Deconvoluting Cathode Performance from Anodic Selectivity Limits: A Multicriteria Methodology for Electrochemical Oxidation Assessment

Abstract

The contribution of the cathode to system-level efficiency in electrochemical oxidation (EO) is rarely isolated from anodic selectivity limitations, even though its influence on cell voltage, hydrogen evolution kinetics, and energy consumption is well recognized. This study presents a multicriteria methodology that deconvolutes cathode performance from these anodic constraints. A stable lead dioxide anode was paired with three cathodes, carbon felt (CF), stainless steel (SS), and titanium dioxide (TiO₂), for Rhodamine B degradation. The methodology combines conventional electrochemical diagnostics, a ten-parameter multicriteria assessment spanning activity, efficiency, and economics, and a sensitivity analysis prioritizing operational metrics. Application revealed that cathode material governs system-level performance through trade-offs between degradation rate and energy consumption: SS minimized cathodic voltage contribution, while CF maximized degradation rate, with sensitivity analysis confirming CF as the optimal practical choice. However, all systems were constrained by a universal limitation: Faradaic efficiencies remained below 0.3% at an applied current of 30 mA, with anode potential well above the oxygen evolution reaction (OER) threshold and more than 99.7% of charge diverted to unwanted water oxidation. Thus, cathode selection modulates cost and yield but cannot resolve the underlying anodic OER limitation. This methodology offers a transferable diagnostic protocol, indicating that future efforts should prioritize integrated system design over single-electrode optimization to overcome EO selectivity limitations.

1. Introduction

Electrochemical oxidation (EO) holds promise for degrading persistent organic pollutants [1,2,3] via potent oxidants, primarily hydroxyl radicals (^●OH), generated at the anode [4,5]. This has driven extensive research into anode materials such as boron-doped diamond [6] and mixed metal oxides [7,8,9] to enhance activity, stability, and selectivity [10,11,12]. However, this anode-centric focus has inadvertently relegated the cathode to a passive role, typically chosen just for its conductivity and cost, rather than being considered as an integral component of a coupled electrochemical system.

Yet, the cathode actively shapes system performance: its material properties determine hydrogen evolution reaction (HER) overpotential [13,14], influence cell voltage [15], modify local pH, and affect mass transport [16,17]. Given this multifaceted role, the cathode has attracted significant interest in electrochemical advanced oxidation processes (EAOPs) [18,19], particularly in configurations where it serves as an active component for H₂O₂ electrogeneration, which is subsequently consumed in Fenton-based systems [20,21]. The extensive body of work has generated detailed knowledge on how individual operational parameters, such as current density, pH, and electrolyte concentration, affect degradation efficiency, with cathode performance typically assessed through isolated metrics (e.g., cell voltage, H₂O₂ yield) [22,23,24] or within membrane-divided cells that physically separate anodic and cathodic processes [25,26]. The breadth of these studies, and the diversity of experimental conditions and metrics employed, underscores the need for a systematic, transferable framework that simultaneously integrates electrochemical activity, degradation kinetics, and energy economics within a single undivided cell, reflecting practical EO operation.

A more fundamental question, however, underlies this gap: can cathode engineering meaningfully improve overall process efficiency when the dominant energy loss stems from the competing oxygen evolution reaction (OER) at the anode? The OER presents a profound selectivity challenge, often consuming the vast majority of the applied electrical charge and limiting Faradaic efficiencies for pollutant oxidation [27,28]. While cathode selection affects cell voltage [29], its role relative to this anodic limitation has not been clearly quantified [30,31].

This study addresses this gap by presenting a multicriteria methodology that quantitatively isolates cathode-specific effects from the fundamental anodic OER bottleneck, integrates multiple performance dimensions into a unified scoring framework, and evaluates the sensitivity of material rankings to application-specific priorities. The novelty of this work lies not in demonstrating that cathode material affects EO performance, a fact already established in the EAOP literature, but in providing a transferable protocol that explicitly separates cathodic from anodic contributions and quantifies their relative significance against the universal baseline of anodic OER dominance. It posits that overcoming the OER limitation requires a paradigm shift, from isolated electrode optimization toward integrated system design that actively circumvents this inherent selectivity limitation. The main hypothesis suggests that while cathode properties have a notable impact on system energy dynamics, the anodic OER remains the primary limiting factor for Faradaic efficiency. Consequently, rational cathode selection must operate within this constrained electrochemical window, guided not by maximizing individual electrode performance, but by optimizing the coupled anode–cathode system.

To address this, it is essential to employ a model system that facilitates precise electrochemical analysis. In this study, a stable lead dioxide anode (PbO₂@TiO₂) was paired with three different types of cathodes: carbon felt (CF), stainless steel (SS), and titanium dioxide (TiO₂), for Rhodamine B (RhB) degradation. RhB is selected as a well-documented model pollutant [32,33], while PbO₂ offers high OER overpotential and stability [34,35]. The methodology integrates conventional electrochemical diagnostics with a multiparameter assessment framework spanning activity, efficiency, and economics, complemented by a sensitivity analysis prioritizing operational metrics. The objective was to quantify how cathode selection influences both kinetics and energy efficiency. The results reveal a striking convergence: despite measurable cathode-induced variations in cell voltage, all systems exhibit fundamentally limited Faradaic efficiencies under high-current operation, with over 99.7% of charge diverted to parasitic OER. This limitation, however, can be partially circumvented through operational rather than material design, as demonstrated by operation at reduced current.

Finally, to synthesize multi-dimensional and often conflicting results into a unified comparative framework, a holistic scoring system was applied. The multicriteria assessment framework employed draws upon established multicriteria decision analysis (MCDA) methodologies commonly used in environmental engineering [36,37]. This holistic approach enables a quantitative comparison that transcends single-parameter rankings and addresses recent calls for system-level assessment in EO research [38]. These findings reframe the challenge: rather than seeking a universally “best” cathode, EO process design must prioritize strategies that bypass or suppress the OER bottleneck, whether through modulated current regimes, reactor engineering, or integrated anode–cathode design.

2. Materials and Methods

2.1. Electrode Materials and Experimental Setup

Three independent electrochemical systems were constructed, each defined by its counter electrode material: carbon felt (CF), stainless steel 316 plate (SS), or a titanium oxide mesh (TiO₂). The working electrode (anode) in all systems was a commercially obtained lead dioxide on titanium oxide substrate mesh (PbO₂@TiO₂) with a geometric area of 2.0 cm² (1 × 2 cm, 1 mm thickness). The cathodes differed in their geometric surface areas: CF had dimensions of 2 × 3 cm with a 3 mm thickness (6.0 cm²); SS measured 3 × 9 cm with a 0.5 mm thickness (27.0 cm²); and TiO₂ was 1 × 2 cm with a 1 mm thickness (2.0 cm²). All electrodes were obtained from the same manufacturer (Baoji Taihengte Metal Co., Ltd., Baoji City, China). Before each experiment, all electrodes were meticulously cleaned by rinsing with ethanol and deionized water.

