Improving the Reproducibility of Oxygen Reduction Reaction Activity Assessment for Pt-Based Electrocatalysts on a Rotating Disk Electrode via Catalytic Layer Optimization

Abstract

The reproducibility of oxygen reduction reaction (ORR) activity assessment for platinum-based electrocatalysts using the rotating disk electrode (RDE) method is critically dependent on the quality of the fabricated catalytic layer. This work presents a comprehensive study on optimizing catalytic ink formulation—specifically the water-to-isopropanol (H₂O:IPA) solvent ratio and the ionomer-to-carbon (I/C) ratio—to achieve a homogeneous catalytic layer and ensure high data reproducibility for monometallic Pt/C and bimetallic PtCu/C catalysts. A key aspect of this research is the implementation of a simple and effective visual inspection method using a benchtop digital microscope to rapidly assess catalytic layer quality, which was shown to correlate directly with electrochemical performance. The optimal ink composition was found to be catalyst-specific. For Pt/C, the highest mass activity of 353 A/g~Pt~ was achieved with a solvent ratio of 1:3 (H₂O:IPA) and an I/C ratio of 0.3. For PtCu/C, the best performance was obtained with the same solvent ratio (1:3) but a higher I/C ratio of 0.4, yielding a mass activity of 491 A/g~Pt~. It was demonstrated that ink compositions leading to layer inhomogeneities, such as aggregates and “coffee-ring” effects, significantly impair mass transport and lead to underestimated ORR activity. The study underscores the absence of a universal ink recipe and establishes that the optimization of ink parameters for each specific catalyst is essential for obtaining reliable and reproducible electrochemical data.

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) represent a promising technology for efficient energy conversion. Their performance critically depends on platinum-based electrocatalysts [1], which facilitate the core electrode reactions: hydrogen oxidation at the anode and the oxygen reduction reaction (ORR) at the cathode. The ORR suffers from slow kinetics, making the development of highly active and stable cathode catalysts a critically important challenge [2,3,4].

A prominent strategy to enhance the catalytic activity of Pt is the alloying of its nanoparticles (NPs) with d-metals (e.g., Co, Ni, Fe, Cu) [5,6,7,8,9]. This strategy not only reduces the cost of the catalysts but also improves their functional characteristics. The activity and stability of Pt/C and PtM/C systems are strongly influenced by NP’ morphology, their distribution on the carbon support, and the strength of their adhesion.

The rapid investigation of the catalyst electrochemical behavior is typically conducted under laboratory conditions in a three-electrode cell using a rotating disk electrode (RDE). However, obtaining reproducible and reliable ORR activity data on the RDE is notoriously difficult, as highlighted in key methodological studies [10,11]. Alongside the catalyst’s composition and structure (primary factors), the quality of the catalytic layer (a secondary factor) plays a significant role in the electrode’s properties [10,11,12]. The parameters for forming a high-quality catalytic layer (CL) include the solvent composition to obtain a stable catalytic suspension (ink) [13,14,15,16,17,18,19,20], the ionomer-to-carbon ratio (I/C) [16,17,20,21,22,23,24], as well as the methods and conditions of deposition (temperature, humidity, atmosphere, electrode rotation rate during the drying of the CL) [25,26,27].

In practice, significant data scatter in metrics like mass activity and half-wave potential often stems from these secondary factors. Issues such as the inhomogeneity of the CL (aggregates, “coffee rings” effect, uneven ionomer distribution) lead to distorted oxygen diffusion fluxes, uncontrolled changes in the triple-phase boundary, and, as a result, underestimation or overestimation of the measured catalyst activity in the ORR. Furthermore, the optimal composition of the catalytic ink (ratio of water/organic solvent, amount of ionomer) depends on the nature of the catalyst (composition of the metal phase, type of carbon support), which precludes the existence of universal recipes [15].

Therefore, methodical optimization of ink formulation and layer deposition is essential for reliable catalyst evaluation. This work presents a comprehensive study on the effect of ink composition for monometallic (Pt/C) and bimetallic (PtCu/C) catalysts provided by PROMETHEUS R&D LLC. Optimal parameters for the water/organic solvent and I/C ratios have been established, ensuring the formation of a homogeneous catalytic layer free of “coffee rings” and aggregates, and yielding high reproducibility in voltametric data. As a result, high mass activities of 276 A/g_Pt for Pt/C and 491 A/g_Pt for PtCu/C were achieved. The obtained results are of practical importance for standardizing catalyst testing methodologies on the RDE as a rapid method for assessing electrochemical characteristics, and for developing advanced materials for PEMFCs.

