# Evaluation of graphite felt activation and aging by electroanalysis at microfiber electrodes

Ranine El-Hage, Vincent Feynerol, Mariela Brites Helú, Rafael Luan Sehn

Canevesi, Alain Celzard, Vanessa Fierro, Liang Liu, Mathieu Etienne

► To cite this version:

Ranine El-Hage, Vincent Feynerol, Mariela Brites Helú, Rafael Luan Sehn Canevesi, Alain Celzard, et al.. Evaluation of graphite felt activation and aging by electroanalysis at microfiber electrodes. Electrochimica Acta, 2024, 485, pp.144033. 10.1016/j.electacta.2024.144033. hal-04788060

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Submitted on 18 Nov 2024

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# Evaluation of graphite felt activation and aging by electroanalysis at microfiber electrodes

Ranine El-Hage<sup>a</sup>, Vincent Feynerol<sup>a</sup>, Mariela Brites Helú<sup>a</sup>, Rafael Luan Sehn Canevesi<sup>b</sup>, Alain Celzard<sup>b,c</sup>, Vanessa Fierro<sup>b</sup>, Liang Liu<sup>a</sup>, Mathieu Etienne<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Electron transfer kinetic

Graphite felt

Microelectrode

AORFB

Activation

## ABSTRACT

Microfiber electrodes derived from graphite felt were used for the evaluation of charge transfer kinetic constants in a ferrocyanide-based electrolyte, which is a model system for redox flow batteries. Cyclic voltammetry experiments were performed in the presence of $10^{-2}$ M or 0.5 M Fe(CN)₆. Within this framework, microfiber electroanalysis allowed for the quantitative assessment of four distinct activation protocols for graphite felt: thermal treatment under CO₂ at 850 °C, nitric acid immersion at 70 °C, exposure to Fenton's reagent at ambient temperature, and treatment with urea under atmospheric conditions at 550 °C. This analytical method enabled a hierarchical categorization of the various activation techniques, aiding in the identification of optimal conditions that yield the highest charge transfer kinetics for the ferrocyanide electrolyte. Additionally, the methodology provided insights into the temporal degradation or aging of these activated materials. These empirical observations are discussed alongside characterization data obtained from X-ray Photoelectron Spectroscopy (XPS), Raman spectrometry, and capacitive current measurements.

## 1. Introduction

Redox flow batteries (RFB) are relevant technologies for renewable energy storage applications due to their ability to cope with the intermittency of these energy sources, as well as their safety and efficiency in storing electrical energy on a large scale for long durations [1–3]. For the moment, all-vanadium redox flow battery (VRFB) is the most developed system with power plants up to 100 MW/400 MWh being installed worldwide [4].

Some recent studies have focused on finding alternative redox couples to vanadium, mainly motivated by discussed issues regarding the cost, scarcity and toxicity of the vanadium electrolyte. The redox systems in question are based on organic compounds such as quinone [5–7], viologen [8–11] or TEMPO derivatives [9–11]. Recently, a study by Di Florio et al. [12] showed that semi-organic RFB could be a valid and environmentally greener alternative to vanadium redox flow battery when choosing a proper production method for the organic electrolyte. This semi-organic redox flow battery consists of an organic negolyte and a metal-based posolyte such as ferrocene or ferrocyanide.

One great advantage of organic or semi-organic redox flow batteries is related to the high reactivity of the redox molecules compared to vanadium species. It is usually considered that the charge transfer kinetics of redox species used in RFB need to be in the order of $10^{-3}$ cm/s for the optimal battery performance. Graphite felts are widely used in RFBs because they allow easy permeation of the electrolyte. Precisely determining the kinetic constants for these materials could be crucial for achieving the highest energy storage efficiency.

However, from the electroanalytical point of view, the intrinsic electrochemical behavior of porous carbon electrode is not easy to analyze [13]. Indeed, the macroporosity and high surface area of these materials can lead to a lack of accuracy in accessing the intrinsic charge transfer kinetics for any redox species, particularly for newly proposed electrolytes. In addition, carbon felt exhibits a variability in its characteristics, either directly from the manufacturer [14,15] or after customized activation protocols, which makes a robust method for accurate quality control of the achieved reactivity useful.

To understand (and thus potentially overcome) the problems usually encountered in the quantitative analysis of electrochemical reactions at

<footnote><sup>a</sup> Université de Lorraine, CNRS, LCPME, Nancy, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, Institut Jean Lamour (IJL), Epinal, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), 75231, Paris</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: mathieu.etienne@cnrs.fr (M. Etienne).</footnote>
the level of porous graphite felts [13,16,17], the present work illustrates the application of electrochemical analysis at the microfiber scale. Several authors have used the microfiber electrode approach as a simplified method (1D geometry) for electrochemical analysis and quantitative evaluation [18–21]. The method we applied here, inspired from the work of Landon-Lane et al. [22–24], Miller et al. [25] and Kroner et al. [26] consists in preparing microfiber electrodes (MFEs) by extracting individual fibers from the corresponding macro-felt. Then, these microfibers are used for the electrochemical analysis in the electrolyte solutions, followed by the simulation of impedance curves and cyclic voltammograms to estimate the charge transfer kinetic constants [13], which can then be used for ranking/evaluating felt activation protocols and their aging.

