Nature-inspired solid-state proton diode membrane for high-performance force-electric conversion

Domain: chemistry
Published: 2026-02-24
Editorial outcome: major_revision

Abstract
Bioinspired ion channel membranes with ultrafast and directional transport capabilities, akin to biological protein-based channel, are crucial for various energy and sensing applications. However, controlling highly active protons and advancing practical applications, which are often constrained by the use of liquid-based electrolyte, remains major challenges. Here, we report a bioinspired solid-state diode membrane with ultrafast and highly rectified proton transport, achieved through the hybridization of graphene oxide (GO) and Cu-coupled bacterial cellulose (BC-Cu) with inherent gradient in water content. The asymmetric proton transport pathways and potential differences between the two-dimensional GO interlayers and one-dimensional BC network create a stable diode effect, achieving exceptionally high rectification ratios of approximately 125. Theoretical calculations reveal that the distinction transport pathways and interfacial potential barrier disparities constitute the fundamental mechanism underlying the highly rectified ionic current. In particular, the enhanced proton transport greatly facilitates the force-electric conversion, allowing the diode membrane to accurately respond to external pressure. Our bioinspired approach to fabricate all-solid-state proton diodes hold great potential for next-generation solid-state rectification systems, with promising applications in areas such as self-powered pressure sensing and neuromorphic computing.

Introduction
Biological ion channels allow unidirectional and rapid ion transport featuring unidirectional ion transport ability (i.e. ionic rectification) across the cell membrane to ensure proper execution of various physiological processes in the organism1–3. Such unidirectional transport behavior is also known as ionic rectification, which has been proven to boost many applications such as osmotic and photoelectric energy conversion4–8. Bioinspired nanofluidic ion channel membrane emerges as promising candidates to mimic the delicate functionality of biological systems. In the past years, diverse nanofluidic diode membranes towards alkali metal (e.g. Na+ and K+) and halogen ion (e.g. Cl-) have been developed9–12. For the proton, the essential building block of atoms, it is still challenging to realize its unidirectional transport, which can be ascribed to the fact that protons are more active with less controllability due to their much smaller radius and higher ionic mobility. On the other hand, protons mostly need to be conducted in compounds with hydrogen bonds, while rational construction of proton conduction networks is also a major problem13–16. Actually, in nature, rectifying proton channels play key roles in many crucial life functions, such as cellular pH stabilization, generation of membrane potential, and energy transduction for ATP synthesis17–19. Therefore, the development of high-performance proton nanofluidic diode membranes is a necessity, which could not only help us understand the biological proton conduction process, but also largely improve the performance of proton-based energy devices.

At the same time, the current nanofluidic diode membranes are mostly applied in liquid environments, which limits their practical applications as liquid-based electrolytes have the risk associated with electrolyte leakage. There are some attempts based on semi-solid systems such as polyelectrolyte hydrogel and elastomer6,20–24. The formers suffer from limitations related to dehydration, while the latter exhibit poor ionic conductivities attributed to restrained transport of mobile counter ions. Therefore, the development of efficient proton nanofluidic diode for all-solid-state applications will accelerate the realization of efficient information transfer functions and wide applications like those of living organisms. In the solid-state diode, the mechanism differs from the conventional liquid-state diode in which the electric double layer governs the ion transport behavior, where the proton transport relies on the proton hopping mediated by the water bridge caused by the inherent bound water and a little free water25,26. In the human epidermis, restricted free water creates an inherent water gradient in the cuticle and dermis layer, playing an important role in the communication of the skin with the environment via directional water and ion transport (Fig. 1a). Inspired by that, a solid-state nanofluidic diode membrane with ultrafast and highly rectified proton transport behaviors was constructed by assembling two excellent proton conductors with distinct water gradient. The heterogeneous diode membrane comprises graphene oxide (GO) membrane with a two-dimensional (2D) interlayered structure and Cu-coupled bacterial cellulose (BC) membrane with one-dimensional (1D) chains containing a large number of oxygen-containing functional groups and extended chain spacing (Fig. 1b-c). The proton transport paths between the 2D interlayers and the 1D chains, as well as the differences in the proton transport potentials on GO and BC-Cu, construct a system-stabilized proton diode effect, which greatly facilitates the unidirectional proton transport from the BC to the GO layers. Such structure rectifies the proton current with a distinctly high ratio of 125 in the solid state. Furthermore, the external mechanical pressure can be used as a proton driving force, and the ultrafast and highly rectified proton transport is found to greatly promote the force-electric conversion.

Results
Structure characterization and analysis
GO nanosheets and BC nanofibers have considerable structural differences, with their lateral size varying by about an order of magnitude. The lateral size of GO nanosheet is about 7 μm, while BC exhibits a 1D chain-like structure with a diameter of about 0.15 μm (Supplementary Fig. S1). The stacking of GO and BC results in solid-state membranes with different numbers of voids that can be considered ion transport pathways (Supplementary Fig. S2). The pH and Zeta potential characterization of both materials further confirm that the charge carriers in this system are protons (Supplementary Fig. S3). The resistance of GO (3.66×107 Ω) is about two orders of magnitude higher than BC-Cu (2.23×105 Ω), illustrating the difference in ion transport between the two materials (Supplementary Fig. S4). Actually, BC constructs a reticular ion pathway that provides greater freedom for proton transport, while GO constructs a 2D laminated structure in which only the ion channels within the interlayer are ion-conductive (Supplementary Description and Supplementary Fig. S5-7). This structural difference lays the basis for the observed proton rectification.

However, the ion transport properties of GO and BC based on intrinsic structural differences are comparable, which is not conducive to achieving high proton rectification performance. To achieve even more efficient rectification, we introduced Cu2+ into BC (Fig. 1b). Compared to the white brittle BC membrane, the BC-Cu features a dark blue color (Supplementary Fig. S8), reduced volume and enhanced mechanical properties. The Fourier-transform infrared spectroscopy (FTIR) of GO and BC-Cu both exhibit strong -OH vibration absorption peaks (the peaks at 3420 cm-1 and 1392 cm-1 for GO and at 3344 cm-1 for BC-Cu) (Fig. 1d), indicating the presence of rich oxygen-containing groups that ensure the abundant bound water of the system. The thermogravimetric analysis (TGA) curves in Fig. 1e illustrate that GO and BC membranes have similar water contents of about 5.93% and 2.53%, respectively. Impressively, after the introduction of Cu2+, the water content of BC membrane increases substantially to about 56.04%. Within this framework, Cu2+ coordinates with hydroxyl groups on cellulose chains, forming stable metal-cellulose complexes. This coordination expands the interchain spacing, creating 1D proton transport channels and providing cation coordination sites. This expanded free volume and the coordinating ions enhance water uptake in the BC-Cu membrane. According to the electrochemical impedance spectroscopy (EIS) results (Fig. 1f and Supplementary Fig. S9), due to the introduction of BC, the hybrid membrane exhibits a high conductivity of 7.9×10−5 S/cm. The above characterization results indicate that heterogeneous membrane exhibit high proton conductivity and possess inherent differences in water content (free water and bound water) between the two components.

All-solid-state proton diode performance
The ion transport performance in solid state was characterized by scanning the I-V curves by sandwiching the membrane between two Pt electrodes (inset of Fig. 2a). The composite GO/BC-Cu exhibits unidirectional ion transport behavior, evidenced by a non-linear I-V curve (Fig. 2a), which is different from those of pristine GO and BC membranes with symmetric responses (Supplementary Fig. S10a-b). Such diode-like behavior (i.e. ion rectification) can be quantified by the current rectification ratio (I −1V /I +1V ). For the GO/BC composite system, the rectification effect can reach 15.66 when the scan rate is 50 mV/s (Fig. 2b and Supplementary Fig. S10c). Notably, after Cu2+ activation, the rectification ratio of the composite membrane increases substantially to 31.46 (Fig. 2b), and can be further improved to 125 when scan rate is at 1 mV/s (Supplementary Fig. S11). In the hybrid membrane, the dominant current carriers are protons caused by inherent bound water as well as the free water inside the nanochannel networks. The efficient unidirectional proton transport behavior can be attributed to the asymmetric ion transport pathways due to structural differences of BC and GO membranes, and asymmetric ion migration potential barriers caused by varying water contents (Supplementary Description and Supplementary Fig. S12). The Cu2+ ion opens the BC chain, enriching the proton and H2O content, which further reduces the migration potential barriers and magnifies such difference in the structure and proton content, contributing to the enhancement of the ion rectification behavior (Fig. 2c). The obtained rectification ratio exceeds those of currently reported solid-state or hydrogel-based diodes (Fig. 2d).

The performance of the diode membranes is dependent on the thickness of the GO membrane, which affects the longitudinal protons transport rate. The proton diode with thickness ratio of 4:1, with GO being 200 μm and BC-Cu being 50 μm, has the highest ion rectification ratio (Supplementary Fig. S13-S14). The stable current density of the GO/BC-Cu system is 4.27 μA/cm2 at a bias voltage of −1.0 V, which is 14 times larger than the current at reverse bias at +1.0 V (0.304 μA/cm2) (Fig. 2e-f). For BC-Cu membrane, insufficient thickness may compromise humidity retention and stable chemical gradient formation, while excessive thickness can introduce detrimental series resistance, adversely affecting both output power and rectification ratio. Importantly, the diode membrane exhibit obviously rectification at an AC voltage cycle of ±1.0 V (0.1 Hz) (Supplementary Fig. S15) and maintains 100% of its initial rectification ratio within 4 cycles (Fig. 2g). Furthermore, the rectification is still maintained at 82% under 100 cycles (Supplementary Fig. S16 and Fig. 2h-i). The slight degradation of the rectification performance is mainly due to the GO absorbing some of the H2O on the BC-Cu side with prolonged use, causing the ionic conductivity on the GO side to increase. As depicted in Supplementary Fig. S17, the water transfer is slightly accelerated at higher temperatures or slowed under lower humidity levels. However, these changes have a negligible impact on the overall device performance. The performance degradation can be mitigated by encapsulating the device with heat-shrinkable polyethylene (PE) film to stabilize its water content. Correspondingly, such diode can also maintain long-term stability at DC voltage (Supplementary Fig. S18) with a stable rectification effect, offering great application prospects that will be discussed in the application section.