2. Results and Discussion

2.1. Microstructure of the Catalysts

The Pt/C and PtCu/C catalysts consist of spherical metal NPs (2–4.5 nm) supported on Vulcan XC-72 carbon black, which has a particle size of 20–50 nm (Figure 1). Analysis of the micrographs reveals that the bimetallic PtCu/C catalyst exhibits a less uniform nanoparticle distribution compared to the monometallic Pt/C, along with the presence of large, isolated particles (Figure 1).

2.2. Effect of Ink Solvent Composition on the Electrochemical Performance of Pt/C Catalyst

At the first stage, the influence of the water-to-isopropanol ratio (H₂O:IPA) in the catalytic ink on the quality of the CL and the electrochemical performance of the electrode was investigated. For this purpose, a series of catalytic inks with H₂O:IPA ratios of 1:3, 1:1, and 3:1 were prepared, while maintaining a constant ionomer-to-carbon ratio (I/C) of 0.3 (

Table 1. Electrochemical performance of the Pt/C electrocatalyst prepared using different ink formulations.

Methodology	Replicate	ECSA, m2/g(Pt)		E1/2, B	Number of ē	Koutecký–Levich Equation			Polarization Curve at 1600 rpm
						Ik, mA	Imass, A/g(Pt)	Isp, A/m2 (Pt)	Ik, mA	Imass, A/g(Pt)		Isp, A/m2 (Pt)
		Absolute Value	Average Value							Absolute Value	Average Value	Absolute Value	Average Value
Stage 1
M0.31	1	66	70 ± 4	0.90	4.2	1.0	277	4.2	1.1	314	353 ± 50	4.8	5.0 ± 0.8
	2	70		0.91	4.2	1.5	374	5.3	1.6	403		5.8
	3	74		0.91	3.5	1.4	351	4.7	1.4	343		4.6
M0.32	1	66	67 ± 1	0.89	3.6	1.0	241	3.7	0.9	229	256 ± 53	3.5	3.8 ± 0.8
	2	67		0.89	4.3	1.0	267	4.0	1.2	309		4.6
	5	67		0.88	4.7	0.7	189	2.8	0.9	230		3.4
M0.33	1	68	68 ± 4	0.88	3.7	0.7	185	2.7	0.7	188	192 ± 4	2.8	2.8 ± 0.3
	3	64		0.89	3.8	0.7	184	2.9	0.8	196		3.1
	5	71		0.88	4.1	0.7	179	2.5	0.8	193		2.7

).

The CL on the RDE was visually inspected using a “G1000” digital microscope. Analysis of the photographs (Figure 2) for the Pt/C catalyst revealed that the most uniform CL was formed using methods M^0.31 and M^0.32, which employ H₂O:IPA ratios of 1:3 and 1:1, respectively (Figure 2b). A significant decrease in CL uniformity was observed when the water content in the ink was increased (M^0.33). Large, isolated aggregates were found at the perimeter of the glassy carbon end face, along with a pronounced “coffee-ring” effect [28] (Figure 2c). The coffee-ring effect is likely caused by centrifugal forces generated during layer formation at 700 rpm [28]. Another contributing factor is the Marangoni effect, driven by a surface tension gradient arising from the different evaporation rates of water and isopropanol at the center and edge of the ink droplet [29,30,31]. This surface tension gradient induces a prolonged capillary flow, transporting catalyst particles toward the droplet’s periphery.

Following the visual inspection of the CL (Figure 2a–c), the electrochemical characteristics of each electrode were investigated (Figure 2d–i). Prior to evaluating the main electrochemical parameters of the catalyst (ECSA and ORR activity), a crucial step is surface activation (Section 2.3). In the literature, this step is also referred to as standardization or conditioning. This process serves to clean the surface from potential contaminants and stabilize the metal nanoparticle surface, thereby ensuring reproducible measurement characteristics [25,32].