All experiments were conducted using a computer-controlled potentiostat (QL 335 DC Power Supply, Aim and Thurlby Thandar Instruments, Huntingdon, Cambridgeshire, UK) in a single-compartment glass cell containing 60 mL of 0.1 M Na₂SO₄ as the supporting electrolyte. The electrolyte composition and concentration were selected based on our previous studies [39,40,41,42], where it was established as a stable and reproducible medium for electrochemical oxidation experiments. The pH was adjusted to 4 using concentrated H₂SO₄ (Sigma-Aldrich, Merck KGaA, Burlington, VT, USA). This value lies near the pKa of the carboxyl group of RhB (~4.3), ensuring that the dye is present predominantly in its zwitterionic form with consistent electrochemical behavior [43], while also maintaining the stability of the PbO₂ anode [44].

2.2. Electrochemical Characterization

Fundamental electrochemical characterization was conducted in a standard three-electrode configuration using an Ag/AgCl electrode (3 M KCl) as the reference. Before degradation experiments, each electrochemical system was stabilized via cyclic voltammetry (CV) between 1 and 3 V vs. Ag/AgCl at a scan rate of 50 mV s⁻¹ until reproducible voltammograms were obtained. The anodic activity, CV @2 V, for each system was quantified as the current density at 2.0 V vs. Ag/AgCl from the final stable CV.

Linear sweep voltammetry (LSV) was performed from 0.5 to 2.2 V vs. Ag/AgCl at 10 mV s⁻¹. The anodic activity, LSV @1.75 V, for each system was quantified as the current density at 1.75 V vs. Ag/AgCl. To enable a direct comparison with steady-state behavior, a separate CV scan was recorded within the same potential range (0.5 to 2.2 V vs. Ag/AgCl at 50 mV s⁻¹), and anodic activity was also estimated at 1.75 V vs. Ag/AgCl (CV @1.75 V). To directly characterize the cathodic half-reaction, separate LSVs were run in the cathodic direction (+0.8 to −1.5 V vs. Ag/AgCl, 10 mV s⁻¹) for each cathode material (CF, SS, TiO₂). Measured current densities in CVs are normalized to the geometric area of the PbO₂@TiO₂ electrode (2 cm²).

Onset potentials for oxygen evolution (E_OER) and hydrogen evolution (E_HER) reactions were determined from derivative analysis of LSV data [45,46]. Methodological details and results are provided in Supplementary Materials (Text S1, Figure S1). The electrochemical window for RhB degradation was calculated as ∆E = E_OER − E_HER. When ΔE is greater, there is a broader range of potential that allows current to be mainly used for degrading RhB while minimizing unwanted gas evolution reactions.

Tafel analysis was conducted to quantify both OER and HER kinetics (Text S2). Tafel slopes (b, mV dec⁻¹) and exchange current densities (j₀, mA cm⁻²) were extracted via linear regression, with fit quality validated by correlation coefficients R² > 0.97 for all cases.

Chronopotentiometry was performed during degradation experiments using a two-electrode configuration at constant current (30 mA), with cell voltage recorded every 1 s. The average cell voltage (${\mathrm{E}}_{\mathrm{ave}}$) was calculated as the arithmetic mean of all recorded voltage values over the entire experimental duration.

2.3. Degradation Experiments and Analysis

Constant-current electrooxidation experiments were performed at 30 mA (j_anodic = 15 mA cm⁻²) for 120 min in a magnetically stirred reactor (800 rpm). The model organic pollutant, Rhodamine B (RhB, Sigma-Aldrich, Merck KGaA, Burlington, VT, USA), was introduced at an initial concentration of 5 ppm. Aliquot samples (0.5 mL) were periodically withdrawn from the reactor and analyzed immediately. RhB concentration was monitored via UV-Vis spectroscopy (Lambda 35, Perkin Elmer, Inc., Shelton, CT, USA) by measuring its characteristic absorbance at 554 nm. All measurements were performed in triplicate, and reported values represent the mean.

The degradation percentage (%) was calculated using Equation (1):

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_t}{{\mathrm{C}}_0}}\times 100\%$$

(1)

where ${\mathrm{C}}_0$ and ${\mathrm{C}}_t$ are the RhB concentrations (ppm) at time zero and time $t$, respectively.

The degradation kinetics were modeled using a pseudo-first-order rate law (Equation (2)):

$$\mathrm{l}\mathrm{n}\left({\displaystyle \frac{{\mathrm{C}}_t}{{\mathrm{C}}_0}}\right)=-\mathrm{k}t$$

(2)

where $\mathrm{k}$ is the pseudo-first-order degradation rate constant (h⁻¹). The value of $\mathrm{k}$ was obtained from the slope of a linear regression of $\mathrm{l}\mathrm{n}\left({\mathrm{C}}_t/{\mathrm{C}}_0\right)$ versus time.

Hydrogen peroxide (H₂O₂) formation during galvanostatic degradation was monitored spectrophotometrically (UV-Vis) using the titanium (IV) oxalate method [47]. The titanium reagent was prepared by dissolving 0.4425 g of potassium titanium (IV) oxalate dihydrate (RhB, Sigma-Aldrich, Merck KGaA, Burlington, VT, USA) in 3.4 mL conc. H₂SO₄ and 8.6 mL deionized water (total 12 mL). At predetermined time intervals (0–120 min), 250 μL aliquots were withdrawn from the reactor and mixed with 125 μL of the titanium reagent. The absorbance (A) of the resulting yellow pertitanic acid complex was immediately recorded at a fixed wavelength of 400 nm.

To enable direct quantitative comparison of H₂O₂ generation dynamics across the three cathode systems, absorbance values were normalized to a common reference. Because significant peroxide accumulation was observed exclusively with the CF cathode, the maximum absorbance recorded in that system, attained at the end of the 120 min treatment, was selected as the reference value, A_ref. The normalized hydrogen peroxide concentration, defined as [H₂O₂]_norm = A/A_ref, is directly proportional to the actual H₂O₂ concentration. It expresses the peroxide accumulated in each cathode system as a fraction of the maximum level generated by the CF cathode over the full experimental duration.

