The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins

Hydrogen, considered to be an alternative fuel to traditional fossil fuels, can be generated by splitting water molecules into hydrogen and oxygen via the use of electrical energy, in a process whose efficiency depends directly on the employed catalytic material. The current study takes part in the relentless search for suitable and low-cost catalysts relevant to the water-splitting field by investigating the electrocatalytic properties of the O2 and H2 evolution reactions (OER and HER) of two metalloporphyrins: Zn(II) 5,10,15,20-tetrakis(4-pyridyl)-porphyrin and Co(II) 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The TEM/STEM characterisation of the porphyrin samples obtained using different organic solvents revealed several types of self-assembled aggregates. The HER and OER experiments performed on porphyrin-modified graphite electrodes in media with different pH values revealed the most electrocatalytically active specimens. For the OER, this specimen was the electrode manufactured with one layer of Co-porphyrin applied from dimethylsulfoxide, exhibiting an overpotential of 0.51 V at i = 10 mA/cm2 and a Tafel slope of 0.27 V/dec. For the HER, it was the sample obtained by drop casting one layer of Zn-porphyrin from N,N-dimethylformamide that displayed a HER overpotential of 0.52 V at i = −10 mA/cm2 and a Tafel slope of 0.15 V/dec.


Introduction
The continuous and rapid increase in the human population worldwide is directly related to the escalating global energy demand. This demand is currently being addressed by using renewable and non-renewable energy sources, with the caveat that non-renewable fuels contribute substantially to the total energy production [1] despite their negative impact on the environment [2]. Instead, the focus should be on the development of ecofriendly technologies for fuels that could be used to avoid a future energy crisis. Hydrogen is regarded as an alternative to the traditional sources that are environmentally harmful due to their combustion products [3]. While it can be produced with technologies that rely on fossil fuels (such as coal gasification, steam reforming, and catalytic partial oxidation), it can also be obtained by employing solar, wind, wave, and tidal energy [1,4]. The specified eco-friendly energy sources are employed to procure hydrogen by powering the water electrolysis process [5], which is responsible for the decomposition of water molecules into hydrogen and oxygen via an electric current [6]. Once generated, hydrogen can be stored, moved, and either reconverted to electrical energy via fuel cells or used to obtain different fuels [7]. Important progress has been made in the water-splitting domain [8][9][10], and one noticeable aspect of this progress concerns the materials utilized to catalyse the two half-cell reactions involved in water electrolysis, namely, the hydrogen evolution reaction (HER), during which H 2 is produced at the cathode as a result of water toanodes by decorating BiVO 4 with graphene oxide and Co-porphyrins, including Co(II) 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin, and found that the photoelectrochemical water-splitting performance depends on the employed Co-porphyrin structure.
Unlike the mentioned studies, the current investigation evaluates the HER and OER electrocatalytic properties of graphite electrodes modified with one, two, and three layers of the specified metalloporphyrins, drop-casted from several organic solvents in electrolyte solutions covering a wide pH range. Furthermore, a morphological analysis focused on the self-assembled aggregates of the two species on a solid support was carried out as well, since the characteristics of these structures can significantly affect the catalytic activity of the electrodes.

Manufacturing of the Working Electrodes
Polyethylene tubes were filled with graphite rods (ø = 6 mm) that were thermally treated (at 180 • C) until they became tightly attached to the tubes and only the rod ends remained bare. During the electrochemical experiments, one rod end was connected to the potentiostat, while the other was modified with each metalloporphyrin and immersed into the electrolyte solution. The modification procedure consisted of several stages. First, the surface of the graphite substrate was polished with silicon carbide paper with grit sizes of 800 and 1200 and felt. Second, the polished surface was washed with double-distilled water, ethanol, and acetone, and left to dry at 23 ± 2 • C. Third, the porphyrin solutions were prepared by dissolving the organic compounds in organic solvents with different polarities. According to Snyder et al. [41] and Szabadai et al. [42], the order of decreasing polarity for the employed solvents is DMSO > DMF > CH 3 CN > PhCN > EtOH > THF > DCM. The 0.15 mM porphyrin solutions were obtained following a 40 min ultrasonication treatment. Fourth, the polished graphite substrates were modified by drop-casting a 10 µL solution volume on their surfaces that was subsequently allowed to dry at 40 • C for 4 h and at 23 ± 2 • C for 20 h. This stage was required to deposit one metalloporphyrin layer on the graphite substrate and was repeated to obtain two and three such layers. The manufactured electrodes were labelled as presented in Table 1.