Figure 4d–f presents the CVs of the activation stage for Pt/C catalysts prepared from different catalytic inks. In all CVs, an initial increase followed by stabilization of the current is observed in both the hydrogen (0.04–0.35 V) and oxygen (0.6–1.0 V) regions (Figure 2d–f). This behavior is attributed to an increase in the nanoparticle surface area, likely caused by platinum dissolution/redeposition [33]. The catalyst surface prepared using method M^0.32 stabilizes the most rapidly (Figure 2e).

Following activation, the CVs exhibit the characteristic profile of a platinum electrode with well-defined hydrogen and oxygen regions. The ECSA value is directly related to the current in the hydrogen region of the CV (Figure 2g). The hydrogen region currents increase in the order: M^0.31 > M^0.32 ≈ M^0.33. The measurements were performed in triplicate for each method, and the average value was calculated from the obtained data. The average ECSA values for each ink composition ranged from 60 to 68 m²/g(Pt), which is within the margin of error for the method q.v.Electrochemical Measurements (Table 1).

The LSV curves obtained for all investigated electrodes exhibit the characteristic S-shaped profile (Figure 2h). The polarization curve for M^0.33 shows a significantly steeper slope in the mixed kinetic-diffusion control region (0.80–0.95 V) compared to M^0.31 and M^0.32 (Figure 2h). These differences are attributed to the quality of the CL formation. Layer inhomogeneity impedes oxygen diffusion through the CL, which consequently affects the shape of the polarization curves and the catalytic activity for the ORR.

The half-wave potential (E₁/₂), a key indicator of catalyst activity [4,25,34,35] shifts to more positive values for ink compositions M^0.31 and M^0.32 compared to M^0.33 (Table 1). The number of electrons transferred and the ORR kinetic parameters were determined using the Koutecký–Levich equation. The kinetic current, specific and mass activity were additionally calculated from a single polarization curve measured at a rotation rate of 1600 rpm (Table 1). The highest ORR activity was demonstrated by the M^0.31 electrode, fabricated with a water-to-isopropanol ratio of 1:3.

Figure S2 presents the results of triplicate experiments for the three catalytic ink preparation methods. It is shown that method M^0.31 ensures the formation of the most uniform CL during repeated applications (Figure S1a). In contrast, a pronounced coffee-ring effect and catalyst aggregation are observed in all replicates for method M^0.33. All obtained linear sweep voltammetry (LSV) curves demonstrate the characteristic S-shape (Figure S1). However, for the layers formed using method M^0.33, a decrease in current in the diffusion region was recorded, which is explained by the hindered transport of oxygen molecules to the active sites due to the non-uniform morphology of the CL (Figure S2d). A comparative analysis of the mass activity in the ORR, presented in the histograms (Figure S1e), confirms that the highest and reproducible values are achieved using method M^0.31.

Analysis of the Tafel slopes for the Pt/C catalyst (Figure S4a) shows that two characteristic linear regions are observed for all layer preparation methods. In the high-potential region (>0.90 V), the slope is approximately 60 mV/dec. This behavior is fully consistent with the classical kinetics of the ORR on Pt NPs in an acidic medium [36]. Method M^0.31 demonstrates a slightly higher activity, which is reflected by a shift in the curve towards more positive potentials in the kinetic region (Figure S1b).

This study demonstrated that the ink composition significantly affects the uniformity of the fabricated CL. The presence of various defects in the catalytic layer such as catalyst aggregates, microcracks, and characteristic “coffee-ring” effects observed by microscopy leads to poor reproducibility of ORR activity. The optimal CL morphology and maximum ORR activity were achieved with a solvent ratio of 1:3 (method M^0.31).

The optimal ionomer content in the CL on RDE depends on the type of carbon support. For instance, the authors of [37] demonstrate the need to vary the ionomer/carbon ratio depending on the nature of the support. Investigating the influence of the support on the layer structure and activity in the ORR is a separate task. Targeted optimization of the ink composition for different supports is considered a promising direction of research.

2.3. Effect of Ink Solvent Composition on the Electrochemical Performance of PtCu/C Catalyst

The introduction of a doping element into the platinum nanoparticle structure can alter the catalyst’s physicochemical properties, consequently necessitating adjustments to the ink composition. In the third stage (

Table 2. Electrochemical performance of the PtCu/C electrocatalyst prepared using different ink formulations.