2.4. Performance Metrics and Evaluation

The three cathodes were deliberately retained in their commercially available form factors: stainless steel as a flat plate (27.0 cm²), carbon felt as a porous fabric (6.0 cm²), and TiO₂ as a mesh (2.0 cm²). This variation reflects the practical reality that different electrode materials are manufactured and deployed in inherently different form factors. The large geometric area of the SS cathode was intentionally preserved to operate it at a low cathodic current density (1.11 mA cm⁻² at 30 mA total current), thereby minimizing parasitic reactions and the formation of corrosion products that could interfere with UV-Vis spectrophotometric monitoring of RhB. The CF and TiO₂ cathodes were used in their own characteristic architectures, which are inherent to their practical application. Thus, the comparison reflects the integrated effect of both material identity and its realistic geometric implementation, which is precisely the system-level assessment this methodology is designed to provide.

To evaluate this integrated performance from both applied and fundamental perspectives, a multicriteria set of metrics was calculated. The total electrical energy consumption per experiment, $E_{\mathrm{total}}$ (Wh), was calculated by integrating the instantaneous power over the operational time required to achieve >90% RhB degradation (Equation (3)). This provides energy metrics relevant to practical, target-driven operation.

$$E_{\mathrm{total}}=\int \mathrm{I}\cdot E(t)dt$$

(3)

where $\mathrm{I}$ is the constant applied current (A) and $E\left(t\right)$ is the time-dependent cell voltage (V). The value of $E_{\mathrm{total}}$ underpins two key application-oriented metrics: the volumetric energy consumption, $E_{\mathrm{vol}}$ (kWh m⁻³), representing the energy consumed to treat a unit volume (Equation (4)); and the specific energy consumption, $E_{\mathrm{sp}}$ (Wh g⁻¹), representing the energy required to degrade a unit mass of pollutant (Equation (5)):

$$E_{\mathrm{vol}}={\displaystyle \frac{E_{\mathrm{total}}}{\mathrm{V}}}$$

(4)

$$E_{\mathrm{sp}}={\displaystyle \frac{E_{\mathrm{total}}}{\Delta {\mathrm{m}}_{\mathrm{RhB}}}}$$

(5)

where $\mathrm{V}$ is the solution volume (m³), and $\Delta {\mathrm{m}}_{\mathrm{RhB}}$ is the mass of RhB degraded (g).

The total electrical charge passed through the system was ${\mathrm{Q}}_{\mathrm{total}}=\mathrm{I}\cdot t$ (C). The Faradaic efficiency (FE) for the oxidation of RhB, assuming a 4-electron transfer for RhB decolorization [48,49,50], was calculated as follows:

$$\mathrm{FE}={\displaystyle \frac{\mathrm{n}\mathrm{F}\cdot (\Delta {\mathrm{m}}_{\mathrm{RhB}}/{\mathrm{M}}_{\mathrm{RhB}})}{{\mathrm{Q}}_{\mathrm{total}}}}\times 100\%$$

(6)

where $\mathrm{n}$ = 4, $\mathrm{F}$ is Faraday’s constant (96,485 C mol⁻¹), and ${\mathrm{M}}_{\mathrm{RhB}}$ is the molar mass of Rhodamine B (479.02 g mol⁻¹).

To explicitly account for geometric differences among the cathodes and to directly assess their contribution to bulk degradation efficiency, a cathodic current utilization efficiency, CUE (h⁻¹ per mA cm⁻²), was defined as a metric that normalizes performance to cathode area (Equation (7)). It relates the observed degradation rate constant ($\mathrm{k}$) to the applied cathodic current density (${\mathrm{j}}_{\mathrm{cathodic}}$), which varied with the geometric area of each cathode.

$$\mathrm{CUE}={\displaystyle \frac{\mathrm{k}}{{\mathrm{j}}_{\mathrm{cathodic}}}}$$

(7)

TOC analysis was performed using the NPOC mode on a TOC-LCPH analyzer (Shimadzu Co., Kyoto, Japan). The Faradaic efficiency for mineralization was calculated as follows:

$${\mathrm{F}\mathrm{E}}_{\mathrm{T}\mathrm{O}\mathrm{C}}={\displaystyle \frac{4\mathrm{F}\cdot (\Delta {\mathrm{m}}_{\mathrm{C}}/{\mathrm{M}}_{\mathrm{C}})}{{\mathrm{Q}}_{\mathrm{total}}}}\times 100\%$$

(8)

where $\Delta {\mathrm{m}}_{\mathrm{C}}$ is the mass of organic carbon removed (g), ${\mathrm{M}}_{\mathrm{C}}$ is the atomic mass of carbon (12.01 g mol⁻¹), and the factor 4 reflects the stoichiometric requirement of four electrons per carbon atom mineralized (C → CO₂).

2.5. Performance Score Calculation

To enable a quantitative comparison of cathode materials, a multi-parameter performance scoring system was developed. The ten selected parameters were chosen to capture three complementary dimensions of system behavior: (i) intrinsic electrochemical activity (CV@2 V, LSV@1.75 V, and ∆E), (ii) degradation kinetics and extent ($\mathrm{k}$, degradation %, and CUE), and (iii) process efficiency and economics (${\mathrm{E}}_{\mathrm{ave}}$, ${\mathrm{E}}_{\mathrm{vol}}$, ${\mathrm{E}}_{\mathrm{sp}}$, and FE). The parameters were drawn exclusively from measurable or readily calculable quantities within a standard electrochemical oxidation experiment. These ensure that the framework remains accessible and transferable without requiring specialized instrumentation, but the parameter set is not intended to be exhaustive or prescriptive. Depending on the specific application, researchers may modify metrics, for instance, replacing degradation % with TOC or COD removal, or adding H₂O₂ yield when peroxide-mediated oxidation pathways are relevant [51]. The methodology itself is thus separable from the specific metrics used in this demonstration case, and its structure accommodates adaptation to different experimental capabilities and treatment objectives.

Parameters were normalized to a 0–10 scale based on their relative performance across the three cathodes. The normalization procedure ensures that all parameters contribute equally to the total score on a common scale. For parameters where higher values indicate better performance (CV @2V and LSV @1.75 V, ∆E, $\mathrm{k}$, degradation percentage, CUE, and FE), the normalized score for cathode $i$ and parameter $j$ was calculated as follows:

$${\mathrm{N}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}_{i,j}={\displaystyle \frac{{\mathrm{X}}_{i,j}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n},j}}{{\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x},j}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n},j}}}\times 10$$

(9)

where ${\mathrm{X}}_{i,j}$ is the raw (original) value, and ${\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n},j}$ and ${\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x},j}$ are the minimum and maximum values for that parameter across the three cathodes.