In fact, the activation of the electrode is an important parameter to consider especially because redox couples are more or less sensitive to surface groups such as carbonyls or oxides. Ferrocyanide is a particularly interesting species because, on the one hand, it is a common posolyte for aqueous organic redox flow batteries [5,27–29] and, on the other hand, its electrochemical behavior has been studied over the years and it was found that the reaction is surface-sensitive [30–32]. Therefore, depending on the activation protocol applied for modifying the characteristics of the graphite felt, or even the side of the material the most exposed to activation [24], the response of the ferrocyanide may vary. Typical activation techniques include, but are not limited to, thermal treatment under air [33,34] or under CO₂ [35], electrochemical treatment [25,36], high- or mild-temperature acid treatment [37,38], as well as methods combining two or more techniques for better results [39–41].

In this work, we propose to study the impact of different activation techniques on the electron transfer kinetic with ferrocyanide, using microfiber electrode electroanalysis. The electrochemical analysis by cyclic voltammetry is then quantitatively analyzed by fitting the cyclic voltammograms (CVs) allowing the charge transfer kinetic constant to be estimated. The comparison of the obtained values allows selecting the most adequate activation for the ferrocyanide redox couple, yielding the highest kinetic constant. In addition, the aging of the different activations with time was also addressed and quantified. The data are discussed while integrating additional characterization from XPS, Raman spectrometry and capacitive measurements.

# 2. Materials and methods

## 2.1. Chemicals and materials

The potassium ferrocyanide solution was prepared by direct dissolution of potassium ferrocyanide trihydrate $K_4[Fe(CN)_6]·3H_2O$ (> 99.5 %, Fluka) and potassium chloride KCl (Omnipur, Merck Millipore) as supporting electrolyte, in ultrapure water. The concentration of active species was fixed at $10^{-2}$ M (unless stated otherwise) in 1 M of KCl. The neutral pH of the studied solution corresponds to a stable electrolyte, used for battery cycling applications [28,42,43]. GFD 4.6 and GFD 4.6 activated graphite felts were purchased from SGL Carbon, Germany.

## 2.2. Activation techniques

Samples of non-activated graphite felt with an area of 2 × 2 cm² were treated with different activation techniques. Each batch of activation was composed of three samples, one for kinetic analysis, one for capacitive measurement and one for spectroscopic analysis (Raman spectroscopy and XPS). After activation, MFEs were prepared from each felt sample and used for the electrochemical analysis. For comparison, MFEs were also prepared from the commercially activated (denoted as CA) and non-activated (denoted as NA, untreated) graphite felts and denoted as CA-MFE and NA-MFE, respectively.

### 2.2.1. Physical activation

Physical activation was performed under a CO₂ atmosphere [35], as follows. The non-activated felt was first heat-treated under a 100 mL/min N₂ flow to the final temperature, 850 °C, at a heating rate of 5 °C/min. This step brings the felt to a temperature at which it can reduce CO₂ gas according to the reaction $CO_2(g) + C(s) → 2 CO(g)$. It is therefore necessary to protect the felt from oxidation during this first stage by means of a continuous flow of inert gas. Once the final temperature was reached, the N₂ flow was replaced by a 50 mL/min CO₂ flow and held for 0.5, 1, 2 or 3 h at 850 °C. During this so-called activation step, some of the original carbonaceous material was progressively removed according to the reaction described above, creating nano-sized pores and increasing the specific surface area. Finally, after this activation time, the CO₂ flow was cut off and replaced again by a 100 mL/min flow of N₂ while the oven was cooled.

### 2.2.2. Activation in solution at low temperature

Two methods of activation, under mild temperature conditions, were tested to activate the graphite felt, activation in nitric acid or with Fenton's reagent.

**Activation with boiling nitric acid.** The felt was activated at 70 °C in 5 M HNO₃ solution for 0.5 and 1 h; the subsequently prepared microfiber electrodes were labelled HNO₃-0.5-MFE and HNO₃-1-MFE, respectively. For both activation times, the felt appeared to be completely wetted by the acid solution. To confirm that the HNO₃ had diffused into the felt, another felt was impregnated with 5 M HNO₃ under vacuum before activation in the same conditions for 1 h. It was referred to as HNO₃-soaked-MFE.

**Fenton's reagent at room temperature.** The experimental protocol for this activation was based on the work of Gao et al. [44], using the parameters they found optimal. After cleaning the felt in EtOH and drying it at 65 °C, the sample was soaked in 20 mL of $FeSO_4$ solution (0.02 M, pH 3), while forcing the solution to impregnate the interior of the entire graphite felt; it was then left to soak for 1 h. After that, 15 mL of 30 % $H_2O_2$ and 5 mL of distilled water were added and the felt was left to react in the mixture for 2 h. Then, a cleaning step in 0.1 M $H_2SO_4$ for 30 min under ultrasonication was necessary to remove the attached $Fe(OH)_3$, and a final rinsing with distilled water was performed until neutral pH was reached. The obtained felt was allowed to dry at 50 °C for 10 h before preparing the MFE, referred as Fenton-MFE.

