The as-prepared NiHC-pz-300 catalyst (12.5 mg) was dispersed in 968 μL absolute ethanol and 32 μL Nafion solution (5 wt%) accompanied by a continuous ultra-sonification to form a homogeneous catalyst ink. Then, 40 μL ink was pipetted onto the double sides of carbon paper, giving a catalyst loading of 1 mg cm−2. The catalyst with lower loadings was prepared by diluting the ink with ethanol. The electrochemical workstation (CHI 660E, Shanghai CH Instruments Co., China) was utilized for the electrochemical studies. The electrochemical measurements were carried out in a typical H-Type cell with three-electrode configuration, which consists of the as-prepared NiHC-pz-300 catalyst electrode as the working electrode, a platinum foil as the auxiliary electrode, and a Ag/AgCl (saturated KCl) as the reference electrode. All measured potentials were converted to the reversible hydrogen electrode (RHE) according to the following equation:
The electrochemical oxidation activity of 25 organic substrates (methanol, ethanol, 2,2,2-trifluoroethanol, benzyl alcohol, 2-propanol, 1,1,1-trifluoro-2-propanol, 1-phenylethanol, benzaldehyde, furfural, ethylene glycol, 1,4-butanediol, 1,6-hexanediol, ethylamine, 1-propylamine, 2,2,2-trifluoroethylamine, benzylamine, 2-propylamine, 1-phenylethylamine, cyclohexanol, cyclohexylamine, urea, glycerol, glucose, 5-hydroxymethylfurfural and 2-aminoethanol) were evaluated in 1 M KOH + 0.1 M substrate. The Linear sweep voltammetry (LSV) curves were scanned at a rate of 5 mV s−1 at room temperature after 5 cyclic voltammetry (CV) cycles at a scan rate of 50 mV s−1. All polarization curves were manually corrected with 90% iR-compensation. For obtaining accurate Tafel slope values, all Tafel plots were iR-corrected. Chronopotentiometric measurements were recorded at a current density of 20 mA cm−2. In order to reduce the impact on the stability of the catalyst due to the changes of substrate concentration, the electrolyte was refreshed every 12 h. Turnover frequencies (TOFs) were calculated from the following equation: where I is the current density in the LSV curve (mA/mg), n is the number electrons needed for the oxidation of one urea molecule (n = 6 (N2) or 12 (NO2−)), F is the Faraday constant of 96485 F/mol, c is the active Ni site density in the catalyst (mol/g).
The electrochemical oxidation activity of 25 organic substrates (methanol, ethanol, 2,2,2-trifluoroethanol, benzyl alcohol, 2-propanol, 1,1,1-trifluoro-2-propanol, 1-phenylethanol, benzaldehyde, furfural, ethylene glycol, 1,4-butanediol, 1,6-hexanediol, ethylamine, 1-propylamine, 2,2,2-trifluoroethylamine, benzylamine, 2-propylamine, 1-phenylethylamine, cyclohexanol, cyclohexylamine, urea, glycerol, glucose, 5-hydroxymethylfurfural and 2-aminoethanol) were evaluated in 1 M KOH + 0.1 M substrate. The Linear sweep voltammetry (LSV) curves were scanned at a rate of 5 mV s−1 at room temperature after 5 cyclic voltammetry (CV) cycles at a scan rate of 50 mV s−1. All polarization curves were manually corrected with 90% iR-compensation. For obtaining accurate Tafel slope values, all Tafel plots were iR-corrected. Chronopotentiometric measurements were recorded at a current density of 20 mA cm−2. In order to reduce the impact on the stability of the catalyst due to the changes of substrate concentration, the electrolyte was refreshed every 12 h. Turnover frequencies (TOFs) were calculated from the following equation: where I is the current density in the LSV curve (mA/mg), n is the number electrons needed for the oxidation of one urea molecule (n = 6 (N2) or 12 (NO2−)), F is the Faraday constant of 96485 F/mol, c is the active Ni site density in the catalyst (mol/g).
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