For parameters where lower values are better (${\mathrm{E}}_{\mathrm{ave}}$, ${\mathrm{E}}_{\mathrm{vol}}$, ${\mathrm{E}}_{\mathrm{sp}}$), the inverse relationship was applied:

$${\mathrm{N}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}}_{i,j}={\displaystyle \frac{{\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x},j}-{\mathrm{X}}_{i,j}}{{\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x},j}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n},j}}}\times 10$$

(10)

The unweighted total score for each cathode was then obtained by summing all ten normalized parameters (Equation (11)), yielding a maximum possible score of 100. Cathodes are ranked in descending order of total score (#1—best, #3—worst).

$${\mathrm{Total} \mathrm{score}}_{i, unweighted}={\displaystyle \sum _{j=1}^{10}}{\mathrm{Normalized} \mathrm{score}}_{i,j}$$

(11)

In the baseline scoring system (Equation (11)), all parameters were assigned equal weight. The equal-weight approach ensures methodological neutrality and transparency, avoiding a priori assumptions about the relative importance of individual metrics, and enables straightforward replication by other researchers. However, in a real-world engineering context, not all parameters carry equal significance, and practical system design is inherently application-specific. Treating low-concentration wastewater in continuous flow, for example, might prioritize energy consumption and throughput, while batch treatment of highly concentrated effluents might emphasize degradation extent and Faradaic efficiency. Different applications would therefore prioritize different metrics, and no single weighting scheme is universally applicable [38].

To assess the sensitivity of cathode ranking to such application-specific priorities, a weighted scoring scheme was applied as a demonstration case. In this scenario, weights were assigned to reflect a typical practical design context in which operational cost and treatment capacity are paramount. Consequently, specific energy consumption (${\mathrm{E}}_{\mathrm{sp}}$) was given the highest weight (3×), followed by degradation rate constant $\mathrm{k}$ (2×), average cell voltage (${\mathrm{E}}_{\mathrm{ave}}$) and Faradaic efficiency (1.5× each), while purely diagnostic parameters (LSV and CV activities) were down-weighted (0.5×). All other parameters retained unit weight. The weighted raw score was calculated as in Equation (12), where $w_j$ represents the weight for parameter $j$.

$${\mathrm{Total} \mathrm{score}}_{i,\mathit{weighted} }={\displaystyle \sum _{j=1}^{10}}{w}_j\cdot {\mathrm{Normalized} \mathrm{score}}_{i,j}$$

(12)

The maximum possible total (raw) weighted score was $\sum w_i\times 10=130$. For direct comparison with the unweighted scores, these values were normalized to a 0–100 scale, according to Equation (13). This normalization preserves the relative contributions of each parameter while enabling a direct comparison of cathode performance under different weighting scenarios.

$${\mathrm{Total} \mathrm{score}}_{i,weighted}^{normailzed}={\displaystyle \frac{{\mathrm{Total} \mathrm{score}}_{i,\mathit{weighted} }}{\sum w_j\times 10}}\times 100$$

(13)

It should be noted that the unweighted scoring serves as a transparent baseline for comparison rather than a definitive ranking. Given the normalization procedure and the absence of replicate-based uncertainty estimates, score differences of less than approximately 5 points on the 0–100 scale should be considered indistinguishable. The weighted scoring, which incorporates application-specific priorities, is the primary decision-support output of the framework.

Data processing, kinetic analysis (including Tafel analysis, reaction onset determination via derivatives, and pollutant degradation kinetics), and visualization were performed using MATLAB R2023a (MathWorks, Natick, MA, USA).

3. Results

Table S1 compiles all electrochemical parameters, kinetic data (with corresponding regression statistics), and performance metrics for each cathode system.

3.1. Cathode-Dependent Modulation of Anodic Response

The fundamental electrochemical response of the PbO₂@TiO₂ anode was first characterized in a three-electrode configuration paired with each cathode. Cyclic voltammetry revealed that the magnitude of the anodic current was strongly dependent on the counter electrode material (Figure 1a). The system with the SS cathode yielded the highest anodic current density across the scanned range. At 2.0 V, the current density reached 27.5 mA cm⁻², which was approximately 54.5% and 45.5% greater than with the CF and TiO₂ cathodes, respectively (Table S1).

This enhancement is attributed to the lower overall cell resistance and favorable interfacial properties of SS, rather than its hydrogen-evolution kinetics. Cathodic LSV revealed a significantly more negative HER onset (E_HER) for SS (−0.77 V vs. Ag/AgCl) compared to CF (−0.23 V) and TiO₂ (−0.42 V) (Figure 1b, Figure S1a). While a more negative onset might initially appear disadvantageous, it in fact provides a wider electrochemical window in which the cathode operates below the HER threshold, suppressing unwanted hydrogen evolution and its associated energy consumption during anodic oxidation. Once HER is initiated, however, SS exhibits the lowest Tafel slope (559.3 mV dec⁻¹), indicating a steeper current–potential dependence that contributes to its stable performance under dynamic conditions (Table S1, Figure S2).

Anodic LSV (Figure 1c and Figure S1b) and subsequent Tafel analysis provided a critical mechanistic insight. While the current magnitude was cathode-dependent, the fundamental kinetic characteristics of the anodic reaction remained consistent. The Tafel slopes were determined to be 487.0, 321.5, and 381.7 mV dec⁻¹ for the CF, SS, and TiO₂ systems, respectively (Table S1, Figure S3). Slopes within this range are characteristic of a multi-step OER on metal oxide anodes where the initial discharge of water to form an adsorbed ^●OH intermediate is rate-limiting [52]. The observed variation of approximately 60–100 mV dec⁻¹ suggests subtle differences in the interfacial environment or surface intermediate coverage, potentially influenced by pH changes or products from the different cathodic reactions, but does not indicate a change in the core reaction pathway. Furthermore, the onset potentials (E_OER) for significant anodic current were virtually identical at ~1.36 V vs. Ag/AgCl for all configurations (Figure 1c and Figure S1b), confirming that the thermodynamics of initiating the anodic process are unaltered by the distant cathode. To enable a direct comparison with steady-state behavior, a separate CV scan was recorded within the same, lower potential range used for anodic LSV; these comparative results are shown in Figure 1d.

3.2. Activity of the Anode and Potential Window for Electrochemical Degradation

Figure 2a presents a detailed comparison of three distinct anodic activity metrics evaluated at characteristic potentials for each cathode material: (i) LSV @1.75 V—current density derived from anodic LSVs at 1.75 V vs. Ag/AgCl, representing steady-state anodic activity; (ii) CV @1.75 V—current density derived from comparative CVs at 1.75 V vs. Ag/AgCl, capturing cyclic voltammetry response; and (iii) CV @2.0 V—CV activity at 2.0 V vs. Ag/AgCl, indicating high-potential performance (Table S1). Results suggest that CV activity effectively reflects the steady-state anodic behavior captured by LSV measurements. This is demonstrated by the fact that the activities measured at 1.75 V vs. Ag/AgCl, LSV @1.75 V, and CV @1.75 V exhibit very similar values, with an average deviation of only 9.8%. The minimal variation across three markedly different cathode configurations (varying 13.5-fold in area) strongly indicates that the anodic response is governed primarily by the working electrode potential, not by the counter electrode geometry. The small residual variation likely originates from differences in ohmic drop and local mass transport, which practically can be inseparable from the cathode material’s physical implementation. Nevertheless, the close agreement substantiates that the three-electrode measurements provide a reliable basis for comparing anodic activity across the systems studied.

