Optimization of electrode preparation conditions by response surface methodology for improved formic acid electrooxidation on Pd/MWCNT/GCE

This work examines the formic acid electrooxidation capabilities of Pd catalysts supported by multiwall carbon nanotubes that were synthesized at varying weight percentages. Advanced surface analysis techniques, namely, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, scanning electron microscopy with X-ray energy dispersive, and elemental mapping, are used to evaluate the Pd/MWCNT. To achieve the highest specific activity for formic acid electrooxidation on Pd/MWCNT, electrode preparation parameters, namely, catalyst slurry amount, ultrasonication duration of catalyst slurry, and electrode drying time, were optimized by response surface methodology-central composite design. Measurements made using cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are used to determine the specific activity and stability of formic acid electrooxidation. The optimum values for the catalyst slurry amount, electrode drying time, and ultrasonication duration of catalyst slurry were determined as 1.84 μl, 45 min, and 37.05 min, while under these optimum conditions, the specific activity on Pd/MWCNT was 2.67 mA cm−2 with a deviation of 6.83%. By optimizing the electrode preparation conditions, a conventional Pd/MWCNT catalyst showed higher performance than many bimetallic catalysts. Optimization of electrode preparation parameters is as important as catalyst design and is an inexpensive and facile method to improve electrocatalytic performance.


Introduction
Electrochemical energy conversion is becoming increasingly important due to the world's increasing energy needs and the need to reduce carbon emissions [1].One of the most promising methods for converting clean energy is the use of fuel cells, which may efficiently and with low impact on the environment transform chemical energy straight into electrical energy [2,3].Formic acid electrooxidation (FAEO) is a key reaction in direct formic acid fuel cells (DFAFCs) that can provide high power densities, as formic acid is a liquid and has a high energy density [4,5].DFAFCs provide a number of benefits over other fuel cells.Formic acid is a convenient and practical fuel source because it is a liquid at ambient temperature and could be stored and transported easily [6].Additionally, DFAFCs can provide continuous power output, making them appropriate for a variety of applications, including electric vehicles, unmanned aerial vehicles, and portable electronic devices [7].However, DFAFCs also have some disadvantages that need to be addressed.One of the main challenges of DFAFCs is their low efficiency, which is attributed to the low durability and activity of the electrocatalysts used for FAEO [8].Another challenge is the crossover of formic acid through the membrane, which can lead to a decrease in performance and membrane degradation [9,10].Furthermore, the expensive electrocatalysts and the complexity of the system also need to be addressed to make DFAFCs commercially viable.Nevertheless, the potential benefits of DFAFCs make them a promising technology for future energy conversion systems.
Dehydration and dehydrogenation are the two recognized mechanisms by means of which FAEO can take place [11,12]: The dehydrogenation pathway is preferable to the dehydration route for FAEO since it involves the direct oxidation of formic acid to CO 2 and H 2 , which are the desired products of the reaction [13,14].In contrast, the dehydration pathway involves the formation of CO, which can adsorb on the surface of the catalyst and block the active sites, resulting in a reduction in the catalyst's total activity [15].CO adsorption can also lead to the poisoning of the catalyst, resulting in reduced performance and stability over time [16].Therefore, optimizing the catalyst composition and preparation parameters is critical to enhancing the dehydrogenation pathway and minimizing the undesirable dehydration pathway.This can lead to the design of extremely efficient and stable catalysts for FAEO and contribute to the development of sustainable energy technologies.
It is crucial to optimize a process in order to achieve optimum efficiency.All other factors that affect the system are typically kept constant while one parameter is being studied.However, this method-based procedure has significant limitations, including the absence of interactive effects of the investigated factors and the failure to produce statistical data illustrating the more in-depth parameters' impact on the response [17].Such processes are also impractical because they involve a lot of trials, which use up extra chemicals and time [18].In order to overcome the issues mentioned above, the process conditions have typically been optimized using a multivariate statistical and mathematical technique.RSM's ability to optimize processes makes it a potent optimization tool.RSM's primary objective is to develop a mathematical model using test findings from the supplied trial data [19].The optimization method uses RSM, which consists of a number of steps.Prior to performing the studies, the experimental strategy is decided upon, along with the desired response and process parameters [20].The model's appropriateness is assessed once a model equation is created using the test data, and the ideal conditions for experiments are then chosen.In order to enhance the electrocatalytic performance of catalysts, the optimization of electrode preparation parameters is a more effective solution than other expensive methods.To our knowledge, however, the literature has not comprehensively examined the impact of electrode preparation procedures and performance testing conditions on FAEO activity.
Herein, a multiwalled carbon nanotube-supported Pd catalyst (Pd/MWCNT) was designed using the NaBH 4 reduction method, and the catalyst was then characterized using X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), elemental mapping, and transmission electron microscopy (TEM).RSM was used in the current study to maximize FAEO kinetics.Electrode preparation parameters for FAEO on Pd/MWCNT-modified glassy carbon electrode (Pd/MWCNT/GCE), namely, the amount of catalyst slurry transferred to the GCE (V s ), the ultrasonication time of catalyst slurry (t u ), and the drying time (t d ) of the Pd/MWCNT/GCE, were optimized using RSM.Cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical behavior of Pd/MWCNT/GCE.

