The advancement of electrochemical H₂O₂ synthesis technology is fundamentally dependent on a robust and standardized evaluation framework [Figure 4]. Such a framework is indispensable for elucidating reaction mechanisms, screening efficient catalysts, and optimizing electrolyzer performance. More importantly, it serves as a critical bridge connecting fundamental laboratory research to industrial-scale applications. The current mainstream evaluation methodology comprises two complementary approaches that form a complete performance assessment chain: the rotary electrode test and the electrolytic cell test combined with H₂O₂ quantification. The two approaches complement each other and form a complete performance evaluation chain. The rotary electrode test can accurately characterize the inherent electrochemical properties of the catalysts by constructing a controllable mass transfer environment, while the electrolytic cell test combines the efficient H₂O₂ quantitative method to comprehensively evaluate the application potential of the system under actual reaction conditions^[115]. This section will systematically elucidate the core principles, operational protocols, data analysis methods, and application boundaries of these two approaches. The aim is to provide a comprehensive theoretical and practical reference for standardized research in H₂O₂ electrosynthesis. As a classic tool for quantifying the performance of catalysts in the laboratory, the rotary electrode relies on the forced convection generated by high-speed rotation, which can effectively eliminate the interference of mass transfer restrictions, so as to accurately obtain the intrinsic activity, selectivity, and kinetic parameters of the catalysts^[116]. The two primary configurations are: rotating disc electrodes (RDE) and rotating ring disc electrodes (RRDE). RDE adopts a three-electrode system, and the working electrode surface has a uniform catalytic layer. When conducting the ORR test, the electrolyte should be kept in an oxygen-saturated state, while the WOR test directly uses water as the reactant. After obtaining the current density-potential curves by linear scanning voltammetry (LSV), key indicators such as the number of transferred electrons, the reaction rate constant, the starting potential and the half-wave potential can be calculated by fitting data at different rotation speeds with the Koutecky-Levich (K-L) equation^[84]:

(4) $$ \begin{equation} \begin{aligned} \frac{1}{j_{\mathrm{D}}}=\frac{1}{j_{\mathrm{K}}}+\frac{1}{j_{\mathrm{Diff}}}=\frac{1}{1000 n F K C_{\mathrm{O}_{2}}}+\frac{1}{1000 B \sqrt{\omega}} \end{aligned} \end{equation} $$

(5) $$ \begin{equation} \begin{aligned} B=0.62 n F C_{\mathrm{O}_{2}} D^{\frac{2}{3}} v^{\frac{1}{6}} \end{aligned} \end{equation} $$

Through linear fitting of 1/jₐ and 1/ω (electrode rotation angular velocity), the number of transferred electrons and the reaction rate constant can be accurately derived. Based on RDE, RRDE adds coaxial annular electrodes to form a disc-ring dual-working electrode structure: The plate electrode reacts to produce H₂O₂, and the ring electrode applies a constant potential to completely oxidize the H₂O₂ diffused to its surface. By synchronously collecting the disk current (i_a) and ring current (i_r), precise quantification of H₂O₂ selectivity and the number of transferred electrons is achievable, making it the gold standard for evaluating catalyst selectivity^[84]. Its core calculation formulas are the H₂O₂ selectivity:

(6) $$ \begin{equation} \begin{aligned} S_{\mathrm{H}_{2} \mathrm{O}_{2}}=\frac{\frac{2 i_{R}}{N_{C}}}{\left|i_{D}\right|+\frac{i_{R}}{N_{C}}} \end{aligned} \end{equation} $$

and the number of transferred electrons:

(7) $$ \begin{equation} \begin{aligned} n=\frac{4\left|i_{D}\right|}{\left|i_{D}\right|+\frac{i_{R}}{N_{C}}} \end{aligned} \end{equation} $$

Where N_C is the collection efficiency of the ring electrode. In order to ensure the accuracy of the test, it is necessary to control the catalyst loading, select suitable electrode materials such as gold rings, and calibrate N_C (ring current collection efficiency). This method can also be expanded to evaluate the 2e^- WOR catalysts, without oxygen supply, expand the potential window of the disk electrode, and directly follow the selective calculation logic of ORR.