Notably, LSV @1.75 V and CV @2.0 V exhibit a consistent trend across all cathode materials, with both metrics showing the same relative performance ranking: SS > TiO₂ > CF. Given this agreement and their complementary nature in characterizing different aspects of electrode performance, LSV @1.75 V (steady-state) and CV @2.0 V (high-potential) will be employed as the primary activity parameters for subsequent system performance evaluation.

Figure 2b illustrates the electrochemical windows (ΔE) defined by the potential difference between the oxygen evolution reaction (E_OER) and hydrogen evolution reaction onset (E_HER) for the three cathode configurations. A wider ΔE reflects a higher HER overpotential at the cathode, which delays cathodic hydrogen evolution and thereby expands the stable operating range of the system before cathodic limitations arise. This is particularly relevant for indirect oxidation pathways involving ^●OH radicals, as these typically require high anodic potentials. Under such conditions, premature HER at the cathode could otherwise limit system performance. Among the tested materials, SS exhibits the widest electrochemical window (ΔE = 2.14 V), followed by TiO₂ (ΔE = 1.77 V) and CF (ΔE = 1.58 V). For easier comparison with literature values reported in different reference scales, all onset potentials converted to both reversible hydrogen electrode (RHE) and standard hydrogen electrode (SHE) scales are provided in Table S2.

3.3. The Activity-Efficiency Paradox in Pollutant Degradation

Under applied, galvanostatic conditions, an unexpected and intriguing performance paradox appeared (Figure 3a). Despite operating at an identical anodic current density, the RhB degradation rates differed dramatically. The system with the CF cathode achieved the fastest degradation, with a pseudo-first-order rate constant $\mathrm{k}$ of 2.774 h⁻¹, corresponding to a half-life of merely 15 min. This was 3.1 times and 4.9 times faster than the rates achieved with the SS (0.884 h⁻¹) and TiO₂ (0.566 h⁻¹) cathodes, respectively (Figure 3b). This outcome reverses the performance hierarchy indicated by the voltametric activity, in which SS demonstrated superior results.

The key to resolving this paradox lies in the operational cathodic current density (${\mathrm{j}}_{\mathrm{cathodic}}$). Due to the different geometric areas of the cathodes, the fixed total current of 30 mA resulted in vastly different ${\mathrm{j}}_{\mathrm{cathodic}}$ values: 5.00 mA cm⁻² for CF, 1.11 mA cm⁻² for SS, and 15.00 mA cm⁻² for TiO₂. The CUE metric was introduced to evaluate the efficiency of translating this cathodic current into bulk chemical degradation. Analysis revealed that the SS cathode achieved the highest CUE of 0.796 h⁻¹ per mA cm⁻² (Figure 3c), indicating superior efficiency in utilizing cathodic current. The superior voltage stability and low ohmic loss of the SS cathode, evidenced by its HER kinetics and most stable chronopotentiometric profile (Figure 4a), minimized the cathodic contribution to the total cell voltage. This allowed a greater fraction of the applied potential to drive the anodic process, resulting in the lowest operational cell voltage observed during galvanostatic degradation (Table S1, Figure 4a). At higher cathodic current densities, mass-transport limitations likely become more pronounced for CF and TiO₂.

The CF cathode maintained a robust CUE of 0.555 h⁻¹ per mA cm⁻², despite its fivefold higher operating ${\mathrm{j}}_{\mathrm{cathodic}}$ compared to SS. In stark contrast, the TiO₂ cathode, forced to operate at an extreme current density, suffered from severe mass transport limitations due to extensive H₂ bubble coverage [53,54], which blocks active sites and increases ohmic resistance [55], resulting in a negligible CUE of 0.038 h⁻¹ per mA cm⁻². Thus, the paradox can be rationalized: the SS cathode is advantageous for charge utilization efficiency, reflecting its stable operation and high HER overpotential, which minimizes energy losses. The CF cathode is optimized for maximum absolute chemical yield, a capability potentially enhanced by its porous structure, which may facilitate the 2-electron oxygen reduction reaction (ORR) to H₂O₂ [56,57].

A control experiment confirmed that adsorption of RhB onto the CF cathode reached approximately 10% within the first 2.5 min and plateaued (Figure S4). Its contribution is rapid, saturable, and small relative to the total degradation extent achieved (more than 90% in 45 min). Since CF represents the most adsorption-prone material, adsorption onto the SS and TiO₂ cathodes can reasonably be assumed negligible. The reported degradation rate constants and CUE values therefore predominantly reflect net electrochemical performance without deconvoluting every individual removal mechanism (adsorption, electrooxidation, and cathodic H₂O₂ generation).

To experimentally verify the hypothesized contribution of cathodic H₂O₂ generation to the observed degradation kinetics, the time-resolved accumulation of hydrogen peroxide was monitored spectrophotometrically in all three cathode configurations during galvanostatic RhB degradation (Figure 4b). The three cathode systems displayed fundamentally different H₂O₂ generation profiles. In the SS and TiO₂ configurations, [H₂O₂]_norm remained at or near the detection limit throughout the experiment, reaching final values of only 3.6% and 14.3% of the CF reference level, respectively. This indicates the absence of sustained, catalytically significant H₂O₂ production on either electrode.

In contrast, the CF cathode exhibited pronounced H₂O₂ accumulation with a distinct three-stage temporal profile: an initial lag phase (0–10 min), followed by rapid accumulation (10–45 min) that coincided with the period of fastest RhB degradation (Figure 3a), and a final slow accumulation phase (45–120 min) after near-complete pollutant removal. By the end of the experiment, the CF system had generated 28-fold and 7-fold more H₂O₂ than the SS and TiO₂ systems, respectively. The observed temporal profile is mechanistically consistent with the in situ generation of H₂O₂ via the two-electron oxygen reduction reaction (ORR) at the porous carbon cathode. The initial lag phase likely reflects the time required to accumulate sufficient dissolved O₂, produced anodically via parasitic OER, for subsequent cathodic reduction. The rapid accumulation during active pollutant degradation suggests that peroxide may serve as a supplementary oxidant, either directly or through secondary reactive species, while the transition to slow accumulation after RhB removal indicates that H₂O₂ behaves predominantly as a stable end-product in the absence of reactive substrate.