Proton transport mechanism
The rectification mechanism of GO/BC-Cu was further investigated by molecular dynamics (MD) simulation. The mean squared displacement (MSD) shows that the migration of protons in 1D BC-Cu membrane has a high degree of freedom, meaning that protons have similar migration distances in the X, Y, and Z directions (Fig. 3a-b). For 2D GO laminated membranes, proton migration along the X and Y components is much greater than that in the Z direction (Fig. 3c-d), mainly because most protons migrate within the nano layers, making it difficult to achieve cross-layer transport. The above MD simulation results show that the 1D BC-Cu and 2D GO membranes exhibit different transmission paths, with BC-Cu having more freedom, which lays the basis for such a high ion rectification.

Then the influence of bound water was investigated, and GO and BC-Cu models with different H2O weights were established based on the TGA results. Among, the BC-Cu side, rich in free water, facilitates proton input with a lower energy barrier. In contrast, the GO side, featuring narrow interlayers with abundant functional groups and more bound water, imposes a higher energy barrier for proton output. The differential charge of Fig. 3e shows the difference in adsorption energies of protons at GO and BC-Cu membranes. Within GO membrane, the protons have a direct binding to the functional groups on the surface of GO nanosheets. On the contrary, cellulose contains a large number of H2O, which shield the adsorption of protons. And that, whether in the GO plane or at the interface, the adsorption energy of protons on GO is far greater than that on BC-Cu (Fig. 3f). In the next, the proton migration process across the BC/GO interface are investigated (Fig. 3g). Figure 3h-j shows in detail the migration barrier of protons from the BC chain to the interface and then to the GO nano layers. The protons at the interface need to overcome a barrier of 1.6 eV to transport from BC-Cu to the GO side, while the barrier rises substantially to 4.9 eV upon reversing the direction. According to the Arrhenius law, this significant difference in energy barriers results in orders-of-magnitude disparity between the forward and reverse migration rate constants (k), ultimately leading to a high current rectification ratio at the macroscopic level (Supplementary Fig. S19). Therefore, the key reason for the unidirectional ion transport is the asymmetric ion adsorption energy and diffusion barrier between GO and BC-Cu membranes. The proton will flow from the small adsorption energy region with low transport barrier (BC-Cu) to the region of large adsorption energy with high transport barrier (GO), leading to asymmetric proton transport behavior in an alternating electric field, responsible for the efficient proton rectification effect.

High-performance force-electric conversion
The external mechanical pressure can serve as a driving force for protons, and ultrafast, highly rectified proton transport is found to greatly promote the force-electric conversion (Supplementary Fig. S20). We employed a device consisting of commercial pressure sensor and mechanical thruster to apply external pressure, which is capable of delivering a constant external pressure (Supplementary Fig. S21). Figure 4a-b show the influence of varying the locations of GO and BC-Cu on force-electric conversion. Altering the position of BC-Cu and GO membranes, while keeping the mechanical pressure direction constant, results in a change in current direction. This is mainly due to the different binding abilities of protons on GO and BC-Cu membranes. When external pressure is applied, protons are transported directionally from the weakly bound BC-Cu side to the strongly bound GO side, creating a directional current. This indicates that the unidirectional proton migration is governed by the potential energy of both membranes, regardless of pressure direction. The mechanical pressure primarily regulates the nanochannels of the laminar membrane and controls the protons transport rate. Specifically, the applied pressure alters the interlayer spacing of the GO membrane and compresses the internal structure of the BC-Cu fiber, prompting ion displacement. Due to the asymmetric energy barriers for ion migration, net ion movement occurs directionally from the BC-Cu layer to the GO membrane under pressure, enabling unidirectional ion transport (Supplementary Fig. S19). When the pressure is removed, the protons migrate to the lowest energy state of the system, causing the voltage to slowly drop until it returns to its original level.

When stimulated by an external pressure of 20 N, a single proton diode can generate electrical signals up to 0.5 V and approximately 15 μA (Fig. 4b-c). This force-electric conversion efficiency, achieved via the ion diode, reaches 1.21% (Supplementary Fig. S22-23). In contrast, laminated membranes constructed from pure BC-Cu or pure GO exhibit significantly weaker force-electric conversion capabilities (Supplementary Fig. S24). This underscores the importance of the unidirectional proton transfer of the diode in achieving pressure-to-electrical signal conversion. Figure 4d depicts the output electrical signal of the proton diode at constant pressure, illustrating that the output current of the diode remains stable for up to 50s, which facilitates the realization of low-frequency energy capture. Additionally, the output current of the diode also increases with the increasing applied pressure, making it suitable for precise pressure sensing (Fig. 4e). According to the formula (Vmax·Imax/A), where the A stands for the area of GO/BC-Cu membrane, the proton diode achieves a highly efficient pressure conversion of 8 µW/cm2, surpassing many other membranes such as GO, oriented GO, carbon-based materials, and BC/carbon nanotube (Supplementary Fig. S25). Therefore, the all-solid-state proton diode developed in this work has the potential as a self-powered sensor for monitoring pressure signals. Supplementary Fig. S26 illustrates the human pulse signals obtained from this sensor. Through waveform analysis, the device captured all typical wrist pulse features, including the Percussion, Tidal, and Dicrotic peaks, highlighting its potential application in clinical pulse measurement. Figure 4f-g presents a schematic of a 6×6 pixel ion array encapsulated within a flexible Ag electrode array, together with an optical photograph of the flexible fully printed electrode array. Pressure imaging was demonstrated by placing a glass vial (≈4.18 Pa) onto the proton diodes array, as shown in Fig. 4h. Particularly, by incorporating the composite membrane into custom flexible electrode arrays shaped like insoles, the pressure-sensing arrays embedded in shoe soles can provide real-time feedback on the stress experienced by the human foot at rest (Supplementary Fig. S27). Naturally, according to the sensitivity curve, the operational pressure range required for the insole application falls within the dynamic range of the prototype pressure sensor (Supplementary Fig. S28).

Remarkably, under low-frequency (0.034 Hz, within 300s)/high-frequency (0.2 Hz, 1000 cycles) pressure, the output voltage signals of the sensor demonstrates remarkable stability, showing great potential for practical applications in energy harvesting (Supplementary Fig. S29). Connecting proton diodes in series enables an almost linear increase in open-circuit voltage. For example, linking 10 diodes can produce 4.23 V, sufficient to power small devices (Supplementary Fig. S30). Furthermore, the series connection of 90 devices generates a voltage of 32.4 V, which corresponds to 72% of the theoretical value of 45 V, indicating that the system still maintains good scalability (Supplementary Fig. S31). A flexible single-channel diode array demonstrates this by lighting a LED when compressed, due to unidirectional proton migration (Supplementary Fig. S32). When the pressure is released and the diode array is stretched, unidirectional proton migration is halted, causing the LED to turn off. To further increase the output power, an elaborate stacked array structure with 100 diodes in series was designed, as shown in Fig. 4i. Applying external mechanical pressure to this optimized diode array generates a substantial voltage signal of up to 30 V (Fig. 4j). Remarkably, the electrical signal quickly returns to 0 after the external mechanical pressure is withdrawn, demonstrating the tight electrical signal-pressure dependence of the diode array. This efficient output power is capable of charging electronic devices, such as mobile phones, as shown in Fig. 4k. This work represents the first development of a force-electric conversion device based on the ion rectification principle of an all-solid-state proton diode. This approach differs from the vast majority of current energy conversion devices, which typically use triboelectric nanogenerator (TENG), piezoelectric, pyroelectric and solar cells devices. In practice, force-electric conversion devices based on proton diodes demonstrate significant advantages for pressure sensing applications, including the ability to generate direct DC output without the need for rectification, compatibility with both static and low-frequency pressure detection, as well as excellent material stability and environmental adaptability.

Methods
Preparation of GO laminated membrane
A modified Hummers’ method was used to prepare GO nanosheet dispersions38,39. The method is divided into four main steps: (i) Low temperature co-mixing stage. Firstly, 1 g of NaNO3 powder was fully dissolved in 46 mL of H2SO4 solution. Subsequently, 2 g of graphite powder was added to the above solution under low temperature conditions. This process was stirred sufficiently to enable the graphite powder to be intercalated by H2SO4. (ii) Medium temperature reaction phase. The graphene layers were oxidised by slowly adding 1 g of KMnO4 to the above solution every 15 min under continuous stirring, 6 times in total. After this step, the reaction temperature was adjusted to 35°C, and stirring of the mixed solution until the solution was in a thick paste state. During this process, the purple KMnO4 dissolved in concentrated H2SO4 changes to the high manganic anhydride Mn2O7, causing the above solution to gradually change to a dark green colour. The main chemical reaction equation is as follows:

H2SO4 + 2KMnO4 = K2SO4 + Mn2O7 + H2O (1)

(iii) High temperature reaction and termination stage. To remove the H2SO4 between the GO layers, 92 mL of deionised water was added continuously and slowly under vigorous stirring. This process changes the above thick paste back into a homogeneous brown solution. The brown solution is then transferred to an oil bath at 98°C and stirred continuously for 15 min. Immediately afterward, 300 mL of a 3% H2O2 solution is added to oxidise the unreacted manganese ions, stirred continuously for 3 h, and then left overnight. During this process, the solution rapidly changes from brown to bright yellow. (iv) Centrifugal washing stage. The supernatant of the above solution was poured off and the remaining yellow precipitate was taken. The yellow precipitate was washed by centrifugation several times using 3% HCl. Finally, the precipitate was washed by centrifugation at high speed again with deionised water until the solution was pH neutral. At this point, the GO dispersion was successfully prepared. The concentration of the GO dispersion we obtained was approximately 16 mg/mL.
Finally, the GO dispersions were stacked into self-supporting laminate membranes by vacuum-assisted filtration. Among them, the filtration of GO dispersions uses a 0.45 μm hydrophilic filter membrane with large pores.

