## 4 Results and Discussion  
### Typical Ionic Current Signals  
Figure 2 shows the measured ionic current signals during the process of contacting different metal sheets with the working electrode in DI water (experimental data can be seen in Table A1 of Appendix A). It can be seen that the ionic current signals are different for different metals, in both direction and magnitude. In detail, the ionic current signals generated by the gold, platinum and copper sheets are in a downwards direction, while those of the aluminum, iron and zinc sheets are in an upwards direction. The magnitudes of the ionic current signals generated are also different for the different metal sheets. Among the six tested metal sheets, the values of the ionic current increase in the sequence of platinum, copper, gold, iron, aluminum and zinc. For example, the ionic current generated by the gold sheet is about \(-\)2.5 \(\upmu\)A, while that generated by the aluminum sheet is about 6.9 \(\upmu\)A. As discussed in Section 2, this is mainly attributed to the different CPDs that generate the different potential difference between the composite working electrode and the sensing electrode. Meanwhile, the CPD depends on the difference in the work functions of the two contacted metals. As the work function of the copper (\(\phi_{Cu}\), 4.53-5.1 eV) working electrode is lower than that of the gold one (\(\phi_{Au}\), 5.1-5.47 eV), the CPD generated by contacting the gold sheet leads to a higher electric potential on the gold-composite working electrode compared with the copper sensing electrode, which is responsible for the negative ionic current. Similarly, as the work function of the copper (\(\phi_{Cu}\), 4.53-5.1 eV) working electrode is higher than that of the aluminum sheet (\(\phi_{al}\), 4.06-4.26 eV), the CPD generated by contacting the aluminum sheet leads to a lower electric potential on the aluminum-composite working electrode compared with the copper sensing electrode, which is responsible for the positive ionic current. Other ionic current signals can also be understood in the same way regarding the work functions of other metals [29]--Pt (5.12-5.93 eV), Fe (4.67-4.81 eV), Al (4.06-4.26 eV) and Zn (3.63-4.35 eV). To confirm that the ionic current is generated between the composite working electrode and the sensing electrode, different immersing media (air or DI water) where the copper working electrode is in contact with the metal sheet are compared. Unlike the experimental system described in Section 2, for the contact position in air, the metal sheet is placed vertically across the air/water interface, where part of the metal sheet is in water and the other part is in air. Then, the metal sheet is contacted with the copper working electrode in air. Figure 3 shows a comparison of the ionic current generated by contacting the metal sheet in air or in DI water (experimental data can be seen in Table A2 of Appendix A). It can be seen that for all the contacted metal sheets, the direction of the generated ionic current signals is the same for both the contact positions in air or DI water, meaning that the electron transfer between the copper working electrode and the metal sheet occurs in air. Then, the ion migration and the ionic current are generated between the composite working electrode and the sensing electrode. Supposing the ionic current is still generated by the copper working electrode and the sensing electrode after contact, as the copper working electrode and the composite working electrode are oppositely charged after the copper working electrode and the metal sheet come into contact, the direction of the ionic current for the contact position in air should be opposite to that for the contact position in DI water. It is thus verified that after the working electrode and the metal sheet come into contact, the ionic current is generated between the composite working electrode and the sensing electrode. Figure 3: Comparison of the ionic current generated by contacting the metal sheets in air or in DI water.