### 2.2.3. Urea activation

The graphite felt was soaked in a urea solution followed by a thermal treatment for nitrogen doping. The experimental protocol was inspired by the work of Ma et al. [40]. It consisted in immersing the felt in a 75 mL solution containing an amount of urea (in this work: 3, 6, 12 or 15 g) under ultrasonication. The samples were then dried at 80 °C before being heat-treated at 550 °C for 1 h under air atmosphere (heating rate 5 °C/min). Urea-activated MFEs were labelled as Urea-xg-MFE, where x is the amount of urea used in the initial solution (e.g., Urea-3 g-MFE corresponds to a GF sample soaked in a 3 g urea/75 g water solution).

After this first urea series, 3 more samples were prepared to discriminate the effect of urea and heat treatment: (i) a piece of felt was soaked in a solution containing 6 g urea in 75 g water, without subsequent thermal activation, and the resulting MFE prepared was labelled Urea-MFE; (ii) a piece of felt was treated at 550 °C under air flow, without prior urea soaking, and the as-prepared MFE was labelled 550C-air-MFE; (iii) a piece of felt was treated at 550 °C under N₂ flow, without prior soaking in urea, and the as-prepared MFE was labelled 550C-N₂-MFE.

## 2.3. Characterization methods and apparatus

The microfiber electrodes (MFE) were prepared by extracting fibers from a piece of graphite felts, non-activated and activated, and placing them on 3D-printed supports as detailed elsewhere [13]. Prior to
electrochemical analysis, MFEs were soaked in 70 % EtOH for 10 min to remove impurities that might be adsorbed on the surface during the electrode preparation process, and then rinsed with ultrapure water. Repeatability of the measurements was ensured by preparing at least three MFEs, extracted from three different locations on each tested felt. In all estimates of kinetic values, a standard deviation value was calculated to reflect the variability of the measurements.

The electrochemical analysis was performed in a three-electrode setup, as shown in Fig. S1 in the Supplementary Materials, using a VSP-300 potentiostat (Bio-Logic) controlled by EC-Lab® software. The counter electrode was a DPP™ composite carbon electrode. The reference electrode was an Ag/AgCl pseudo-reference, made by coating a silver wire (Ø = 0.25 mm) with an AgCl deposit. Raman spectra were collected at six different measurement spots per sample with a Renishaw inViaTMQontor®microspectrometer equipped with a confocal microscope and an Olympus X50 objective. A 532 nm exciting radiation was used with a laser power of 1 %. X-Ray Photoelectron Spectroscopy (XPS) was performed using a KRATOS Axis Ultra X-Ray Photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromated AlKa X-Ray source (hg=1486.6 eV) operated at 150 W.

The capacitance of the graphite felts can be estimated by the method of capacitive current measurement. The experimental setup was a customized PVC cell, a sample of graphite felt of 1 cm diameter (working electrode) was positioned inside a 3D-printed frame so that the WE was centered between two larger pieces of felt constituting the counter electrodes, with their respective centers aligned on a horizontal line. The experiment consists here in plotting cyclic voltammograms with the graphite felt disk in 1 M KCl supporting electrolyte, in a potential range where no faradic reaction takes place, between -0.1 and 0.1 V (vs. Ag/AgCl wire) at different scan rates. In this work, 7 scan rates were used: 10, 20, 50, 75, 100, 150 and 200 mV/s. Then, by plotting the current as a function of the scan rate I = f(v), the capacitance C in Farad can be estimated according to Eq (1):

$$I_{(A)} = C_{(F)} \times \nu_{(V/s)} \quad (1)$$

C can be used for the determination of the electrochemical surface area (ECSA) if the specific capacitance is known. However, the value of specific capacitance can vary from 10 µF/cm² for an untreated felt, to a range between 40 and 100 µF/cm² after activation [13,18,22,25]. For this reason, we preferred not estimating ECSA and extracting only the capacitance from the different materials having the same geometric surface area, i.e. 0.78 cm², the area projected on a plane surface.

## 2.4. Modelling of cyclic voltammograms

The mathematical model used for the simulation of cyclic voltammograms plotted for the microfiber electrodes is based on the model presented by Compton et al. [45], where the electrode is considered as an “infinitely long cylinder” (ignoring the effect of the tip), with a 1D concentration profile in cylindrical coordinates.

The fitting of the experimental cyclic voltammetry data was done by taking the experimental values of *time* and *potential* as input variables, and determining the corresponding current response as a function of two optimized parameters. These parameters are the electron transfer kinetic constant k⁰ and the geometric surface area A, in order to include the measurement error of the fiber lengths as well as the variability of their radii r⁰. The calculated current points at corresponding input potentials were compared to experimental current points using the least square minimization method. The whole fitting procedure was coded with MATLAB® and the model is described in detail in our previous work [13].

The fixed parameters to perform the fitting are the diffusion coefficient D assumed to be the same for the ferrocyanide and the generated ferricyanide and equal to 7.5 × 10⁻⁶ cm²/s, the fibers radius r⁰ = 4.5 × 10⁻⁴ cm taken as an average value from SEM images [13], the formal

potential of the redox system E⁰, = 0.24 V (vs. Ag/AgCl pseudo-reference) determined from cyclic voltammograms plotted with a graphite felt piece and the charge transfer coefficient α = 0.5.

