Sulfur filling activates vacancy-induced C–C bond cleavage in polyol electrooxidation

ABSTRACT Using the electrochemical polyol oxidation reaction (POR) to produce formic acid over nickel-based oxides/hydroxides (NiOxHy) is an attractive strategy for the electrochemical upgrading of biomass-derived polyols. The key step in the POR, i.e. the cleavage of the C–C bond, depends on an oxygen-vacancy-induced mechanism. However, a high-energy oxygen vacancy is usually ineffective for Schottky-type oxygen-vacancy-rich β-Ni(OH)2 (VSO-β-Ni(OH)2). As a result, both β-Ni(OH)2 and VSO-β-Ni(OH)2 cannot continuously catalyze oxygen-vacancy-induced C–C bond cleavage during PORs. Here, we report a strategy of oxygen-vacancy-filling with sulfur to synthesize a β-Ni(OH)2 (S-VO-β-Ni(OH)2) catalyst, whose oxygen vacancies are protected by filling with sulfur atoms. During PORs over S-VO-β-Ni(OH)2, the pre-electrooxidation-induced loss of sulfur and structural self-reconstruction cause the in-situ generation of stable Frenkel-type oxygen vacancies for activating vacancy-induced C–C bond cleavage, thus leading to excellent POR performances. This work provides an intelligent approach for guaranteeing the sustaining action of the oxygen-vacancy-induced catalytic mechanism in electrooxidation reactions.