TEM/STEM Characterisation of Metalloporphyrin Specimens
Through the process of molecular self-assembly, porphyrins in general and metalloporphyrins in particular have the ability to form well-defined aggregates that result from the non-covalent interactions between spontaneously associated molecules [30]. The self-assembly process can take place in solution or at interfaces, and the aggregates often have different properties from the molecules comprising them [47,48], including properties relevant to the water-splitting domain [49]. In order to outline the morphology of the structures developed from the self-assembly of the ZnP and CoP molecules, a TEM study was conducted on the specimens obtained by applying the metalloporphyrins on nonpolar carbon films from organic solvents with different polarity. The recorded images are presented in Figures 1 and 2, and they reveal several types of architectures.

TEM/STEM Characterisation of Metalloporphyrin Specimens
Through the process of molecular self-assembly, porphyrins in general and metalloporphyrins in particular have the ability to form well-defined aggregates that result from the non-covalent interactions between spontaneously associated molecules [30]. The self-assembly process can take place in solution or at interfaces, and the aggregates often have different properties from the molecules comprising them [47,48], including properties relevant to the water-splitting domain [49]. In order to outline the morphology of the structures developed from the self-assembly of the ZnP and CoP molecules, a TEM study was conducted on the specimens obtained by applying the metalloporphyrins on nonpolar carbon films from organic solvents with different polarity. The recorded images are presented in Figures 1 and 2, and they reveal several types of architectures.  The image recorded on the ZnP specimen obtained using DMSO as a solvent ( Figure  1a) shows irregular formations with micrometric lengths and widths, sprinkled with small particles sized between 30 and 180 nm. Irregular aggregates were also observed when the solvent was DMF (Figure 1b), but they were mixed with cuboid-like structures. While the length and width of most of the identified structures were micrometric, for some of the architectures resembling cuboids these dimensions were in the submicrometric range. Another similarity between this specimen and the previously discussed one consisted in the presence of small particles on the surface of the assemblies, while a difference was provided by the existence of crevices on the surface of some of the structures from the former sample. When applied from benzonitrile, the ZnP molecules organized as aggregates resembling cuboids and square plates (Figure 1c and inset). The measurements conducted for several cuboids revealed widths between 1.38 and 1.7 μm and thicknesses in the 0.2-0.44 μm range. It has been reported that complex porphyrin architectures comprise simpler ones, resulting from the side-by-side or face-to-face positioning of the molecules [50]. In the case of 3D aggregates-such as the ones observed for this specimen-these two types of assemblies, referred to as J-type and H-type arrangements, are both present [51].
In the case of the ZnP samples obtained using the solvents with the lowest polarity, the molecules no longer interacted to create irregular structures and cuboids but some very different aggregates instead (Figures 1d,e). Thus, when THF was employed as a solvent, a discontinuous metalloporphyrin layer covered the carbon film of the TEM grid. Ring-shaped formations revealed the substrate and appeared brighter than the surrounding ZnP deposits, probably due to a higher concentration of molecules. The ring diameters were in the 30-150 nm range. The inset in Figure 1d shows a magnified image of one of the rings (ø = 150 nm). When ZnP was applied from DCM, the observed aggregates were The image recorded on the ZnP specimen obtained using DMSO as a solvent (Figure 1a) shows irregular formations with micrometric lengths and widths, sprinkled with small particles sized between 30 and 180 nm. Irregular aggregates were also observed when the solvent was DMF (Figure 1b), but they were mixed with cuboid-like structures. While the length and width of most of the identified structures were micrometric, for some of the architectures resembling cuboids these dimensions were in the submicrometric range. Another similarity between this specimen and the previously discussed one consisted in the presence of small particles on the surface of the assemblies, while a difference was provided by the existence of crevices on the surface of some of the structures from the former sample. When applied from benzonitrile, the ZnP molecules organized as aggregates resembling cuboids and square plates (Figure 1c and inset). The measurements conducted for several cuboids revealed widths between 1.38 and 1.7 µm and thicknesses in the 0.2-0.44 µm range. It has been reported that complex porphyrin architectures comprise simpler ones, resulting from the side-by-side or face-to-face positioning of the molecules [50]. In the case of 3D aggregates-such as the ones observed for this specimen-these two types of assemblies, referred to as J-type and H-type arrangements, are both present [51].