Methodology	Replicate	ECSA, m2/g(Pt)		E1/2, B	Number of ē	Koutecký–Levich Equation			Polarization Curve at 1600 rpm
						Ik, mA	Imass, A/g(Pt)	Isp, A/m2 (Pt)	Ik, mA	Imass, A/g(Pt)		Isp, A/m2 (Pt)
		Absolute Value	Average Value							Absolute Value	Average Value	Absolute Value	Average Value
Stage 2
M0.31	1	31	29 ± 2	0.92	3.9	1.9	340	9.1	2.1	337	324 ± 46	10.9	11.0 ± 1.3
	2	29		0.91	4.4	1.2	304	9.1	1.4	357		12.3
	3	28		0.9	4.5	1.2	306	9.1	1.1	278		9.9
M0.32	1	30	29 ± 2	0.9	4.3	0.8	189	5.1	0.9	222	193 ± 29	7.4	6.7 ± 0.9
	2	27		0.88	4.0	0.6	171	5.1	0.7	185		6.9
	3	30		0.88	3.4	0.8	183	5.1	0.7	173		5.8
M0.33	1	28	29 ± 3	0.9	3.7	1.0	253	8.1	1.0	245	226 ± 22	8.8	7.7 ± 1.1
	2	32		0.9	3.7	0.9	213	5.1	0.9	228		7.1
	3	28		0.89	3.3	0.9	206	6.1	0.9	204		7.3
Stage 3
M0.31	1	31	29 ± 2	0.92	3.9	1.9	340	9.1	2.1	337	324 ± 46	10.9	11.0 ± 1.3
	2	29		0.91	4.4	1.2	304	9.1	1.4	357		12.3
	3	28		0.9	4.5	1.2	306	9.1	1.1	278		9.9
M0.41	1	28	29 ± 2	0.92	3.9	1.9	491	16.1	2.0	523	462 ± 61	18.7	16.0 ± 2.7
	2	28		0.92	4.2	1.3	318	10.1	1.8	438		15.6
	3	31		0.92	4.1	1.5	377	11.1	1.7	424		13.7
M0.71	1	30	29 ± 1	0.87	3.2	0.6	155	4.1	0.6	153	131 ± 22	5.1	4.5 ± 0.6
	2	28		0.84	2.6	0.6	137	3.1	0.5	112		4.0
	3	29		0.87	2.5	0.6	156	4.1	0.5	129		4.4

), we investigated the effect of the solvent ratio (water-to-isopropanol) on the quality of the CL and the electrochemical characteristics of the PtCu/C catalyst.

Visual inspection of the CLs prepared with different solvent ratios at a constant Nafion content for the PtCu/C catalyst revealed that increasing the water content in the ink reduces layer uniformity. Photographs demonstrate that as the water content increases from method M^0.31 to M^0.33, the size of aggregates at the RDE periphery increases (Figure 3a–c). Consequently, the most uniform CL was achieved using method M^0.31 with a water-to-isopropanol ratio of 1:3.

Figure 3d–f presents the CVs of the surface activation stage for the PtCu/C catalyst with different H₂O:IPA ratios. During activation, the CVs develop the characteristic profile of a platinum electrode, indicating surface enrichment of the bimetallic nanoparticles with platinum and the formation of a secondary core–shell structure [33,38,39]. A decrease in current is observed in the hydrogen and double-layer (0.35–0.60 V) regions, while an increase occurs in the oxygen region. Similar to the Pt/C material, the ink composition and CL quality for PtCu/C did not significantly affect the ECSA values (Figure 3g).

Improved CL uniformity on the RDE correlates with enhanced ORR activity. Visual inspection of the RDE surface confirmed that the CL prepared using method M^0.31 exhibits the highest uniformity, with the corresponding catalyst demonstrating the highest mass activity of 340 A/g(Pt) and the smallest data scatter (

Table 3. Composition of catalytic inks.

