(a) FEs of carbon monoxide (CO), methane (CH4), ethylene (C2H4), formate (HCOO−), ethanol (EtOH), acetate (AcO−) and n-propanol (n-PrOH) at various applied cathodic potential for NGQDs. (b) FE of CO2 reduction products for pristine GQDs. (c) Selectivity to CO2 reduction products for NRGOs. (d) Tafel plots of partial current density of CO2 reduction versus applied cathodic potential for three nanostructured carbon catalysts. The error bar represents the s.d. of three separate measurements for an electrode.