3.4. Comprehensive Energy Assessment

The analysis of energy in the galvanostatic experiments (

Table 1. Key operational, energy, and efficiency metrics for galvanostatic RhB degradation on the PbO₂@TiO₂ anode paired with CF, SS, and TiO₂ cathodes at 30 mA constant current.

Cathode	${\mathbf{E}}_{\mathbf{a}\mathbf{v}\mathbf{e}}$, V	${\mathbf{E}}_{\mathbf{vol}}$, kWh m−3	${\mathbf{E}}_{\mathbf{sp}}$, Wh g−1	FE, %
CF	3.353	1.245	251.4	0.295
SS	3.091	3.097	734.9	0.092
TiO2	3.673	3.668	1073.0	0.077

) highlights the practical significance of the kinetic results and reveals a profound, unifying limitation. The CF system exhibited superior degradation kinetics, achieving the shortest treatment time (~90% removal in ~60 min, Figure 3a) and the lowest volumetric energy consumption (1.245 kWh m⁻³). Although it operated at a higher cell voltage (3.353 V), CF required the least specific energy to destroy a unit mass of RhB (251.4 Wh g⁻¹), establishing it as the most efficient cathode for targeted contaminant removal. In contrast, the SS cathode delivered the lowest average cell voltage (3.091 V) despite displaying the most negative HER onset potential (Figure 1b) and consequently the highest kinetic overpotential for hydrogen evolution (Figure S2). This apparent contradiction indicates that under sustained galvanostatic operation, factors such as electrode conductivity, interfacial contact resistance, and bubble-induced resistance dominate the overall cell voltage, outweighing the influence of intrinsic cathode kinetics. Therefore, when scaling up under constant-current conditions, SS may offer enhanced energy efficiency due to its advantageous ohmic and interfacial characteristics.

However, the most critical finding is the extremely low apparent Faradaic efficiency (FE < 0.3%) across all systems at 30 mA, indicating that over 99.7% of the charge was consumed by unwanted side reactions. This calculation assumes a 4-electron transfer ($\mathrm{n}$ = 4) for RhB decolorization [58], an assumption that, while commonly employed, carries inherent uncertainty: RhB degradation proceeds through multiple sequential steps (de-ethylation, chromophore cleavage, ring opening), and the effective number of electrons transferred per decolorized molecule is neither an integer nor constant across conditions. The reported values are therefore best interpreted as apparent Faradaic efficiencies, suitable for comparative rather than absolute purposes. Since complete mineralization would require substantially more electrons per molecule, these values also represent an upper-bound estimate. Nevertheless, even if the true n were an order of magnitude larger, the FE would remain below 3%, and the relative comparison among cathode systems, where $\mathrm{n}$ is applied consistently, remains valid.

To assess whether the low decolorization-based Faradaic efficiencies genuinely reflect limited mineralization, total organic carbon (TOC) analysis was performed on the PbO₂@TiO₂/CF system. After 120 min, TOC removal reached 63%, corresponding to a mineralization ${\mathrm{F}\mathrm{E}}_{\mathrm{T}\mathrm{O}\mathrm{C}}$ on the order of 2%, roughly seven times higher than the decolorization-based value, yet still confirming that over 98% of the charge is consumed by OER. This indicates that a fraction of the charge contributes to oxidation beyond chromophore cleavage, but that anodic selectivity limitations dominate regardless of the diagnostic metric employed.

Electrochemical diagnostics clearly show the connection between the operational conditions used and the observed low efficiency: when operating at high current (30 mA) in galvanostatic mode, the anode potential is forced significantly positive of the OER onset to sustain the current, resulting in water oxidation becoming the primary reaction.

3.5. The Cost of High-Current Operation: Holistic Performance Under OER Dominance

A comprehensive evaluation of the cathode materials conducted at an intentionally elevated level of 30 mA provides quantifiable insight into the performance trade-offs that emerge when the system is forced to operate extensively within the OER-dominated regime (Figure 5). Under the equal-weight scoring scheme, the radar chart (Figure 5a) and aggregated scores (blue bars in Figure 5b, Table S1) reveal complementary profiles: the SS cathode dominates in fundamental electrochemical metrics and efficiency parameters (anodic activity, ΔE, CUE, and ${\mathrm{E}}_{\mathrm{ave}}$) due to its lower cell voltage and superior voltage stability during prolonged operation, reflecting low interfacial degradation and minimal bubble-induced resistance, while the CF cathode leads in applied performance metrics (degradation rate, degradation percentage, ${\mathrm{E}}_{\mathrm{vol}}$, ${\mathrm{E}}_{\mathrm{sp}}$, and FE), likely aided by its porous structure, albeit at a higher operational voltage. Aggregation of these normalized scores yields total performance rankings of 62.8/100 for SS, 62.3/100 for CF, and 10.9/100 for TiO₂ (Figure 5b, Table S1).

The 0.5-point difference between CF and SS in the unweighted ranking is within the resolution limit of the methodology and indicates that, in the absence of application-specific priorities, these two materials are essentially indistinguishable. This equivalence is instructive in itself: without a defined design objective, there is no universal “best” cathode. The analysis rather offers quantitative guidelines for rational selection of the cathode based on the primary system objective: Choose the CF cathode if minimizing operational energy cost per volume of treated wastewater is the priority (process economics). Choose the SS cathode when the goal is to operate safely at high anodic potentials close to the OER onset (electrochemical stability). The operational cathodic current density, a parameter directly controlled by the anode-to-cathode area ratio, is thus revealed as a critical, adjustable design variable.

To assess the robustness of this ranking under real-world design priorities, a sensitivity analysis was performed by applying differential weights to the ten performance parameters. The weighted scenario was designed to reflect an application context in which operational costs and treatment throughput are the primary concerns, acknowledging that different applications would warrant different weighting schemes [59]. As described in Section 2.5, specific energy consumption (${\mathrm{E}}_{\mathrm{sp}}$) was weighted most heavily (3×), followed by degradation rate constant $\mathrm{k}$ (2×), average cell voltage (${\mathrm{E}}_{\mathrm{ave}}$) and Faradaic efficiency (1.5× each). LSV and CV activities, which probe the intrinsic anodic response and are largely insensitive to cathode material as shown in Section 3.2, were down-weighted (0.5×). All other parameters retained unit weight.