The evaluation of the quality of fitting was done by visual comparison of the fitted and experimental curves (see the detailed procedure in the Supplementary Materials). We would like to clarify that the choice of fixed and free parameters in this work was mainly based on practical concerns of balancing the computation time, fitting quality and reasonable physical meaning. This could be eventually altered in other works.

# 3. Results and discussion

## 3.1. Evaluation of the optimal activation technique

The first experiments were carried out on microfibers from commercially activated (CA-MFE) and non-activated graphite felt (NA-MFE). These results are used as a reference for comparison with the performance obtained on microfiber electrodes prepared from the lab-activated felts. A first step was to compare the experiments in a concentrated ferrocyanide solution (0.5 M in 1 M KCl) and in a diluted solution (10⁻² M in 1 M KCl) for choosing the conditions applicable in a quantitative analysis (Fig. 1). As it was expected, the electrochemical reaction was more rapid and the system more reversible on the activated material (CA-MFE) than on the non-activated (NA-MFE): the curve exhibits the shape of a classical reversible system while the curve plotted on NA-MFE leans towards higher potential values. In fact, the half-wave potential (E₁/₂) increases from 0.34 V on the CA-MFE to 0.61 V on the NA-MFE for the 0.5 M solution (Fig. 1A), and from 0.26 V to 0.61 V in the 10⁻² M solution (Fig. 1B). To further compare the two conditions, the current responses were normalized with the K₄Fe(CN)₆ concentrations, showing significant differences in the electrochemical responses from diluted to concentrated solutions (see Fig. S2 in the Supplementary Materials). It might be related to the concentration dependencies of the diffusion constants or to migration phenomena. For this reason, quantitative analysis and determination of the heterogeneous charge transfer kinetic constants were performed only in the dilute (10⁻² M) ferrocyanide solutions by CV fitting (Fig. 1B – dashed lines) based on the existing model.

The values of the kinetic constant (average of three replicates) are 3.27 × 10⁻³ cm/s (± 1.05 × 10⁻³) for CA-MFE and 3.67 × 10⁻⁵ cm/s (± 5.35 × 10⁻⁵) for NA-MFE (all k⁰ values are reported in Table 1). These values confirm that the material activation has a significant impact on electrochemical ferrocyanide oxidation kinetics. It should be noted that the fitting of the cyclic voltammetry of the non-activated fiber is not optimal as visualized by the shape of the curve and suggested by the value of the standard deviation, showing a lack of accuracy in the exact value of the constant. Nevertheless, the method allows determining a reliable order of magnitude for k⁰, which is seen to be at least 100 times lower with non-activated than with activated material. These values will serve as reference for comparing the effect of different activation protocols in the next sections.

### 3.1.1. Physical activation under CO₂

Physical activation under CO₂ atmosphere aims to enhance the specific surface area of the graphite felt. The presence of CO₂ at high temperature produces micropores [35], thus increasing potentially the amount of active sites on the material and thereby its reactivity. The k⁰ estimated from the raw data are presented in Fig. 2A and Table 1 (Cyclic voltammograms are reported in Fig. S3 in the Supplementary Materials). The reversibility of the electrochemical reaction is improved but in different proportions depending on the activation time. Increasing the treatment time from 0.5 to 2 h leads to a gradual improvement of the charge transfer kinetics for ferrocyanide oxidation, with an increase from 6.7 × 10⁻⁴ cm/s to 3.2 × 10⁻³ cm/s. However, increasing the treatment time to 3 h had a negative impact by decreasing the kinetic of
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** CVs plotted for microfiber electrodes, extracted from (a) commercially activated and (b) non-activated graphite felts, in potassium hexacyanoferrate solution. (A) [K₄[Fe(CN)₆]] = 0.5 M and (B) [K₄[Fe(CN)₆]] = 10⁻² M (compared to their corresponding fitting (dashed curves)), both in 1 M KCl; WE = MFE; RE = Ag/AgCl pseudo-reference wire; CE = graphite composite rod; v = 100 mV/s.</figcaption></figure>

the electron transfer reaction. Although reactivity under CO₂ is better controlled than under air (O₂), excessive exposure to high temperature resulted in deeper surface modification, which is not suitable for a surface-sensitive compound such as ferrocyanide. Therefore, the optimal treatment time under CO₂ at 850 °C appears to be 2 h.

### 3.1.2. Acid treatment in 5 M HNO₃

A low-temperature treatment was also chosen to modify the carbon surface. The concentrated acid at its boiling point can oxidize the carbon surface, creating additional active sites and increasing the specific surface area. Indeed, this activation treatment led to an improvement in kinetics compared to the non-activated fiber. The kinetic constant are presented in Table 1 (cyclic voltammograms are illustrated in Fig. S4 in the Supplementary Materials).

The obtained values confirm that the system is improved compared to NA-MFE but does not reach the performance observed on CA-MFE. Indeed, the optimal treatment time in HNO₃ is 1 h without forced soaking, which leads to a value of k⁰ of 6.94 × 10⁻⁴ cm/s, which can still be considered in the lower range of the kinetics. The acid treatment creates oxygen groups on the surface of the fibers as seen by Di Blasi et al. [46]. The ferrocyanide reaction is reported to be surface-sensitive, but the sensitivity to surface oxides is the subject of contradictory statements, [47][30,31]. In other words, the sensitivity to surface oxides cannot be totally excluded but the limited improvement observed in the k⁰ value could also come from etching of the carbon surface in contact with the acid, leading to an increase in the electrochemically active area.