In the case of the ZnP samples obtained using the solvents with the lowest polarity, the molecules no longer interacted to create irregular structures and cuboids but some very different aggregates instead (Figure 1d,e). Thus, when THF was employed as a solvent, a discontinuous metalloporphyrin layer covered the carbon film of the TEM grid. Ringshaped formations revealed the substrate and appeared brighter than the surrounding ZnP deposits, probably due to a higher concentration of molecules. The ring diameters were in the 30-150 nm range. The inset in Figure 1d shows a magnified image of one of the rings (ø = 150 nm). When ZnP was applied from DCM, the observed aggregates were island-shaped and displayed a tendency to organize into circular arrangements (Figure 1e) due to their helical growth. The TEM/STEM characterisation of the samples obtained using solutions of CoP dissolved in various organic solvents outlined a diversity of self-assembled architectures as well. Thus, when DMSO was employed as a solvent, the metalloporphyrin molecules formed cell-like 2D aggregates with submicrometric dimensions that co-existed with submicrometric particles (Figure 2a). The drop-casting of the Co-porphyrin from DMF ( Figure 2b) resulted in discontinuous micrometric islands surrounded by the carbon film substrate patterned with porphyrin-based spots (inset in Figure 2b). When acetonitrile was used as a solvent, a discontinuous porphyrin layer covered the TEM grid and different types of arrangements were observed (Figure 2c), including irregular structures and cell-like formations (inset in Figure 2c)-all in the submicrometric domain. Figure 2d presents an image recorded for the specimen obtained by applying the Co-porphyrin species from benzonitrile. The molecules organized in patterns resembling flat crystalline bundles with micrometric lengths. The inset shows a fragment from such an arrangement. Micron-sized ring-like aggregates were obtained by drop-casting the CoP from ethanol (Figure 2e). For exemplification purposes, the inset in Figure 2e shows a magnified ring. Its thickness varied between 50 and 260 nm, being inconstant like the thickness of the other observed structures of the same type. The area between these architectures consisted in the carbon substrate covered to a high degree with irregularly shaped formations. Interestingly, the deposited material was noticed both outside and inside the rings, and this is important for identifying the de-wetting mechanism involved in their formation. According to the literature, there are two such mechanisms that explain the creation of ring-like assemblies, referred to as "coffee-stain" and "pinhole" [52,53]. The fact that a significant amount of material was located inside the rings is suggestive of the coffee-stain mechanism. As for the material between the rings, the pattern of the deposition is indicative of spinodal de-wetting [52]. When the Co-porphyrin was applied from THF, the identified aggregates were shaped as bundles of quasi-spherical structures (Figure 2f and inset). The performed measurements evidenced dimensions between 70 and 300 nm. Since the partially formed quasi-spheres observed during the investigation were not hollow, it is likely that the complete assemblies were also filled with metalloporphyrin molecules. These remarks are in agreement with other studies [54]. A general conclusion regarding the TEM/STEM analysis of the ZnP and CoP specimens is that a variety of aggregates were obtained by applying the two species on nonpolar carbon substrates from organic solvents with different polarity. Furthermore, a comparison among the samples that resulted by drop-casting ZnP and CoP from the same solvent revealed, in all cases, the presence of very different architectures. These are probably due to the interaction between the solvent, the substrate, and the elements specific to each metalloporphyrin (such as their substituents, their central metal ion, and their hydrophilic/hydrophobic balance).
The morphology of the aggregates is relevant for the electrochemical water-splitting field, since it has been reported that inhomogeneous electrode surfaces provide more catalytically active sites than homogeneous ones, and can thus lead to an improved charge transfer at the electrolyte/electrode interface during the OER and HER [55].  Figures S3 and S4. The curves recorded on the most electrocatalytically active metalloporphyrin-modified electrodes in the neutral, alkaline, and acidic media were selected from the four figures and are rendered in Figure 3. Figure 3a shows the overlapped voltammograms obtained on G ZnP-PhCN-1 and G CoP-CH3CN-1 , which were identified as the most catalytically active electrodes in the neutral solution. G ZnP-PhCN-1 was also evidenced as having the highest catalytic activity for the OER in the alkaline medium ( Figure 3b) and it attained the highest i value in the considered E range. The LSV traced on G CoP-DMSO-1 in an acidic environment can be seen in Figure 3c. This electrode was the most active in the specified medium and it reached the highest i value in the studied potential range. A comparison between the curves from Figure 3 reveals that the modified electrodes investigated in 0.1 M KCl solution displayed the lowest OER catalytic activity. Thus, no further experiments were performed concerning their electrocatalytic properties for the specified half-cell reaction in the neutral medium. The electrodes that exhibited the highest catalytic activity in alkaline and acidic environments were subsequently evaluated in more concentrated electrolyte solutions. metalloporphyrin-modified electrodes in the neutral, alkaline, and acidic media were selected from the four figures and are rendered in Figure 3. Figure 3a shows the overlapped voltammograms obtained on GZnP-PhCN-1 and GCoP-CH3CN-1, which were identified as the most