To facilitate the conversion of the substrate, 50 mM alcohol was selected for the electrolysis process.For the identification and quantification of reaction intermediates and products in AORs, the catalyst was painted on carbon paper (CP; 1 cm 2 ) in order to increase the electrolysis efficiency of AORs.Products analysis.The substrates and the products were quantified using HPLC equipped with a refractive detector and a 7.8 × 300 mm Coregel-87H3 column.Column temperature was kept at 60 °C and mobile phase was sulfuric acid 5 mM with a flow rate of 0.5 mL min -1 .After electrolysis, 50 µL of the electrolyte was taken out, and its pH was adjusted to neutral by adding 0.5 M H 2 SO 4 .The filtered sample was injected into the HPLC for retention time (30 min) analysis.Organic compounds were characterized by 1 H NMR and 13 C NMR.Typically, 400 μL of electrolyte after electrolysis was mixed with 200 μL of D 2 O.The conversion rate, selectivity, and Faradic efficiency are calculated as follows: Conversion rate (%) = (mol of nucleophile consumed) / (mol of the nucleophile at the beginning) × 100% Selectivity (%) = (mol of product) / (mol of consumed nucleophile) × 100% Faradic efficiency (%) = (mol of product) × n × F / (total passed charge) × 100% n = 2 for the electrooxidation of R-CH 2 OH to R-CHO, R-CHO/R-CH(OH) 2 to R-COOH, and the cleavage of C-C bond.n = 4 for the electrooxidation of R-CH 2 OH to R-COOH.n = 6 for the electrooxidation of R-CHOH-CH 2 OH to R-COOH and HCOOH.n = 8 for the electrooxidation of CH 2 OH-CHOH-CH 2 OH to HCOOH.F is the Faraday constant (96485 C mol -1 ).
Theoretical methods.All the spin-polarized calculations were performed within the framework of DFT using the projector-augmented plane wave (PAW) technique as implemented in the Vienna Ab Initio Simulation Package (VASP 5.4) [1].The generalized gradient approximation (GGA) method parameterized by the Perdew-Burke-Ernzerhof (PBE) functional was used to account for the exchange-correlation energy [2,3].The kinetic energy cutoff was set to be 450 eV in the plane-wave expansion.We constructed (2×2) supercells of β-Ni(OH) 2 (001) surfaces, one without vacancy and another with oxygen vacancies, each consisting of five atomic layers.The bottom two atomic layers remained fixed, and the topmost three atomic layers and adsorbed species underwent relaxation.To avoid interlayer interaction, the vacuum spacing in a direction perpendicular to the plane of the structure is set to 15 Å.The Brillouin zone was sampled using a 3 × 5 × 1 Monkhorst-Pack k-point mesh for geometry optimization calculation.The systems were relaxed until the energy and force reached at 1×10 -6 eV cell -1 and 0.02 eV Å -1 , respectively.The Gaussian smearing width was set to 0.2 eV.To describe the effect of van der Waals interactions, the DFT-D3 empirical correction method was applied [4].The free energy for the dehydrogenation (E H ) was calculated according to: E H = E V-H + 1/2 E H2 -E slab where E V-H , E slab , and E H2 represent energy for relaxed slab after H atom desorption, relaxed slab, and gas phase H 2 , respectively.The free energy for the adsorption energies (E ads ) was calculated according to: E ads = E slab-x -E slab -E x where E slab-x represents energy for relaxed slab after x molecule adsorption, and E x represents the energy for gas phase x.Coordination-unsaturated Ni cations with lower valence states result from oxygen vacancies [10].The Ni K-edge XANES spectrum of S-V O -β-Ni(OH) 2 reveals that both the adsorption edge and pre-edge peak are shifted back to high energies, indicating that oxygen vacancies have been filled in S-V O -β-Ni(OH) 2 [11].S3.     two semicircles appear at the potential above 1.35 V (Figure S12a).The radius of the first semicircle (representing the high-frequency electrochemical reaction) decreases gradually at potentials above 1.35 V (Figure S12b).The radius of the second semicircle (representing the low-frequency electrochemical reaction) significantly decreases at the potential above 1.5 V (Figure S12c).The Bode plot visualizes the different frequency-dependent of these two electrochemical steps (Figure S12d).As to the OER system based on S-V O -β-Ni(OH) 2 , the high-frequency electrochemical reaction involves the electrooxidation of Ni 2+ -OH to Ni 3+ -O bond containing electrophilic lattice oxygen, and the low-frequency electrochemical reaction is OER involving the generation of electrophilic adsorbed oxygen species (Figure S12e) [12].S16a).According to TEM image, β-Ni(OH) 2 nanosheets after PAOR are still regular hexagonal nanosheets, and the lattice structure remains intact (Figure S16b and S16c).Anodic polarization curve shows that reaction current of PAOR over β-Ni(OH) 2 increases sharply at potentials above about 1.40 V (Figure S16d).In situ Raman spectra proved that Ni 2+δ species could not be accumulated during PAOR over β-Ni(OH) 2 (Figure S16e).The morphology and crystal structure of β-Ni(OH) 2 can remain unchanged without the formation of Ni 2+δ O x H y species during PAOR (Figure S16f).Thirdly, after the sufficient reaction between Ni 2+δ species and ethylene glycol, the electrode was rinsed with deionized water, and then transferred to the electrolyte (1 M KOH) (Figure S39a).We repeated the cycle described above twenty times to increase the concentration of reaction product in the solution.As shown in Figure S39b, a small number of formic acid was generated in the solution after the reaction between Ni 2+δ species and ethylene glycol.This result fully proves that Ni 2+δ species (e.g., Ni 3+

Figure
Figure S6 Ni K-edge XANES spectra of β-Ni(OH) 2 , V SO -β-Ni(OH) 2 , and S-V O -β-Ni(OH) 2 .XANES was measured to determine the electronic and local structures of the samples [9].Both adsorption edge and pre-edge peak in the Ni K-edge XANES spectra of V SO -β-Ni(OH) 2 show obvious peak-shifts toward the low energy regions, compared with that of β-Ni(OH) 2 .Coordination-unsaturated Ni cations with lower valence states result from oxygen vacancies[10].The Ni K-edge XANES spectrum of S-V O -β-Ni(OH) 2 reveals that both the adsorption edge and pre-edge peak are shifted back to high energies, indicating that oxygen vacancies have been filled in S-V O -β-Ni(OH) 2[11].