### 3.1.3. Activation by Fenton's reagent at room temperature

The next treatment was done with Fenton's reagent (CVs are presented in Fig. S5 in the Supplementary Materials) but it was rapidly discarded from the study because the obtained k⁰ value was very low, 1.63 × 10⁻⁶ cm/s. Activation using Fenton's reagent seems to be here detrimental to the oxidation of ferrocyanide, contrary to what was observed previously for the vanadium system [44]. The only benefit that we observed was an increase in current density on the Fenton-activated fiber compared to the non-activated one.

### 3.1.4. Urea activation

The combination of low-temperature soaking in urea and high-temperature treatment could dope the fiber surface with nitrogen atoms and increase the specific surface area of the electrode at the same time. This activation provides the opportunity to evaluate the impact of nitrogen on the electrochemical oxidation of ferrocyanide.

Cyclic voltammograms plotted for fibers extracted from urea treated graphite felts as well as curves fitted by simulation are presented in the Supplementary Materials (Fig. S6 and S7). The shape of the CVs suggests a very fast and reversible kinetic reaction of ferrocyanide on the treated fibers. It is confirmed by the k⁰ values reported in Fig. 2B and

Table 1 with the corresponding standard deviations for the different treatment conditions.

One parameter considered to optimize this treatment was the amount of urea used for impregnating the graphite felt. As shown in Fig. 2B, k⁰ increases with higher amounts of urea but this improvement is not statistically significant as suggested by the standard deviation values of the 4 samples. On the other hand, the measured kinetics are largely higher than those obtained by both the high-temperature activation under CO₂ and the commercial activation, meaning that the presence of nitrogen groups might have an overall positive impact on the subsequent ferrocyanide reaction.

Since the protocol includes heat treatment under air, which is well known to increase the amount of surface oxides and to improve felt properties, different control experiments were applied to discern the role of each step. The results are summarized in Fig. 3. As it could be expected, the immersion of the felt in urea without thermal treatment did not improve the reactivity of the material and on the contrary led to degradation of the electron transfer kinetic when compared to the non-activated graphite felt (see Table 1 and Fig. 3, curve a). The final thermal step is thus the key to activation, by allowing nitrogen fixation or creation of nitrogen-based functional groups on the fiber surface. Similarly, thermal activation under nitrogen atmosphere (Fig. 3, curve b) shows a reactivity slightly lower than the non-activated material. Thermal activation under air is a classical activation protocol and curve c of Fig. 3 illustrates performances comparable to that of the commercially activated felt (see k⁰ values in Table 1), with a reversible ferricyanide/ferrocyanide redox reaction and a half-wave potential E₁/₂ at 0.28 V. This could partly explain the improved performance previously observed with full activation (Fig. 3, curve d), but qualitatively, from the shapes of the curves and the values of E₁/₂ (0.24 V for UME-6 g-MFE), it appears that mixed activation with urea still yields a better response for the ferrocyanide reaction, which is confirmed by the estimated k⁰, that is higher when urea is present on the graphite felt before the thermal treatment. At this stage of the discussion, it is necessary to include additional data related to the prepared materials in an attempt to clarify the role of oxygen, nitrogen, defects and surface area on the electron transfer kinetics.

## 3.2. Raman spectroscopy, capacitance and XPS analysis

### 3.2.1. k⁰ versus defects on the graphite surface

Raman spectroscopy is known to be an adequate technique for the analysis of graphite, given that it is a very polarizable material. In fact, two distinctive bands are obtained in the Raman analysis of carbon at 1st order. At ~ 1360 cm⁻¹ is found the D band, characteristic of the disordered graphitic material and defects in its covalent structure and at ~ 1582 cm⁻¹ is found the G band, characteristic of graphite itself.

The analysis of the Raman spectra can thus be based on a comparison
<caption>**Table 1**</caption>

Summary table of the different activation treatments and their impact on ferrocyanide reactivity, presented in the form of values of intrinsic kinetic constant k⁰ (cm/s) and the I<sub>D</sub>/I<sub>G</sub> ratio of the corresponding Raman analysis, with their respective values of standard deviation (SD), calculated from three repetitions for k⁰ and six measurements for Raman, in addition to the slope of the I = f(v) curves representing the capacitance of the felt. A<sub>fit</sub>/A<sub>meas</sub> is the mean ratio for three microfiber electrodes between the calculated surface area from the fitting of experimental data with the model (A<sub>fit</sub>) and the measured surface area (A<sub>meas</sub>).

