Nano-size Electrolytic Manganese Dioxide synthesis and characterization for sensing applications

Acidic gases, such as HCl, are used as precursors in CVD processes and as etching gases in semiconductor processing. Moreover, CO2 attracts attention as a main source of global warming. Many methods have been developed to detect such acidic gases in environment; in particular, electrochemical sensors have been intensively studied because of their unique features. Several studies [l-3] were made using an acidic-gas sensor based on the amperometry of lead oxide, manganese dioxide, iodine and Platinum oxide, but it is important to develop more reliable amperometric sensors in the industrial purposes. Direct electrode oxidation of HF or HCl is difficult, because of their high redox potentials. So, we have investigated amperometric detection system based on electroactive manganese dioxide to detect the acidic gases. The electroactive manganese dioxide phases used in the work were prepared by electrodeposition, and hence were designated as electrolytic manganese dioxide (EMD). We prepared EMD in an electrolysis cell based on a 300 mL Pyrex beaker in a temperature controlled bath. The anode in the cell onto which the EMD was deposited consisted of two 14 cm2 titanium sheets interspaces with one similarly sized titanium cathode. The electrolysis current passed through the cell was set to 50 A/m2. The electrolyte used was based on an aqueous solution of MnSO4 and H2SO4. We select the specific conditions from the literature to achieve highest BET surface area around 50 m2/g. The bath temperature, MnSo4 and H2So4 molarity was chosen to be 90C, 0.7 molar and 0.5 molar respectively. The EMDs were characterized by electron microscopy studies (Fig 1a.) and XRD (Fig 1b.). The dry electrode ingredients were combined by lightly grinding together a mass mixture of the active material (manganese dioxide), conductive agent and binder using a ceramic 429 mortar and pestle (5 min). This powder was then made into an ink by adding terpineol in a weight ratio of 40:1 solvent to solid. The ink was sonicated for 30 min until evenly dispersed. Electrode was screen printed on a gas-permeable membrane. Fabricated electrode have been used as working electrode in an miniature sized amperometric three electrode electrochemical cell consisting of Ag/AgCl reference electrode, Pt Counter electrode and Nh4Cl aqueous Electrolyte. The reaction current was expected to depend on the H+ concentration at the electrode surface. We compared the characteristics of sensors made of working electrodes composed of EMD and commercial MnO2 for detection of HCl. Figure 2 shows the comparison of the sensitivity of the sensors made of commercial MnO2 and EMD based working electrode to the 10 ppm HCl flow with 0.7 Lpm flow rate. The data acquired as it can be seen EMD based sensors are more than 5 time sensitive than their commercial analogous.