Synthesis of Pd/MWCNT
Electrocatalysts containing varying amounts of Pd supported by MWCNT were prepared by the conventional NaBH 4 method [22].First of all, the analytical grade K 2 PdCl 4 used as a precursor was dissolved in 10 ml of distilled water, and the determined amount of MWCNT was added to the solution.It was then stirred on a magnetic stirrer for 2 h, and the NaBH 4 solution was added dropwise.The final solution was stirred for 24 h, and after the reduction was complete, it was filtered, washed, and dried at 90 °C.

Characterization of Pd/MWCNT
Analytical methods, namely, XRD, XPS, SEM-EDX, elemental mapping, and TEM, were used to characterize the as-prepared catalyst.XRD analysis was used to examine the catalyst's crystallinity and crystal size (Panalytical Empyrean).The chemical state of Pd was determined by using Specs-Flex XPS.The particle distribution on support and the average particle size of the catalysts were investigated using the TEM imaging method (Hitachi HighTech HT7700).The elemental content and morphological characteristics of the catalyst were examined through SEM-EDX with elemental mapping (Zeiss Sigma 300).

Electrochemical measurements
The CH Instrument 660E Potentiostat/Galvanostat was used for electrochemical measurements.Electrochemical measurements were carried out by connecting the conventional three-electrode system consisting of the working electrode, reference electrode, and counter electrode to the CH Instrument 660E device in 0.5 M H 2 SO 4 + 1 M HCOOH solution at a scan rate of 100 mVs −1 .Before the electrochemical experiments, the glassy carbon electrode was cleaned with alumina powder, an ethanol/water mixture, and acetone.For the modification of the working electrode, a catalyst slurry was prepared by dissolving 3 mg of Pd/MWCNT in 1 ml of Nafion in an ultrasonic water bath for t u minutes.The amount of prepared catalyst slurry (V s ) was dropped onto the glassy carbon electrode.Then, it was dried at room temperature for the specified time (t d ).