<table><thead><tr><th>Activation</th><th></th><th>k<sup>0</sup> (cm/s)<br>(± SD)</th><th>Raman I<sub>D</sub>/I<sub>G</sub><br>(± SD)</th><th>Capacitance (F)</th><th>A<sub>fit</sub>/A<sub>meas</sub></th></tr></thead><tbody><tr><td rowspan="2">Pristine felt</td><td>Non-activated</td><td>3.7 (±<br>5.4) ×<br>10<sup>-5</sup></td><td>1.11 (±<br>0.06)</td><td>3.40 × 10<sup>-3</sup></td><td>0.8</td></tr><tr><td>Activated</td><td>3.3 (±<br>1.1) ×<br>10<sup>-3</sup></td><td>1.40 (±<br>0.07)</td><td>1.55 × 10<sup>-2</sup></td><td>1.0</td></tr><tr><td rowspan="5">Thermal activation<br>under CO<sub>2</sub><br>atmosphere at<br>850 °C</td><td>0.5 h</td><td>6.7 (±<br>4.2) ×<br>10<sup>-4</sup></td><td>1.19 (±<br>0.15)</td><td>4.70 × 10<sup>-3</sup></td><td>0.8</td></tr><tr><td>1 h</td><td>1.7 (±<br>0.5) ×<br>10<sup>-3</sup></td><td>1.13 (±<br>0.07)</td><td>3.80 × 10<sup>-3</sup></td><td>0.8</td></tr><tr><td>2 h</td><td>3.2 (±<br>6.9) ×<br>10<sup>-3</sup></td><td>1.29 (±<br>0.08)</td><td>4.80 × 10<sup>-3</sup></td><td>0.9</td></tr><tr><td>3 h</td><td>7.8 (±<br>2.7) ×<br>10<sup>-4</sup></td><td>1.22 (±<br>0.09)</td><td>4.80 × 10<sup>-3</sup></td><td>0.8</td></tr><tr><td>1 h</td><td>6.9 (±<br>1.9) ×<br>10<sup>-4</sup></td><td>0.98 (±<br>0.10)</td><td>5.00 × 10<sup>-3</sup></td><td>1.0</td></tr><tr><td rowspan="2">Activation in 5 M<br>HNO<sub>3</sub> at 70 °C</td><td>soaked</td><td>6.0 (±<br>1.8) ×<br>10<sup>-4</sup></td><td>1.23 (±<br>0.08)</td><td>4.60 × 10<sup>-3</sup></td><td>0.7</td></tr><tr><td>3 g</td><td>9.8 (±<br>2.6) ×<br>10<sup>-3</sup></td><td>1.40 (±<br>0.20)</td><td>1.26 × 10<sup>-2</sup></td><td>1.0</td></tr><tr><td rowspan="3">Activation in urea<br>solution (room<br>temperature then<br>550 °C under air)</td><td>6 g</td><td>8.5 (±<br>3.6) ×<br>10<sup>-3</sup></td><td>1.21 (±<br>0.17)</td><td>1.12 × 10<sup>-2</sup></td><td>1.0</td></tr><tr><td>12 g</td><td>1.4 (±<br>0.3) ×<br>10<sup>-2</sup></td><td>1.21 (±<br>0.10)</td><td>1.02 × 10<sup>-2</sup></td><td>1.1</td></tr><tr><td>15 g</td><td>1.5 (±<br>0.2) ×<br>10<sup>-2</sup></td><td>1.20 (±<br>0.07)</td><td>1.44 × 10<sup>-2</sup></td><td>1.1</td></tr><tr><td rowspan="3">Additional analysis<br>for urea<br>activation</td><td>Urea 6 g –<br>no heat</td><td>2.9 (±<br>0.48) ×<br>10<sup>-6</sup></td><td>1.09 (±<br>0.07)</td><td>–</td><td>1.0</td></tr><tr><td>No urea –<br>550 °C air</td><td>2.3 (±<br>0.57) ×<br>10<sup>-3</sup></td><td>1.33 (±<br>0.13)</td><td>–</td><td>0.9</td></tr><tr><td>No urea –<br>550 °C N<sub>2</sub></td><td>1.4 (±<br>0.24) ×<br>10<sup>-6</sup></td><td>0.95 (±<br>0.09)</td><td>–</td><td>0.9</td></tr></tbody></table>

of the intensity ratio of these two bands, I<sub>D</sub>/I<sub>G</sub>. In graphite-like carbons, a higher value of this ratio means a higher amount of defects in the nanotexture of the material, suggesting changes induced by an external treatment, whether thermal, mechanical or chemical. The graphite felts of this study were analyzed by Raman spectroscopy starting with the non-activated and the commercially activated ones, illustrated in Fig. S8 in the Supplementary Materials, to serve as a reference for comparison with the felts activated in this work. The I<sub>D</sub>/I<sub>G</sub> ratios obtained are 1.12 for the non-activated felt and 1.4 for the commercially activated one. It is well known that the activation creates more defects on the surface of the fibers, leading also to a higher specific surface area (confirmed by BET analysis of the N<sub>2</sub> adsorption: 500 vs. 1400 cm²/cm³ for NA and CA graphite felts, respectively) and to the creation of additional sites for the electrochemical reaction to occur. In parallel to Raman analysis, estimation of the capacitance of these samples was also done. Here, the

capacitance was not converted to electrochemical surface area because of the uncertainties about the specific capacitance that should be used, but one can consider that a much higher capacitance is related to a much higher surface. Table 1 reports the Raman I<sub>D</sub>/I<sub>G</sub> ratio and the capacitance in F along with the kinetic and Fig. 4 evaluate simple correlations between the two parameters and the kinetic of electron transfer.