catalytically active electrodes in the neutral solution. GZnP-PhCN-1 was also evidenced as having the highest catalytic activity for the OER in the alkaline medium ( Figure 3b) and it attained the highest i value in the considered E range. The LSV traced on GCoP-DMSO-1 in an acidic environment can be seen in Figure 3c. This electrode was the most active in the specified medium and it reached the highest i value in the studied potential range. A comparison between the curves from Figure 3 reveals that the modified electrodes investigated in 0.1 M KCl solution displayed the lowest OER catalytic activity. Thus, no further experiments were performed concerning their electrocatalytic properties for the specified half-cell reaction in the neutral medium. The electrodes that exhibited the highest catalytic activity in alkaline and acidic environments were subsequently evaluated in more concentrated electrolyte solutions.  Figure 4. The HER overpotential values were determined at i = -10 mA/cm 2 -a current density value at which ηH2 is often specified [43,56,57]-as: 1.05 V for GZnP-DMF-3 and 0.89 V for GCoP-EtOH-1 in the neutral medium, 0.67 V for GZnP-DMF-1 in the alkaline medium, and 0.6 V for GCoP-EtOH-1 in the acidic electrolyte solution. Since the electrodes investigated in the neutral environment exhibited the highest ηH2 values, they were not additionally studied in terms of their HER catalytic activity. The remaining two electrodes were subsequently tested in strong alkaline and strong acidic media.  Figure 4. The HER overpotential values were determined at i = -10 mA/cm 2 -a current density value at which η H2 is often specified [43,56,57]as: 1.05 V for G ZnP-DMF-3 and 0.89 V for G CoP-EtOH-1 in the neutral medium, 0.67 V for G ZnP-DMF-1 in the alkaline medium, and 0.6 V for G CoP-EtOH-1 in the acidic electrolyte solution. Since the electrodes investigated in the neutral environment exhibited the highest η H2 values, they were not additionally studied in terms of their HER catalytic activity. The remaining two electrodes were subsequently tested in strong alkaline and strong acidic media.

The OER and HER Electrocatalytic Activities of the ZnP-and CoP-Modified Electrodes
As a general observation, the results of the OER and HER experiments carried out in the 0.1 M electrolyte solutions on all the ZnP-and CoP-based electrodes outline the fact that their electrocatalytic activity depended on the solutions drop-casted on the surface of the graphite substrate, obtained using solvents with different polarities. Furthermore, for most electrodes, the number of applied layers had a noticeable impact on their catalytic performance.  . The experimental data were used in the Randles-Sevcik equation [58,59], and the calculated values of the two parameters are displayed in Table 2. Each value is an average obtained from the experiments performed on two electrodes of the same type and is rendered with the standard deviation. A high diffusion coefficient is a desirable electrochemical property for an electrode, since the higher the value, the faster the diffusion. The obtained diffusion coefficient values are higher than the tabulated ones published by Konopka et al. [60], determined using a thin-layer electrochemical cell with twin working Pt electrodes, but they are similar to the value estimated for a previously reported metalloporphyrin-modified electrode [44]. As can be seen in Table 2, the diffusion coefficient values for the porphyrin-based electrodes are higher than that of G0, which means that the self-assembled aggregates present on the substrate surface enhance diffusion. It should also be pointed out that in the case of GZnP-DMF-1 and GZnP-DMF-3, both the EASA and diffusion coefficient values are higher for the former, meaning that the drop-casting of more than one Zn(II) porphyrin layer from  6 ]. The experimental data were used in the Randles-Sevcik equation [58,59], and the calculated values of the two parameters are displayed in Table 2. Each value is an average obtained from the experiments performed on two electrodes of the same type and is rendered with the standard deviation. 0.325 ± 0.007 9.35 × 10 -6 ± 0.14 × 10 -6 G ZnP-PhCN-1 0.8 ± 0.06 6.15 × 10 -5 ± 0.24 × 10 -5 G ZnP-DMF-3 0.422 ± 0.0014 1.525 × 10 -5 ± 0.007 × 10 -5 G ZnP-DMF-1 0.942 ± 0.015 7.7 × 10 -5 ± 0.24 × 10 -5 G CoP-CH3CN-1 0.948 ± 0.07 8.375 × 10 -5 ± 0.62 × 10 -5 G CoP-EtOH- 1 1.19 ± 0.08 5.7 × 10 -5 ± 0.35 × 10 -5 G CoP-DMSO-1 0.94 ± 0.04 7.62 × 10 -5 ± 0.72 × 10 -5 A high diffusion coefficient is a desirable electrochemical property for an electrode, since the higher the value, the faster the diffusion. The obtained diffusion coefficient values are higher than the tabulated ones published by Konopka et al. [60], determined using a thin-layer electrochemical cell with twin working Pt electrodes, but they are similar to the value estimated for a previously reported metalloporphyrin-modified electrode [44]. As can be seen in Table 2, the diffusion coefficient values for the porphyrin-based electrodes are higher than that of G 0 , which means that the self-assembled aggregates present on the substrate surface enhance diffusion. It should also be pointed out that in the case of G ZnP-DMF-1 and G ZnP-DMF-3 , both the EASA and diffusion coefficient values are higher for the former, meaning that the drop-casting of more than one Zn(II) porphyrin layer from DMF leads to samples with worsened electrochemical properties. Therefore, the electrodemanufacturing procedure should involve the application of a single metalloporhyrin coat. Regarding the diffusion coefficient values calculated for the specimens obtained using one layer, they are within the same order of magnitude and the differences between them are not substantial, indicating that the properties of the self-assembled structures do not have a major impact on the parameter in question.