Methodology Designation	H2O:IPA	I/C	VH2O, μL	VIPA, μL	VNafion 5%, μL
Pt/C
Stage 1
M0.31	1:3	0.3	492.5	1492.5	15
M0.32	1:1		992.5	992.5	15
M0.33	3:1		1492.5	492.5	15
PtCu/C
Stage 2
M0.31	1:3	0.3	492.5	1492.5	15
M0.32	1:1		992.5	992.5	15
M0.33	3:1		1492.5	492.5	15
Stage 3
M0.31	1:3	0.3	492.5	1492.5	15
M0.41		0.4	490.0	1490.0	20
M0.71		0.7	483.0	1483.0	34

, Figure S2a,b). As the water content in the ink increases, the half-wave potential of the LSV shifts, reaching 0.91 V for composition M^0.31 (Figure 3h).

Since the electron transfer numbers for M^0.32 and M^0.33 were below 3.8, the kinetic current, specific and mass activity were additionally calculated from a single polarization curve measured at a rotation rate of 1600 rpm (Table 2). Further analysis revealed that method M^0.31 enables the highest ORR activity for the PtCu/C catalyst.

2.4. Influence of Ionomer-to-Carbon Ratio in Catalytic Inks on the Electrochemical Properties of PtCu/C Catalyst

The previous section demonstrated that the most uniform CL forms using method M1. Nevertheless, some inhomogeneity and isolated catalyst aggregates remain present in the layer. Therefore, this solvent composition was selected for further investigation of the ionomer content effect in Stage 3 (Table 3). Analysis of the RDE surface with the deposited CL revealed that the most uniform layer forms at an I/C ratio of 0.4 (M^0.41) (Figure 4a–c).

Increasing the ionomer content in the catalytic ink intensifies the coffee-ring effect. With a further increase in the I/C ratio to 0.7, the formation of a Nafion film is observed, appearing as blue inclusions on the RDE surface (white arrows in Figure S3a). This film negatively impacts oxygen diffusion, consequently reducing the material’s ORR activity. We believe this effect is associated with a specific interaction between IPA and Nafion.

Figure 4d–f shows that the evolution of the CV profiles during the activation of the PtCu/C catalyst is similar for different I/C ratios. Current stabilization in all regions is achieved within a comparable number of cycles for all ink formulations (M^0.31—80 cycles, M^0.41—81 cycles, M^0.71—81 cycles). Analysis of the CVs further demonstrated that the ECSA value is independent of the catalytic layer quality (Figure 4g, Table 2).

The study of ORR activity identified an optimal CL composition that enables the highest performance for the PtCu/C material. When the I/C ratio is decreased from 0.7 to 0.4, the voltammogram develops a more pronounced S-shape, and an increase in current is observed in the diffusion-limited region, indicating improved reactant mass transport (Figure S3c). Further decreasing the I/C ratio from 0.4 to 0.3 does not alter the diffusion-limited current; however, a 10 mV negative shift in the half-wave potential is observed. The CL fabricated using method M^0.41 yielded the highest half-wave potential (0.92 V) and ORR mass activity (491 A/g(Pt)) for the PtCu/C catalyst (Table 2).

Analysis of the Tafel slope for the PtCu/C catalyst shows that the results obtained with all CL methods (Figure S4b) are characterized by a slope of about 60 mV in the potential region above 0.85 V. It is worth noting that the dependencies in the E-ln(I_k) coordinates show that method M^0.31 yields the highest catalyst activity in the potential region below 0.90 V. An increase in the amount of Nafion in the catalytic suspension leads to an increase in catalyst activity in the potential region above 0.90 V (Figure S4c). The highest activity was shown by the PtCu/C catalyst layer prepared using method M^0.41.The study demonstrated that the ionomer content in the catalytic ink affects the ORR activity of PtCu/C but does not influence its ECSA. Increasing the I/C ratio from 0.3 to 0.4 enhanced the mass activity by a factor of 1.4 while maintaining high measurement reproducibility. Notably, the optimal ink composition differed between Pt/C and PtCu/C catalysts. This indicates that the composition of platinum-based nanoparticles is a critical factor influencing the formation of the catalytic layer on the RDE.

3. Materials and Methods

The investigation focused on Pt/C and PtCu/C electrocatalysts supplied by PROMETHEUS R&D LLC (Rostov-on-Don, Russia), which were synthesized via patented methods [40]. Both catalysts utilized Vulcan XC-72 (Cabot Corporation, Boston, MA, USA) carbon black as a support. The platinum mass loading was 40 wt.% for Pt/C and 41 wt.% for PtCu/C.

