Efficient Electrocatalytic Membrane For Removing Oxosalt From Water:Efficacy And Mechanism

Water pollution and water resource crisis caused by the harmful oxosalts（such as nitrates,bromates,chlorates,etc.）have aroused extensive attention due to the risk towards the ecological environment and human health.As a promising electrochemical process for the water treatment,the electrocatalytic reduction technology can achieve the selective removal of oxosalt ions in wastewater.The harmful oxosalts such as nitrate,bromate and chlorate can be efficiently removed and converted into nitrogen gas,bromide and chloride,respectively,via the direct electron transfer at the cathode or the atomic hydrogen（H*）mediated indirect reduction.However,the electric field repulsion and hydrogen evolution reaction during the cathodic reduction process might lead to the low Faradaic reduction efficiency,slow reaction rate and high energy consumption for the removal of oxosalts.Thus,the construction of spatial confinement and the rational optimization of catalytic structure will be the effective pathway to enhance the electron transfer efficiency between oxosalts and electrocatalytic materials.This can alleviate the adverse impact of the electric field repulsion and hydrogen evolution reaction on the oxosalts removal.In view of recent advances and the above-mentioned challenges,we try to fabricate the electrocatalytic membrane for enhancing the oxosalts removal,and reveal the underlying mechanisms.The molecular structure of oxosalts is predicted to evaluate the reaction energy barrier in the reduction process.Furthermore,three electrocatalytic membranes including 2D-layered composite membrane,Co-Cu O_xelectrocatalytic filter,and electrochemical ceramic membrane were fabricated to remove nitrate and bromate.Under this circumstance,the diffusion and mass transfer of oxosalt in two-dimensional nano-water channels were explored.The relationship between removal rate and energy consumption under different voltages and membrane fluxes was illustrated.The mechanism of direct electron transfer and H*-mediated indirect reduction on the electrochemical reduction of oxosalts was investigated,to provide scientific reference and technical support for the novel electrocatalytic membrane technology.Firstly,the molecular characteristics of bromate,nitrate,and chlorate were accurately identified by theoretical chemical calculation.The electrocatalytic activity of different electrode materials was then evaluated by the calculation of Gibbs free energy.We explored the the dissociation energies of oxysalt hydrate ions under the acidic,neutral and alkaline conditions.The dissociation energies of oxysalt hydrate ions under the neutral condition is similar to that of alkaline condition.Meanwhile,the reduction of oxosalts will be enhanced under the acidic condition.The catalytic activity of different metals for electrochemical reduction of oxosalts was explored from the perspective of thermodynamics.To enhance the reaction rate and decrease the energy consumption of nitrate reduction,we fabricated a mechanically flexible 2D-MXene（Ti₃C₂Tx）membrane with multilayered nanofluidic channels to weaken the adverse impact of electric field repulsion on the nitrate mass transfer.Positron annihilation spectroscopy and X-ray photoelectron spectroscopy（XPS）confirmed the existence of defects on the surface of MXene nanosheets,which may provide active sites for nitrate adsorption and reduction.Nitrite will be the main product of nitrate reduction at a lower cathodic voltage.The highest selectivity of nitrogen gas and Faradaic efficiency can be achieved at the cathodic voltage of-0.8 V（vs Ag/Ag Cl）.To further investigate the diffusion behavior of nitrate in the nanofluidic channels,we collected the CV curves of the membrane with filtration（flow-through mode）and without filtration and obtained the corresponding diffusion coefficients using the Randles–Sevcik equation.The diffusion coefficient of nitrate with filtration is much higher than that without filtration.These results indicated the significance of the multilayered nanofluidic channels in the enhancement of nitrate mass transfer and subsequent energy-efficient reduction.Under the flux of 100 L m^-2h^-1,the MXene composite membrane can achieve the optimum nitrate removal,N₂selectivity,and Faradaic efficiency,which were 73.6±6.4%,82.8±5.1%,and 72.6±4.7%,respectively.The corresponding energy consumption was 0.28 k Wh m^-3.Density functional theory（DFT）calculations revealed that the low-coordinated titanium exposed by the defects of MXene nanosheets has a strong electrochemical selectivity for nitrate reduction.Moreover,the hydrogen evolution reaction on MXene nanosheets may be suppressed,and thus exerting an insignificant impact on the nitrate reduction.To improve the N₂selectivity of nitrate electrochemical reduction,an electrochemically active metal filter with the three-dimensional structure was synthesized on the surface of copper filter by the hydrothermal/solvothermal method.The Co-Cu O_xnanoarrays can tune the oxygen vacancy distribution of copper and optimize the coordination structure of the catalytic interface,improving the selectivity of the copper-based material for the conversion of nitrate to N₂.The extended X-ray absorption fine structure spectroscopy（EXAFS）and XPS confirmed that the presence of abundant oxygen vacancies on Co–Cu O_xwas due to the change in the electron density of the Cu atom and a decrease of the coordination numbers of Cu–O via cobalt doping.This structure has lower transfer resistance and stronger electrochemical selectivity than other cobalt-copper oxides.At a cathodic potential of-1.1 V（vs Ag/Ag Cl）,the Co–Cu O_xfilter showed 95.2%nitrate removal and 96.0%N₂selectivity at an influent nitrate concentration of 20 mg-N L^–1.Meanwhile,the energy consumption and FE were 0.60 k W h m^–3and 53.5%,respectively,at a permeate flux of 60 L m^–2h^–1.Theoretical calculations and electrochemical tests indicated that the oxygen vacancies suppress the contribution of H*to nitrate reduction,leading to a thermodynamically favorable process to N₂via the direct electron transfer.To achieve a high reaction kinetic constant and reaction efficiency for the low concentration bromate removal,an electrochemical ceramic membrane with metal heteroatom interface of Ru and Ni was fabricated by using the magnetron sputtering deposition.The analyses of scanning electron microscopy（SEM）and XPS showed that the Ru and Ni were homogeneously dispersed on the membrane surface,endowing a large number of active sites for bromate reduction during the membrane filtration.CV curves and electron spin resonance（ESR）spectra indicated that the electrochemical ceramic membrane can produce H*and subsequently react with bromate.The doping of nickel on the ruthenium crystal structure could significantly improve the electrochemical activity of ruthenium when the mass ratio of ruthenium to nickel was 4/1.The relationship between bromate removal efficiency and energy consumption under different p H values was demonstrated.Moreover,the results of the continuous-flow experiments illustrated that the membrane could achive 91.3%of bromate removal at 5 m A cm^-2under the membrane flux of 40 L m^-2h^-1.The corresponding energy consumption is 10.7 k Wh m^-3.The contribution of H*to the bromate removal was quantified by the quenching experiments.The membrane filtration process can effectively improve the utilization of H*at the Ru Ni metal-catalyzed interface,facilitating the efficient bromate removal.