The cyclic voltammetry data resulting from studying the modified electrodes were further employed to represent the plots of the anodic and cathodic peak current densities vs. the square root of the scan rate ( Figure 5). Without exception, the absolute values of the peak current densities increase with the scan rate, and this behaviour is typical of a diffusion-controlled electron transfer process [59].
DMF leads to samples with worsened electrochemical properties. Therefore, the electrode-manufacturing procedure should involve the application of a single metalloporhyrin coat. Regarding the diffusion coefficient values calculated for the specimens obtained using one layer, they are within the same order of magnitude and the differences between them are not substantial, indicating that the properties of the self-assembled structures do not have a major impact on the parameter in question.
The cyclic voltammetry data resulting from studying the modified electrodes were further employed to represent the plots of the anodic and cathodic peak current densities vs. the square root of the scan rate ( Figure 5). Without exception, the absolute values of the peak current densities increase with the scan rate, and this behaviour is typical of a diffusion-controlled electron transfer process [59].

OER and HER Investigations in Strong Alkaline and Acidic Electrolyte Solutions
The anodic polarisation curve obtained for GZnP-PhCN-1 in 1 M KOH solution is presented in Figure 6a. A comparison with the LSV traced on the same electrode but in a lower pH electrolyte (Figure 3b) reveals that for all the recorded current density values the OER overpotential was smaller at the higher concentration. In addition, for the same potential range in which the voltammogram from Figure 3b was obtained, the increase in the concentration resulted in a higher maximum current density value. Such observations have been previously reported for other electrodes modified with water-splitting catalytic materials [46,61], including porphyrins [55]. The OER kinetics at the interface between the GZnP-PhCN-1 electrode and the 1 M KOH electrolyte solution were also studied. In order to represent the Tafel plot (Figure 6b), the current density was normalised by the calculated EASA value of the investigated sample (iEASA). The Tafel equation [62] was used to determine a Tafel slope value of 0.2 V/dec.

OER and HER Investigations in Strong Alkaline and Acidic Electrolyte Solutions
The anodic polarisation curve obtained for G ZnP-PhCN-1 in 1 M KOH solution is presented in Figure 6a. A comparison with the LSV traced on the same electrode but in a lower pH electrolyte (Figure 3b) reveals that for all the recorded current density values the OER overpotential was smaller at the higher concentration. In addition, for the same potential range in which the voltammogram from Figure 3b was obtained, the increase in the concentration resulted in a higher maximum current density value. Such observations have been previously reported for other electrodes modified with water-splitting catalytic materials [46,61], including porphyrins [55]. The OER kinetics at the interface between the G ZnP-PhCN-1 electrode and the 1 M KOH electrolyte solution were also studied. In order to represent the Tafel plot (Figure 6b), the current density was normalised by the calculated EASA value of the investigated sample (i EASA ). The Tafel equation [62] was used to determine a Tafel slope value of 0.2 V/dec.   (Figure 3c) to 0.5 M H2SO4 resulted in a smaller OER overpotential and higher current density values. ηO2 was determined at i = 10 mA/cm 2 , in accordance with other studies [43,63], and was found to be 0.51 V. This value is smaller than the one calculated for the GZnP-PhCN-1 specimen (of 0.56 V), studied in the 1 M KOH solution, and it indicates a higher OER electrocatalytic activity. However, the Tafel slope for GCoP-DMSO-1, obtained from the Tafel plot presented in Figure 6d, was higher than the one determined for GZnP-PhCN-1, which points to the faster OER kinetics of the latter electrode [64].
Both GZnP-PhCN-1 and GCoP-DMSO-1 were also evaluated in terms of their electrochemical stability. Thus, a 6 h chronoamperometric test was performed for each electrode at the potential value corresponding to i = 10 mA/cm 2 . The recorded current density-time curves are shown in Figures 6e and 6f, and they indicate the relative stability of the samples under the experimental conditions. However, in the case of the GZnP-PhCN-1 electrode, the voltammogram traced after the stability test (inset in Figure 6e) increasingly differed in shape compared to the initial LSV as the current density increased, while it can be seen that the G′CoP-DMSO-1 curve, obtained using the GCoP-DMSO-1 electrode, does not deviate as much from the one recorded before the experiment (inset in Figure 6f).