It should be noted that the rotary electrode test depends on idealized conditions and forced convection, which is significantly different from the actual operation of electrolytic cells involving natural mass transfer and restricted electrode structure, and often overestimates the H₂O₂ selectivity of catalysts. In contrast, the electrolytic cell test is closer to the actual application scenario, because it comprehensively considers the electrode structure, mass transfer efficiency, electrolyte properties, and other engineering factors. Combined with accurate H₂O₂ quantitative analysis, this test can comprehensively evaluate the H₂O₂ production capacity of the electrode under actual working conditions^[117]. Its core evaluation index is FE, which directly reflects the charge utilization efficiency in reactions^[31]. The calculation formula is:

(8) $$ \begin{equation} \begin{aligned} F E=\frac{n F M_{\mathrm{H}_{2} \mathrm{O}_{2}}}{\int_{0}^{t} I(t) d t} \times 100\% \end{aligned} \end{equation} $$

where n = 2 (corresponding to the two-electron reaction), M_(H2O2) is the molar amount of generated H₂O₂, and I(t) is the current as a function of time. H₂O₂ quantification methods should be selected on demand: indirect spectrophotometry is simple to operate and highly anti-interferent, suitable for quantifying low to medium concentration H₂O₂ in complex electrolyte systems; titration has high accuracy and low detection limit, mostly used for offline precise quantification; chemiluminescence and fluorescence methods exhibit excellent sensitivity for trace H₂O₂ analysis, only limited by reagent costs or external light interference; colorimetric test strip and refractometry methods are fast to operate, suitable for on-site rapid screening or preliminary evaluation with low accuracy requirements, but have relatively large errors. In practical research, a synergistic strategy of rotating electrode screening and electrolytic cell verification is usually adopted: high activity and high selectivity catalyst candidates are quickly identified via rotating electrode testing, then fabricated into practical electrodes. Their actual performance is evaluated through electrolytic cell testing combined with appropriate quantification methods, and finally, catalyst structures or electrolytic cell parameters are optimized based on test results, forming a closed-loop research and development process of screening-verification-optimization to facilitate the efficient iteration of H₂O₂ electrosynthesis technology.

The H₂O₂ yield rate is another important performance metric, which is a comprehensive embodiment of FE, current density, and stability. Based on the amount of H₂O₂ produced per unit catalyst or electrode area within a unit time, the performance of the catalysts can be directly compared. Recently, the Zhang team reported that they applied a flow cell to evaluate BBL-PcNi as a 2e^- ORR catalyst for H₂O₂ production, which achieved the yield rate of 9.7 mol g^-1 h^-1 at 135 mA cm^-2[114]. Subsequently, they employed a membrane electrode assembly-solid-state electrolyte (MEA-SSE) electrolyzer to verify its capability for continuous production of pure medical-grade H₂O₂ under practical conditions. In the MEA-SSE configuration, BBL-PcNi achieved continuous operation for over 6 h with a low cell voltage of 2 V. Furthermore, the accumulated concentration of H₂O₂ in the electrolyte, its decomposition rate, and the long-term stability of the system ultimately determine the practical productivity. Therefore, the H₂O₂ yield rate of catalysts needs to be further evaluated and compared, as it is a prerequisite for practical applications.

Current literature predominantly emphasizes FE and productivity, but it lacks systematic accounting for actual production costs. According to the economic estimation in Siahrostami’s study, which is based on the International Renewable Energy Agency (IRENA) green electricity price (0.05 USD/kWh) and an experimental productivity of 15 μmol/min, the base costs of electrochemically producing high-concentration H₂O₂ are lower, respectively, than those of the conventional anthraquinone process^[7]. This indicates that the electrochemical route is already cost-competitive at the fundamental level. However, this estimation is based on the following conservative assumptions: First of all, the long-term stability cost of the 2e^- WOR catalyst under high potential was not included. Then, the energy consumption for membrane separation required to concentrate the dilute solution (< 5%) to the commercially relevant concentration of 35% was not accounted for. The differences in catalyst preparation costs arising from the possible use of different anode and cathode catalysts in a 2e^- system were not considered. We have therefore given this further thought. IRENA predicts that the green electricity price could drop to 0.02-0.03 USD/kWh by 2030, and the share of electricity cost in the total H₂O₂ production cost will decrease further from the current 30%-50%. Against this background, catalyst lifetime, membrane cost, and separation cost will become the dominant factors governing economic viability. Consequently, future economic assessments should establish a full life-cycle cost model that incorporates catalyst replacement intervals (target: > 8,000 h), membrane lifetime (target: > 5,000 h), and energy consumption for concentration (target: < 0.5 kWh/kg H₂O₂). In particular, our analysis reveals that if a single bifunctional catalyst can be used in a 2e^- system to simultaneously drive both the anodic and cathodic reactions, the costs for catalyst preparation and replacement can be substantially reduced, making this the most economically promising technical pathway for the future.

