Studies on the mechanism and properties of self-powered $\mathrm{H_{2}O_{2}}$ electrochemical sensors based on a fuel cell configuration with nanozymes and enzyme mimics

$\mathrm{H_{2}O_{2}}$ is not only widely used in medicine and domestic fields, but of great interest for industrial, ecological, and analytical applications, it is also an important molecule that exists in biological systems as it can be either harmful or involved in regulation of different cellular functions. Thus, the detection of $\mathrm{H_{2}O_{2}}$ is of great interest. To date, different detection methods have been proposed, among all methods, electrochemical detection has drawn enormous attentions due to the advantages in simplicity, fast response, selectivity, and sensitivity. However, the flexibility and application of conventional electrochemical sensors are hindered due to the external power supply/modulation system that is essential in voltammetric and amperometric methods as it powers and modulates the sensors. Unlike conventional electrochemical sensors, self-powered electrochemical sensors (SPES) convert chemical energy of the analyte directly into electrical energy and generate signals without external power supply and modulation. Thus, the SPES offer energy-saving property, simplicity in device, and better compatibility for applications in wearable equipment, in vivo, and in situ measurements. The dual redox property of $\mathrm{H_{2}O_{2}}$ makes it possible to develop $\mathrm{H_{2}O_{2}}$ SPES using $\mathrm{H_{2}O_{2}}$ analyte as both an oxidant and a reductant by fabricating a membrane-less galvanic cell. However, the catalytic materials for different reaction pathways of $\mathrm{H_{2}O_{2}}$ at the cathode and anode in a one-compartment cell are crucial for the characteristics of the $\mathrm{H_{2}O_{2}}$ SPES. To achieve broad operation conditions, iron phthalocyanine (FePc), an enzyme mimic, and Prussian blue (PB), a nanozyme, were used as cathode catalysts because enzyme mimics and nanozymes overcome the shortcomings of natural enzymes but possess relatively high catalytic activity, and the properties can be easily tuned through modifications. The multienzymatic activities of modified PB catalysts were evaluated to investigate the influence of modifications. The properties of PB catalyst modified by Mn-doping and reduced graphene oxide interaction, as well as FePc catalyst modified by graphene nanoplatelets were investigated through structural and electrochemical analysis. The results revealed that the multienzymatic properties of PB could be tuned by doping with a suitable transition-series metal because the active metal center played a key role. And it was also possible to regulate the properties by changing the interface. Improved catalyst dispersion and better electrochemical performance of PB and FePc were achieved via carbonaceous supporting materials. The open-circuit potential tests demonstrated the feasibility of this self-powered system. The modification of the cathodes helped to achieve better analytical characteristics of the $\mathrm{H_{2}O_{2}}$ SPES due to enhanced electrochemical properties. However, oxygen showed significant influence on the properties of the cathodes modified by FePc-based materials and Ni anode, which required further studies on anode materials as well as cathode materials with better selectivity. The characteristics of the $\mathrm{H_{2}O_{2}}$ SPES under the control of external variable load resistances indicated a requirement of optimization in operation parameters. Nevertheless, the practical applicability of the $\mathrm{H_{2}O_{2}}$ SPES was verified by determination of $\mathrm{H_{2}O_{2}}$ in blood serum, and the tests showed good reproducibility and recovery rate at low concentration of $\mathrm{H_{2}O_{2}}$. Meanwhile, since the $\mathrm{H_{2}O_{2}}$ SPES were based on the principle of fuel cell, the results and outcomes could contribute to the development of $\mathrm{H_{2}O_{2}}$ fuel cells. Although the proposed $\mathrm{H_{2}O_{2}}$ SPES are still at the early stage of development and there are many issues to be investigated, a series of functional studies highlight the implementation, simplicity, and potential of this self-powered system in detecting hydrogen peroxide.