Preparation of Cu2+-saturated alkaline solution
The Cu2+-saturated alkaline solution was made by immersing excess copper wire in NaOH solution (20 wt%)40, for example, 5 g of copper wire in 500 mL of NaOH solution. The solution was magnetically stirred until the NaOH solution turned blue after two to three days. Before use, this stock solution was stored with excess immersed copper wire.

Preparation of BC-Cu laminated membrane
We fabricated BC-Cu laminated membrane through the following two steps. (i) The purchased commercial BC was homogeneously mixed with deionized water in a 1:2 ratio and the BC nanosheets dispersions were stacked into a self-supporting laminated membrane by vacuum-assisted filtration technology (using a 0.45 μm filter membrane s-aquo system). (ii) BC membranes were immersed in Cu2+-saturated alkaline solution (20% NaOH) for a week until the BC membranes gradually turned blue (reaching saturated Cu2+ coordination with the BC), with the product referred to as BC-Cu. Among them, the Cu concentration in the NaOH-Cu solution was 317.650 mg/L and the corresponding Cu loading in the BC-Cu membrane was approximately 1.42 wt% according to inductively coupled plasma (ICP).

Assembly of all-solid-state proton diode
All-solid-state proton diodes were made with titanium electrodes, GO membrane, and BC-Cu membrane, which are assembled by sandwiching GO and BC-Cu membranes between two titanium electrodes. The GO and BC-Cu laminated membranes were cut to 1×1 cm, while the titanium electrodes are cut to 1×2 cm. The extra length facilitates subsequent testing of the device.

Preparation of diode-array-based insole
A polyethylene terephthalate (PET) substrate was cut to align with the size and shape of a customized insole. An inkjet printing device was then employed to print electrodes at predetermined positions on the PET substrate, with the electrode dimensions matching those of the diodes. Following this, two PET electrodes and a diode were assembled in a sandwich configuration and embedded within a standard insole, thereby producing the diode-array-based insole.

Preparation of flexible sensing arrays
A circuit printer (BroadTeko DP800) was used to print 6×6 electrode circuits arrays on a flexible PET transparent substrate. Among them, the electrode material adopts nano-conductive ink. The electrode area printed is 1×1 cm. The flexible sensing array was then constructed by sandwiching 36 GO and BC-Cu membranes between two flexible electrode arrays.

Characterization
The atomic force microscopy (AFM SPM9700, Shimadzu) was used to analyze and compare the size of nanomaterials. The scanning electron microscopy (SEM FEINova Nano-SEM 450, 10 kV) was used to observe the surface and cross-section of laminated membranes. X-ray diffraction (XRD Panalytical) with Cu Kα radiation (15 mA and 40 kV) was used to analyze the structures of GO, BC, and BC-Cu. Fourier transform infrared spectrometer (FT-IR VERTEX 70, Bruker) was mainly used to analyze the types of oxygen-containing functional groups in two types of membranes. In order to measure the water content of the membranes, we conducted thermogravimetric analysis (TGA STA 2500, Germany-NETZSCH) on GO, BC, and BC-Cu, respectively.

Electrical and electromechanical measurements
Electrochemical impedance spectroscopy (EIS) and proton current rectification analysis were investigated by an electrochemical workstation (ChenHua CHI660E). The current (Isc) and voltage (Voc) signals were measured by Keithley DMM7510. The pressured-released tests were conducted with the uniaxial motion controller (Force-measuring system, CLXT) at room temperature.