The development of concerted H₂O₂ production by a sustainable paired-electrosynthesis strategy commonly hinges on the design of advanced asymmetric catalytic electrode pairs. This strategy involves engineering the anode catalysts to tackle the more challenging 2e^- WOR. In a pioneering study, Xia et al.^[103] employed a surface modification approach by coating carbon fiber paper (CFP) with a hydrophobic PTFE polymer to locally confine generated O₂ gas near active sites. This confinement alters interactions with oxygen intermediates, boosting the selectivity and activity of the 2e^- WOR pathway. When coupled with an oxidized carbon nanotube (O-CNT) cathode as the 2e^- ORR catalyst, this system achieved an exceptional H₂O₂ FE_cell of 153%^[103]. The mechanism study shows that the direct 2e^- WOR pathway occurs on the carbon-based electrode rather than the indirect over-carbonate-mediated pathway. This conclusion is supported by evidence such as ¹⁸O isotope markers. The universality of this local gas-limit concept has been further verified after applying it to a typical OER catalyst, i.e., foam nickel, enabling the generation of H₂O₂. It is worth noting that in a sodium carbonate electrolyte, the generated H₂O₂ can form solid sodium percarbonate, which is convenient for storage and transportation, and highlights its application potential in organic synthesis.

In a parallel approach, Ling et al. functionalized a CFP anode with a self-assembled monolayer (SAM) of (3-Mercaptopropyl) triethoxysilane (MPTES)^[118]. The modification constructs a highly ordered interface and optimizes the microenvironment around the active sites, which is conducive to generating the 2e^- WOR pathway. The resulting SAM-modified anode (S-CFP) achieved a H₂O₂ selectivity of 62.1% in sodium carbonate electrolyte. In order to build a practical system, the anode was coupled with the natural air diffusion cathode (NADE) based on carbon cloth. The cathode can passively use oxygen in the atmosphere without forcing aeration with intense energy consumption. In a membrane-separated electrolyzer, this paired system delivered a total H₂O₂ FE of 152.9% and a production rate of 38 μmol cm^-2 min^-¹, demonstrating the viability of decentralized H₂O₂ production. This strategy highlights the potential of metal-free catalysts for decentralized H₂O₂ generation with low energy consumption. A common strategy highlighted in these works involves boosting the 2e^- WOR on the anode through precise interfacial microenvironment engineering. This is combined with leveraging established, high-performance 2e^- ORR catalysts on the cathode to assemble an effective asymmetric electrode pair for concurrent H₂O₂ generation.

Supplementary to carbon materials, investigations confirm that the metal-based electrocatalysts are equally effective for the 2e^- WOR. Zhu et al. investigated H₂O₂ production on a Pt electrode in carbonate solutions using in-situ Raman spectroscopy [Figure 5A] and ¹⁸O isotope labeling, revealing that carbonate coupling is the dominant pathway [Figure 5B]^[119]. These researchers also suggest that during electrochemical H₂O₂ production from water oxidation in carbonate electrolyte, the vast majority of H₂O₂ is generated via the coupling of two adsorbed CO₃^2- species to form a peroxodicarbonate intermediate (C₂O₆^2-), which subsequently dissociates to yield H₂O₂. In contrast, the conventionally presumed *OH-*OH coupling pathway contributes negligibly, while the CO₃^2--*OH cross-coupling pathway accounts for approximately 7% of the total contribution^[119]. This mechanism highlights the critical role of carbonate adsorption on the Pt surface in suppressing competing reactions and enhancing the selectivity of the 2e^- WOR. Their study showed that 93% of H₂O₂ originates from CO₃^2- coupling via a C₂O₆^2- intermediate, with minimal contribution from OH-mediated routes. The Pt electrode achieved high selectivity, exceeding 90% FE in 2 M K₂CO₃ under optimized potentials [Figure 5C]. This work highlights the key role of carbonate adsorption in inhibiting competitive reactions and achieving selective H₂O₂ electrosynthesis. By coupling the commercial ORR cathode with the matching anode in the flow cell configuration [Figure 5D], the synchronous generation of H₂O₂ by paired electrosynthesis was realized. When the flow cell operated at a current density of 1 A cm^-2, it showed an FE_cell of up to 150% for H₂O₂, and the required cell voltage was only 2.3 V [Figure 5E].