Response surface methodology
The drying time of the electrode, the ultrasonication time of the catalyst slurry dropped on the GCE, and the amount of catalyst slurry are symbolized as t d , t u , and V s , respectively, while these parameters are coded with central the composite design (CCD) as A, B, and C, respectively.Optimization was applied to the independent parameters t d , t u , and V s with a central composite design to determine the maximum specific activities for FAEO.Table 1 shows the specific activity values for FAEO at experimental points determined by the central composite design.To determine the specific activity values for FAEO on Pd/MWCNT, the current values corresponding to the oxidation peaks were normalized to the physical area of the GCE (Figure S1).The low, medium, and high levels of the parameters examined in the CCD applied with the Design Expert software are shown as −1, 0, and +1, respectively.A total of 20 experiments were performed for each reaction with 6-point repeat experiments, and the specific activity values were entered into the experimental plan determined by RSM-CCD and analyzed with Design Expert 7.0.
The lattice constants of 7 wt.%, 10 wt.%, and 20 wt.% Pd/MWCNT catalysts are shown in Table S1.Accordingly, the lattice constants of 7 wt.%, 10 wt.%, and 20 wt.% Pd/ MWCNT were calculated as 3.91 Å, 3.94 Å, and 3.89 Å, respectively, and showed 1.11%, 0.35%, and 1.39% deviations from the standard lattice constant values (JCPDS card no.01-087-0641).It has been observed that lattice contraction occurs when the Pd loading rate is increased from 7 to 20 wt.%.The decreases in lattice constant at increasing Pd loading rates may be due to the agglomeration of Pd nanoparticles.In addition, the crystal sizes of 7 wt.%, 10 wt.%, and 20 wt.% Pd/MWCNT were calculated as 4.93 nm, 5.37 nm, and 6.22 nm, respectively, by applying the Debye-Scherer equation to the diffraction peak (1 1 1).The lower mass activity and higher crystal size of 20 wt.% Pd/ MWCNT than expected indicate that the Pd nanoparticles are agglomerated.Figure S2 shows the Williamson-Hall plots of 7 wt.%, 10 wt.%, and 20 wt.% Pd/MWCNT.Lattice strains of 7 wt.%, 10 wt.%, and 20 wt.% Pd/MWCNT were determined as 0.03164, 0.00742, and 0.01127, respectively.The optimum value of the lattice stress has a significant effect on the activity of the catalyst as it changes the d-band center of the Pd surface [23].The distance between the d-band center and the fermi level has a decisive role in the bond strength of the HCOO-Pd intermediate formed during the FAEO reaction.To achieve high electrocatalytic activity, the d-band center position, which influences the bond strength between HCOO-Pd, should be tuned [24].Therefore, 10 wt.% Pd/MWCNT exhibited the optimum lattice strain value and d-band center position in this study and showed higher activity for the FAEO reaction than other metal loading rates [24].XPS analysis was performed on 10 wt.% Pd/MWCNT to determine the chemical state of Pd in the catalyst.After background removal using Shirley's method, the Gaussian method is used to fit all of the XPS curves.All spectra's binding energies were fixed in relation to the C 1s binding energy of 284.6 eV (Figure S3b). Figure S3b shows the core level C 1s spectrum of 10 wt.% Pd/MWCNT.Deconvoluted peaks at 284.5 and 287.3 eV binding energies were attributed to C-C and C-OH, respectively [25].C 1s, Pd 3d, Pd 3p, and O 1s peaks were observed in the general survey of 10 wt.% Pd/MWCNT (Figure S3a).The highresolution Pd 3d spectrum is presented in Figure S3c.In Figure S3c, peaks indicating the presence of Pd° and PdO x were observed at 341.1 and 344.4 eV binding energies as a result of the deconvolution of the Pd 3d 1/2 peak [26].The Pd 3d 5/2 peak consists of two peaks with 335.2 and 336.6 eV and is attributed to the presence of Pd in the structure in the form of Pd° and PdO x [27].It was concluded that the results obtained from the XPS analysis were highly compatible with the XRD.
The surface morphology and elemental composition of 10 wt.% Pd/MWCNT were analyzed by the SEM-EDX method.As can be seen from Fig. 2a, b, the Pd nanoparticles were homogeneously dispersed on the MWCNT.Figure 2c-e shows SEM images of 10 wt.% Pd/MWCNT at 100 nm scale.It can be clearly seen that the nano-sized Pd particles are dispersed on the MWCNT without agglomeration.The elemental mapping results of C, Pd, and O containing 10 wt.% Pd/ MWCNT are given in Fig. 2f-h, while the overlay images of these elements are presented in Fig. 2i.The homogeneous distribution of Pd nanoparticles was also confirmed by the elemental mapping images of 10 wt.% Pd/MWCNT (Fig. 2h).EDX analysis was performed to confirm the elemental composition of 10 wt.% Pd/MWCNT (Fig. 2j).The EDX result of 10 wt.% Pd/MWCNT shows that the catalyst consists of carbon, oxygen, and palladium.In addition, elemental percentages for C, O, and Pd were determined as 83.63 wt.%, 7.99 wt.%, and 8.44 wt.%, respectively.The peak of oxygen observed in the EDX spectrum is due to PdO.
The morphology, particle size, and distribution of 10 wt.% Pd/MWCNT were investigated by TEM.The image of the MWCNT network is clearly visible in Fig. 3.It has been determined that Pd nanoparticles have spherical shapes and nanometric scales.In addition, Pd adhered to both the outer and inner walls of the MWCNT and showed a homogeneous distribution on the support material.The sizes of the particles observed in each TEM image were determined with ImageJ software, and the corresponding particle size distribution histograms are given in the inset.While it was observed that Pd nanoparticles were dispersed in a narrow size range, namely, 4-8 nm, the average particle size of 300 randomly selected particles was found to be 4.51 nm.