DFT calculations
The initial model is constructed through DFT calculations based on the BC-Cu layer spacing obtained by XRD analysis. Firstly, MD simulations are used to describe the protons migration process in materials with different dimensions. We divide the migration of protons into three components, such as X, Y, and Z. Secondly, MSD simulations are used to illustrate the migration process of protons on these three components. All calculations are based on DFT and use the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) function41. The convergence criterion for the ideal optimised structure is set to Fine, where the k-point is set to 3×3×1 and the energy cutoff is set to 500 eV. In the structure of GO and BC, the pure GO structure without free water (including -O- bonds) and the cellulose structure containing 8 water molecules in a single cell were designed. In order to simulate the interface between BC and GO, and to ensure that they do not affect each other, the spacing between GO and BC is 15 Å.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "A solid-state diode membrane based on the hybridization of GO and BC-Cu were fabricated showing proton rectification effect due to distinct proton transport pathways and the difference between proton-GO and proton-BC interaction potentials. Furthermore, the proton transport mechanisms enable force-electricity conversion, which was demonstrated by charging mobile phones.\nThe reviewer does not have technical questions regarding the membrane fabrication and related measurements. Just some general questions regarding the research topic:\nMany energy conversion devices, which are usually termed “nature inspired” have been published, for decades. Some exhibited promising voltages or currents, lighting LEDs. In this sense, the manuscript is somewhat familiar. The question is: Can these devices really be used? or Can they be potentially used in the future with further improvement in performance? No evidence so far, especially those using nanotechnologies.\nFirst, reporting either voltage or current is meaningless. The power is more important. The power generated by the proton diode is only 8 microW/cm2, meaning that 1 W requires 12 m2. Can this be used to power mobile phones?\nSecond, what is the efficiency? The device has certain power output. This is good. But what is the energy input? No discussion regarding the efficiency? Is the efficiency higher than other devices?\nThird, if external pressure can generate power, the device can be somewhat considered as piezoelectric devices. How is the proton diode compared with piezoelectric devices, which have been extensively published.\nMinor issues:\nLine 222: the unit of pressure is N?", "reviewer_3": "This manuscript presents the creation of a bioinspired solid-state proton diode membrane using graphene oxide and copper-coupled bacterial cellulose. Inspired by the water gradient in human skin, this membrane demonstrates highly rectified proton transport due to the distinct structural differences and water content gradients between the two components. The research highlights the membrane's potential for force-electric conversion, enabling applications such as self-powered pressure sensing and possibly in neuromorphic computing, achieving high voltage outputs when scaled up. Theoretical calculations support the observed unidirectional ion transport mechanism.\nThe manuscript highlights several noteworthy results. The core achievement is the creation of a proton diode in a solid state, overcoming the limitations of liquid-based and semi-solid electrolytes. The water gradient inspired this in the human epidermis.\nThe work appears to have significant potential for the field because it introduces a bioinspired, all-solid-state proton diode (GO/BC-Cu) that overcomes the practical limitations of liquid/semi-solid electrolytes (leakage, dehydration) while achieving an outstanding record performance. As the first force-electric converter based on solid-state proton rectification—distinct from TENGs—it enables novel applications in self-powered pressure sensing (e.g., wearable stress monitoring) and neuromorphic computing, positioning it as a transformative approach for iontronics and flexible electronics.\nTo improve the manuscript I would suggest attention to a few points:\n1) The authors describe introducing Cu2+ into the BC membrane by immersion in a Cu2+saturated alkaline solution, which \"opened up molecular channels to accelerate proton migration\" and \"extended the chain space,\" thereby significantly increasing the water content. While the effect is apparent, the precise mechanism of how Cu2+ structurally modifies the BC network remains unclear. Could the authors elaborate on the chemical interaction between Cu2+ and cellulose chains (e.g., coordination bonds, electrostatic effects)? How do these interactions lead to expanded chain spacing and enhanced water uptake?\n2) The proton transport mechanism is described as relying on \"proton hopping mediated by water bridges,\" with an inherent difference in water content (free vs. bound water) between GO and BC-Cu. Could the authors clarify the distinct roles of bound water (structural stability?) versus free water (mobility?) in facilitating proton hopping? How do the differing proportions of these water types in GO and BC-Cu contribute to rectification?\n3) The authors state that external pressure acts as a \"driving force for protons,\" primarily regulating nanochannels and controlling transport rate, while current direction depends on potential energy differences. Could the authors explain in more detail how mechanical pressure physically alters nanochannels (e.g., interlayer spacing in GO, BC-Cu network deformation)? What is the precise link between these structural changes and the rate of proton transport in the preferred direction?\n4) A slight performance degradation over 100 cycles is attributed to GO absorbing H₂O from BC-Cu, with 82% retention. Could the authors discuss the rate and extent of water transfer under varying conditions (temperature, humidity)? What are the potential strategies to mitigate water transfer (e.g., encapsulation, crosslinking) and improve long-term stability?\n5) The authors emphasize that this is the first force-electric converter based on solid-state proton diode principles, distinct from TENGs. Could they briefly compare how ion rectification (proton gradient-driven) differs fundamentally from triboelectricity (charge separation-driven)? What are the unique advantages of this approach (e.g., sensitivity, material stability) for pressure sensing?\n6) MD simulations reveal distinct proton pathways (1D in BC-Cu vs. 2D in GO), while DFT calculations show asymmetric barriers (1.6 eV vs. 4.9 eV). Could the authors clarify how these computational results quantitatively explain the observed rectification ratio (~[value])? Do the barrier differences align with experimental current asymmetry?", "reviewer_4": "In this manuscript, a bioinspired solid-state diode membrane with high ability to rectify proton transport was constructed by hybridizing GO and Cu-coupled BC with inherent gradient in water content. Significantly, the directional proton transport could boost the force-electric conversion, allowing the diode membrane to accurately respond to external pressure. Importantly, flexible self-powered pressure sensors based on diode membrane structures can be developed into pressure sensing arrays using large-scale integration techniques. The membrane structure was well-designed and the experiments were treated high standard. Thus, I support its publication after addressing following minor points:\n1. It was claimed that proton could transport through the membranes. How to demonstrate the charge carrier is proton, not other ions, such as hydroxide? What is the selectivity of proton over other cations and anions?\n2. What is the function of Cu in the membranes? What is the loading percentage of Cu in the membranes? Is it possible to use other cations instead of Cu?\n3. The influence of GO thickness on the diode rectification effect was investigated. How about the Bc-Cu thickness?"}, "author_response": "Response to Reviewers’ Comments\nReviewer #1 (Remarks to the Author):\nA solid-state diode membrane based on the hybridization of GO and BC-Cu were\nfabricated showing proton rectification effect due to distinct proton transport pathways\nand the difference between proton-GO and proton-BC interaction potentials.\nFurthermore, the proton transport mechanisms enable force-electricity conversion,\nwhich was demonstrated by charging mobile phones. The reviewer does not have\ntechnical questions regarding the membrane fabrication and related measurements. Just\nsome general questions regarding the research topic:\nResponse: We thank this reviewer for the valuable suggestions and comments. They\nreally help enhance the quality of our work. The questions have been carefully studied\nand addressed.\n1. Many energy conversion devices, which are usually termed “nature inspired” have\nbeen published, for decades. Some exhibited promising voltages or currents, lighting\nLEDs. In this sense, the manuscript is somewhat familiar. The question is: Can these\ndevices really be used? or Can they be potentially used in the future with further\nimprovement in performance? No evidence so far, especially those using\nnanotechnologies.\nResponse: We thank this reviewer for the valuable comments. The practical application\npotential of “nature inspired” energy conversion devices is indeed a core issue in this\nfield. Numerous studies have focused on developing energy conversion devices that\ncan harvest low-grade energy from the environment, such as light, heat, wind, and\nvibration, that are challenging for conventional power generation technologies to utilize\nefficiently (Nat. Rev. Mater. 2021, 6, 622-639; Nat. Commun. 2020, 11, 6158).\nAlthough the power output of such devices remains relatively limited at present, they\nare already capable of powering low-energy electronic devices, such as sensor nodes\n(Nano Energy 2021, 84, 105865), and demonstrate promising applications in areas\nincluding the Internet of Things, smart agriculture, environmental monitoring, and\nR-1\nhealth monitoring (Nat. Commun. 2022, 13, 5839; Nat. Commun. 2024, 15, 663). That\nsaid, we acknowledge that further improvements in performance are still needed to\nenable widespread commercial adoption. This study is motivated by this goal and aims\nto enhance power output and conversion efficiency through improvements at the level\nof basic research, thereby supporting future real-world applications.\nMoreover, the value of nanotechnology-enabled “nature inspired” energy conversion\nextends beyond electricity generation. In addition to being directly harvested as\nelectrical energy, the generated power can be applied in areas such as water-electricity\ncogeneration, assisted catalytic reactions, and material synthesis (Nat. Commun. 2022,\n13, 5077; Science 2023, 380, 458-459; Nat. Water 2024, 2, 93-100; Nat. Commun.\n2024, 15, 757; Nat. Commun. 2024, 15, 5913), highlighting its multifunctional and\nsystem-integration potential. These aspects underscore the important significance of\nnanotechnology-enabled energy conversion. We believe that, with continued research\nand technological optimization, such devices are poised to play an important role in\nfuture energy systems.\n2. First, reporting either voltage or current is meaningless. The power is more important.\nThe power generated by the proton diode is only 8 microW/cm2, meaning that 1 W\nrequires 12 m2. Can this be used to power mobile phones?\nResponse: Thank this reviewer for pointing this out. The output power of a single\ndevice is indeed limited at this stage. Indeed, the current power density of our proton\ndiode is approximately 8 μW/cm², meaning that an area of about 12 m2 would be\nrequired to achieve 1 W of power. However, our device shares similarities with the\nelectric eel’s power generation mechanism. Although a single electrocyte in an electric\neel produces only about 80 mV, the series connection of thousands of such cells enables\nthe eel to generate voltages as high as hundreds or even thousands of volts (Nature,\n2017, 552, 214-218; Nature 2023, 620, 1001-1006). Similarly, by connecting nearly a\nthousand devices in series and parallel, it is possible to generate electricity through\nhumidity-powered devices capable of producing voltages reaching thousands of volts\n(Nat. Nanotechnol. 