In addition, Li et al. pioneered the development of a Sn-doped CuWO₄/Sn anode catalyst prepared by pyrolysis^[120]. The X-ray diffraction (XRD) spectroscopy [Figure 5F] and high-resolution transmission electron microscopy (HRTEM) [Figure 5G] analysis confirmed that the catalyst has a pure wolframite CuWO₄ structure with uniform element distribution [Figure 5H]. The hydrophobic CC/CuWO₄/Sn electrode achieved 72% H₂O₂ FE at 2.5 V vs. RHE [Figure 5I], and the yield rate reached 11.6 μmol cm^-2 min^-1. In the full-cell system coupled with a nitrogen-doped carbon nanotube air self-respiration cathode, the total FE under 100 mA cm^-2 reached 161%, showing efficient paired-electrosynthesis performance [Figure 5J]. DFT calculations indicate that atomic-level doping of Sn modulates the adsorption energies of key intermediates (especially *OH) on the CuWO₄ surface, thereby facilitating the formation and coupling of *OH toward H₂O₂ production. Concurrently, the high-energy-barrier 4e^- WOR pathway is suppressed, thereby enhancing selectivity for the 2e^- WOR [Figure 5K]. Moreover, Wang et al. synthesized a single atomic Ni-doped TiO₂ (Ni_xTi_1-xO_2-y) anode through a solvothermal method, which displays a rutile structure with Ni atoms substituting Ti sites^[16]. The anode delivered a FE of 70% for H₂O₂ at 300 mA cm^-2 in weakly alkaline media, and when paired with an O-CNT cathode, the full-cell reached 146% FE at 240 mA cm^-2. Specifically, DFT calculations suggest that the incorporation of Ni atoms into the TiO₂ lattice weakens the excessively strong adsorption of *OH, while simultaneously enhancing the kinetic favorability of the *OH-*OH coupling pathway, thereby improving the selectivity of the 2e^- WOR. Jia et al. synthesized an oxygen-doped nanodiamond (O-ND) cathode through acid washing and heat treatment, which removes the surface graphite layer and increases the sp³ carbon content^[121]. The cathode delivered an FE of 87.1% for H₂O₂ via 2e^- ORR in 0.1 M KOH electrolyte, demonstrating excellent stability over 24 h. The full-cell reached a combined FE of 161.1% at a voltage of 1.8 V. DFT calculations revealed that the oxygen doping modulates the local charge distribution of sp³ carbon, optimizing the *OOH adsorption energy for 2e^- ORR, while the Cu₂O component in Cu-OND exhibits an optimal d-band center that balances *OH adsorption, thereby enhancing the selectivity of the 2e^- WOR pathway. Overall, these studies have advanced this field by demonstrating the efficient and on-site production of H₂O₂ using rich resources.

Assembling asymmetric catalytic electrode pairs usually includes two steps, namely optimizing the WOR catalyst and then coupling it with the standard carbon-based anode catalyst. Through strategic methods such as local oxygen limitation, atomic doping, and interface engineering, the WOR performance can be effectively optimized, so as to improve its selectivity and activity. These modifications can regulate the adsorption behavior of key intermediates (such as *OH), making them more inclined to the 2e^- pathway rather than the competitive response^[16]. In the end, this kind of electrode pair can achieve synergy, which verifies the feasibility of high-yield and distributed H₂O₂ production.