3.1. Preparation of Catalytic Inks

The catalytic ink is an aqueous-organic suspension. Deionized water and isopropanol (IPA) in various ratios were used as solvents (Table 3).

The catalyst ink was prepared by adding a specified amount of IPA, deionized water, and a 5% aqueous-alcoholic Nafion^® D520 (The Chemours Company, Wilmington, NC, USA) emulsion to 4.0 mg of the catalyst sample (Table 3, Figure 5, Steps 1–3). The resulting suspension was then mixed on a magnetic stirrer for 5 min (Figure 5, Step 4). This was followed by a dispersion sequence consisting of ultrasonication in a water bath for 10 min and magnetic stirring for 5 min (Figure 5, Step 5). This two-step sequence was repeated once, resulting in a total preparation time of 30 min for a homogeneous ink (Figure 5, Step 6). The temperature of the water in the ultrasonic bath was maintained below 20 °C. Only freshly prepared inks were used for the investigation.

Catalytic inks were prepared according to different methodologies (Table 3). The mass-based ionomer-to-carbon ratio (I/C) was kept constant at 0.3 for all formulations.

3.2. Fabrication of the Catalytic Layer on the Rotating Disk Electrode (RDE)

Prior to the catalytic ink deposition, the glassy carbon end face of the RDE was polished using an alumina slurry (“Allied” aluminum oxide suspension) in accordance with the Pine Instruments manufacturer’s recommendations [41,42]. To remove any residual polishing particles, the electrode was subsequently ultrasonicated in deionized water and rinsed with IPA (a degreasing step).

The prepared catalytic inks were used to deposit a thin catalytic layer onto the glassy carbon electrode. Immediately before applying the ink aliquot, the RDE surface was wiped with deionized water (Figure 5, Step 7).

A 2 µL aliquot of the catalytic ink was pipetted while the ink was continuously stirred. A droplet of this specified volume was deposited onto the polished and cleaned glassy carbon electrode (5 mm diameter, 0.196 cm² geometric area). After the ink application, the electrode was dried by rotating it at 700 rpm in ambient air for 10–15 min until the droplet was completely dry (Figure 5, Steps 8 and 9), as recommended in [26]. Ambient humidity was maintained above 30% using a humidification system (Kitfort KT-2808 (Saint Petersburg, Russia)), and the temperature was kept between 21 and 24 °C. A schematic of the setup for catalytic layer fabrication is presented in Figure S5 (see Supplementary Materials).

After the first ink droplet had dried, a second droplet of the required volume was applied. The volume was calculated to achieve a total platinum loading on the RDE between 19.0 and 21.0 µg(Pt)/cm².

The quality of the deposited catalytic layer was visually inspected using a benchtop digital microscope with LED illumination (“G1000”, China) (Figure 5, Step 10).

3.3. Electrochemical Measurements

3.3.1. Determination of Electrochemical Active Surface Area (ECSA)

Prior to the electrochemical measurements, the electrolyte (0.1 M HClO₄) was saturated with argon for 40 min. The catalyst surface was then activated by potential cycling between 0.04 and 1.00 V for 100 cycles at a scan rate of 200 mV/s. All potentials in this work are reported versus the reversible hydrogen electrode (RHE) (Figure 6b).

Following the surface activation, two cycles of voltammograms (CVs) were recorded in the potential range of 0.04 to 1.00 V at a scan rate of 20 mV/s under an argon atmosphere. The ECSA was determined from the second CV cycle by calculating the electric charge associated with hydrogen desorption (Q_des) and adsorption (Q_ads), as shown in Figure 6c [43].

The ECSA was calculated using Equation (1):

$$\mathrm{ECSA} = {\displaystyle \frac{0.5\ast (Q_{ads}+Q_{des})}{210\ast m_{Pt}}}$$

(1)

where:

• ECSA is the electrochemical active surface area of platinum, m²/g;
• Q_ads is the electric charge associated with hydrogen adsorption (after double-layer correction), μC;
• Q_des—is the electric charge associated with hydrogen desorption (after double-layer correction), μC;
• 210 μC/cm² is the charge required for the adsorption of a hydrogen monolayer on a smooth polycrystalline Pt surface.