Fig. 4A reports the evolution of the k⁰ with the I<sub>D</sub>/I<sub>G</sub> ratio. While a higher ratio is observed when the kinetic is higher, a correlation cannot be clearly extracted from this plot. The highest kinetic was observed for an I<sub>D</sub>/I<sub>G</sub> ratio around 1.2 but a large variability in kinetic responses is observed at this specific ratio. The highest I<sub>D</sub>/I<sub>G</sub> ratio was observed with the commercial activation and the sample treated with 3 g urea, and are associated to relatively high k⁰ values. So, it is clear that this set of data shows that the introduction of defects plays indeed a major role in the improvement of the kinetic of electron transfer but the observation of the I<sub>D</sub>/I<sub>G</sub> ratio alone does not allow the prediction of the reactivity.

### 3.2.2. k⁰ versus capacitance and aging of the material

The fitting routine was adjusting both k⁰ and the surface area of the fiber to reach the best agreement between the model and the experimental data. Table 1 is reporting the mean ratio between the fitted surface area (A<sub>fit</sub>) and the measured surface area (A<sub>meas</sub>, considering a radius of 4.5 µm and the length determined under optical microscope). The detailed set of data is also reported in Table S1 in Supplementary Materials. In fact, the difference between the measured and the fitted surface area was relatively limited, the fitted value was usually a bit lower or very close to the measured value. Moreover, no clear relationship could be identified between the A<sub>fit</sub>/A<sub>meas</sub> ratio and the measured capacitance (see Fig. S9 in Supplementary Materials). On the contrary, a relatively good correlation exists between k⁰ and capacitance (see Fig. 4B) when considering only the treatment performed in the laboratory, which confirm that the increase in capacitance (and surface area) is a key parameter for improving the apparent electron transfer kinetic.

However, despite this good correlation with the laboratory treatments, the kinetic of the commercially activated material was showing a relatively low kinetic for the measured capacitance (point surrounded by a dashed circle). We thought that this relatively low reactivity could be due to some aging of the material between the moment of the activation at the industrial facility and the moment at which this reactivity evaluation was done (but we do not know how much time passed between these two moments).

To confirm that, we performed evaluation of the kinetic parameter k⁰ along with aging time for three different activation protocols, activation under CO₂ at 850 °C for 2 h, in boiling HNO₃ for 1 h and by impregnation with 6 g urea and thermal treatment at 550 °C. k⁰ was determined just after activation, and after 1.5 months and 3 months (the samples were stored in sealed plastic bags). The absolute values of the kinetic parameter are given in Table 2 and the relative values are plotted in Fig. 5 (raw data are reported in the Supplementary Materials, Fig. S10). Indeed, we observe that the reactivity of the material is rapidly decreasing with time. The most sensitive one was the sample treated under CO₂, whose reactivity decreased by more than 10 times. The relative decrease was less pronounced for the urea and the HNO₃ activated materials but the kinetic constant was only 30 % of the initial value after 3 months. So great care must be taken to provide surface characterization at the same moment as the kinetic when analyzing the correlation between these different parameters.

### 3.2.3. XPS analysis

The final argument for this discussion was obtained by X-ray photoelectron spectroscopy (XPS). Fig. 6 shows the XPS spectra for three different felts at the moment of the electrochemical characterization: non-activated felt (curve a), commercially activated felt (curve b), and urea-activated felt (6 g, 550 °C under air) (curve c). The corresponding surface chemical compositions are presented in Table 3. The values
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Evolution of the intrinsic kinetic constant of ferrocyanide as a function of the activation conditions of the initial graphite felt: (A) at 850 °C under CO₂ atmosphere and (B) in urea; k⁰ values estimated by fitting CVs plotted for microfiber electrodes extracted from the corresponding felts (raw data are reported in Figures S2, S5 and S6).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (A) Cyclic voltammograms plotted in 10⁻² M K₄[Fe(CN)₆] on microelectrodes extracted from SGL felts treated (a) 6 g urea without thermal treatment, (b) treatment at 550 °C under N₂, (c) treatment of graphite felt at 550 °C in air, and (d) modification of graphite felt with 6 g urea followed by a thermal treatment at 550 °C in air. (B) Evolution of the intrinsic kinetic constant of ferrocyanide oxidation from these different conditions. WE = MFE; RE = Ag/AgCl pseudo-reference wire; CE = graphite composite rod; v = 100 mV/s.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (A) Evolution of the k⁰ with the I<sub>D</sub>/I<sub>G</sub> ratio. (B) Evolution of the k⁰ with the capacitance. All data are reported in Table 1.</figcaption></figure>

**Table 2**

Kinetic constant values estimated from CV fitting directly after activation (t = 0), 1.5 months after activation, and 3 months after activation. (a) CO₂, (b) HNO₃, (c) Urea treatment.