A symmetric catalytic electrode pair employs the same material at both the cathode and anode to couple the 2e^- ORR and the 2e^- WOR for concerted H₂O₂ electrosynthesis. This configuration not only simplifies the device structures but also has unique operational advantages. When the activity of the catalysts decreases due to overreduction or peroxidation, the polarity of the switching electrodes can regenerate the active structures in situ, so as to achieve continuous operation without replacing the electrodes. However, the central material challenge is non-negligible, since one catalyst must remain selective and stable across strongly different interfacial environments (cathodic reductive vs. anodic oxidative potentials), while concurrently suppressing the competing four-electron pathways (4e^- ORR to H₂O and 4e^- WOR to O₂). Building on this concept, this subsection highlights that the bifunctional catalysts are used on both electrodes to drive the two half-reactions in a concerted manner. The following representative studies illustrate how tuning the adsorption behaviors of *OOH and *OH intermediates via defect, facet, and interfacial regulation, which can simultaneously promote the 2e^- ORR and the 2e^- WOR while suppressing competing 4e^- pathways, thereby enabling efficient coupled H₂O₂ production.

Defect engineering becomes an effective route to bifunctionality in symmetric electrode pairs, such as oxygen vacancies (O_v), which provide a direct lever to reshape local electronic structures of the electrocatalysts and thus the binding strengths of *OOH and *OH. The first example is TiO_2-x nanocrystals obtained by hydrogen plasma etching, where “defects with spatial roles” were explicitly demonstrated^[122]. The inner oxygen vacancies stabilize *OOH during 2e^- ORR and inhibit its dissociation, whereas surface lattice distortion promotes *OH coupling during 2e^- WOR. In-situ Raman spectroscopy demonstrated that the characteristic peaks of the TiO_2-x surface remained nearly unchanged within the ORR potential window. The post-reaction decrease in oxygen vacancy concentration indicated that the residual oxygen vacancies are located in the inner layer rather than on the surface. During the WOR process, the Raman signals gradually attenuated above 2.2 V, revealing that a slight surface distortion of TiO_2-x is the origin of the high selectivity. DFT calculations further confirmed that surface and subsurface oxygen vacancies are readily healed by reaction intermediates, while only the near-surface oxygen vacancies can stably persist and optimize *OOH adsorption. Experimental tuning of the oxygen vacancy concentration additionally showed that the ORR performance correlates positively with the oxygen vacancy concentration, whereas the WOR performance is independent of it. This bifunctional partitioning translated to a 2e^- ORR H₂O₂ selectivity of 94.5% in 0.1 M KOH and a 2e^- WOR selectivity up to 55% at 2.1 V vs. RHE. In a 2e^- electrolyzer, TiO_2-x delivered 20 mmol L^-1 h^-1 of H₂O₂ at 2.0 V with a FE_cell of 134%. Importantly, in situ Raman spectroscopy evidenced that the anatase (101) plane remains stable under operation to improve defect-site durability under anodic bias. Subsequent studies pushed defect chemistry from qualitative toward quantitatively controlled and stability-validated. In the oxygen-deficient D-PSFZ perovskite oxides [Figure 6A], Zn²⁺ doping induces δ ≈ 0.14-0.15 oxygen vacancies that regulate Fe/Zn electronic structures, strengthening *OOH binding while increasing the adsorption energy of *O intermediates to thermodynamically inhibit 4e^- ORR/OER. D-PSFZ achieved > 99% FE for 2e^- ORR over 0.05-0.45 V vs. RHE (0.1-2.0 M KHCO₃), retained 98% FE after 500 h, and reached 80% FE for 2e^- WOR at 2.15 V vs. RHE [Figure 6B]^[25]. In a membrane-free electrolyzer, the FE_cell reached 163% at 2.1 V and 50 mA cm^-2 (remaining 150% after 100 h) [Figure 6C]. This study elegantly unifies the selectivity for the two key intermediates, *OOH and *OH, into a thermodynamic suppression of their common decomposition intermediate, *O, by engineering an oxygen-deficient perovskite oxide rich in oxygen vacancies. DFT calculations reveal that Zn-doping-induced oxygen vacancies modulate the electronic structure of adjacent B-site cations, simultaneously optimizing the binding energies of both *OOH and *OH to promote the 2e^- ORR and 2e^- WOR, respectively. The crucial insight lies in introducing ΔG_*O as a more accurate selectivity descriptor, which unveils that *O binding is energetically highly unfavorable on the vast majority of surface active sites. This fundamentally prohibits O-O bond scission, robustly steering the reaction towards H₂O₂. Leveraging this intrinsic mechanism and the synergistic carbonate effect, the bifunctional catalyst achieves up to 99% ORR selectivity and 80% WOR selectivity in a membrane-free symmetric electrolyzer. In another work, amorphous TiO_x embedded in mesoporous carbon hollow spheres (TiO_x@MCHS) combined abundant O_v with low-coordinated Ti³⁺ (average coordination number 5.1) [Figure 6D-F], enabling 55.5% FE for 2e^- WOR at 2.6 V vs. RHE [Figure 6G], alongside 99.3% FE for 2e^- ORR at 0.2 V vs. RHE with 104 mA cm^-2 [Figure 6H]^[123]. When deployed in a membrane-free flow cell coupling synthesis system, the H₂O₂ yield rate reached 108.3 μmol cm^-2 min^-1 at a constant current of 240 mA, and the total FE was 145% at a cell voltage of 2.5 V [Figure 6I]. Additionally, the FE only slightly decreased from 114% to 107% after 7 test cycles, indicating excellent stability in both structure and catalytic performance [Figure 6J].