Figure
Figure S10 (a) Photograph of fresh S-V O -β-Ni(OH) 2 and the S-V O -β-Ni(OH) 2 after OER suspensions.(b, c) TEM and HR-TEM images of S-V O -β-Ni(OH) 2 nanosheets after OER.(d) Anodic polarization curve of S-V O -β-Ni(OH) 2 in the OER system.After OER, the color of S-V O -β-Ni(OH) 2 changed from turquoise to black due to the formation of black Ni 2+δ O x H y species

Figure S11
Figure S11 EDX images of the S-V O -β-Ni(OH) 2 electrode after pre-electrooxidation.It proves that, during electrooxidation over S-V O -β-Ni(OH) 2 , pre-electrooxidation can cause the irreversible loss of S occurs.

Figure S12
Figure S12 Nyquist plots (a-c), Bode plots (d), and potential-dependent behavior (e) of the S-V O -β-Ni(OH) 2 electrode at different potentials in 1 M KOH.Nyquist plots of S-V O -β-Ni(OH) 2 in 1 M KOH show only one semicircle with a larger radius at the potential below 1.35 V, while two semicircles appear at the potential above 1.35 V (Figure S12a).The radius of the first semicircle (representing the high-frequency electrochemical reaction) decreases gradually at

Figure
Figure S15 (a) Anodic polarization curves of the fresh S-V O -β-Ni(OH) 2 and the used S-V O -β-Ni(OH) 2 electrode after 24 hours of air exposure in 1 M KOH with 50 mM ethanol.(b) Long-term test (four consecutive electrolysis) of PAOR on S-V O -β-Ni(OH) 2 /CP electrode in 1 M KOH with 50 mM ethanol at the potential of 1.45 V.The fresh S-V O -β-Ni(OH) 2 exhibited excellent PAOR performance, and the used S-V O -β-Ni(OH) 2 electrode after 24 hours of air exposure also showed excellent PAOR performance.Besides, the PAOR performance of S-V O -β-Ni(OH) 2 remained stable during consecutive electrolysis, indicating the stability of S-V O -β-Ni(OH) 2 in the PAOR system.

Figure
Figure S16 (a) Photograph of fresh β-Ni(OH) 2 and the β-Ni(OH) 2 after PAOR suspensions.(b, c) TEM and HR-TEM images of β-Ni(OH) 2 nanosheets after PAOR.(d) Anodic polarization curve of β-Ni(OH) 2 in the PAOR system.(e) In situ Raman spectra of β-Ni(OH) 2 in the PAOR system (1 M KOH with 0.5 M ethanol) at different potentials.(f) Schematic illustration showing the surface species evolution of the β-Ni(OH) 2 electrode in the PAOR system.The characterization before and after PAOR was performed to identify the catalyst function of primary alcohol electrooxidation reaction over catalysts.The potential of electrolysis and electrolysis times are 1.45 V RHE and one

Figure
Figure S17 (a) Photograph of fresh S-V O -β-Ni(OH) 2 and the S-V O -β-Ni(OH) 2 after PAOR suspensions.(b, c) TEM and HR-TEM images of S-V O -β-Ni(OH) 2 nanosheets after PAOR.(d) Anodic polarization curve of S-V O -β-Ni(OH) 2 in the PAOR system.(e) In situ Raman spectra of S-V O -β-Ni(OH) 2 in the PAOR system at different potentials.(f) Schematic illustration showing the surface species evolution of the S-V O -β-Ni(OH) 2 electrode in the PAOR system.The PAOR system of S-V O -β-Ni(OH) 2 differs vastly from the OER system.The color of the S-V O -β-Ni(OH) 2 electrode after PAOR remained turquoise, suggesting that the S-V O -β-Ni(OH) 2 electrode may remain unchanged during PAOR (FigureS17a).TEM images show that S-V O -β-Ni(OH) 2 nanosheets after PAOR still are regular hexagonal nanosheets with a complete lattice structure (FigureS17b and S17c).These results indicate that the morphology and crystal structure of S-V O -β-Ni(OH) 2 remained nearly constant during PAOR.Anodic polarization curve shows that reaction current of PAOR over S-V O -β-Ni(OH) 2 increases sharply at potentials above about 1.35 V (FigureS17d).In situ Raman spectra proved that Ni 2+δ species could not be accumulated during PAOR over S-V O -β-Ni(OH) 2 (FigureS17e).The generation of Ni 2+δ O x H y species cannot occur during PAOR over S-V O -β-Ni(OH) 2 (FigureS17f).

