I performed three electrode test for MoS2 electrode using aqueous Na2SO4 electroyte in stable potential range. When i performed XPS of samples cycled for 100 and 500 cycles, i observed formation of SEI interphase on the surface. However, the previous literature suggests that SEI formation can only take place in non-aq electrolytes.
Can someone please enlighten me these results?
Hello Pranjala Tiwari
SEI is only associated with non-aqueous electrolyte, as the water decomposition products such as O2/H+ at cathode & H2/OH- at anode can not be deposited on electrode surface in solid state and create a covering interface.
However, recent reports introduce SEI concept in aqueous electrolyte associated with 'superconcentration of Electrolyte ions'. Therefore the electrochemcial potential window of electrode can be easily extended from 1.23 V (water decomposition potential) to 3-4 V with excellent stability. Remember that, water decomposition does not contribute for aqueous SEI formation. The two ways to from aqueous SEI are reduction of
(i) dissolved O2 / CO2
(ii) anion clusters/complexes present in electrolyte
Though, several important aspects are responsible to form aqueous SEI such as chemical structure, concentration, reduction products, solubility , and also preparation conditions.
Please read the attached article for details.
SEI is just an interface, it can occur even if we dip a metal in water. No wonder it can occur in your sample. I guess it should be a very very thin layer as compared to non aq electrolytes.