Electrochemical evaluation
Before RSM experiments, the optimum Pd loading ratio for FAEO was determined among 3 wt., 5 wt., 7 wt., 10 wt., and 20 wt.% Pd/MWCNTs.Figure 4a shows the voltammograms of Pd/MWCNTs in 0.5 M H 2 SO 4 at a 100 mVs −1 scan rate.It was observed in Fig. 4a that all Pd loading ratio catalysts exhibited a distinct hydrogen adsorptiondesorption region in the potential range of −0.2 and 0 V [28,29].In the potential range of 0 V and 0.6 V, no electrochemical activity occurred, and this potential range was explained by the double-layer region [30,31].The areas of the obtained voltammograms increased with increasing metal loading ratios, and the highest voltammogram area was reached with 20 wt.% Pd loading.This is explained by the number of electrochemically active sites increasing with the increase in Pd content.Also, the peak observed at a potential of about 0.49 V in the reverse scan was attributed to the reduction of PdO [32,33].Electrochemically active surface areas (ECSA) of catalysts at varying metal loading ratios were calculated by using the area of the PdO reduction peak (Eq.( 3)) [29].
where Q is the Coulomb charge obtained PdO reduction peak (Fig. 4a), 0.424 mC cm −2 represents the charge needed for PdO monolayer reduction, and Pd m symbolizes the quantity of Pd.As can be seen in Table 2, the electrochemical active surface area of 3 wt., 5 wt., 7 wt., 10 wt., and 20 wt.% Figure 4b, c shows the voltammograms of Pd/MWCNT in a 0.5 M H 2 SO 4 +1 M HCOOH solution in terms of mass and specific activity, respectively.The results in Fig. 4a, b are summarized in Table 2.The electrochemical parameters of the anodic peak indicating the formic acid dehydrogenation (direct pathway) reaction occurring at a potential of approximately 0.1 V are given in Table 2.Although no significant changes were observed in the peak and onset potentials of the catalysts, 10 wt.% and 20 wt.% Pd/MWCNT exhibited more negative onset potentials than those of other metal loading ratios [34].Therefore, 10 wt.% and 20 wt.% Pd/MWCNT catalysts have higher activity for FAEO through the direct pathway (formic acid dehydrogenation) than other metal loading ratios.Specific activities for 3 wt.%, 5 wt.%, 7 wt.%, 10 wt.%, and 20 wt.% Pd/MWCNT at around 0.2 V potential were determined as 0.18, 0.68, 0.22, 1.53, and 3.91 mA cm −2 while mass activities were calculated as 48.1, 58.4,26.7, 252.0, and 184.0 mA mg −1 Pd (Table 2).A total of 20 wt.% Pd/ MWCNT and 10 wt.% Pd/MWCNT exhibited the highest specific activity and mass activity for FAEO, respectively.When the Pd loading ratio was increased from 3 to 10 wt.%, the mass activity increased 4.25 times, while when it was increased to 20 wt.%, a sharp decrease in mass activity attributable to agglomeration was observed.Therefore, RSM experiments for FAEO on Pd/MWCNT were carried out with 10 wt.% Pd/MWCNT.