Figure S18
Figure S18 EDX images of the S-V O -β-Ni(OH) 2 electrode after PAOR.It proves that, during PAOR over S-V O -β-Ni(OH) 2 , pre-electrooxidation can cause the irreversible loss of S occurs as well.

Figure S19
Figure S19 Nyquist plots (a), Bode plots (b), and potential-dependent behavior (c) of the β-Ni(OH) 2 electrode at different potentials in 1 M KOH with 0.5 M ethanol.There is only a high-frequency interface for the electrochemical behavior of β-Ni(OH) 2 in the electrooxidation of ethanol (the model PAOR system; electrolyte: 1 M KOH with 0.5 M ethanol) (FigureS19a).As shown in Nyquist and Bode plots of the PAOR on β-Ni(OH) 2 , the radius of semicircle and the phase angle of the high-frequency electrochemical step decrease as the potential increases at a potential above 1.4 V (FigureS19a and S19b).It is worth noting that the low-frequency electrochemical step, i.e., the electrochemical generation of electrophilic adsorbed oxygen species, does not work during the PAOR on β-Ni(OH) 2 (FigureS19a and S19b).These results suggest that the electrochemical step of the PAOR on β-Ni(OH) 2 is the high-frequency electrochemical step, i.e., the electrooxidation of Ni 2+ -OH to Ni 3+ -O bond containing electrophilic lattice oxygen, instead of the electrochemical generation of electrophilic adsorbed oxygen species.We assume that the Ni 3+ -O bond containing electrophilic lattice oxygen can seize the hydrogen atom of alcohols to generate carboxylic acid products and Ni 2+ -OH bonds, thus avoiding the accumulation of Ni 3+ -O bonds and Ni 2+δ O x H y species[12].In brief, due to the synergy of the electrooxidation of Ni 2+ -OH to Ni 3+ -O and the hydrogen atom transfer (HAT) from alcohols to Ni 3+ -O bond, PAOR takes place at the high-frequency interface, without the accumulation of Ni 3+ -O bonds at the low-frequency interface (FigureS19c).

Figure S20
Figure S20 Nyquist plots (a-c), Bode plots (d), and potential-dependent behavior (e) of the S-V O -β-Ni(OH) 2 electrode at different potentials in 1 M KOH with 0.5 M ethanol.For the Nyquist plots of S-V O -β-Ni(OH) 2 in 1 M KOH with 0.5 M ethanol, there are two semicircles during the ethanol electrooxidation (the model PAOR system) at a potential above 1.4 V (FigureS20a).The radiuses of two semicircles decrease as the potential increases at a potential above 1.4 V, suggesting that both the high-frequency and low-frequency electrochemical steps play important roles in the ethanol electrooxidation (PAOR) on S-V O -β-Ni(OH) 2 (FigureS20b and S20c).The Bode plots show an intuitive presentation of two different frequency-dependent electrochemical steps during the PAOR on S-V O -β-Ni(OH) 2 (FigureS20d).Therefore, both the electrophilic lattice oxygen species (Ni 3+ -O bond) and the electrophilic adsorbed oxygen species (e.g., S-V O -Ni 2+δ -OH ads ) function as the redox mediator to catalyze the dehydrogenation of R-CH 2 OH to R-COOH during the PAOR on S-V O -β-Ni(OH) 2 (FigureS20e)[12].