The results of the HER investigations performed on GZnP-DMF-1 in a strong alkaline medium and on GCoP-EtOH-1 in a strong acidic medium are presented in Figure 7. The cathodic polarisation curves obtained on these two electrodes are shown in Figures 7a,c, Table 3, and allow for the following observations: (a) the more   (Figure 3c) to 0.5 M H 2 SO 4 resulted in a smaller OER overpotential and higher current density values. η O2 was determined at i = 10 mA/cm 2 , in accordance with other studies [43,63], and was found to be 0.51 V. This value is smaller than the one calculated for the G ZnP-PhCN-1 specimen (of 0.56 V), studied in the 1 M KOH solution, and it indicates a higher OER electrocatalytic activity. However, the Tafel slope for G CoP-DMSO-1 , obtained from the Tafel plot presented in Figure 6d, was higher than the one determined for G ZnP-PhCN-1 , which points to the faster OER kinetics of the latter electrode [64].
Both G ZnP-PhCN-1 and G CoP-DMSO-1 were also evaluated in terms of their electrochemical stability. Thus, a 6 h chronoamperometric test was performed for each electrode at the potential value corresponding to i = 10 mA/cm 2 . The recorded current density-time curves are shown in Figure 6e,f, and they indicate the relative stability of the samples under the experimental conditions. However, in the case of the G ZnP-PhCN-1 electrode, the voltammogram traced after the stability test (inset in Figure 6e) increasingly differed in shape compared to the initial LSV as the current density increased, while it can be seen that the G CoP-DMSO-1 curve, obtained using the G CoP-DMSO-1 electrode, does not deviate as much from the one recorded before the experiment (inset in Figure 6f).
The results of the HER investigations performed on G ZnP-DMF-1 in a strong alkaline medium and on G CoP-EtOH-1 in a strong acidic medium are presented in Figure 7. The cathodic polarisation curves obtained on these two electrodes are shown in Figure 7a,c, while the Tafel plots determined for them are displayed in Figure 7b,d. The η H2 and Tafel slope values are rendered in Table 3, and allow for the following observations: (a) the more concentrated electrolyte solutions improved the HER catalytic activity of the samples, which is reflected in the smaller η H2 values; (b) the overpotential decrease was more significant for the ZnP-modified electrode than for the CoP-modified one, making it the most electrocatalytically active specimen with respect to the HER; and (c) the Tafel slope value for G ZnP-DMF-1 was notably smaller than the one determined for G CoP-EtOH-1 , evidencing more favourable HER kinetics. Nevertheless, the slope is >120 mV/dec, which may indicate the occurrence of secondary processes, such as reduction reactions at the catalytic material's surface or electrochemical H 2 absorption [65]. Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 20 concentrated electrolyte solutions improved the HER catalytic activity of the samples, which is reflected in the smaller ηH2 values; (b) the overpotential decrease was more significant for the ZnP-modified electrode than for the CoP-modified one, making it the most electrocatalytically active specimen with respect to the HER; and (c) the Tafel slope value for GZnP-DMF-1 was notably smaller than the one determined for GCoP-EtOH-1, evidencing more favourable HER kinetics. Nevertheless, the slope is >120 mV/dec, which may indicate the occurrence of secondary processes, such as reduction reactions at the catalytic material's surface or electrochemical H2 absorption [65].  Since the data outlined the Zn metalloporphyrin-based electrode as the most catalytically performant for the HER, the sample was further studied in terms of its electrochemical stability. As the i-time curve shows (Figure 7e), the −10 mA/cm 2 current density was reached after 28 min. The i-value continued to increase to −9.5 mA/cm 2 , and it subsequently decreased back to −10 mA/cm 2 (after 290 min). This value remained almost constant until the end of the experiment. The inset in Figure 7e presents the LSVs obtained before and after the chronoamperometric test, and it can be noticed that there is no significant difference between them until i = −33 mA/cm 2 . The changes of shape at the lower current densities can, at least in part, be attributed to the strong H2 evolution.  Since the data outlined the Zn metalloporphyrin-based electrode as the most catalytically performant for the HER, the sample was further studied in terms of its electrochemical stability. As the i-time curve shows (Figure 7e), the −10 mA/cm 2 current density was reached after 28 min. The i-value continued to increase to −9.5 mA/cm 2 , and it subsequently decreased back to −10 mA/cm 2 (after 290 min). This value remained almost constant until the end of the experiment. The inset in Figure 7e presents the LSVs obtained before and after the chronoamperometric test, and it can be noticed that there is no significant difference between them until i = −33 mA/cm 2 . The changes of shape at the lower current densities can, at least in part, be attributed to the strong H 2 evolution.