Our group synthesized porous bismuth oxide nanorods enriched with oxygen vacancies (O_v-Bi₂O₃-EO) via in situ electrochemical oxidation of a bismuth-based metal-organic framework (MOF) precursor [Figure 7A-C]^[28]. The introduction of oxygen vacancies modulates the electronic structure of bismuth atoms, DFT calculations reveal that the introduction of oxygen vacancies significantly reduces the ΔG_*OOH adsorption from 0.456 to 0.296 eV, thermodynamically promoting the 2e^- ORR, while the ΔG_OH is much higher than ΔG_H2O2, rendering the dissociation of *OOH to *O energetically highly unfavorable and thus effectively suppressing the 4e^- ORR pathway^[28]. Concurrently, O_v-Bi₂O₃ lowers the energy barrier for *OH oxidation to H₂O₂ from 1.519 to 1.380 eV, whereas the barrier for the competing 4e^- WOR is as high as 1.801 eV, granting a thermodynamic preference to the 2e^- WOR. In situ Raman spectroscopy further confirms the preferential emergence of *OH intermediate signals at elevated potentials. The origin of this performance lies in the oxygen-vacancy-induced optimization of the Bi p-band center from -2.36 to -2.04 eV, which endows the catalyst with an optimal intermediate adsorption strength.

Under alkaline conditions, O_v-Bi₂O₃-EO delivered an H₂O₂ selectivity as high as 90% for the 2e^- ORR at 0.4 V vs. RHE [Figure 7D], with a production rate of 0.465 mmol cm^-2 h^-1 in a flow cell configuration^[28]. In a 2 M KHCO₃ electrolyte, the same catalyst achieved an FE of 78.8% for the 2e^- WOR at 2.9 V vs. RHE, corresponding to a yield rate of 2.01 mmol cm^-2 h^-1 [Figure 7E]. Notably, when implemented in a flow cell configured for paired electrosynthesis, the integrated system exhibits an overall FE of 154.8% and a total H₂O₂ output of 4.3 mmol cm^-2 h^-1 [Figure 7F]. After 12 h of continuous operation, the accumulated H₂O₂ concentration remained at 159.1 mM, indicating good operational stability [Figure 7G]. These results collectively affirm the strong practical potential of O_v-Bi₂O₃-EO for efficient and scalable electrochemical H₂O₂ production.

Despite these successes, defect strategies face some inherent questions, such as defect groups may be heterogeneous, dynamically variable, and difficult to clearly attribute to a single active site, especially under oxidative potentials. This limitation has prompted research to turn to more structurally certain regulatory methods, the most prominent of which is crystal surface engineering. In this strategy, the adsorption geometry is dominated by a clear crystalline surface, not a set of defects in the statistical sense. For example, CeO₂ nanocubes (NCs) that predominantly expose (100) facets have been reported as an efficient bifunctional electrocatalyst for H₂O₂ production via both the 2e^- ORR and the 2e^- WOR^[124]. Electrochemical measurements showed that the CeO₂ NCs achieved an H₂O₂ FE of 95.3% and a production rate of 632 mmol g^-1 h^-1 for the 2e^- ORR, significantly outperforming facet-unspecified CeO₂ nanodots (NDs)^[124]. Mechanistic studies combining spectroscopy tests indicated that the enhanced activity and selectivity were mainly governed by the crystal facet effect rather than oxygen vacancies^[124]. When deployed in a flow cell reactor for paired H₂O₂ electrosynthesis, the CeO₂ NCs attained a total H₂O₂ yield rate of 4.6 mol g^-1 h^-1, underscoring their practical potential for sustainable oxidant production.