Figure
Figure S21 (a) Current density versus time of the β-Ni(OH) 2 electrode (0 to 50 s: 1.5 V; 50 to 100 s: open-circuit condition; 100 to 150 s: 1.0 V) in 1M KOH.(b) Corresponding schematic diagram showing the catalytic cycle including the electrooxidation of Ni 2+ -OH to Ni 3+ -O and the electroreduction of Ni 3+ -O to Ni 2+ -OH in the electrochemical testing.We carried out the multi-potential step chronoamperometric measurement to research the Ni 2+ /Ni 3+ redox couple for the β-Ni(OH) 2 electrode in 1 M KOH.The electrochemical testing of multi-potential step includes three stages (Figure S21a).1.5 and 1.0 V were carried out at the first 50 seconds (0 to 50 s) and the third 50 seconds (100 to 150 s), respectively, and the β-Ni(OH) 2 electrode was under an

Figure
Figure S22 (a) Current density versus time of the β-Ni(OH) 2 electrode (0 to 50 s: 1.5 V; 50 to 100 s: open-circuit condition; 100 to 150 s: 1.0 V) in adjustable electrolytes (0 to 60 s: 1 M KOH; 60 to 150 s: 1 M KOH with 0.5 M ethanol).(b) Synchronous in situ Raman spectra of the β-Ni(OH) 2 electrode during the electrochemical testing.(c) Corresponding schematic diagram showing the catalytic cycle including the electrooxidation of Ni 2+ -OH to Ni 3+ -O and the spontaneous reaction between Ni 3+ -O and R-CH 2 OH.As mentioned above, the electrocatalyst function of the PAOR on β-Ni(OH) 2 is composed of the electrooxidation of Ni 2+ -OH to Ni 3+ -O and the HAT between Ni 3+ -O and R-CH 2 OH (Figure S22c).To investigate the electrocatalyst function, the HAT process should be separated from the electrooxidation of Ni 2+ -OH to Ni 3+ -O bond through the electrochemical testing of multi-potential step and synchronous in situ Raman spectra with tunable electrolytes (1 M KOH with/without 0.5 M ethanol) (Figure S22a and S22b).For the β-Ni(OH) 2 electrode, Ni 3+ -O bonds were formed and accumulated at 1.50 V in 1 M KOH (0 to 50 s), and the generated Ni 3+ -O bonds still existed in 1 M KOH under an open-circuit condition (0 to 50 s).However, after adding 0.5 M ethanol to 1 M KOH at 60 s (open-circuit conditions: 50 to 100 s), all accumulated Ni 3+ -O bonds spontaneously reacted with ethanol to form Ni 2+ -OH bonds, and no Ni 3+ -O bond was identified at the third 50 seconds with the potential of 1.0 V. Hence, the electrocatalyst function of the PAOR on β-Ni(OH) 2 , i.e., LOM-HAT, includes two processes (Figure S22c): (1) the electrooxidation of Ni 2+ -OH to Ni 3+ -O bonds containing electrophilic lattice oxygens (Ni 2+ -OH + OH -= Ni 3+ -O + H 2 O + e -), and (2) spontaneous HAT reaction between alcohols and Ni 3+ -O bond (defined as lattice oxygen-induced HAT: LO-HAT: Ni 3+ -O + H ethanol + e - circuit = Ni 2+ -OH + product).

Figure
Figure S24 (a) Current density versus time of the S-V O -β-Ni(OH) 2 electrode (0 to 50 s: 1.5 V; 50 to 100 s: open-circuit condition; 100 to 150 s: 1.0 V) in adjustable electrolytes (0 to 60 s: 1 M KOH; 60 to 150 s: 1 M KOH with 0.5 M ethanol).(b) Simultaneous in situ Raman spectra of S-V O -β-Ni(OH) 2 during electrochemical testing.(c, d) Corresponding schematic diagrams showing the catalytic cycles including the electrochemical generation of S-V O -Ni 3+ -O and the spontaneous reaction between S-V O -Ni 3+ -O and R-CH 2 OH (c), and the electrochemical generation of S-V O -Ni 2+δ -OH ads and the spontaneous reaction between S-V O -Ni 2+δ -OH ads and R-CH 2 OH (d).Via combining the multi-potential step chronoamperometric measurement and in situ Raman spectra of S-V O -β-Ni(OH) 2 , the accumulated Ni 2+δ species containing electrophilic lattice/adsorbed oxygen (e.g., Ni 3+ -O bond, S-V O -Ni 3+ -O bond, and S-V O -Ni 2+δ -OH ads ) were