The study of the ZnP-and CoP-modified electrodes in terms of their OER and HER electrocatalytic activities revealed that G CoP-DMSO-1 and G ZnP-DMF-1 were, overall, more electrocatalytically performant than the other investigated modified samples. The surfaces of these electrodes were analysed by SEM before and after testing their electrochemical stability in order to examine the effect of the experimental conditions on the morphology of the porphyrin aggregates.

SEM Characterisation of the G CoP-DMSO-1 and G ZnP-DMF-1 Electrodes
The SEM micrographs recorded on the surface of G CoP-DMSO-1 and G ZnP-DMF-1 are displayed in Figure 8. The images obtained on the surface of the Co-porphyrin-modified electrode before (Figure 8a and inset) and after ( Figure 8b) the chronoamperometric study show aggregates with similar morphology. It can also be noted that the shape of these assemblies resembles the cell-like 2D structures revealed during the TEM/STEM study (Figure 2a) but are much bigger in size. In the case of the G ZnP-DMF-1 electrode, the structures present on its surface before the stability experiment (Figure 8c and inset) were similar to those following the test (Figure 8d and inset), as well as to the ones outlined during the TEM/STEM characterisation (Figure 1b). The results of the SEM analysis indicate that the metalloporphyrin aggregates that self-assembled on the graphite substrate's surface did not undergo any significant morphological modifications in the course of the electrochemical stability test and this is a benefit for long-time usage. The study of the ZnP-and CoP-modified electrodes in terms of their OER and HER electrocatalytic activities revealed that GCoP-DMSO-1 and GZnP-DMF-1 were, overall, more electrocatalytically performant than the other investigated modified samples. The surfaces of these electrodes were analysed by SEM before and after testing their electrochemical stability in order to examine the effect of the experimental conditions on the morphology of the porphyrin aggregates.

SEM Characterisation of the GCoP-DMSO-1 and GZnP-DMF-1 Electrodes
The SEM micrographs recorded on the surface of GCoP-DMSO-1 and GZnP-DMF-1 are displayed in Figure 8. The images obtained on the surface of the Co-porphyrin-modified electrode before (Figure 8a and inset) and after ( Figure 8b) the chronoamperometric study show aggregates with similar morphology. It can also be noted that the shape of these assemblies resembles the cell-like 2D structures revealed during the TEM/STEM study (Figure 2a) but are much bigger in size. In the case of the GZnP-DMF-1 electrode, the structures present on its surface before the stability experiment (Figure 8c and inset) were similar to those following the test (Figure 8d and inset), as well as to the ones outlined during the TEM/STEM characterisation (Figure 1b). The results of the SEM analysis indicate that the metalloporphyrin aggregates that self-assembled on the graphite substrate's surface did not undergo any significant morphological modifications in the course of the electrochemical stability test and this is a benefit for long-time usage.

Additional Observations Concerning the G CoP-DMSO-1 and G ZnP-DMF-1 Electrodes
The water-splitting electrocatalytic properties of metalloporphyrins have been the subject of several studies that have revealed the central metal cation as the catalytic centre [21].
For the two species that are the subject of the current study, the catalytic centres are the Zn 2+ and Co 2+ cations located in the Zn-N 4 and Co-N 4 sites of the porphyrin macrocycles. However, the catalytic performance of porphyrins is not provided by the metal ion alone, but by other factors as well. One of them is the previously specified ability of porphyrins to self-assemble into aggregates. In this respect, the SEM analysis of both the G CoP-DMSO-1 and G ZnP-DMF-1 samples revealed the unevenness of the depositions covering the graphite substrate. The disordered distribution of the ZnP structures (together with their edges and defects, such as the crevices observed on some of them) and the discontinuous CoP-based coating outline the inhomogeneous nature of the electrode surfaces, and inhomogeneities have been reported to increase the number of catalytically active sites exposed to the electrolyte solution [66,67]. Furthermore, the EASA value is directly proportional to the number of active sites involved in electrochemical reactions [68], and the calculated values for the two specimens are among the highest found in the study.