<table><thead><tr><th>Electrode treatment</th><th>t = 0</th><th>t = 1.5 months</th><th>t = 3 months</th></tr></thead><tbody><tr><td>CO₂ - 2h</td><td>3.2 × 10<sup>-3</sup> cm/s</td><td>6.4 × 10<sup>-4</sup> cm/s</td><td>9.9 × 10<sup>-5</sup> cm/s</td></tr><tr><td>HNO₃ - 1h</td><td>6.9 × 10<sup>-4</sup> cm/s</td><td>3.9 × 10<sup>-4</sup> cm/s</td><td>1.9 × 10<sup>-4</sup> cm/s</td></tr><tr><td>Urea - 6 g</td><td>8.5 × 10<sup>-3</sup> cm/s</td><td>5.1 × 10<sup>-3</sup> cm/s</td><td>2.3 × 10<sup>-3</sup> cm/s</td></tr></tbody></table>

show an increase in the amount of surface oxygen with activation, from around 5.6 with non-activated graphite to 7.3 % for the commercial activation and to 6.02 % for the urea activation. So, despite the thermal treatment in air at 550 °C, the impregnation with urea limits the amount of oxygen on the surface of the graphite, replaced by nitrogen species that was detected on the surface. The nature of the functional groups could not be determined on the magnification of the N₁₅ signal (Fig. 6B) but the presence of graphitic carbon nitride is suggested in the literature [40].
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Relative variation of the k° constant with time after activation. (a) 2 h activation under CO₂ at 850 °C, (b) 1 h activation in HNO₃ and (c) activation with 6 g urea and thermal treatment at 550 °C in air. Absolute values are reported in Table 2.</figcaption></figure>

The type of carbon bonds was also modified by the activation. It can be seen that the amount of C sp² and C—O bonds on the urea-activated felts was much lower than the two other felts tested, which can be explained by the formation of nitrogen-based bonds but also the appearance of more C—H bonds. The comparison of these results with the corresponding values of k⁰ in Table 1 confirms that indeed the oxidation of ferrocyanide is surface-sensitive, but not dramatically dependent on the presence of oxygen-based functional groups on the surface of graphite felt.

## 4. Conclusion

Cyclic voltammetry of individual microfiber electrodes extracted from carbon felts provides a powerful tool to quantitatively compare and screen different activation states and protocols, and select the optimal ones based on the highest kinetic constant. This technique was used here to investigate the impact of the kind of activation treatment on the kinetics of ferrocyanide oxidation (solution concentration ranging from 10⁻² to 0.5 M), by determining charge transfer kinetic constants k⁰ at 10⁻² M. This approach confirmed that commercial activation increases the kinetics by about 100 times compared to a non-activated fiber. Among the different activations tested in this study, a mixed activation involving soaking in a urea solution followed by heat treatment at 550 °C gave the best performances. The kinetic constant increased to a value of 1.52 × 10⁻² cm/s, almost 5 times that of a fiber extracted from a

commercially activated felt, 3.27 × 10⁻³ cm/s. It was also found that felt activation leads to the creation of surface defects, but the number of these defects does not correlate with the kinetics increase. The amount of oxygen on the surface seems not to be correlated to the reactivity. At the opposite, a clear correlation can be observed between the capacitance of the material and k⁰, the highest kinetic being observed with the material displaying the highest capacitance, with the remarkable exception of the commercial activation that was activated some time before analysis. In fact, we observed that the time of aging of these materials had a dramatic influence on k⁰ that decreased significantly three months after the surface treatment was done. These different results confirm the relevance of microfiber electrode for fine characterization of graphite felt reactivity with redox flow battery electrolytes. The next stage of such investigation could be the characterization of the evolution of graphite felt during battery cycling.

### CRediT authorship contribution statement

**Ranine El-Hage:** Writing – original draft, Investigation. **Vincent Feynerol:** Resources, Methodology. **Mariela Brites Helú:** Writing – review & editing, Methodology. **Rafael Luan Sehn Canevesi:** Investigation. **Alain Celzard:** Writing – review & editing, Validation, Supervision, Methodology. **Vanessa Fierro:** Writing – review & editing, Validation, Funding acquisition, Conceptualization. **Liang Liu:** Writing – review & editing, Validation, Supervision, Conceptualization. **Mathieu Etienne:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

<caption>**Table 3**</caption>

Atomic concentration and carbon type (relative amounts expressed in %) of various graphite felt samples, obtained by XPS analysis.

<table><thead><tr><th rowspan="2"></th><th colspan="3">Atomic concentration (%)</th><th colspan="3">Carbon type (%)</th></tr><tr><th>C</th><th>O</th><th>N</th><th>C sp²</th><th>C-H</th><th>C—O</th></tr></thead><tbody><tr><th>Non-activated</th><td>94.37</td><td>5.63</td><td>—</td><td>74.17</td><td>14.33</td><td>11.54</td></tr><tr><th>Commercially activated</th><td>92.17</td><td>7.32</td><td>—</td><td>73.60</td><td>16.38</td><td>10.02</td></tr><tr><th>Urea-activated</th><td>93.26</td><td>6.02</td><td>&lt; 0.5</td><td>66.29</td><td>25.83</td><td>7.88</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** (A) XPS spectra of (a) non-activated, (b) commercially activated and (c) urea-activated (6 g urea + 550 °C under air) graphite felts; (B) N1s XPS spectrum of the urea-activated graphite felt. Atomic concentrations are reported in Table 3.</figcaption></figure>
## Data availability

Data will be made available on request.

## Acknowledgements

This work was carried out as part of the HIGREEW project, which received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement no. 875613, and it was also supported by the European Regional Development Fund (ERDF) (TALISMAN and TALISMAN2 projects). The SMI platform is acknowledged for Raman spectroscopy and XPS analysis.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2024.144033.

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