Interfacial engineering aims to create interfacial regions with distinct electronic structures by constructing heterojunctions^[125]. A representative example is the NiZnO_x-C dual-site catalyst fabricated via the Kirkendall effect [Figure 8A], which simultaneously regulates (100) plane exposure and heterointerface density^[102]. Here, NiO_x clusters are dispersed within a ZnO_x matrix to generate an abundant NiO_x/ZnO_x heterointerface. Oxygen vacancies and built-in electric fields at these interfaces facilitate water dissociation, producing reactive *H/*OH species and improving interfacial charge transfer, features that directly benefit anodic kinetics and suppress parasitic O₂ evolution. Crucially, the catalyst implements an explicit site assignment, where the Zn sites dominate 2e^- ORR, while the Ni sites dominate 2e^- WOR [Figure 8B], thereby easing the conflicting adsorption requirements for *OOH versus *OH. In situ ATR-FTIR and EPR spectroscopies confirmed the preferential adsorption of *OOH and *OH at the respective active sites, while DFT calculations revealed that interfacial charge transfer upshifts the d-band centers of both Zn and Ni, optimizing the adsorption energies of the key intermediates. This interface-driven water activation and dual-site spatial separation strategy enables a single catalyst to rapidly synthesize H₂O₂ at both electrodes simultaneously, achieving selectivity exceeding 90% for both 2e^- ORR and 2e^- WOR. The FE for H₂O₂ production increased with current from 400 to 1,200 mA, peaking at 148.5% at 1,000 mA [Figure 8C]. At 1 A, the cell voltage remained stable near 3.5 V, while the FE_cell reached 150.9% before stabilizing at ~140% [Figure 8D]. In 0.1 M Na₂SO₄, this design reached 96.7% H₂O₂ selectivity for 2e^- ORR and 93.6% for 2e^- WOR. In flow-cell tests, the FE_cell reached 145.8% under simulated alternating current (AC) operation with sustained H₂O₂ accumulation (33,987 mg L^-1) after 4 h [Figure 8E and F]. This work is strategically important because it demonstrates how symmetry-specific operational modes (e.g., AC-like switching or polarity reversal) can synergize with interface-rich catalysts to maintain activity and mitigate degradation.

Except for oxides, Yang et al. prepared mesoporous carbon-doped boron nitride (C-doped BN) by high-temperature annealing of organic resin and silica composites in NaBH₄/NaNH₂ molten salts^[126]. Carbon substitution at B/N sites generates CB and CN defect interfaces, which narrows the BN band gap from 3.97 to 1.67 eV and improves electronic conductivity, while also providing active sites for both the 2e^- ORR and the 2e^- WOR^[126]. The defect active sites introduced by carbon doping respectively optimize the adsorption of *OOH and the conversion of *OH. In 2 M KHCO₃, the H₂O₂ FE for the 2e^- ORR reached 93%-98% with a yield rate of 1,650 mmol g^-1 h^-1, and the 2e^- WOR yield rate was 551 mmol g^-1 h^-1. When applied to an H-type cell in a coupling mode, the cathodic H₂O₂ yield rate reached 2,750 mmol g^-1 h^-1, achieving efficient coupling generation of H₂O. This system serves as a conceptual bridge, retaining a defect-interface logic while moving toward more chemically describable motifs than oxygen-vacancy ensembles in metal oxides.