The results reveal a dramatic shift in rankings when practical considerations are prioritized (orange bars in Figure 5b, Table S3). While SS and CF were nearly indistinguishable in the unweighted analysis (62.8 vs. 62.3 points), CF emerges as the preferred option under practical weighting, achieving 77.0 points compared to 51.8 for SS. This 25.2-point margin reflects CF’s superior performance in degradation kinetics ($\mathrm{k}$) and competitive energy consumption (${\mathrm{E}}_{\mathrm{sp}}$), both of which carry high weights. Conversely, SS loses its advantage as its strengths in diagnostic parameters (anodic activity, ΔE) are down-weighted. TiO₂ remains the lowest-ranked cathode under both schemes, with its weighted score dropping to 5.4 points due to poor performance across all practically significant parameters. It should be noted that TiO₂ is employed here exclusively as a cathode, operating under HER conditions. The well-documented oxidative surface chemistry of TiO₂, including its capacity for adsorption and charge-transfer-mediated oxidation of aromatics [60,61], is relevant to its behavior as an anode or photocatalyst but does not manifest under the cathodic polarization conditions applied in this study.

The ten evaluation parameters can be broadly grouped into three families: electrochemical diagnostics (LSV, CV, and ΔE), degradation kinetics ($\mathrm{k}$, degradation %, and CUE), and energy economics ${\mathrm{E}}_{\mathrm{ave}}$ (${\mathrm{E}}_{\mathrm{vol}}$, ${\mathrm{E}}_{\mathrm{sp}}$, and FE). The convergence of CF and SS under equal weights reflects a balance among these families, in which SS’s advantages in electrochemical diagnostics and cell voltage compensate for CF’s superiority in kinetics and energy efficiency. Under an energy-prioritized weighting scheme, where ${\mathrm{E}}_{\mathrm{sp}}$ and ${\mathrm{E}}_{\mathrm{ave}}$ receive the highest weights, the gap between CF and SS would narrow as SS’s lower cell voltage becomes more influential. Conversely, a kinetics-prioritized scheme would further widen CF’s lead, reflecting its 3.1-fold higher degradation rate constant. The sensitivity of the ranking to these alternative scenarios confirms that the multicriteria framework is not prescriptive; it transparently translates user-defined design objectives into a quantitative material ranking, and the outcome depends systematically on which performance dimensions are prioritized.

The analysis clarifies the practical consequence of the OER bottleneck: when the anode operates well above its onset potential, it essentially acts like a water electrolyzer. In this scenario, the choice of cathode only affects voltage and, consequently, the energy cost.

3.6. Circumventing the OER Bottleneck: Operational Optimization and Mechanistic Validation

To directly test the hypothesis that dominant water oxidation at 30 mA decreases overall process efficiency, controlled comparative experiments were performed under moderate conditions (Figure 6a). Both the CF and SS systems were evaluated at reduced current and elevated initial RhB concentration (Tables S4 and S5). For CF, reducing the current from 30 to 8 mA and increasing the concentration from 5 to 10 ppm preserved rapid degradation kinetics ($\mathrm{k}$ from 2.774 to 2.253 h⁻¹) while cutting specific energy consumption by 91% (from 502.9 to 43.1 Wh g⁻¹) and increasing Faradaic efficiency 7.3-fold (from 0.149% to 1.084%). For SS, a more aggressive optimization, 30 to 2.5 mA, 5 to 20 ppm, yielded a 50-fold increase in Faradaic efficiency (from 0.092% to 4.584%) and a 98% reduction in specific energy consumption (from 734.9 to 9.7 Wh g⁻¹), while the degradation rate constant improved slightly (from 0.884 to 0.992 h⁻¹). Both systems achieved these gains at substantially lower cell voltages, confirming that the OER bottleneck can be mitigated through operational rather than material design. This stark contrast underscores that the key to energy-efficient EO lies not in maximizing current but in optimizing the operational window to maximize charge utilization toward the target reaction.

The differing optimization profiles of the two systems reflect their distinct degradation architectures. CF retained a clear kinetic advantage but showed a more modest FE improvement, consistent with its dual-pathway mechanism: at high current, parasitic OER supplies dissolved O₂ for cathodic H₂O₂ generation (Section 3.3), so reducing the current limits both anodic OER competition and the cathodic O₂ supply. In SS, where H₂O₂ generation is negligible, efficiency gains from current reduction translate directly into improved charge utilization.

The CUE metric was introduced (Section 2.4) to normalize degradation kinetics for differences in cathodic current density arising from the varying geometric areas of the three cathodes. To empirically validate CUE, the SS cathode was additionally tested at a reduced area matching CF (6.0 cm²) under baseline conditions (30 mA, 5 ppm RhB; Table S6). Reducing the SS area decreased CUE 5.5-fold (from 0.796 to 0.145 h⁻¹ per mA cm⁻²), while the average cell voltage remained nearly unchanged, indicating that the low operating voltage of SS is an intrinsic material property.

When compared at equal geometry and equal cathodic current density, CF outperformed SS by a factor of 3.8 in both degradation rate constant (2.774 vs. 0.727 h⁻¹) and CUE (0.555 vs. 0.145 h⁻¹ per mA cm⁻²), consistent with CF’s kinetic superiority being a genuine material-specific effect rather than an artifact of geometry and supporting the use of CUE as a meaningful normalized parameter within the multicriteria framework.

The kinetic advantage of CF is mechanistically linked to cathodic H₂O₂ generation. However, H₂O₂ may contribute to degradation either directly or via secondary ^●OH formed through homolytic cleavage. To distinguish between these pathways, radical scavenging experiments were performed on the optimized CF system (10 ppm RhB, 8 mA) using methanol (MeOH, 100 mM), which quenches both ^●OH and SO₄^●−, and tert-butanol (TBA, 250 mM), a selective ^●OH scavenger (Figure 6b). MeOH reduced the degradation rate constant from 2.253 to 0.252 h⁻¹ (88.7% inhibition), while TBA reduced it to 0.282 h⁻¹ (87.5% inhibition). The small difference between the two scavengers indicates a negligible contribution from sulfate radicals. Kinetic deconvolution attributes the degradation to ^●OH-mediated oxidation (87.5%), direct anodic electron transfer (11.3%), and SO₄^●−-mediated oxidation (1.2%). At the end of the 90 min treatment, total degradation reached 96.8%.

The dominance of ^●OH is consistent with the established PbO₂ oxidation mechanism [62], although the scavenging experiment cannot distinguish anodic ^●OH from ^●OH derived from H₂O₂ homolysis. When viewed alongside the H₂O₂ generation data and the geometric control experiments, the results strongly suggest that a significant fraction of the ^●OH flux in the CF system derives from the cathodic H₂O₂ pathway. The SS system, which lacks this pathway, achieves substantially lower degradation rates despite operating via the same anodic ^●OH mechanism. The dual-pathway architecture of CF, anodic ^●OH coupled with cathodic H₂O₂ and its secondary radicals, thus offers a unified mechanistic explanation for its superior performance across all operating regimes.

