Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.

An electrochemical approach is introduced for the versatile carboxylation of multi-layered graphene in 0.1 M Bu4NBF4/MeCN. First, the graphene substrate (i.e., graphene chemically vapor-deposited on Ni) is negatively charged at -1.9 V versus Ag/AgI in a degassed solution to allow for intercalation of Bu4N(+) and, thereby, separation of the individual graphene sheets. In the next step, the strongly activated and nucleophilic graphene is allowed to react with added carbon dioxide in an addition reaction, introducing carboxylate groups stabilized by Bu4N(+) already present. This procedure may be carried out repetitively to further enhance the carboxylation degree under controlled conditions. Encouragingly, the same degree of control is even attainable, if the intercalation and carboxylation is carried out simultaneously in a one-step procedure, consisting of simply electrolyzing in a CO2-saturated solution at the graphene electrode for a given time. The same functionalization degree is obtained for all multi-layered regions, independent of the number of graphene sheets, which is due to the fact that the entire graphene structure is opened in response to the intercalation of Bu4N(+). Hence, this electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. On a more general level, this approach will provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups.

A general and simple solvent-free procedure using direct heating of a ball-milled mixture of L-histidine−Fe 2 O 3 −FeCl 3 is developed for the synthesis of iron-and nitrogen-doped porous carbon electrocatalysts. Through adjustment of the reactant ratios and the pyrolysis temperature, a series of electrocatalysts are easily obtained with varying activities for electrochemical CO 2 reduction reaction (CO 2 RR). The electrocatalyst synthesized from L-histidine−Fe 2 O 3 − FeCl 3 at a 4:1:0.25 component ratio at 1000 °C exhibits the highest Faradaic efficiency of 83% for CO 2 -to-CO conversion at a small overpotential (360 mV) in aqueous media. The use of a number of characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy, and nitrogen sorption experiments, reveals that both Fe 2 O 3 and FeCl 3 contribute to the iron doping and formation of porosity. As a result, they are both crucial to produce the optimal CO 2 RR electrocatalyst. Correlation of the CO 2 RR activity with the carbon structure suggests that the degree of graphitization of the carbon electrocatalysts plays an important role in their CO 2 RR performance.

In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the I D / I G ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale.

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