An even more explicit approach is provided by a conductive MOF, Wang et al. prepared a nitrogen-bridged dual-atom Ni-Sn catalyst (NiSn-HITP) via a post-synthetic metallization strategy using the conductive MOF Ni-HITP as the support^[127]. In NiSn-HITP, the atomically dispersed Ni and Sn sites are precisely engineered such that Sn predominantly serves as the active center for 2e^- ORR, while Ni facilitates 2e^- WOR^[127]. Crucially, the Ni-N-Sn coordination motif induces orbital coupling that upshifts the Ni d-band center and enhances the density of electronic states near the Fermi level, thereby simultaneously optimizing the adsorption energies of *OOH at Sn sites and *OH at Ni sites. The incorporation of Sn precisely regulates the electronic structure of Ni sites through orbital coupling among the Ni 3d orbitals, ligand N 2p orbitals, and Sn 5p orbitals, shifting the d-band center of Ni upward from -1.560 to -1.513 eV. Meanwhile, the adsorption energies of key intermediates are optimized. The adsorption energy of *OOH on Sn sites is reduced to 0.80 eV, suggesting thermodynamic stability, and the desorption barrier of *OH on Ni sites decreases from 1.72 to 1.41 eV, thereby suppressing the side reactions of 4e^- ORR to H₂O and 4e^- WOR to O₂^[127]. DFT calculations reveal that the formation of *OOH at Sn sites is exergonic (-0.80 eV), while the desorption barrier for *OH coupling to H₂O₂ at Ni sites is markedly reduced to 1.41 eV. In situ ATR-FTIR spectroscopy confirms the accumulation of *OOH and *OH intermediates during the ORR and WOR processes, respectively, validating the dual-pathway mechanism. This complementary dual-site catalysis enables breakthroughs in both FE and production rate in a single electrolytic cell. In 0.1 M KOH, NiSn-HITP reached an H₂O₂ selectivity up to 98.8% for the 2e^- ORR at 0.2 V vs. RHE, with a yield rate of 17.9 mol g^-1 h^-1 in the GDE system. Notably, the FE remained nearly 100% after a 15-h stability test^[127]. In 2 M KHCO₃, the H₂O₂ FE for 2e^- WOR reached 66.9% at 2.1 V vs. RHE, with an overpotential of 0.69 V at 10 mA cm^-2 and a yield rate of 1.3 mol g^-1 h^-1. When applied in a membrane-free flow cell for coupled synthesis, the total FE reached 171.5% at a cell voltage of 3.0 V, and the total H₂O₂ yield rate was 9.1 mol g^-1 h^-1. After 15 h of stable operation at 50 mA cm^-2, the H₂O₂ concentration reached 0.61 wt%, demonstrating excellent practical application potential. Although the FE promoted by bifunctional catalysts has now exceeded 170%, practical large-scale H₂O₂ production via 2e^- ORR//WOR still requires a catalyst that can operate at the lowest possible potential, or deliver a current density of at least 200 mA cm^-2, while simultaneously achieving near-perfect selectivity and high FE, and maintaining its performance for a long duration (> 100 h). However, as shown in Figure 9, existing catalysts struggle to simultaneously meet these requirements. For instance, Ni-HTIP exhibits an exceptionally high H₂O₂ FE (> 170%), yet the corresponding potential is 3 V vs. the counter electrode and the current density under catalytic conditions is only 50 mA cm^-2, such performance falls far short of practical production demands. This is not an isolated case; this deficiency is common among the bifunctional catalysts we have investigated. Increasing the operating potential is indeed a strategy to raise the current density, thereby enhancing the rate of H₂O₂ generation. However, this approach often introduces significant competitive reactions, such as 1e^- and/or 4e^- pathways, and can even promote the decomposition of H₂O₂^[128]. Under highly oxidative conditions, the durability of the anode may also be compromised. Therefore, future research directions should take this practical issue into account to promote the industrial-scale mass production of electrocatalytic H₂O₂.

For symmetric catalytic electrode pairs that need to simultaneously catalyze both the WOR and ORR, and to selectively regulate the conversion of the key intermediates *OH and *OOH, the existing literature has primarily developed strategies such as bifunctional active sites, precise regulation of oxygen vacancies, and interfacial micro-environment engineering. Based on these strategies, the design of symmetric catalytic electrode pairs can address the challenges of bifunctional catalysis while fully leveraging their unique advantages in structural simplicity and operational stability. Viewed through the strategy evolution, the symmetric catalytic electrode pairs advanced along a clear development route. The next-step designs are hybrids that fuse determinism across length scales: In the first place, facet-constrained defect construction places oxygen vacancies or low-coordination sites at crystallographically defined locations. Second, interface engineering combined with single-/dual-atom motifs, interfaces for water activation and charge mediation, and atomic motifs for precise *OOH/*OH tuning. Methodologically, operando characterization that resolves how *OOH/*OH energetics evolve under alternating redox polarization will be essential to transform these concepts into design rules, ensuring that half-reaction selectivity reliably translates to high-rate, long-life symmetric devices.