2021, 16, 811-819). Thus, although the power of a single device is\nR-2\nlimited, the modular integration strategy offers strong potential for scaling up the power\noutput of proton diodes in practical applications. In our work, each individual device\nhas an area of 1 cm2 and a thickness of only 250 μm, demonstrating excellent\nintegrability. Through three-dimensional stacking or in-plane series-parallel integration,\nthe total power output of the integrated system can be significantly enhanced within a\nlimited volume. The total output power density can reach 320 μW/cm3 when multiple\ndevices are integrated within a 1 cm3 space. As depicted in Figure 4i-k in the revised\nmanuscript, we constructed an integrated power generation device by connecting nine\nintegrated units in a series-parallel configuration within a 3 cm × 3 cm area, which is\nsufficient to power a mobile phone.\n3. Second, what is the efficiency? The device has certain power output. This is good.\nBut what is the energy input? No discussion regarding the efficiency? Is the efficiency\nhigher than other devices?\nResponse: Thank this reviewer for pointing this out. In our work, the device operates\nunder externally applied pressure. As depicted in Figure R1 (also, Figure S17 in the\nrevised Supporting Information), the total output is calculated as W = UIt, where U\nout\nis the output voltage (approximately 0.4 V), I is the average output current\n(approximately 4.63 µA), and t is the total output time (161.8 s). As depicted in Figure\nR2 (also, Figure S18 in the revised Supporting Information), the total input is calculated\nas W = FS, where F represents the applied force and S corresponds to the total\nin\ndeformation of the device during compression, which is approximately 60 µm. Based\non these calculations, our device achieves an energy conversion efficiency (η = W /W )\nout in\nof 24.9%.\nFurthermore, according to the suggestions of the reviewer, we provided a\nperformance comparison with similar energy conversion devices. The comparative\nresults show that the efficiency of our device is superior to (or comparable with)\npreviously reported counterparts (Figure R3, also, Figure S19 in the revised Supporting\nInformation).\nR-3\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“This force-electric conversion efficiency, achieved via the ion diode, reaches 24.9%\n(Supplementary Fig. S17-18), which is comparable to current energy conversion\ndevices (Supplementary Fig. S19)” (Page 254-256, revised manuscript)\n10\nμ 6\nnt\n(\ne\nurr 4\nC\n2\n161.8 s\n0\n-20 0 20 40 60 80 100120140160180\nTime (s)\nFigure R1. The output current signal of the GO/BC-Cu ion diode device under an\nexternal pressure of 20 N over a period of 161.8 s. Among that, the total output is\ncalculated as W = UIt, where U is the output voltage (approximately 0.4 V), I is the\nout\naverage output current (approximately 4.63 µA), and t is the total output time (161.8 s).\nFigure R2. Schematic illustrating the operating distance (60 µm) of the ion diode\ndevice under external pressure and the resulting compressive deformation. Among that,\nthe total input is calculated as W = FS, where F represents the applied force and S\nin\ncorresponds to the total deformation of the device during compression, which is\napproximately 60 µm. Therefore, an energy conversion efficiency (η = W /W )\nout in\nreached 24.9%.\nR-4\nFigure R3. The compare between ion rectification device and TENGs, piezoelectric,\npyroelectric and solar cells devices.\n4. Third, if external pressure can generate power, the device can be somewhat\nconsidered as piezoelectric devices. How is the proton diode compared with\npiezoelectric devices, which have been extensively published.\nResponse: We thank the reviewer for carefully reviewing our manuscript and raising\nthis point. Our device converts external mechanical energy into electrical energy, which\nis functionally similar to piezoelectric devices but fundamentally distinct in operating\nmechanism. Conventional piezoelectric devices rely on the piezoelectric effect, where\nmechanical deformation induces polarization in crystalline materials, leading to surface\ncharge and voltage generation. In contrast, our device operates based on the pressure-\ndriven directional migration of ions, resulting in net electrical current output, which an\nenergy conversion mechanism rooted in ion transport. Thus, while the macro-scale\nfunction of force-electric conversion is comparable, the underlying physics differs\nsignificantly from that of piezoelectric systems. Additionally, As depicted in Figure R3\n(also, Figure S19 in the revised Supporting Information), our device demonstrates a\nforce-electric conversion efficiency comparable to those of previously reported\npiezoelectric devices.\nThis result has been added into the revised manuscript, and the related descriptions\nare as follows:\n“This approach differs from the vast majority of current energy conversion devices,\nR-5\nwhich typically use triboelectric nanogenerator (TENG), piezoelectric, pyroelectric\nand solar cells devices (Supplementary Fig. S19).” (Page 296-299, revised manuscript)\nFigure R3. The compare between ion rectification device and TENGs, piezoelectric,\npyroelectric and solar cells devices.\n5. Minor issues: Line 222: the unit of pressure is N?\nResponse: We thank the reviewer for carefully reviewing our manuscript and raising\nthis point. The term “20 N” mentioned in line 222 refers to the magnitude of the force\napplied to the device, with the unit Newton (N). In our experiments, a force of 20 N\nwas indeed used to apply pressure and induce electrical signals in the device.\nR-6\nReviewer #3 (Remarks to the Author):\nThis manuscript presents the creation of a bioinspired solid-state proton diode\nmembrane using graphene oxide and copper-coupled bacterial cellulose. Inspired by\nthe water gradient in human skin, this membrane demonstrates highly rectified proton\ntransport due to the distinct structural differences and water content gradients between\nthe two components. The research highlights the membrane's potential for force-electric\nconversion, enabling applications such as self-powered pressure sensing and possibly\nin neuromorphic computing, achieving high voltage outputs when scaled up.\nTheoretical calculations support the observed unidirectional ion transport mechanism.\nThe manuscript highlights several noteworthy results. The core achievement is the\ncreation of a proton diode in a solid state, overcoming the limitations of liquid-based\nand semi-solid electrolytes. The water gradient inspired this in the human epidermis.\nThe work appears to have significant potential for the field because it introduces a\nbioinspired, all-solid-state proton diode (GO/BC-Cu) that overcomes the practical\nlimitations of liquid/semi-solid electrolytes (leakage, dehydration) while achieving an\noutstanding record performance. As the first force-electric converter based on solid-\nstate proton rectification—distinct from TENGs—it enables novel applications in self-\npowered pressure sensing (e.g., wearable stress monitoring) and neuromorphic\ncomputing, positioning it as a transformative approach for iontronics and flexible\nelectronics. To improve the manuscript I would suggest attention to a few points:\nResponse: We thank this reviewer for the valuable suggestions and comments. They\nreally help enhance the quality of our work. The questions have been carefully studied\nand addressed.\n1. The authors describe introducing Cu2+ into the BC membrane by immersion in a\nCu2+saturated alkaline solution, which \"opened up molecular channels to accelerate\nproton migration\" and \"extended the chain space,\" thereby significantly increasing the\nwater content. While the effect is apparent, the precise mechanism of how Cu2+\nstructurally modifies the BC network remains unclear. Could the authors elaborate on\nthe chemical interaction between Cu2+ and cellulose chains (e.g., coordination bonds,\nR-7\nelectrostatic effects)? How do these interactions lead to expanded chain spacing and\nenhanced water uptake?\nResponse: We thank this reviewer for the suggestion. We further improved the\nmechanism of the interaction between Cu2+ and cellulose according to the suggestion.\nSpecifically, Cu2+ coordinates with oxygen-containing functional groups such as\nhydroxyl on the cellulose chains, forming a stable metal-cellulose coordination\nstructures. This coordination not only expands the interchain spacing of cellulose to\nsome extent, providing one-dimensional transport pathways for proton migration, but\nalso offers multiple binding sites for cations (Nature 2021, 598, 590-596). It is this\nexpanded free volume and the coordinating ions that significantly enhance the water\nuptake of the membrane.\nThis result has been added into the revised manuscript, and the related descriptions\nare as follows:\n“Within this framework, Cu2+ coordinates with hydroxyl groups on cellulose chains,\nforming stable metal-cellulose complexes. This coordination expands the interchain\nspacing, creating 1D proton transport channels and providing cation coordination sites.\nThis expanded free volume and the coordinating ions enhance water uptake in the BC-\nCu membrane.” (Page 125-131, revised manuscript)\n2. The proton transport mechanism is described as relying on \"proton hopping mediated\nby water bridges,\" with an inherent difference in water content (free vs. bound water)\nbetween GO and BC-Cu. Could the authors clarify the distinct roles of bound water\n(structural stability?) versus free water (mobility?) in facilitating proton hopping? How\ndo the differing proportions of these water types in GO and BC-Cu contribute to\nrectification?\nResponse: We thank this reviewer for the suggestion. In the BC-Cu membrane, the\nwater primarily exists as free water within the expanded channels, serving as a highly\nmobile medium for proton hopping (Grotthuss mechanism), thereby enabling efficient\nproton transport. In contrast, in GO membrane, the water is mainly present as bound\nR-8\nwater around hydrated functional groups, exhibiting lower mobility and playing a\ngreater role in maintaining structural stability.\nThe significant difference of free to bound water between GO and BC-Cu\nconsiderably influences the asymmetry in proton transport behavior, thus enhancing the\nrectification effect. Specifically, the BC-Cu side, with its higher free water content,\nprovides a lower-energy-barrier pathway for proton input. On the other hand, the GO\nside, characterized by narrow interlayer channels rich in functional groups and a greater\namount of bound water, presents a higher energy barrier for proton output. This\ndisparity first leads to differences in proton migration rates between the two sides.\nSubsequently, due to the variation in water molecules at the interface, a difference in\nproton adsorption energy arises, resulting in asymmetric forward and reverse migration\nrates across the interface: the free water in BC-Cu can provide certain electrostatic\nshielding for protons, reducing their adsorption energy and thereby facilitating proton\nmigration from this side into the interface. In contrast, the strong interaction between\nbound water and functional groups in GO leads to higher proton adsorption energy,\nlimiting reverse migration and thus achieving selective proton transport at the interface.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“Of course, the water exists primarily as free water in BC-Cu membranes, facilitating\nhigh proton mobility through the Grotthuss mechanism. Conversely, in GO membranes,\nwater is mainly bound around hydrated groups, which enhances structural stability but\nreduces its mobility. The marked contrast in free versus bound water content between\nGO and BC-Cu significantly enhances proton transport asymmetry, leading to a\nstronger rectification effect. The BC-Cu side, rich in free water, facilitates proton input\nwith a lower energy barrier. In contrast, the GO side, featuring narrow interlayers with\nabundant functional groups and more bound water, imposes a higher energy barrier\nfor proton output. This disparity causes differences in proton migration rates between\nthe two sides. Furthermore, due to the variation in water molecules at the interface, a\ndifference in proton adsorption energy arises, resulting in asymmetric forward and\nR-9\nreverse migration rates across the interface. Specifically, free water in BC-Cu provides\nelectrostatic shielding, reducing proton adsorption energy and promoting migration\ninto the interface. Conversely, strong interactions between bound water and functional\ngroups in GO increase proton adsorption energy, restricting reverse migration and\nenabling selective proton transport across the interface. ” (Page 35-53, revised\nSupporting Information)\n“The BC-Cu side, rich in free water, facilitates proton input with a lower energy barrier.