CV measurements for ECSA determination were repeated at least three times.

The error of the method is no more than 10%.

3.3.2. Determination of Activity in the ORR

Following the ECSA measurement, a background voltammogram was recorded using linear sweep voltammetry (LSV) in the potential range of 0.1 to 1.1 V. The measurement was performed under an argon atmosphere at an RDE rotation rate of 1600 rpm and a scan rate of 20 mV/s. LSV measurements should be performed by scanning from low to high potentials, as recommended by the authors of [44], since the Pt surface remains in a reduced state for a longer period, enabling more accurate measurement of ORR activity before the onset of irreversible oxide formation.

Subsequently, the electrolyte was replaced with a freshly prepared solution and saturated with oxygen for 40 min while rotating the RDE at 700 rpm. Following saturation, ORR polarization curves were recorded under identical LSV conditions at various rotation rates: 400, 900, 1600, and 2500 rpm (Figure 6d).

The ORR activity measurements were repeated at least three times to ensure reproducibility.

To account for the ohmic drop (iR compensation), the measured potential was corrected using Equation (2) [34,35]:

𝐸(RHE) = 𝐸_set + 𝐸_ref + 𝐸_pH − iR

(2)

where:

• E_set is the potential set value, V;
• E_ref is the reference electrode potential, V;
• E_pH is the adjustment for pH of the solution; and
• iR is the ohmic potential drop equal to the product of the current strength by the resistance of the cell. This resistance was no greater than 25 Ω.

The contribution of processes occurring in the deaerated electrolyte (Ar atmosphere) was accounted for by subtracting a background curve from the voltammograms recorded at each rotation rate. This background curve was measured on the same electrode in an Ar atmosphere at a rotation rate of 1600 rpm.

To plot the ORR polarization curves, the current was recalculated using Equation (3):

I = I(O₂) − I(Ar)

(3)

where:

• I(O₂) is the current for the ORR at a given potential, obtained from the measurement in an oxygen-saturated electrolyte.
• I(Ar) is the background (capacitive) current at a given potential, obtained from the measurement in an argon-saturated electrolyte.

The activity of an electrode towards the ORR is characterized by its kinetic current. The kinetic current represents the current flow in the absence of mass-transfer limitations. The measured current at the disk electrode (i) is the total current, encompassing all processes occurring at the electrode. The kinetic current, which excludes the diffusional contribution, can be determined using the Koutecký–Levich Equations (4)–(6) [4,28,45]:

$${\displaystyle \frac{1}{i}}={\displaystyle \frac{1}{i_k}}+{\displaystyle \frac{1}{i_d}}$$

(4)

$$i_d=0.62nFC{D}^{2/3}{\omega }^{1/2}{v}^{-1/6}$$

(5)

$${\displaystyle \frac{1}{i}}={\displaystyle \frac{1}{i_k}}+{\displaystyle \frac{1}{0.62nFC{D}^{\frac{2}{3}}{\omega }^{\frac{1}{2}}{v}^{-\frac{1}{6}}}}={\displaystyle \frac{1}{i_k}}+{\displaystyle \frac{1}{B{\omega }^{1/2}}}$$

(6)

where:

• i is the measured current at the disk electrode,
• i_k is the kinetic current,
• i_d is the diffusion-limited current,
• ω is the electrode rotation rate (rad/s),
• n is the number of electrons transferred in the electrochemical reaction,
• F is the Faraday constant (C/mol),
• D is the diffusion coefficient of oxygen (cm²/s),
• υ is the kinematic viscosity of the electrolyte (cm²/s),
• C is the bulk concentration of oxygen in the solution (mol/cm³).

To account for mass-transport limitations, the investigation must be conducted on a RDE at various rotation rates. To determine the kinetic current density (1/j_k), measurements are performed at several RDE rotation rates (400, 900, 1600, and 2500 rpm). The data are then plotted in Koutecký–Levich coordinates (1/J vs. ω⁻¹/²), where J is the current density normalized to the geometric area of the RDE.