Figure
Figure S27 (a, d) 1 H NMR spectra for electrolytes before and after electrolysis (1 M KOH with 50 mM R-CH 2 OH) for the electrooxidations of n-propyl alcohol (a) and benzyl alcohol (d).(b, e) 13 C NMR spectra for electrolytes before and after electrolysis for the electrooxidations of n-propyl alcohol (b) and benzyl alcohol (e).Conversion rates, selectivities, and Faradaic efficiencies for the electrooxidations of n-propyl alcohol (c) and benzyl alcohol (f) on S-V O -β-Ni(OH) 2 .For the S-V O -β-Ni(OH) 2 electrode, the electrooxidation products of CH 3 -CH 2 OH, CH 3 -CH 2 -CH 2 OH, and C 6 H 5 -CH 2 OH are CH 3 -COOH, CH 3 -CH 2 -COOH, and C 6 H 5 -COOH, respectively (Figures S14 andS27).As to different PAOR systems based on S-V O -β-Ni(OH) 2 , the conversion rates of primary alcohols and the selectivities of R-COOH are close to ~100%.Due to the low electrolytic efficiency for the PAOR system with a low concentration of nucleophile, Faradaic efficiencies are relatively low, yet above 84%.

Figure
Figure S28 (a) Electrochemical and non-electrochemical steps in the PAOR on S-V O -β-Ni(OH) 2 .(b) Schematic diagram showing the electrooxidation of CH 3 -CH 2 OH on S-V O -β-Ni(OH) 2 .The PAOR on S-V O -β-Ni(OH) 2 comprises the electrochemical step, electrocatalyst-induced non-electrochemical step, and electrocatalyst-irrelevant non-electrochemical step (Figure S28a).Electrochemical step include (1) the electrochemical generation of Ni 3+ -O and (2) the electrochemical generation of S-V O -Ni 2+δ -OH ads .Electrocatalyst-induced non-electrochemical steps include (1) LO-HAT and (2) AO-HAT.The electrocatalyst-irrelevant non-electrochemical step is reversible hydration reaction of R-CHO.On the S-V O -β-Ni(OH) 2 electrode, CH 3 -CH 2 OH can be electrochemically oxidized to CH 3 -COOH due to the synergistic effect between the LOM-HAT/S-V O -AOM-HAT and hydration of CH 3 -CHO (Figure S28b).In the first step, CH 3 -CH 2 OH loses two hydrogen atoms due to LOM-HAT and S-V O -AOM-HAT, resulting in CH 3 -CHO.Afterwards, a nucleophilic attack by water molecules on CH 3 -CHO leads to the formation of aldehyde hydrates (CH 3 -CH(OH) 2 ).In the end, CH 3 -CH(OH) 2 undergoes two steps HAT processes to generate CH 3 -COOH.

Figure
Figure S30 (a) Anodic polarization curves of β-Ni(OH) 2 in the POR system (in 1 M KOH with 50 mM polyols, e.g., ethylene glycol, phenethyleneglycol, and glycerol).(b) Linear sweep voltammetry (LSV) curves of the β-Ni(OH) 2 electrode (the second LSV curve was measured in 1 M KOH with 50 mM ethylene glycol, and the others were measured in 1 M KOH).β-Ni(OH) 2 is unable to catalyze POR involving the cleavage of C-C bonds, leading to poor POR performances (FigureS30a).The β-Ni(OH) 2 electrode surface can be passivated during the ethylene glycol electrooxidation (the model POR system).Compared with the 1st LSV curve of β-Ni(OH) 2 in OER system, there is no Ni 2+ /Ni 3+ oxidation peak for the LSV curve of the passivated β-Ni(OH) 2 electrode in the OER system (the 3rd LSV curve), and the OER performance of the passivated β-Ni(OH) 2 electrode is significantly lower than that of the fresh β-Ni(OH) 2 electrode.

Figure S31
Figure S31 EDX images of the β-Ni(OH) 2 electrode after POR.Obviously, the β-Ni(OH) 2 nanosheets after POR were encapsulated by polymers.It is evident from this result that an insoluble passivation film can be formed in POR over β-Ni(OH) 2 .