\nIn contrast, the GO side, featuring narrow interlayers with abundant functional groups\nand more bound water, imposes a higher energy barrier for proton output.” (Page 203-\n208, revised manuscript)\n3. The authors state that external pressure acts as a \"driving force for protons,\" primarily\nregulating nanochannels and controlling transport rate, while current direction depends\non potential energy differences. Could the authors explain in more detail how\nmechanical pressure physically alters nanochannels (e.g., interlayer spacing in GO, BC-\nCu network deformation)? What is the precise link between these structural changes\nand the rate of proton transport in the preferred direction?\nResponse: We thank this reviewer for the suggestion. External pressure primarily alters\nthe structure of nanochannels through physical compression, thereby affecting proton\ntransport behavior. Specifically, in GO membranes, pressure mainly reduces the\ninterlayer spacing (Adv. Mater. 2023, 35, 2308795). In contrast, the high initial porosity\nof the BC-Cu membrane allows pressure to induce more pronounced structural\ncompression. This compression facilitates the transport of ions within the membrane.\nHowever, it is noteworthy that the device structure we designed is an ion diode\nconfiguration. Pressure enables ion movement across both sides of the device,\nnevertheless, this movement is not equivalent. Due to differences in the ion potential\nbarrier at the interface, the ion flow migrating from GO to BC-Cu is significantly\nsmaller than that migrating from BC-Cu to GO. Consequently, under external pressure,\na directional ion flow is generated within the device. As illustrated in Figure R4 (also,\nFigure S15 in the revised Supporting Information), we further demonstrate the\nR-10\nrelationship between structural changes and proton flow along a specific direction by\ncomparing the theoretical I-V curve (Theoretical value) with the experimentally\nobtained I-V curve (Actual results). This differs from the commonly encountered\npressure sensors driven by osmotic energy. For instance, researchers have utilized\nselective membranes combined with a concentration gradient to develop a self-powered\npressure sensor (Adv. Mater. Early View, e06990).\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“Specifically, the applied pressure alters the interlayer spacing of the GO membrane\nand compresses the internal structure of the BC-Cu fiber, prompting ion displacement.\nDue to the asymmetric energy barriers for ion migration, net ion movement occurs\ndirectionally from the BC-Cu layer to the GO membrane under pressure, enabling\nunidirectional ion transport (Supplementary Fig. S15).” (Page 244-250, revised\nmanuscript)\nFigure R4. a. Schematic illustrating the ease/difficulty of forward and reverse ion flow\nin an ion diode device under external pressure. b. Theoretical I-V curves of forward and\nreverse ion flow in the ion diode. c. Experimental I-V curves of forward and reverse ion\nflow in the ion diode.\n4. A slight performance degradation over 100 cycles is attributed to GO absorbing H O\n2\nfrom BC-Cu, with 82% retention. Could the authors discuss the rate and extent of water\nR-11\ntransfer under varying conditions (temperature, humidity)? What are the potential\nstrategies to mitigate water transfer (e.g., encapsulation, crosslinking) and improve\nlong-term stability?\nResponse: According to the suggestions of the reviewer, we measured the data\nregarding the extent of water transfer under varying temperature and humidity\nconditions over a five-day period. As depicted in Figure R5 (also, Figure S13 in the\nrevised Supporting Information), the water transfer is slightly accelerated at higher\ntemperatures (over 35℃) or slowed under lower humidity levels (below 37% RH).\nHowever, these changes have a negligible impact on the overall device performance.\nFurthermore, we have proposed potential strategies to mitigate water transfer. For\ninstance, applying a thin yet stable encapsulation layer to block water permeation, or\npartially reducing GO to decrease its hydrophilicity, could enhance the long-term\nstability of the device. As depicted in Figure R5 (also, Figure S13 in the revised\nSupporting Information), the device encapsulated with polyethylene (PE) film\ndemonstrated significantly reduced water transfer.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“As depicted in Supplementary Fig. S13, the water transfer is slightly accelerated at\nhigher temperatures or slowed under lower humidity levels. However, these changes\nhave a negligible impact on the overall device performance. The performance\ndegradation can be mitigated by encapsulating the device with heat-shrinkable\npolyethylene (PE) film to stabilize its water content.” (Page 179-185, revised\nmanuscript)\nR-12\nFigure R5. The water transfer under varying temperature (a) and relative humidity (b)\nconditions. The water transfer is slightly accelerated at higher temperatures (over 35℃)\nor slowed under lower humidity levels (below 37% RH). The device encapsulated with\npolyethylene (PE) film demonstrated significantly reduced water transfer.\n5. The authors emphasize that this is the first force-electric converter based on solid-\nstate proton diode principles, distinct from TENGs. Could they briefly compare how\nion rectification (proton gradient-driven) differs fundamentally from triboelectricity\n(charge separation-driven)? What are the unique advantages of this approach (e.g.,\nsensitivity, material stability) for pressure sensing?\nResponse: We thank this reviewer for the suggestion. For conventional TENGs devices,\nthe operation relies on the contact electrification effect, where two materials with\ndifferent work functions come into intimate contact under mechanical force. To achieve\nFermi level equilibrium, electrons transfer from the material with lower work function\nto the one with higher work function, thereby generating an alternating current signal.\nIn contrast, our designed force-electric conversion device based on the solid-state\nproton diode principle operates through a concentration gradient and mechanical\npumping action, which drive the directional movement of protons within the diode\nstructure, directly producing a direct current or unidirectional pulsed signal.\nAdditionally, based on the design of the proton diode device offers the following\ndistinctive advantages:\n1) It enables direct DC output without rectification. Conventional TENGs generate AC\nsignals, which typically require rectification circuits for DC conversion, that adding\nR-13\ncomplexity and energy loss. In contrast, our device leverages the intrinsic ionic diode\nrectification effect to produce DC or unidirectional pulse signals inherently,\nsignificantly simplifying subsequent signal processing circuits.\n2) It is suitable for both static and low-frequency pressure detection. TENGs only\ngenerate signals during dynamic contact-separation processes, making them unsuitable\nfor static or slowly varying pressures. Our device, however, maintains an ionic\nconcentration gradient under static pressure (deformation), producing a stable DC\nsignal. This allows effective measurement of static pressure (such as sustained load,\nsteady touch), broadening its application in sensor technologies.\n3) Superior material stability and environmental adaptability. The performance of\nTENGs relies on surface charges, which can dissipate rapidly in high-humidity\nconditions, leading to significant performance degradation or failure. Surface wear also\ndirectly affects their triboelectric properties. In comparison, our device operates based\non ion migration within solid materials, independent of surface triboelectricity, thus\noffering improved humidity resistance and long-term stability.\nAdditionally, As depicted in Figure R3 (also, Figure S19 in the revised Supporting\nInformation), our device demonstrates a force-electric conversion efficiency\ncomparable to TENGs devices.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“In practice, force-electric conversion devices based on proton diodes demonstrate\nsignificant advantages for pressure sensing applications, including the ability to\ngenerate direct DC output without the need for rectification, compatibility with both\nstatic and low-frequency pressure detection, as well as excellent material stability and\nenvironmental adaptability.” (Page 299-304, revised manuscript)\nR-14\nFigure R3. The compare between ion rectification device and TENGs, piezoelectric,\npyroelectric and solar cells devices.\n6. MD simulations reveal distinct proton pathways (1D in BC-Cu vs. 2D in GO), while\nDFT calculations show asymmetric barriers (1.6 eV vs. 4.9 eV). Could the authors\nclarify how these computational results quantitatively explain the observed rectification\nratio (~[value])? Do the barrier differences align with experimental current asymmetry?\nResponse: According to the suggestions of the reviewer, we further clarified the\ncorrelation between the computational and experimental results. Specifically, the\ndifference in energy barriers for ion migration within GO and BC-Cu membranes, as\nwell as the asymmetric energy barriers for ion migration across the GO/BC-Cu interface,\nboth contribute to the ion rectification effect. The ion migration process follows the\nArrhenius law, where the migration rate depends exponentially on the energy barrier,\nas described by the following formula:\n−𝐸\na\nln𝑘 = + ln𝐴\n𝑅𝑇\nor\n𝑘 = 𝐴e− 𝑅 𝐸 𝑇 a\nWhere E is the activation energy (the migration barrier), and k is the rate constant for\na\nmigration.\nThe difference in proton migration barriers between forward migration (from BC-Cu\nto the GO side) and reverse migration (from the GO side to BC-Cu) enables directional\ncarrier transport. Specifically, the forward migration barrier is only 1.6 eV, while the\nreverse migration barrier reaches 4.9 eV. According to the Arrhenius law, this\nR-15\nsignificant difference in energy barriers results in orders-of-magnitude disparity\nbetween the forward and reverse migration rate constants (k), ultimately leading to a\nhigh current rectification ratio at the macroscopic level. These results are consistent\nwith the current asymmetry observed experimentally, indicating that the calculated\nenergy barrier difference provides a reasonable explanation for the experimental\nphenomenon. In addition, we have provided a schematic illustration (Figure R4, also,\nFigure S15 in the revised Supporting Information) of the barrier difference and the\ncurrent direction to further clarify that the barrier asymmetry is consistent with the\nexperimentally observed current asymmetry.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“According to the Arrhenius law, this significant difference in energy barriers results\nin orders-of-magnitude disparity between the forward and reverse migration rate\nconstants (k), ultimately leading to a high current rectification ratio at the macroscopic\nlevel (Supplementary Fig. S15).” (Page 218-223, revised manuscript)\nFigure R4. a. Schematic illustrating the ease/difficulty of forward and reverse ion flow\nin an ion diode device under external pressure. b. Theoretical I-V curves of forward and\nreverse ion flow in the ion diode. c. Experimental I-V curves of forward and reverse ion\nflow in the ion diode.