Optimization of electrode preparation conditions
GCE was modified with 10 wt.% Pd/MWCNT under the conditions given in Table 1 to determine the optimum electrode preparation conditions for FAEO, and electrochemical measurements were performed in a 0.5 M H 2 SO 4 +1 M HCOOH solution.Voltammograms obtained from CV measurements performed at determined experimental points for the optimization of V s , t d , and t u independent parameters are given in Figure S1, and the relevant specific activity values were analyzed with the Design Expert 7.0 software.Experimental results from Design Expert 7.0 show that FAEO on 10 wt.% Pd/MWCNT could be well explained by a quadratic model.Accordingly, the specific activity values for FAEO on 10 wt.% Pd/MWCNT in terms of coded and actual values are given in Eqs. ( 4) and ( 5).The statistical analysis of the quadratic equation (Eq.( 1)) proposed by Design Expert 7.0 was evaluated by analysis of variance, and the ANOVA of the 10 wt.% Pd/MWCNT modified GCE electrode is presented in Table 3.The F value and p-value of the quadratic model proposed by the software were determined as 12.23 and 0.0003, respectively.The model p-value being less than 0.05 indicates that the proposed model is significant in the 95% confidence interval.In addition, the p-values of model parameters A, B, C, AB, AC, and BC were determined as 0.0024, 0.0001, 0.0039, 0.0030, 0.0420, and 0.0078, respectively.In this direction, it was concluded that the effects of V s , t u , and t d parameters and their binary interactions on FAEO were statistically significant.The p-values of the terms A 2 , B 2 , and C 2 in the model are 0.2129, 0.1148, and 0.4143, respectively, and are not statistically significant as they are greater than 0.1.However, these model terms were seen to make a positive contribution to the model due to their closeness to the 0.1 p-values and were not removed from the model.In this study, statistical parameters such as adequate precision, lack of fit, PRESS and regression coefficient (R 2 ), and model fit plots were used to test the suitability of the model.The lack of fit F value found at 1.27 in the ANOVA test is statistically insignificant, indicating that the proposed model is in good agreement with the experimental data.In addition, PRESS and adequate precision values for the model were determined as 82.84 and 15.5, respectively.The relatively (5) Specific activity for FAEO = +0.91837  The plots provided by Design Expert 7.0 are shown in Fig. 5 to test the compatibility of the proposed quadratic model with the experimental data.The distribution of actual and estimated specific activity values for FAEO on 10 wt.% Pd/MWCNT modified GCE is given in Fig. 5a.As can be seen in Fig. 5a, the experimental points are scattered around the diagonal.The scattering of the experimental data around the diagonal reveals that the proposed model represents the experimental points well [35].Figure 5b shows the internally studentized residual vs. normal probability distribution.It is desired that the residues be distributed around the straight line determined by the software, and the distribution observed in Fig. 5b fits this description quite well.The very good distribution of the residuals around the straight line is attributed to the unsystematic random distribution of the errors [36].The distribution of residuals versus estimated values is shown in Fig. 5c.As can be seen in Fig. 5c, the residues are randomly distributed between the reference values indicated by the red lines.Randomly distributed points within the reference limits are explained by the non-constant variance [2].The distribution of residuals according to the experimental order given in Table 1 is presented in Fig. 5d to test systematic experimental errors.It was determined that the residuals were not distributed in a certain order according to the experiment run number.The fact that the residuals are not dispersed within a trend indicates the absence of systematic errors and that the specific activity values for FAEO on Pd/MWCT modified GCE are time-independent.
The model parameters and Fig. 5 show parallelism with each other, indicating that the proposed model has good predictive power within the studied parameters.
Figure 6 shows the binary interactions of the independent parameters V s , t d , and t u .The binary interaction of t u and V s parameters for FAEO on 10 wt.% Pd/MWCNT/GCE is presented in Fig. 6a.Specific activity increased from 1.25 to 5.66 mA cm −2 when t u was increased from 1 to 60 min under conditions where V s and t d were 5.25 μl and 20.5 min, respectively.In addition, it was observed that the rate of increase in specific activity after 30.5 min t u value was 3.2 times that of 0-30.5 min.It was concluded that the inability to provide a homogeneous catalyst slurry at low t u values and the ability to obtain a uniform catalyst slurry after a 30.5 min t u value are the main reasons for the increased specific activity.The nanoparticles well dispersed in Nafion increased the charge transfer efficiency.The 3d response surface plots of V s and t d are presented in Fig. 6b.When the amount of catalyst slurry transferred onto the GCE was increased from 0.5 to 5.25 μl, the specific activity increased from 0.30 to 1.98 mA cm −2 (t u = 30.5 and t d = 20.5 min).After this value, when V s was increased up to 10 μl, the specific activity continued to increase slowly until 2.80 mA cm −2 and reached a constant value.The increase in the specific activity for FAEO as V s increased was attributed to the presence of more active sites for the reaction on the 10 wt.% Pd/MWCNT modified GCE.The nearly constant specific activity observed at catalyst slurry amounts greater than 8 μl V s may be due to mass transfer limitation because of excess catalyst slurry.In Fig. 6c, the effect of the t d parameter on the specific activity was examined under the conditions of 30.5 min t u and 5.25 μl V s .By increasing the drying time of the 10 wt.% Pd/MWCNT/GCE electrode system from 1 to 30 min, the specific activity increased from 0.32 to 2.62 mA cm −2 and remained constant until the 40th min.
