active sites that impeded the electroreduction of O(2). Remarkably, the extent of decline was found to vary with the specific single crystal surface, following the order of Pt(111) > PtSn(111) > Pt(110) ∼ Pt(100). Consistent behaviors were also observed in Tafel analysis and in electrochemical impedance spectroscopic measurements. Within the present experimental context, Pt(110) was found to be the optimal crystal surface for ORR in phosphoric acid fuel cells with the smallest charge transfer resistance, whereas the poisoning effects of phosphate anion adsorption were the most pronounced on Pt(111), most likely because the phosphate anions primarily adsorbed on the 3-fold sites on the Pt(111) faces, as manifested in in situ X-ray absorption spectroscopic measurements.