## 1 Introduction  
Underwater electric signal sensing [1, 2] shows great potential for electric-field-sensing-based target detection [3], positioning [4], signal transmission [5] and communications [6], among others. Nevertheless, underwater equipment such as underwater manipulators, autonomous underwater vehicles and workstations rely heavily on electric power supply [7, 8]. Therefore, it is of great importance to develop self-powered sensing methods for underwater activities. Based on working principles, the electric current or electric voltage signals generated for sensing can be generally derived from three methods: piezoelectric, triboelectric and piezoelectric. The piezoelectric method is based on the piezoelectric effect, where a voltage difference across the piezoelectric material is generated as it is pressed [9]. Thus, the mechanical energy is directly converted into electricity. Piezoelectric materials, such as quartz, PZT, ZnO, PVDF and piezoelectric composites [10, 11], are widely used for electricity generation on land. However, these materials must respond to strain caused by various stimuli, and the high hydrostatic pressure in deep-sea environments can easily deform them, limiting their effectiveness in such conditions. Underwater electric current generated for sensing based on the triboelectric effect have also been proposed. Compared to land-based triboelectric nanogenerators (TENGs), underwater TENGs should be specially designed to adapt to the complexity of the underwater environment [12]. To further optimize the underwater performance of TENGs, diverse designs including origami structure [13], electrode geometry [14], triboelectric layer innovation [15], biomimetic structure (seaweed, jellyfish, sea anemones, etc.) [16; 17; 18] and electric double layer (EDL)-based structure [19] have been proposed. Nevertheless, as mobile ions in water can neutralize the electrostatic charges of TENGs [20], waterproof structures including shell, encapsulation, and coatings are necessary to avoid this side effect [21]. Distinct from the piezoelectric effect or triboelectric effect, using electrons as charge carriers, electric currents can also be generated through the piezoionic effect. Ionic transportation is utilized to produce an electrical output when pressure is applied [22]. Several underwater applications, such as piezoionic elastomers and TPU film-based tactile sensors, have been developed using the second type of piezoionic process [23; 24]. Nevertheless, the piezoionic materials have to withstand long-term soaking in an aquatic environment [25]. Moreover, the mechanical strength is insufficient to work under high hydrostatic pressure conditions, such as in the deep sea [26]. While the methods of electric current signal-based sensing show some potential in underwater applications, the shortcomings are obvious. Contact electrification (CE) provides a possible solution. The CE of contact and separation mode refers to the charge transfer between the two different materials [27; 28], where net positive charges accumulate on one material and net negative charges on the other [29]. For a metal/metal contact pair, electrons are transferred between the two metal surfaces, resulting in a contact potential difference (CPD) between the two contacted metals. From the pioneering investigations of the electron transfer mechanism between metal and metal by [PERSON] [30] and [PERSON] [31], it was revealed that the work function is the minimum work needed to extract the electrons from the Fermi level of a solid carrying a net charge [32; 33]. Therefore, electrons will flow from the metal with a smaller work function to the metal with a larger work function until the equilibrium of the Fermi level is achieved [34]. Based on the above-mentioned mechanism, the electric potential difference of the electrodes can be determined via the modulation of the electrode distance by through applying the scanning Kelvin probe (SKP) method [35].