Figure S33
Figure S33 Anodic polarization curves of β-Ni(OH) 2 , V SO -β-Ni(OH) 2 , and S-V O -β-Ni(OH) 2 in 1 M KOH with 50 mM glycerol.Both β-Ni(OH) 2 and V SO -β-Ni(OH) 2 cannot catalyze the electrooxidation of glycerol to formic acids involving the C-C bond cleavage, and both β-Ni(OH) 2 and V SO -β-Ni(OH) 2 can be passivated in the POR system with glycerol substrate.S-V O -β-Ni(OH) 2 exhibits an excellent performance for the electrooxidation of glycerol to formic acids, and the passivation of electrode cannot be observed during glycerol electrooxidation over S-V O -β-Ni(OH) 2 .

Figure
Figure S35 (a) Anodic polarization curves of the fresh S-V O -β-Ni(OH) 2 and the used S-V O -β-Ni(OH) 2 electrode after 24 hours of air exposure in 1 M KOH with 50 mM ethylene glycol.(b) The long-term test (three consecutive electrolysis) of POR on S-V O -β-Ni(OH) 2 /CP electrode in 1 M KOH with 50 mM ethylene glycol at the potential of 1.45 V.The fresh S-V O -β-Ni(OH) 2 exhibited excellent POR performance, and the used S-V O -β-Ni(OH) 2 electrode after 24 hours of air exposure also showed excellent EGOR performance.Besides, the EGOR performance of S-V O -β-Ni(OH) 2 remained stable during consecutive electrolysis, indicating the sustained effect of oxygen vacancy-induced catalytic mechanism during EGOR on S-V O -β-Ni(OH) 2 .

Figure S36
Figure S36 EDX images of the S-V O -β-Ni(OH) 2 electrode after POR.As shown in the EDX images of S-V O -β-Ni(OH) 2 nanosheets after POR, the passivation film containing polymers is not observed.Hence, S-V O -β-Ni(OH) 2 cannot be passivated during POR.

Figure S38
Figure S38 Nyquist plots (a), Bode plots (b), and potential-dependent behavior (c) of the S-V O -β-Ni(OH) 2 electrode at different potentials in 1 M KOH with 0.5 M ethylene glycol.For the Nyquist plots of S-V O -β-Ni(OH) 2 in 1 M KOH with 0.5 M ethylene glycol, there are two semicircles during the ethylene glycol electrooxidation (the model POR system), and the radiuses of two semicircles decrease as the potential increases at a potential above 1.4 V (Figure S38a).Consequently, both the electrophilic lattice oxygen species (e.g., Ni 3+ -O) and the electrophilic adsorbed oxygen species (e.g., S-V O -Ni 2+δ -OH ads ) function as the redox mediator to catalyze the oxidation of R-CHOH-CH 2 OH to R-COOH and HCOOH accompanied with the C-C bond cleavage during the POR on S-V O -β-Ni(OH) 2 (Figure S38c).

