Electrochemical quartz crystal microbalance (EQCM) is the combination of electrochemistry and quartz crystal microbalance, which was generated in the eighties. Typically, an EQCM device contains an electrochemical cells part and a QCM part. Two electrodes on both sides of the quartz crystal serve two purposes. Firstly, an alternating electric field is generated between the two electrodes for making up the oscillator. Secondly, the electrode contacting electrolyte is used as a working electrode (WE), together with a counter electrode (CE) and a reference electrode (RE), in the potentiostatic circuit constituting the electrochemistry cell. Thus, the working electrode of electrochemistry cell is the sensor of QCM. As a high mass sensitive in-situ measurement, EQCM is suitable to monitor the dynamic response of reactions at the electrode–solution interface at the applied potential. When the potential of a QCM metal electrode changes, a negative or positive mass change is monitored depending on the ratio of anions adoption on the electrode surface and the dissolution of metal ions into solution. EQCM calibration The EQCM sensitivity factor K can be calculated by combing the electrochemical cell measured charge density and QCM measured frequency shift. The sensitivity factor is only valid when the mass change on the electrode is homogenous. Otherwise, K is taken as the average sensitivity factor of the EQCM. where is the measured frequency shift (Hz), S is the quartz crystal active area (cm2), ρ is the density of quartz crystal, is the quartz crystal shear modulus and is the fundamental quartz crystal frequency. K is the intrinsic sensitivity factor of the EQCM. In a certain electrolyte solution, a metal film will deposited on the working electrode, which is the QCM sensor surface of QCM. The charge density () is involved in the electro-reduction of metal ions at a constant current , in a period of time (). The active areal mass density is calculated by where is the atomic weight of deposited metal, z is the electrovalency, and F is the Faraday constant. The experimental sensitivity of the EQCM is calculated by combing and . EQCM application Application of EQCM in electrosynthesis EQCM can be used to monitor the chemical reaction occurring on the electrode, which offers the optimized reaction condition by comparing the influence factors during the synthesis process. Some previous work has already investigated the polymerization process and charge transport properties, polymer film growth on gold electrode surface, and polymerization process