Effect Of Ni（OH）₂ Morphology On Electrocatalytic Dechlorination Of Clofibric Acid By Palladium/Nickel Foam Electrode

Pharmaceuticals and personal care products（PPCPs）,as a new type of trace organic pollutants,have potential risks to human health and the ecological environment,due to their bioaccumulation and“pseudo-persistence”（persistent in the environment due to constant emissions）.Clofibric acid（CA）is a typical chlorinated PPCPs,as the main active metabolite of fibrates and the intermediate of drug synthesis.It is commonly found in municipal and pharmaceutical wastewater,and it is difficult to biodegrade and cannot be completely removed during conventional wastewater treatment.Electrocatalytic hydrodechlorination（ECH）is an efficient method for the removal of chlorinated organics under mild conditions,which has great application prospects for the removal of chlorinated PPCPs in water.Pd/Ni foam electrode is one of the most widely used cathodes in ECH research.Pd²⁺is reduced to Pd⁰during the impregnation process,and the increased concentration of Ni²⁺in the impregnation solution leads to the formation of Ni（OH）₂.In this paper,the formation of Ni（OH）₂in the preparation of Pd/Ni foam electrode and ECH degradation performance of CA was investigated,and by controlling the electrode preparation process,the Ni（OH）₂formed was conducive to the electrocatalytic dechlorination of CA.The Pd/Ni foam electrode was prepared by the impregnation method.The effects of Pd loading and reaction conditions（initial concentration of CA,working potential,initial p H of catholyte,reaction temperature,and concentration of electrolyte Na₂SO₄）on the dechlorination of CA were systematically investigated.The results showed that the degradation of the CA by the electrode followed the first-order kinetics.98%of CA(the initial concentration was 5 mg L^-1)could be efficiently removed after 120min reaction under the optimal conditions(0.20 mg cm^-2Pd loading,10 mmol L^-1Na₂SO₄,-0.85 V vs Ag/Ag Cl cathode potential,p H=4.6,and T=303 K).The poor control over the impregnation process of Pd/Ni foam electrode results in unstable ECH activity.Herein,Ni（OH）₂coatings were observed on the surface of Pd/Ni foam electrode during conventional impregnation.Detailed analysis revealed that large numbers of Ni²⁺would be firstly replaced into impregnation solution,and then deposited onto electrode with aqueous OH^-.Specifically,impregnation time was identified as the key parameter to control the synthesis of Pd/Ni foam electrode.By prolonging impregnation time,continuous deposition of Ni（OH）₂would gradually increase the anti-corrosive property and the electrochemically active surface area of the electrode.However,Ni（OH）₂coatings covered the Pd catalyst,which had negative impacts on interfacial charge transfer and atomic H*adsorption,and aggravated the HER side reaction,thus affecting the degradation efficiency of CA.By tuning impregnation time to 15 min,the surface of the electrode had suitable Pd loading and less Ni（OH）₂coatings,which showed higher ECH activity than impregnation for 10min,0.5 h,1 h,2 h,6 h and 24 h.And,the ECH performance of Pd/Ni foam electrode decreased gradually with the prolonging of impregnation time.The deposition of Ni（OH）₂on the surface of Pd/Ni foam electrode can be avoided by complexing the Ni²⁺in the impregnation solution.When CH₃COONa was added to the bath,the Ni（OH）₂coatings decreased,and the distribution of Ni（OH）₂changed,forming a structure with Ni（OH）₂growing on the Ni foam substrate and interweaving with the Pd catalyst.Ni（OH）₂positively promoted the electrochemical activity and ECH activity of the electrode with this structure.Ni（OH）₂promotes the evolution of H*,then H*can be quickly adsorbed by the Pd catalyst for ECH reaction,thus promoting the dechlorination of CA.With the same Pd loading and impregnation time,the efficiency and apparent reaction rate of CA degradation at Pd/Ni foam electrode with CH₃COONa added during impregnation were 1.3 times and 6.5 times higher than those without CH₃COONa,respectively.Therefore,adding the complexing agent（CH₃COONa）into the impregnation solution can improve the ECH activity of Pd/Ni foam electrode and achieve a faster and more efficient dechlorination effect of CA.