Figure
Figure S39 (a) Schematic diagram showing the verification test for the spontaneous reaction between Ni 2+δ species and R-CH 2 OH-CH 2 OH.(b) 13 C NMR spectra of the electrolyte (1 M KOH with 50 mM EG) before and after the twenty-fold cycles of reaction between EG and Ni 2+δ species.We carried out the product detection for spontaneous reactions between Ni 2+δ species and R-CHOH-CH 2 OH.Firstly, the controlled potential electrolysis of S-V O -β-Ni(OH) 2 loading on carbon paper (S-V O -β-Ni(OH) 2 /CP; 1 cm 2 ) in 1 M KOH was run at 1.5 V for 200 s to enrich Ni 2+δ species on the electrode.Secondly, the S-V O -β-Ni(OH) 2 /CP electrode containing Ni 2+δ species was transferred to the solution (1 M KOH with 50 mM ethylene glycol) to oxidize ethylene glycol.Thirdly, after the sufficient reaction between Ni 2+δ species and ethylene glycol, the electrode was rinsed with deionized water, and then transferred to the electrolyte (1 M KOH) (FigureS39a).We repeated the cycle described above twenty times to increase the concentration of reaction product in the solution.As shown in FigureS39b, a small number of formic acid was generated in the solution after the reaction between Ni 2+δ species and ethylene glycol.This result fully proves that Ni 2+δ species (e.g., Ni3+ -O and S-V O -Ni 2+δ -OH ads ) can spontaneously catalyze the oxidative C-C bond cleavage of ethylene glycol to generate formic acid.Given that Ni 3+ -O bond cannot be used for C-C bond cleavage, S-V O -Ni 2+δ -OH ads is the only specie that is able to catalyze the cleavage of C-C bond spontaneously.
Figure S39 (a) Schematic diagram showing the verification test for the spontaneous reaction between Ni 2+δ species and R-CH 2 OH-CH 2 OH.(b) 13 C NMR spectra of the electrolyte (1 M KOH with 50 mM EG) before and after the twenty-fold cycles of reaction between EG and Ni 2+δ species.We carried out the product detection for spontaneous reactions between Ni 2+δ species and R-CHOH-CH 2 OH.Firstly, the controlled potential electrolysis of S-V O -β-Ni(OH) 2 loading on carbon paper (S-V O -β-Ni(OH) 2 /CP; 1 cm 2 ) in 1 M KOH was run at 1.5 V for 200 s to enrich Ni 2+δ species on the electrode.Secondly, the S-V O -β-Ni(OH) 2 /CP electrode containing Ni 2+δ species was transferred to the solution (1 M KOH with 50 mM ethylene glycol) to oxidize ethylene glycol.Thirdly, after the sufficient reaction between Ni 2+δ species and ethylene glycol, the electrode was rinsed with deionized water, and then transferred to the electrolyte (1 M KOH) (FigureS39a).We repeated the cycle described above twenty times to increase the concentration of reaction product in the solution.As shown in FigureS39b, a small number of formic acid was generated in the solution after the reaction between Ni 2+δ species and ethylene glycol.This result fully proves that Ni 2+δ species (e.g., Ni3+ -O and S-V O -Ni 2+δ -OH ads ) can spontaneously catalyze the oxidative C-C bond cleavage of ethylene glycol to generate formic acid.Given that Ni 3+ -O bond cannot be used for C-C bond cleavage, S-V O -Ni 2+δ -OH ads is the only specie that is able to catalyze the cleavage of C-C bond spontaneously.

Figure S43
Figure S43 Schematic diagram showing possible reaction pathways of the electrooxidation of CH 2 OH-CHOH-CH 2 OH.In the glycerin electrooxidation reaction (GOR), the first step has four possibilities: (1) Hydroxymethyl dehydrogenation to produce glyceraldehyde (GLA), (2) Secondary hydroxyl dehydrogenation to yield dihydroxyacetone (DHA), (3) Cleavage of one C-C bond to produce glycolic aldehyde hydrate (GAH) and FALH, and (4) Cleavage of two C-C bonds to form one FA molecule and two FALH molecules [16-18].

Figure S45
Figure S45 Reaction pathway for the electrooxidations of H 2 C(OH) 2 to HCOOH on S-V O -β-Ni(OH) 2 .The S-V O -AOM involving C-C bond cleavage preferentially occurs during the CH 2 OH-CH 2 OH electrooxidation on S-V O -β-Ni(OH) 2 , and the key reaction intermediate is H 2 C(OH) 2 .For H 2 C(OH) 2 , BDFEs of the C-H bond and the O-H bond are 92.7 and 96.4 kcal mol -1 , respectively.Hence, for the oxidation of H 2 C(OH) 2 to HCOOH, the first HAT step (HAT 1 ) is the dehydrogenation of H 2 C(OH) 2 to H·C(OH) 2 , and the second step (HAT 2 ) is the dehydrogenation of H·C(OH) 2 to HCOOH.

Table S4 .
Elemental quantification obtained by EDX spectrum analysis of S-V O -β-Ni(OH) 2 after pre-oxidation.

Table S6 .
Elemental quantification obtained by EDX spectrum analysis of β-Ni(OH) 2 after POR.

Table S7 .
Elemental quantification obtained by EDX spectrum analysis of S-V O -β-Ni(OH) 2 after POR.