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A variety of atomically dispersed transition-metal-anchored nitrogen-doped carbon (M−N−C) electrocatalysts have shown encouraging electrochemical CO 2 reduction reaction (CO 2 RR) performance, with the underlying fundamentals of central transition-metal atom determined CO 2 RR activity and selectivity yet remaining unclear. Herein, a universal impregnation-acid leaching method was exploited to synthesize various M− N−C (M: Fe, Co, Ni, and Cu) single-atom catalysts (SACs), which revealed d-orbital electronic configuration-dependent activity and selectivity toward CO 2 RR for CO production. Notably, Ni−N−C exhibits a very high CO Faradaic efficiency (FE) of 97% at −0.65 V versus RHE and above 90% CO selectivity in the potential range from −0.5 to −0.9 V versus RHE, much superior to other M−N−C (M: Fe, Co, and Cu). With the d-orbital electronic configurations of central metals in M−N−C SACs well elucidated by crystal-field theory, Dewar−Chatt−Duncanson (DCD) and differential charge density analysis reveal that the vacant outermost dorbital of Ni 2+ in a Ni−N−C SAC would benefit the electron transfer from the C atoms in CO 2 molecules to the Ni atoms and thuseffectively activate the surface-adsorbed CO 2 molecules. However, the outermost d-orbital of Fe 3+ , Co 2+ , and Cu 2+ occupied by unpaired electrons would weaken the electron-transfer process and then impede CO 2 activation. In situ spectral investigations demonstrate that the generation of *COOH intermediates is favored over Ni−N−C SAC at relatively low applied potentials, supporting its high CO 2 -to-CO conversion performance. Gibbs free energy difference analysis in the rate-limiting step in CO 2 RR and hydrogen evolution reaction (HER) reveals that CO 2 RR is thermodynamically favored for Ni−N−C SAC, explaining its superior CO 2 RR performance as compared to other SACs. This work presents a facile and general strategy to effectively modulate the CO 2to-CO selectivity from the perspective of electronic configuration of central metals in M−N−C SACs.

One of the most fundamental and yet elusive collective phases of an interacting electron system is the quantum Wigner crystal (WC), an ordered array of electrons expected to form when the electrons' Coulomb repulsion energy eclipses their kinetic (Fermi) energy. In low-disorder, twodimensional (2D) electron systems, the quantum WC is known to be favored at very low temperatures (T ) and small Landau level filling factors (ν), near the termination of the fractional quantum Hall states. This WC phase exhibits an insulating behavior, reflecting its pinning by the small but finite disorder potential. An experimental determination of a T vs ν phase diagram for the melting of the WC, however, has proved to be challenging. Here we use capacitance measurements to probe the 2D WC through its effective screening as a function of T and ν. We find that, as expected, the screening efficiency of the pinned WC is very poor at very low T and improves at higher T once the WC melts. Surprisingly, however, rather than monotonically changing with increasing T , the screening efficiency shows a well-defined maximum at a T which is close to the previously-reported melting temperature of the WC. Our experimental results suggest a new method to map out a T vs ν phase diagram of the magnetic-field-induced WC precisely.

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