The optimization of the parameters t u , V s , and t d was carried out to maximize the specific activity for FAEO using Design Expert 7.0 software.It is aimed to maximize the specific activity at minimum t u , t d , and V s values, and the suggested 10 solutions as a result of numerical optimization are given in Table 4.With these inputs, the optimum values for the parameters V s , t d , and t u were determined as 1.84 μl, 45 min, and 37.05 min, respectively, while the model response value at these points was determined as 2.50 mA cm −2 .At these optimum conditions, the specific activity for FAEO on 10 wt.% Pd/MWCNT was 2.67 mA cm −2 with a deviation of 6.83% (Fig. 7a).Consistent with the fit tests of the model, it The results of CV, CA, and EIS performed under optimum conditions are presented in Fig. 7. Figure 7a shows CV profiles of 50 cycles at a 100 mV s −1 scan rate in a 0.5 M H 2 SO 4 + 1 M HCOOH.In the forward scan, the peak observed at 0.22 V potential is attributed to formic acid dehydrogenation [37], while the shoulder peak detected at 0.55 V potential indicates that FAEO occurs via the dehydration pathway [38].It is known that the peaks observed at high potentials for FAEO indicate poisoning caused by carbonaceous compounds accumulating on active sites [39,40].In this direction, although the shoulder peak at 0.55 V potential indicates catalyst poisoning, the high intensity of the peak caused by formic acid dehydrogenation indicates a high toxicity tolerance of 10 wt.% Pd/ MWCNT.The peak at 0.29 V potential in the reverse scan shows overall FAEO on the clean electrode surface [29].The peak intensity in the forward scan (2.67 mA cm −2 ) is greater than the current density in the reverse scan (2.27 mA cm −2 ), confirming the high CO poisoning tolerance of 10 wt.% Pd/MWCNT.Specific activities for FAEO on 10 wt.% Pd/MWCNT were determined as 2.67 mA cm −2 and 0.77 mA cm −2 for Cycle 1 and Cycle 50, respectively.The significant decrease in electrocatalytic activity was  attributed to the carbonaceous intermediates deposited on the electrode surface at each cycle [41].As mentioned above, the carbonaceous intermediates formed by the formic acid dehydration reaction, which was detected as a shoulder peak at 0.55 V potential, were strongly bound to the active sites of the Pd/MWCNT catalyst and caused the catalyst to be exposed to CO poisoning with the increasing cycle number [16].In the chronoamperometric curves of the 10 wt.% Pd/MWCNT, the initial current drops due to CO poisoning were observed in the first 20 s and were consistent with the CV results.After 20 s, the 10 wt.% Pd/ MWCNT exhibits an almost constant current (Fig. 7d).A total of 10 wt.% Pd/MWCNT showed long-term stability at optimum conditions, and CA and CV results of 10 wt.% Pd/ MWCNT show parallelism.Figure 7c shows the EIS curves of 10 wt.% Pd/MWCNT in a 0.5 M H 2 SO 4 + 1 M HCOOH solution at a scan rate of 100 mV s −1 .As the radius of the EIS profiles decreases, the charge transfer resistance decreases, and the electrocatalytic activity increases [25].
In this direction, the charge transfer kinetics and electrocatalytic activity of 10 wt.% Pd/MWCNT increased with the increment of applied potential.It was concluded from the trend of the impedance profile that FAEO on 10 wt.% Pd/MWCNT proceeds through charge transfer kinetics.The corresponding Bode plots are shown in Figure S4a and b.The phase angles observed in Fig. 2a were found to be less than 90 and explained by non-ideal capacitive behavior [42].It was observed that the relaxation time decreased with increasing potential and was attributed to the porous structure of the Pd/MWCNT/GCE surface (Figure S4a) [43].It was concluded that the porous structure increased the kinetics of the FAEO reaction with increasing potential [44].In addition, the slope of the Bode plots from Figure S4b was determined to be approximately 0.83 with a correlation coefficient of 0.993.The almost identical slope in the high-frequency region is a result of resistive behavior, while the nearly constant slope in the mid-frequency region is due to pseudocapacitive behavior [45].Synthesis procedures and surface properties of anode catalysts for high electrocatalytic performance are of primary importance.However, it was noted that the modification of the electrode system prepared with catalysts facilely increased the electrocatalytic activity.An extremely limited number of studies have been reported in the literature on the optimization of electrode preparation conditions to increase electrochemical activity.With the electrode systems Pt/CNT/GCE and Pd/CNT/GCE previously reported by our working group, electrocatalytic performances close to those of complex synthesis procedures and more expensive catalyst systems were observed [36,46].It is seen in Table 5 that electrode preparation parameters are not specified in detail in most of the studies reported in the literature.Geng et al. prepared dual-phase PdCu by a one-pot wet-chemical method.The specific activity for FAEO was reported as 1.91 mA cm −2 with the electrode prepared under 12.4 μg cm −2 Pd load, 5 μl V s , and 15 min t u electrode preparation conditions [47].The Pd/FeP anode catalyst was prepared via the thermal annealing method by Bao et al., and they reached a specific activity of 1.75 mA cm −2 for FAEO at 5 μl V s , 141.5 μg cm −2 Pd load, and 30 min t u parameter values [41].In another study, Hu et al. reported that the specific activities of PdAg/C, Pd-Au/C, Pd-Cu/C, Pd-Pt/C, Pd-Ru/C, and Pd/C at 50 μl V s and a 10 wt.% Pd loading rates were 0.8, 1.8, 3.2, 3.3, 1.5, and 0.9 mA cm −2 , respectively.The researchers attributed the higher specific activity of bimetallic catalysts than Pd/C to the electronic state alteration of the Pd surface with the addition of the second metal [49].In addition, the specific activities of trimetallic electrocatalysts, namely, PdSnAg/C (V s = 2 μl and Pd load = 40 μg cm −2 ) and Pd 70 Ag 20 Ni 10 (V s = 5 μl, t u = 10, t d = 30, and Pd load = 10 wt.%) for FAEO were reported as 3.59 and 1.6 mA cm −2 , respectively [51,52].The 10 wt.% Pd/MWCNT/GCE reported in the current study is competitive with the studies reported on FAEO in the literature.It was concluded that optimization of the preparation conditions of the electrodes using the reported catalysts will have a booster effect on the electrocatalytic activity of FAEO.It has been concluded that these benefits will make a positive contribution to the commercialization of DFAFCs.

