Functional Materials For Capacitive Mixing To Extract Salinity Gradient Energy

As an important branch of ocean energy(also called blue energy),salinity gradient energy(SGE),i.e.the Gibbs free energy released from the controlled mixing process of two solutions with different concentration,generally referring to the energy produced at estuaries where river water meets sea water,is a kind of enormous renewable energy source but untapped yet.Capacitive mixing(CapMix)is an emerging and low cost technology which can generate voltage and current via an electrochemical capacitor in which salt water and fresh water alternately flow through,and thereby converting SGE into electricity.At present,symmetric or asymmetric electric double layer capacitors that based on electric double layer theory,and membrane capacitors that using ion exchange membranes have been employed to extract SGE.However,their electricity generation performance from salinity gradients need to be enhanced.To solve this problem,in this paper,we created new capacitor types and new functional materials for CapMix.As a result,we greatly increased the electricity generation performance and promoted CapMix to a new and higher level.The main contents and conclusions are followed:(1)A novel technique called hybrid capacitive mixing which employs a membrane-less hybrid capacitor consisting of a battery-type electrode(Na4Mn9O18,NMO)and a capacitive-type electrode(activated carbon,AC)is proposed to extract SGE.The electrochemical tests show that the electrolyte concentration has obvious effect on the electrochemical performance of electrode materials.When the electrolyte switched from 500 mM(HC)to 20 mM(LC)NaCl aqueous solutions,the open circuit potential(OCP)of NMO and AC electrodes decreased 89 mV and increased 55 mV,respectively.The concentration-dependent voltage and peak short circuit current(SCC)density of the NMO//AC hybrid capacitor can attain 110 mV and 0.2 A m-2,respectively.The feasibility of the hybrid capacitive mixing technique was demonstrated by a four-step cycle supported with external power source.(2)Positively charged quaternized poly(4-vinylpyridine)coated activated carbon(AC-QPVP),and negatively charged nitric acid oxidized activated carbon(AC-HNO3)were prepared as electrode materials for an asymmetric capacitor.As the LC solution entered,the OCPs of AC-QPVP and AC-HNO3 electrodes decreased 70 mV and increased 80 mV,respectively.The concentration-dependent voltage and peak SCC density of the AC-QPVP//AC-HNO3 asymmetric capacitor can attain 150 mV and 13.2 A m-2,respectively.Without external power source and ion exchange membranes,this device was able to output average power density of 65 mW m-2(external resistance =100 ?)This indicated that using oppositely charged carbon materials prepared by chemical modification method efficiently enhanced the electricity generation performance from salinity gradients of asymmetric capacitive mixing.(3)Concentration capacitor,a novel CapMix device,which combines the electric double layer potential and the membrane voltage was created.It consists of two capacitive electrodes,a membrane,current collectors and double channels where the HC and LC solutions alternately flow through.Graphene hydrogel(GH)was prepared by the hydrothermal reduction method.Its concentration-dependent voltage was-81.2 mV,which did not changed much with the reactant concentration,hydrothermal duration or the addition of H2O2,but decreased greatly when the concentration of CTAB(a kind of cationic surface active agent)increased.This indicated that the high concentration-dependent voltage of GH is mainly owing to its strong negative charge.Using G-H(as the symmetric electrodes)and an filtration membrane(FM)or an anion exchange membrane(AEM),GH//FM//GH and GH//AEM//GH concentration capacitors were constructed and tested.The concentration-dependent voltage and peak SCC density of them can reach 166.7 and 288.5 mV,13 and 37 A m-2,respectively.When external resistance increases,voltage of one concentration capacitor increases but current decreases,leading to a maximum value for power density.As external resistance is 180 ?,the maximum average power densities output via two-step cycles of the GH//FM//GH and GH//AEM//GH concentration capacitors reached up to 141.4 and 482.4 mW m-2,respectively.Concentration capacitor is a type of promising CapMix technique with simple structure,superhigh performance and low cost.