The analysis proceeds as follows: on the voltammograms measured at each rotation rate, the current (I) is determined at a fixed potential of E = 0.9 V. This current is then divided by the geometric area of the RDE to obtain the current density (J). A plot is constructed with the four data points (current density for each rotation rate). By extrapolating the resulting linear fit to the y-axis (where ω = ∞, ω⁻¹/² = 0, indicating the absence of diffusion limitations), the intercept (b) is obtained. This intercept corresponds to the inverse kinetic current density (1/J_k) under conditions free from mass-transport limitations (Figure 6e).

The kinetic parameters were calculated using Equations (7) and (8):

j_k = 1/b

(7)

i_k = j_k × A

(8)

where:

• j_k is the kinetic current density (A/cm²),
• b is the y-intercept of the linear fit in the Koutecký–Levich plot (A⁻¹ cm²),
• i_k is the kinetic current (A),
• A is the geometric area of the RDE (0.19625 cm² for a 5 mm diameter disk).

To avoid potential inaccuracies from limited oxygen diffusion due to catalytic layer inhomogeneity, which can cause the Koutecký–Levich equation to yield underestimated values, the electrochemical activity was further evaluated by normalizing the current at a selected potential to the diffusion current at a rotation rate of 1600 rpm [44,46].

The kinetic current is then calculated using Equation (9):

$${\mathrm{i}}_{\mathrm{k}} = {\displaystyle \frac{i^{0.9}\ast i^{dif}}{|i^{dif}-i^{0.9}|}}$$

(9)

where:

• i^0.9 is the current measured at a potential of 0.9 V vs. RHE;
• i^dif is the diffusion-limited current.

Furthermore, it is necessary to determine the half-wave potential (E₁/₂). The ORR activity is typically quantified at a potential of 0.90 V vs. RHE.

It should be noted that since approximately 2005, this calculation protocol has been widely adopted by international research groups for determining the kinetic current [47,48,49].

Since around 2015, following key publications [25,43,50], standard practice has evolved to include:

• Current correction for ohmic drop (iR compensation) and background subtraction using measurements in an argon or nitrogen (Ar/N₂) atmosphere.
• The preparation of thin, uniform catalytic layers with low platinum group metal loading to minimize mass-transport limitations within the layer itself.

4. Conclusions

Visual inspection of the catalytic layers and analysis of the voltammograms for Pt/C and PtCu/C catalysts demonstrated that the composition of the catalytic ink significantly influences the formation of the catalytic layer and the ORR activity. The following conclusions were drawn from this study:

• A method for the visual assessment of catalytic layer quality was proposed, enabling the rapid and cost-effective determination of optimal catalytic ink preparation parameters without the need for expensive equipment (e.g., SEM or AFM). This approach is based on analyzing the macroscopic homogeneity of the layer and the absence of defects (aggregates, coffee rings), which correlates with electrochemical activity. Visualization using a digital benchtop microscope proved highly effective for the preliminary screening of catalytic layer quality prior to electrochemical measurements, thereby reducing sample preparation time and minimizing non-representative experiments.
• The highest quality catalytic layer for the Pt/C catalyst was formed using an ink preparation method with a solvent ratio of H₂O:IPA = 1:3 and an I/C ratio of 0.3. This method ensures the most uniform distribution of the catalytic material on the RDE surface, leading to the highest mass activity of 353 A/g(Pt).
• The best results for the PtCu/C catalyst were achieved using ink preparation method M^0.41 with a solvent ratio of H₂O:IPA = 1:3 and an I/C ratio of 0.4. This ink composition facilitates the formation of a uniform catalytic layer. The mass activity of PtCu/C under these conditions is 491 A/g(Pt), which is 25–30% higher than the values obtained with other investigated ink formulations.

The conducted research demonstrates the absence of a universal approach to catalytic ink preparation applicable to all types of catalysts. The optimal composition must be individually tailored for each specific catalytic material (Pt/C, PtCu/C, etc.) based on experimental data.

If low electrochemical performance is observed for a catalyst, we recommend not discarding it. Carefully examine its microstructure. If this parameter is inconsistent with the electrochemical characteristics, it is advisable to optimize the catalytic ink composition and the catalytic layer formation conditions. Adjusting the solvent composition, ionomer concentration, or deposition regime can significantly improve layer uniformity and material activity.

Future studies will benefit from the implementation of quantitative image analysis techniques, as recommended in the literature, to provide an objective metric for catalytic layer homogeneity and further minimize subjective assessment.