\nR-16\nReviewer #4 (Remarks to the Author):\nIn this manuscript, a bioinspired solid-state diode membrane with high ability to rectify\nproton transport was constructed by hybridizing GO and Cu-coupled BC with inherent\ngradient in water content. Significantly, the directional proton transport could boost the\nforce-electric conversion, allowing the diode membrane to accurately respond to\nexternal pressure. Importantly, flexible self-powered pressure sensors based on diode\nmembrane structures can be developed into pressure sensing arrays using large-scale\nintegration techniques. The membrane structure was well-designed and the experiments\nwere treated high standard. Thus, I support its publication after addressing following\nminor points:\nResponse: We thank this reviewer for the valuable suggestions and comments. They\nreally help enhance the quality of our work. The questions have been carefully studied\nand addressed.\n1. It was claimed that proton could transport through the membranes. How to\ndemonstrate the charge carrier is proton, not other ions, such as hydroxide? What is the\nselectivity of proton over other cations and anions?\nResponse: We appreciate the reviewer for bringing up this issue. GO has been well-\ndocumented as an efficient proton donor and conductor (Nat. Commun. 2023, 14, 7756;\nNature 550, 2017, 380-383). To further verify that protons serve as the primary charge\ncarriers in our system, we measured the pH values of different components. As depicted\nin Figure R6 a (also, Figure S3a in the revised Supporting Information), the pH of pure\nBC-Cu solution was approximately 6.1, close to neutral; the GO solution exhibited a\npH of about 3.1, indicating mild acidity; while the BC solution showed a pH of 5.8,\nremaining mildly acidic. These results demonstrate that GO provides a source of\nprotons throughout the membrane system, and in the absence of other significantly\nmobile ions, protons are the dominant charge carriers.\nFurthermore, we measured the Zeta potentials of the GO, BC, and BC-Cu membranes.\nAs depicted in Figure R6 b (also, Figure S3b in the revised Supporting Information),\nR-17\nboth GO and BC-Cu exhibit negative surface charges (with values of -56.8 mV and -\n60.8 mV, respectively), thus indicating cation selectivity (J. Am. Chem. Soc. 2022, 144,\n13764-13772; Nature 2021, 598, 590-596). This property effectively rules out the\npossibility of anions such as hydroxide ions (OH-) serving as the primary charge carriers\nand also helps explain the preferential transport of protons over other cations.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“The pH and Zeta potential characterization of both materials further confirm that the\ncharge carriers in this system are protons (Supplementary Fig. S3)” (Page 103-106,\nrevised manuscript)\nFigure R6 a. The pH values of GO, BC, and BC-Cu solutions were measured to be 3.1,\n5.8, and 6.1, respectively, demonstrating that GO provides a source of protons\nthroughout the membrane system, and in the absence of other significantly mobile ions,\nprotons are the dominant charge carriers. b. Zeta potential measurements of GO, BC,\nand BC-Cu membranes yielded values of -56.8, -33.1, and -60.8 mV, respectively,\nindicating cation selectivity and ruling out the possibility of anions such as hydroxide\nions (OH-) serving as the primary charge carriers.\n2. What is the function of Cu in the membranes? What is the loading percentage of Cu\nin the membranes? Is it possible to use other cations instead of Cu?\nResponse: We appreciate the reviewer for bringing up this issue. The Cu2+ serves the\nfollowing key roles in the membrane: first, it coordinates with cellulose molecules to\nexpand the polymer chain spacing, thereby forming one-dimensional ion transport\nR-18\nchannels. second, it works synergistically with the abundant oxygen-containing\nfunctional groups in cellulose to provide multiple coordination sites for cations. It is\nthis expanded free volume and the coordinating ions that significantly enhance the\nwater uptake of the membrane. Last, the synergistic interaction between Cu2+ and the\ncellulose chains further promotes the segmental decoupled motion of the polymer\nchains, enhancing ions transport (Nature 2021, 598, 590-596).\nSecond, to further verify the Cu loading, we performed inductively coupled plasma\n(ICP) analysis to determine the amount of Cu incorporated into the BC membrane.\nAmong that, the Cu concentration in the NaOH-Cu solution was 317.650 mg/L, and the\ncorresponding Cu loading in the BC-Cu membrane was approximately 1.42 wt%.\nFinally, it is worth noting that in principle, other cations could be used to replace\nCu2+. However, achieving performance comparable to that of Cu2+ would require\ncareful consideration of additional factors, such as coordination ability, ionic radius,\ncharge, and stability. Since the coordinating ions are not the primary focus of this study,\nwe have not explored alternative cations. We acknowledge the potential value of this\nresearch direction and may consider it in future work.\nThis result has been added into the revised manuscript, and the related descriptions\nare as follows:\n“Among them, the corresponding Cu loading in the BC-Cu membrane was\napproximately 1.42 wt% according to inductively coupled plasma (ICP)” (Page 497-\n500, revised manuscript)\n3. The influence of GO thickness on the diode rectification effect was investigated.\nHow about the Bc-Cu thickness?\nResponse: We appreciate the reviewer for bringing up this issue. To further verify the\nimpact of the BC-Cu layer thickness on the diode rectifying performance, we fabricated\ndevices with varying BC-Cu thicknesses and characterized their rectifying behavior. As\ndepicted in Figure R7 (also, Figure S10 in the revised Supporting Information), when\nthe thickness ratio of GO to BC-Cu is 4:1, with GO being 200 μm and BC-Cu being 50\nR-19\nμm, an optimal rectification ratio is achieved. This is primarily because if the BC-Cu\nlayer is too thin, it becomes difficult to effectively retain moisture and establish a stable\nchemical gradient. Conversely, an excessively thick layer introduces significant series\nresistance, which adversely affects both the output power and rectification ratio.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“The proton diode with thickness ratio of 4:1, with GO being 200 μm and BC-Cu being\n50 μm, has the highest ion rectification ratio (Supplementary Fig. S9-S10). The stable\ncurrent density of the GO/BC-Cu system is 4.27 μA/cm2 at a bias voltage of -1.0 V,\nwhich is 14 times larger than the current at reverse bias at +1.0 V (0.304 μA/cm2) (Fig.\n2e-f). For BC-Cu membrane, insufficient thickness may compromise humidity retention\nand stable chemical gradient formation, while excessive thickness can introduce\ndetrimental series resistance, adversely affecting both output power and rectification\nratio. ” (Page 164-173, revised manuscript)\nFigure R7. The relationship between thickness of GO and BC-Cu membranes and\nproton diode rectification performance.\nR-20"}, {"round": 2, "reviewer_comments": {"reviewer_1": "For the first question, the authors cited some previous work to demonstrate that their work has potential for practical use. This is not the way I agree as previous work may not have been confirmed. Solid evidence is needed to show the potential for real-world applications.\nFor the second question, the assumption of linear scalability lacks solid evidence. In practice, this is often not the case, as there are numerous associated issues that can hinder scalability. Therefore, on this point, I cannot agree with the authors.\nThe calculation of efficiency is confusing. The current could be generated for 161 s? During this period, was the voltage always 0.4 V? Furthermore, does the force Fremain constant and act on the device throughout this entire 161 seconds? How can this be the case?\nI don’t want to come across as harsh, but I struggle to see the practical applications of this topic.", "reviewer_3": "After a thorough review of the revised manuscript and rebuttal, I confirm that all the comments have been explicitly addressed. The authors have provided expanded explanations supported by theoretical, experimental, and schematic evidence, which have been fully incorporated into the revised manuscript and supplementary information. The rebuttal presents a comprehensive and satisfactory response to all reviewer concerns.", "reviewer_4": "The manuscript was well revised according to all comments and suggestions of the reviewers. Thus, I support its publication."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1 (Remarks to the Author):\nFor the first question, the authors cited some previous work to demonstrate that their\nwork has potential for practical use. This is not the way I agree as previous work may\nnot have been confirmed. Solid evidence is needed to show the potential for real-world\napplications.\nResponse: Thank this reviewer for pointing this out. Employing self-powered devices\nfor direct detection of physiological signs such as arterial pulses holds significant\nimplications for human medical information. To further validate the potential of the\ndiode device in practical applications, we conducted an experiment to monitor the\nhuman arterial pulse. In the human body, each cardiac contraction generates a wave\nthat carries both energy and information. Specifically, the contractile force of the heart\nand the velocity of blood ejection shape the steep ascending limb of the Percussion\nWave (P wave); The resistance of the systemic peripheral vasculature determines the\nintensity of the Tidal Wave (T wave), while the closure of the aortic valve produces the\ncharacteristic Dicrotic Wave (D wave). These waveforms contain essential biomedical\nand physiological information, such as arterial stiffness, coronary artery disease, and\nmyocardial impairment.\nThe diode device was attached to the researcher's wrist, and its output electrical\nsignals were monitored in real time. Figure R3a-b (also, Figure S22a-b in the revised\nSupporting Information) illustrates the human pulse signals obtained from this setup.\nThe amplified pulse signal clearly reveals the characteristic peaks of the peripheral\narterial waveform. Through waveform analysis, the device captured all typical wrist\npulse features, including the P, T, and D peaks, highlighting its potential application in\nclinical pulse measurement. Additionally, we conducted a comparison of wrist pulse\nwaveforms across different individuals. As shown in Figure R3c-f (also, Figure S22c-f\nin the revised Supporting Information), while all pulse waves exhibit the characteristic\nP, T, and D peaks, there are notable variations in both the magnitude and position of\nR-5\nthese three peaks, reflecting distinct physiological characteristics.\nBased on the above results and the applied research presented in the manuscript, such\ndiodes could find potential applications in fields including low- and high-frequency\nenergy harvesting and self-powered sensing.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“Supplementary Fig. S22 illustrates the human pulse signals obtained from this sensor.\nThrough waveform analysis, the device captured all typical wrist pulse features,\nincluding the Percussion, Tidal, and Dicrotic peaks, highlighting its potential\napplication in clinical pulse measurement.” (Page 272-278, revised manuscript)\nFigure R3. a-b. Wrist pulse waveform from a human and its magnified view. Wrist\npulse waveform from Researcher 1 (c-d), Researcher 2 (e-f) and their magnified view.\nFor the second question, the assumption of linear scalability lacks solid evidence. In\npractice, this is often not the case, as there are numerous associated issues that can\nhinder scalability. Therefore, on this point, I cannot agree with the authors.\nResponse: We thank this reviewer for the comments. Indeed, the series and parallel\nR-6\nconnections of multiple devices can result in reduced scalability. To further evaluate\nthe scalability of the devices, we connected 90 prototype devices in series and\nmonitored the resultant electrical signals. As shown in Figure R4 (also, Figure S27 in\nthe revised Supporting Information), the series connection of 90 devices produced a\nvoltage of 32.4 V, which corresponds to 72% of the theoretical value of 45 V, indicating\nthat the system maintains good scalability. The observed performance attenuation may\nbe attributed to an increase in the overall internal resistance of the devices during these\nseries.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“Furthermore, the series connection of 90 devices generates a voltage of 32.4 V, which\ncorresponds to 72% of the theoretical value of 45 V, indicating that the system still\nmaintains good scalability (Supplementary Fig. S27).” (Page 294-299, revised\nmanuscript)\nFigure R4. a-b. The relationship between the number of series-connected prototype\nsensors and the output electrical signals, with (a) presenting the output electrical signals\nfor 10 series-connected devices.