Conclusion
The Pd/MWCNT catalyst was synthesized by the NaBH 4 reduction method at varying Pd ratios.It was determined from the XRD results of 10 wt.% Pd/MWCNT that fcc cubic Pd was successfully reduced onto MWCNT, and the crystal size of Pd nanoparticles was determined as 4.06 nm.SEM-EDX and elemental mapping analysis methods revealed that Pd nanoparticles were homogeneously dispersed on MWCNT.The average particle size of Pd nanoparticles was determined as 4.51 nm from the TEM results.While the optimum electrode preparation parameters V s , t u , and t d for FAEO on 10 wt.% Pd/MWCNT/GCE were determined as 1.84 μl, 45 min, and 37.05 min, respectively, the desirability and the experimental specific activity value at optimum points were found to be 0.945 and 2.67 mA cm −2 , respectively.The closeness of the specific activity values obtained from the experimental and model results shows that the proposed quadratic model is a good predictor for FAEO on 10 wt.% Pd/MWCNT/GCE.In addition, PRESS, adequate precision, and R 2 values determined as 82.84, 15.5, and 0.92, respectively, confirm the viability of the proposed model.CV, CA, and EIS measurements for FAEO on 10 wt.% Pd/MWCNT/GCE were performed under optimized conditions.It was determined from the CV results that FAEO proceeds with the dehydrogenation pathway, and the experimental-specific activity value showed a deviation of 6.83% from the model value.According to CA and EIS, it was concluded that 10 wt.% Pd/MWCNT showed long-term stability for FAEO and that the reaction progressed with charge transfer kinetics.Thanks to the optimization of the electrode preparation conditions, the 10 wt.% Pd/MWCNT catalyst has become competitive with the ternary catalysts in the literature for FAEO.Accordingly, optimization of electrode preparation parameters is an extremely effective way to design catalysts with high activity and stability.

