Utilizing solar energy to fix CO 2 with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by low selectivity and activity, especially in CO 2 electrocatalytic reduction. Here, we report transition-metal atoms coordinated in a graphene shell as active centers for aqueous CO 2 reduction to CO with high faradic efficiencies over 90% under significant currents up to $60 mA/mg. We employed three-dimensional atom probe tomography to directly identify the single Ni atomic sites in graphene vacancies. Theoretical simulations suggest that compared with metallic Ni, the Ni atomic sites present different electronic structures that facilitate CO 2 -to-CO conversion and suppress the competing hydrogen evolution reaction dramatically. Coupled with Li + -tuned Co 3 O 4 oxygen evolution catalyst and powered by a triple-junction solar cell, our artificial photosynthesis system achieves a peak solar-to-CO efficiency of 12.7% by using earth-abundant transition-metal electrocatalysts in a pH-equal system.

Proton exchange membrane fuel cell (PEMFC) is one of the most promising green power sources, in which perfluorinated sulfonic acid ionomer-based membranes (e.g., Nafion) are widely used. However, the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions. In this work, the high-performance sulfonated carbon nanotubes/Nafion composite membranes (Su-CNTs/Nafion) for the PEMFCs were prepared and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied. Microstructure evolution in Nafion membranes during water uptake was investigated by positron annihilation lifetime spectroscopy, and results strongly showed that the Su-CNTs or CNTs in Nafion composite membranes significantly reinforced Nafion matrices, which influenced the development of ionic-water clusters in them. Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the mass formation of proton-conducting pathways (water channels) along the Su-CNTs. In particular, these pathways along Su-CNTs in Su-CNTs/Nafion membranes interconnected the isolated ionic-water clusters at low humidity and resulted in less tortuosity of the water channel network for proton transportation at high humidity. At a high temperature of 135 °C, Su-CNTs/Nafion membranes maintained high proton conductivity because the reinforcement of Su-CNTs on Nafion matrices reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.

The free volumes and proton conductivities of Nafion membranes were investigated at different humidities by positron annihilation lifetime spectroscopy (PALS) and using an electrochemical workstation, respectively. The results showed that the variation in o-Ps lifetime τ was closely associated with the microstructure evolution and the development of hydrophilic ion clusters in Nafion membranes as a function of water uptake, regardless of metal oxide additives. In particular, with increasing relative humidity, the maximum value of τ in the Nafion membranes corresponded to the formation of numerous water channels for proton transportation. Numerous well-connected water channels in Nafion-TiO hybrid membranes could be formed at a much lower relative humidity (∼40% RH) than in the pristine one (∼75% RH), due to the better water retention ability of the Nafion-TiO membranes. Further, a percolation behavior of proton conductivity at high water uptake in Nafion membranes was observed, which showed that the percolation of ionic-water clusters occurred at the water uptake of ∼4.5 wt%, and ∼6 wt% was basically enough for the formation of a well-connected water channel network.

The electrochemical CO2 reduction reaction (CO2RR) on Pd-based electrodes to dissolved formate and/or gaseous CO is largely dependent on potential and the electrode material, yet there is a lack of molecular-level insights into this dependence. Herein, in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with differential electrochemical mass spectrometry (DEMS), gas chromatography (GC), and nuclear magnetic resonance (NMR) measurements is applied to investigate the CO2RR on Pd and Pd-B film electrodes, providing a direct observation of the role of surface CO as well as the B-doping effect at varied potentials. Comprehensive spectrometric results reveal that at lower overpotentials, CO gradually accumulates on both Pd electrode surfaces poisoning the dominant formate pathway, while at higher overpotentials, surface CO forms facilely with linearly bonded CO (the minor surface CO species) acting as an active precursor and bridge-bonded CO (the major surface CO species) as a spectator toward the gaseous CO product. Moreover, B-doping in Pd hinders CO formation and promotes formate production on the Pd-B electrode for the CO2RR as compared to that on the pristine Pd electrode at all of the overpotentials under investigation.