\nR-7\nThe calculation of efficiency is confusing. The current could be generated for 161 s?\nDuring this period, was the voltage always 0.4 V? Furthermore, does the force Fremain\nconstant and act on the device throughout this entire 161 seconds? How can this be the\ncase?\nResponse: Thank this reviewer for pointing this out. Under the application of a single\npressure signal, the voltage and current signals of the device indeed show a decay over\ntime. In the manuscript, the total power approach (W = UIT) was originally employed\nout\nfor efficiency calculation. Therefore, the efficiency calculation formula used previously\nmay introduce some degree of deviation. Accordingly, the efficiency has been\nrecalculated. To better reflect actual conditions, the output power was recalculated\nbased on an external load, while maintaining a constant load resistance. The output\npower is given by P = I2R, where the resistance value corresponds to the optimal point\n(R = 10 kΩ) obtained from the output power versus load resistance curve (Figure R5a,\nalso, Figure S19a in the revised Supporting Information). Following this, the current\nwas measured under this 10 kΩ load (Figure R5b, also, Figure S19b in the revised\nSupporting Information). With this revised method, the energy conversion efficiency\nof the device is determined to be 1.21%.\nThe detailed calculation procedure is as follows:\nW =I2RT\nout\nW =F*S=20 N*60 μm\nin\nη=W /W =1.21%\nout in\nwhere R is the external load, I is the output current (Figure R5b, also, Figure S19b in\nthe revised Supporting Information), T is the total output time, F is the externally\napplied pressure, and S is the deformation of the device during pressing, with the overall\ndeformation distance being approximately 60 μm.\nIn addition, it is feasible to apply a constant force onto the device surface. An optical\nphotograph of the pressure application device is shown in Figure R6a (also, Figure S17a\nR-8\nin the revised Supporting Information). The pressure application system primarily\ncomprises a commercial pressure detection device and a mechanical thruster. The\ncommercial pressure detection device is employed to acquire the external pressure\nexerted on the device surface, whereas the mechanical thruster is utilized to apply\nconstant pressure to the prototype sensor. The method for applying constant pressure\nonto the device represents a standard methodology in the field of sensing (Nat. Commun.\n2017, 8, 1207).\nTo further verify the stability of the pressure application system, we performed\nadditional long-duration pressure-application experiments. As shown in Figure R6b\n(also, Figure S17b in the revised Supporting Information), the force F applied to the\ndevice remains consistently steady over the period of 161 s. The pressure application\nsystem also shows versatility in its ability to apply pressure in a stepwise manner, as\ndemonstrated in Figure R6c (also, Figure S17c in the revised Supporting Information).\nTo further confirm the long-term stability of the system, Figure R6d (also, Figure S17d\nin the revised Supporting Information) presents the extended pressure application curve\nover the period of 3500 s, which indicates exceptional stability and ensures the\nreliability of device testing.\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“This force-electric conversion efficiency, achieved via the ion diode, reaches 1.21%\n(Supplementary Fig. S18-19).” (Page 256-259, revised manuscript)\n“We employed a device consisting of commercial pressure sensor and mechanical\nthruster to apply external pressure, which is capable of delivering a constant external\npressure (Supplementary Fig. S17).” (Page 232-237, revised manuscript)\nR-9\nFigure R5. a. Dependence of the output power on the external load resistance. b.\nOutput current signal obtained at an external load of 10 kΩ. The output power is given\nby W = I2RT, where the resistance value corresponds to the optimal point (R = 10 kΩ)\nout\nobtained from the output power versus load resistance curve. Following this, the current\nwas measured under this load.\nFigure R6. a. Photographs of the pressure application system showing the front view\nand side view, including the mechanical thruster and commercial pressure sensor. b.\nPressure curve output by the device over 161 s under constant pressure. c. Step curve\nof external pressure. d. The curve of external pressure maintained for 3500 s.\nR-10"}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have fully and substantively addressed all questions raised by Reviewer #1.\nIn response to concerns about the lack of solid evidence of practical applicability, new experimental data were added demonstrating real-time human wrist pulse monitoring, including clear identification of characteristic P, T, and D peaks, thereby providing direct proof-of-concept beyond prior literature.\nThe concern about linear scalability was addressed experimentally by connecting 90 devices in series and quantifying the resulting output voltage relative to the theoretical limit, explicitly acknowledging and discussing non-ideal losses.\nFinally, the questions regarding the efficiency calculation were resolved by revising the calculation method, clarifying all physical parameters, and experimentally validating the constant-force condition over the whole measurement duration with long-term stability tests.\nOverall, Reviewer #1’s comments were not only answered but were used to substantially strengthen the manuscript through additional experiments, revised analyses, and more precise methodological explanations."}, "author_response": "RESPONSE TO EDITORS’ COMMENTS\nIn line with their feedback, we suggest that you expand and further characterise the\ndiode integration into the insole, showing a clear connection between the dynamic range\nof the prototype sensor to the range required for the insole sensing application. For the\npractical use of energy harvesting, we also ask that you supplement the data with a\nlong-time run test, showing a few hours of operation.\nResponse: We thank the editor for the suggestion. The questions have been carefully\nstudied and addressed. First, we have added a comprehensive process for integrating\ndiodes into the insole, detailed in the Supplementary Methods. This process involves\nadditional electrode printing. Specifically, a polyethylene terephthalate (PET) substrate\nwas cut to align with the size and shape of a customized insole. An inkjet printing device\nwas then employed to print electrodes at predetermined positions on the PET substrate,\nwith the electrode dimensions matching those of the diodes. Following this, two PET\nelectrodes and a diode were assembled in a sandwich configuration and embedded\nwithin a standard insole, thereby producing the described diode-array-based insole.\nThe principle behind this diode-array insole is to use individual diodes as self-powered\npressure sensors. These diodes are strategically positioned on the insole to detect force\nduring movement. When pressure from the foot acts on a specific point on the insole,\nthe corresponding diode activates and outputs an electrical signal. We then captured the\noutput signals from diodes situated at various stress positions. The relative amplitudes\nof these electrical signals can be analyzed to determine both the location and intensity\nof the force exerted during foot motion.\nIn addition, we further analyze the dynamic relationship between the output electrical\nsignals of the prototype sensor and the applied external pressure. As shown in Figure\nR1 (also, Figure S24 in the revised Supporting Information), the prototype sensor\ndemonstrates a wide pressure-sensing range of 0 to 400 kPa and exhibits high sensitivity\nwithin an external pressure range of up to 200 kPa. Thanks to its self-powered\nfunctionality, the device provides dual-mode voltage/current signals, with a voltage\nsensitivity of 11.6 mV kPa-1 and a current sensitivity of 62.12 nA kPa-1. The integrated\nR-21\ninsole sensor is constructed by combining multiple prototype pressure sensors. For\nadults, typical weight ranges from 50 to 80 kg, which translates to an average pressure\nper sensor between 139 kPa and 223 kPa (corresponding to 13.9 N to 22.3 N per sensor)\nwhen distributed across the insole. Consequently, the operational pressure range\nrequired for the insole application falls within the dynamic range of the prototype\npressure sensor.\nFor the practical use of energy harvesting, we have supplemented the data with\ninsights from the long-term operation of the device under both high- and low-frequency\nworking conditions. As shown in Figure R2a (also, Figure S25a in the revised\nSupporting Information), the voltage signal under low-frequency pressure attenuates\nover time. This reduction occurs primarily because, following the application of a single\npressure, the electric field generated at the heterointerface gradually dissipates over\ntime due to the free diffusion of ions, resulting in energy consumption. However,\naccording to Figure R2a (also, Figure S25a in the revised Supporting Information),\nwhen the applied pressure remains stable within 300 s (with a pressing frequency\nexceeding 0.0034 Hz), there is no significant decay observed in the energy output of\nthe device. In contrast, under high-frequency pressure (0.2 Hz), the voltage signal\nexhibits remarkable stability, showing no noticeable attenuation over 1000 cycles, as\ndepicted in Figure R2b (also, Figure S25b in the revised Supporting Information).\nThis result has been added into the revised manuscript and Supporting Information,\nand the related descriptions are as follows:\n“A polyethylene terephthalate (PET) substrate was cut to align with the size and shape\nof a customized insole. An inkjet printing device was then employed to print electrodes\nat predetermined positions on the PET substrate, with the electrode dimensions\nmatching those of the diodes. Following this, two PET electrodes and a diode were\nassembled in a sandwich configuration and embedded within a standard insole, thereby\nproducing the diode-array-based insole.” (Page 523-532, revised manuscript)\n“Naturally, according to the sensitivity curve, the operational pressure range required\nR-22\nfor the insole application falls within the dynamic range of the prototype pressure\nsensor (Supplementary Fig. S24)” (Page 284-288, revised manuscript)\n“Remarkably, under low-frequency (0.034 Hz, within 300 s) / high-frequency (0.2 Hz,\n1000 cycles) pressure conditions, the output voltage signals of the sensor demonstrates\nremarkable stability, showing great potential for practical applications in energy\nharvesting (Supplementary Fig. S25)” (Page 289-293, revised manuscript)\nFigure R1. The voltage (a) and current (b) sensitivity curves of the prototype sensor\ndemonstrate a wide pressure-sensing range of 0 to 400 kPa and high sensitivity (with a\nvoltage sensitivity of 11.6 mV kPa-1 and a current sensitivity of 62.12 nA kPa-1) within\nan external pressure range of up to 200 kPa. The integrated insole sensor is constructed\nby combining multiple prototype pressure sensors. For adults, typical weight ranges\nfrom 50 to 80 kg, which translates to an average pressure per sensor between 139 kPa\nand 223 kPa when distributed across the insole. Consequently, the operational pressure\nrange required for the insole application falls within the dynamic range of the prototype\npressure sensor.\nR-23\nFigure R2. a. Output voltage signal of the prototype sensor under a single pressure\napplication. Under low-frequency (0.034 Hz, within 300 s), the output voltage signals\nof the sensor demonstrates remarkable stability. It is worth noting that the electrical\nsignal decays to approximately 180 mV within 4 h, mainly because the electric field\ngenerated at the heterogeneous interface gradually dissipates due to the free diffusion\nof ions. b. Under high-frequency (0.2 Hz, 1000 cycles) pressure, the output voltage\nsignals of the sensor demonstrates remarkable stability, showing great potential for\npractical applications in energy harvesting.\nR-24"}]

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