# Development and Validation of a Capacitor-Current Circuit Model for Evaporation-Induced Electricity
## 4 Results and Discussion  
### Materials Characterization and Device Fabrication  
Figure 2 shows the microstructure of beech wood, highlighting its highly aligned microchannels with an average diameter of approximately 50 \(\upmu\)m. This alignment facilitates efficient water flow through the wood during evaporation, which is essential for generating a streaming current. The interconnected channels support continuous water flow, enabling ion transport and charge separation. Beech wood was selected for its optimal pore size, which, as demonstrated in previous work, reaches a balance between small and large pores to achieve the best device performance [20].  
The wood was chemically modified using citric acid, and FTIR spectra were used to confirm this modification. As shown in Figure 3, characteristic peaks at 1730 cm\({}^{-1}\) and 1156 cm\({}^{-1}\) corresponding to the stretching of -C-O and -C-O bonds. The presence of these bonds indicates successful esterification of hydroxyl groups on the wood surface, which increases both hydrophilicity and surface charge density. These enhancements improved water flow and boosted the evaporation-induced electricity output.  
The contact angle measurement in Figure 4 further demonstrates the enhanced hydrophilicity following CA treatment. The observed contact angle is significantly below 90\({}^{\circ}\), confirming strong hydrophilicity [21]. This property is essential for the device's operation, allowing the wood to absorb and transport water efficiently through its microchannels. Con  
Figure 2: SEM Image of the wood structure. SEM image displaying the aligned microchannels in the beech wood sample with an average channel diameter of approximately 50 \(\upmu\)m. This microstructure facilitates efficient water evaporation and ion transport, essential for the device’s performance.  
tinuous water flow sustains ion movement within the channels, enabling stable electricity generation in evaporation-induced nanogenerators.  
Figure 5 shows the zeta potential measurement, indicating a negative surface charge of \(-17.1\) mV. These surface charges facilitate ion transport along the wood's microchannels, enabling the generation of streaming current driven by water evaporation. As shown in Equation (1), the zeta potential plays a critical role in generating streaming current.  
The electricity output measurement setup of the wood-based nanogenerator is demonstrated in Figure 6. Figure 6a shows a single device connected to a Keithley multimeter  
Figure 4: Contact angle measurement. Contact angle measurement showing the hydrophilicity of the CA-treated wood.  
Figure 5: Zeta potential measurement. Zeta potential measurement of the CA-treated wood, showing a negative surface charge of \(-17.1\) mV.  
Figure 3: FTIR spectra of the wood after citric acid treatment. FTIR spectra showing characteristic peaks at \(1730\) cm\({}^{-1}\) (–C=O) and \(1156\) cm\({}^{-1}\) (–C–O), indicating successful esterification of hydroxyl groups on the wood surface. These modifications enhance the surface charge and hydrophilicity, promoting better ion transport.  
for \(I_{\text{SC}}\) measurements. Figure 6b displays a setup with multiple devices, demonstrating scalability for series connected devices, while Figure 6c shows parallel connected devices with an enhanced current output. This simple setup showcases measurement of the generated current from single and scalable devices, highlighting the feasibility of this system for practical applications.