Another property of metalloporphyrins relevant to their water-splitting catalytic performance is the electronegativity of the metal centre. The higher the electron affinity of the metal cation in the M-N 4 site of the porphyrin macrocycle is, the stronger its electropositive doping effect on the carbon support becomes [38]. The electronegativities of both Zn and Co were higher than those of the C atoms comprising the graphite substrate, and this facilitated the charge transfer at the porphyrin-graphite interface. The OER and HER catalytic activity of metalloporphyrins is also affected by the electron-donating or electronwithdrawing properties of the functional moieties serving as their substituents [21]. ZnP has four electron-withdrawing pyridyl groups, while CoP has four electron-withdrawing hydroxyphenyl groups, due to their meta positioning. The HER experimental results revealed a ZnP-modified electrode as the most catalytically active, instead of a CoP-based one, despite the fact that Co is more electronegative than Zn and that the OHgroups form intermolecular hydrogen bonds contribute to charge transport. Even in the case of the OER study, the differences between the G CoP-DMSO-1 and G ZnP-PhCN-1 samples were not very significant. The explanation for these observations likely includes the effects of the meso-substituents. The electron-withdrawing moieties are known to decrease the macrocycle's electron density, leading to improved stability during catalysis and a positive shift in reduction potential. The outcome is a decreased energy cost for H 2 evolution and an increased oxidising power in the case of OER catalysts [21,69].
The expected pH influence is in line with the obtained results. It is expected that the OER's catalysis was more adequately achieved for the Co-porphyrin in a strong acidic medium because the four hydroxyl functional groups grafted on the porphyrin ring increase their tendency to form hydrogen bonds due to the substitution in the meta position. In a recent paper, it was noticed that the enhanced catalytic activity was due to the Coporphyrin-OH intermediate [24].
Another possible explanation is the electronic effect of the substituents on the electronic structure at the cobalt centre [24]. Due to the already-known electron attractive property of OH groups in the meta position of the meso-phenyls [70] and to the surrounding hydrogen bonds, the electron densities on the Co ions decreased, and thus the nucleophilic attack of a water molecule was facilitated [19].
In the same manner, the highest HER catalytic activity can also be expected in an alkaline solution for the electrode covered with Zn-porphyrin because the four pyridyl groups grafted on the porphine ring are stable in this medium, but generate pyridinium N + H in an acidic environment, thus promoting dimerization and an undesired uniform and compact self-aggregation due to London's dispersion force being the main binding force [71]. In addition, the positional disorder of the pyrrole NH atoms was demonstrated in pyridyl-substituted porphyrins [72] in normal and basic pH. Instead, the distorted porphyrin molecules obtained in acidic medium by the protonation of the N-pyridyl groups were packed more efficiently in adjacent stacks because of the weak C-H . . . N hydrogen bonds formed between β-pyrrole moieties and the pyridyl groups of the molecules. This type of compact arrangement diminishes the number of catalytically active sites in acidic media.
In order to compare the OER and HER electrocatalytic activity of the G CoP-DMSO-1 and G ZnP-DMF-1 specimens with other reported electrodes manufactured using Zn-and Co-porphyrins, a literature survey was conducted and presented in Table S1 from the Supplementary Material File that outlined a disproportionate number of studies performed on cobalt metalloporphyrins, as opposed to zinc-metalated ones, which have been more the subject of photocatalytic water-splitting investigations [49,73]. The findings indicate that the η H2 value for G ZnP-DMF-1 is either similar to or smaller than the values reported for electrodes modified only with Zn-and Co-porphyrins, while also being similar to several of the ones published for more complex Co-porphyrin-based catalysts. The same situation is observed when contrasting the η O2 value for G CoP-DMSO-1 with the ones exhibited by other electrodes manufactured using Co-porphyrins. The Tafel slopes determined for the two samples are among the highest identified, which suggests that in future water-splitting studies of these metalloporphyrins they should be utilized in combination with other materials (nickel phosphite, carbon black, and perovskites) so as to obtain a synergistic effect.

Conclusions
Two symmetrically substituted metalloporphyrins were investigated as heterogeneous catalysts for the two half-cell reactions involved in the water-splitting process in aqueous electrolyte solutions with different pH values. The porphyrin solutions obtained using organic solvents with different polarities were drop-casted on TEM grids and on graphite supports. The TEM/STEM characterisation revealed several types of self-assembled porphyrin aggregates, resulting from the interplay between the solvent's polarity, the nonpolar carbon film substrate, and the properties of the differently substituted Zn-and Co-porphyrins. The study of the OER and HER electrocatalytic properties of the manufactured metalloporphyrin-based graphite samples revealed the most performant electrodes for each environment in which they were evaluated. The best overall results concerning OER catalysis were observed in a strong acidic medium for the sample manufactured by applying one layer of Co-porphyrin from DMSO, while the highest HER catalytic activity was evidenced in a strong alkaline solution on the electrode made with one layer of Znporphyrin drop-casted from DMF. In the first case, a η O2 value of 0.51 V (at i = 10 mA/cm 2 ) and a Tafel slope value of 0.27 V/dec were determined, and in the second case, a η H2 value of 0.52 V (at i = −10 mA/cm 2 ) and a Tafel slope value of 0.15 V/dec were obtained. The stability of the two electrodes was verified electrochemically and by an SEM analysis.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.