4. Discussion

The results demonstrate that the counter electrode in electrochemical oxidation acts as an active regulator of system-level performance, primarily through its influence on the hydrogen evolution reaction overpotential, which determines the cell voltage and energy consumption. This observation aligns with earlier EAOP studies documenting that cathode material and geometry influence energy consumption and current efficiency and that operational parameters, especially current density, often govern overall treatment efficiency [18,21]. However, the present methodology extends these findings by quantifying this influence within a unified multicriteria framework that simultaneously accounts for parameters rarely assessed together (degradation kinetics, energy costs, and Faradaic efficiency) and by explicitly demonstrating that cathode optimization cannot overcome the anodic OER bottleneck.

This study offers direct electrochemical evidence that cathode engineering has a secondary effect compared to the anodic oxygen evolution reaction. The OER onset potential of the PbO₂@TiO₂ anode (~1.36 V) provides a fixed thermodynamic benchmark. The catastrophic Faradaic efficiencies observed at high currents are a direct consequence of operating the anode at potentials far exceeding this threshold, a condition that is largely invariant to the distant cathode material. Thus, while the cathode influences energy efficiency, the anode’s OER onset ultimately limits the maximum achievable oxidation efficiency.

The complementary performance profiles of SS and CF cathodes, revealed by the multi-parameter assessment, reflect distinct underlying mechanisms. The chronopotentiometric stability, combined with the lowest operational cell voltage, indicates that SS benefits from minimal interfacial degradation and reduced resistive losses, while its delayed HER onset postpones hydrogen evolution, thereby minimizing H₂-induced mass transport limitations that could hinder ^●OH generation and RhB oxidation. The CF cathode demonstrates a greater absolute degradation rate, attributed to its ability to generate H₂O₂ via cathodic ORR. Radical scavenging experiments further indicate that ^●OH is the dominant reactive species, consistent with a dual-pathway mechanism in which anodically generated ^●OH is supplemented by ^●OH derived from cathodic H₂O₂ homolysis. However, this advantage operates within the same fundamental charge-utilization constraint imposed by the anode, as reflected in the persistently low Faradaic efficiencies for both decolorization and TOC mineralization.

The holistic performance synthesis underscores that there is no universally superior cathode. Instead, the choice represents a strategic decision based on the primary process objective by understanding the OER limitation. If the target pollutant is easily oxidized (i.e., it reacts at relatively low anodic potentials), CF is the preferred cathode because it minimizes energy consumption and maximizes degradation rate. However, if the pollutant demands high anodic potentials close to the OER onset, SS offers a distinct advantage: its higher HER overpotential delays cathodic hydrogen evolution, resulting in a wider stable electrochemical window. This provides a larger safety margin for operating at elevated anodic potentials by ensuring that cathodic hydrogen evolution does not become limiting, allowing the system to maintain stable current even when the anode is driven deep into the OER region. Thus, choosing the optimal cathode depends on the pollutant’s oxidation potential and the intended trade-off between energy efficiency and operational potential range.

The sensitivity analysis further refines this picture by testing the robustness of cathode rankings under practically relevant weighting scenarios. When specific energy consumption (${\mathrm{E}}_{\mathrm{sp}}$) and degradation rate ($\mathrm{k}$) are weighted to reflect their direct impact on operational costs, CF emerges as the clear leader, while SS loses its advantage as its strengths in diagnostic parameters are down-weighted. These results demonstrate that while SS offers a balanced profile across all measured parameters, CF is the superior choice when degradation rate and energy efficiency are the primary design criteria, the most relevant considerations for real-world wastewater treatment applications. The sensitivity analysis thus reinforces the central message: cathode selection must be guided by application-specific priorities, and the equal-weight framework provides a transparent baseline from which such tailored decisions can be made. Finally, a natural refinement of the framework would be the incorporation of formal uncertainty propagation, which would allow meaningful differences in total scores to be distinguished from experimental variability.

These insights have important implications for the advancement of electrochemical oxidation processes. Current findings indicate that further incremental optimization of cathode materials within conventional cell configurations will yield diminishing returns against the dominant OER loss. Consequently, a more innovative approach is required. Future research should concentrate on strategies that address this intrinsic anode–cathode coupling limitation [63,64]. Promising directions include (i) developing anode materials featuring intrinsically higher OER overpotentials to favor pollutant oxidation versus water splitting [65]; (ii) engineering cathodes to function as active components for targeted functions, such as in situ H₂O₂ generation or localized pH regulation [14,16]; (iii) optimizing operational parameters, particularly interfacial current densities, to manage radical production and unwanted reactions [66,67]; (iv) designing new reactor configurations that physically or temporally decouple the primary ^●OH generation zone from the bulk pollutant stream [68]; and (v) employing advanced operational modes like pulsed electrolysis to dynamically regulate surface conditions and enhance charge efficiency [69]. In these integrated system architectures, the cathode may be engineered to balance HER onset while enabling additional synergistic functions, thereby contributing to overall process optimization [70].

5. Conclusions

This study presented a multicriteria methodology for deconvoluting cathode performance from anodic selectivity limits in electrochemical oxidation. Building on the established understanding that cathode material influences EO system performance, this framework provides a systematic integration of electrochemical, kinetic, and economic metrics with a sensitivity analysis that adapts to application-specific design priorities. Applied to three contrasting cathode materials (CF, SS, and TiO₂), paired with a PbO₂ anode, the framework integrated conventional electrochemical diagnostics, a ten-parameter assessment spanning activity, efficiency, and economics, and a sensitivity analysis prioritizing operational metrics.

The deconvolution yielded two distinct insights. First, the cathode material measurably governs system-level performance through trade-offs between degradation rate and energy consumption: SS minimized cell voltage, while CF maximized degradation kinetics. Sensitivity analysis, reflecting a practical design scenario in which operational costs are paramount, confirmed CF as the optimal choice, demonstrating how the framework adapts to application-specific priorities. Second, and more critically, the analysis isolated a universal anodic limitation: across all cathode configurations, the overwhelming majority of applied charge was consumed by parasitic oxygen evolution, constraining Faradaic efficiencies to near-negligible levels regardless of cathode identity.

The central conclusion is that cathode engineering, while necessary, is inherently insufficient to overcome the OER bottleneck. Adjusting the cathode modulates cost and reaction yield but cannot resolve the fundamental anodic selectivity problem. The methodology presented here provides a transferable diagnostic protocol for evaluating electrochemical oxidation systems, with both the parameter set and weighting scheme designed to be adapted rather than adopted prescriptively. Its application to the present case demonstrates that meaningful progress in EO efficiency will require integrated system designs that actively circumvent OER, rather than incremental single-electrode optimization within conventional configurations.