Fig. 3
Fig. 3 TEM images of 10 wt.% Pd/MWCNT at a 200 nm, b 100 nm, c 200 nm and d-f 100 nm scale and corresponding particle size distribution histogram

Fig. 4
Fig. 4 Cyclic voltammograms of Pd/MWCNT in a 0.5 M H 2 SO 4 and b, c 0.5 M H 2 SO 4 +1 M HCOOH in terms of mass and specific activity

Fig. 5 a
Fig. 5 a Predicted versus actual b normal probability distribution of residuals, c residuals versus predicted, and d internally studentized residuals versus run number for the specific activity toward FAEO on 10 wt.% Pd/ MWCNT/GCE

Fig. 6
Fig. 6 Response surface plots of a t u and V s , b t d and V s , and c t d and t u for FAEO on 10 wt.% Pd/ MWCNT/GCE

Fig. 7 a
Fig. 7 a-b CV, c EIS, and d CA profiles of 10 wt.% Pd/ MWCNT under optimized conditions

Table 2
Electrocatalytic features of Pd/MWCNTs for FAEO + 0.045185 * V s − 0.085626 * t u + 0.029042 * t d + 7.53977E − 003 * V * s t u + 0.00801246 * V * s t d + 0.00215921 * t * u t d − 0.020506 * V 2 s + 0.00107839 * t 2 u − 0.00190070 * t 2 d low predicted residual sum of square (PRESS) value and an adequate precision value greater than 4 indicate that the proposed model has good predictive power in the investigated parameter range.In addition, the R 2 value for the quadratic model was determined as 92%, indicating that the model has the power to explain 92% of the experimental data.