# Metal-free Advanced Energy Materials for the Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Ramesh Singh, John Douglin, Vipin Kumar, Polina Tereshchuk, Pietro Santori, Eduardo Ferreira, Gregory Jerkiewicz, Paulo Ferreira, Amir Natan, Frederic Jaouen, et al.

► To cite this version:

Ramesh Singh, John Douglin, Vipin Kumar, Polina Tereshchuk, Pietro Santori, et al. Metal-free Advanced Energy Materials for the Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells. Applied Catalysis B: Environmental, 2024, 357, pp.124319. 10.1016/j.apcatb.2024.124319. hal-04740656

HAL Id: hal-04740656

https://hal.science/hal-04740656v1

Submitted on 12 Jan 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Metal-free Advanced Energy Materials for the Oxygen
Reduction Reaction in Anion-Exchange Membrane Fuel
Cells

Ramesh K. Singh<sup>a,b</sup>, John C. Douglin<sup>a</sup>, Vipin Kumar<sup>c,d</sup>, Polina Tereshchuk<sup>c</sup>, Pietro G. Santori<sup>e</sup>, Eduardo B. Ferreira<sup>f</sup>, Gregory Jerkiewicz<sup>f</sup>, Paulo J. Ferreira<sup>g,h,i</sup>, Amir Natan<sup>c,d</sup>, Frédéric Jaouen<sup>e</sup>, and Dario R. Dekel<sup>a,j*</sup>

<sup>a</sup>The Wolfson Department of Chemical Engineering, Technion – Israel Institute of Technology, Haifa 3200003, Israel

<sup>b</sup>CO<sub>2</sub> Research and Green Technologies Centre, Vellore Institute of Technology (VIT), Vellore 632014, Tamil Nadu, India

<sup>c</sup>Department of Physical Electronics, Tel Aviv University, Tel Aviv 69978, Israel

<sup>d</sup>The Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 69978, Israel

<sup>e</sup>ICGM, Univ. Montpellier, CNRS, ENSCM, 1919 route de Mende, 34293 Montpellier, France.

<sup>f</sup>Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario, K7L 3N6, Canada

<sup>g</sup>International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga s/n, Braga 4715-330, Portugal

<sup>h</sup>Mechanical Engineering Department and IDMEC, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

<sup>i</sup>Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, USA.

<sup>j</sup>The Nancy & Stephen Grand Technion Energy Program (GTEP), Technion – Israel Institute of Technology, Haifa 3200003, Israel

<footnote>*Corresponding author: dario@technion.ac.il</footnote>
**Abstract**

The success of the next generation of anion-exchange membrane fuel cells (AEMFCs) depends on the development of active, reliable, and economical oxygen reduction reaction (ORR) catalysts. Here, we synthesize a series of ultra-low-cost metal-free ORR catalysts by doping a common pristine graphite precursor with chemically singular-type heteroatoms, namely I, S, N, or B, using single-step planetary ball milling technique. All dopedgraphites show substantially enhanced ORR performance relative to the pristine (undoped) graphite. Among all the tested catalysts, N-graphite exhibited the highest ORR onset potential of 0.87 V vs. reversible hydrogen electrode. These results are supported by density functional theory calculations. The ORR catalysts also exhibit remarkable stability as evaluated through electrochemical tests. Most importantly, the AEMFCs prepared using these ultra-low-cost doped graphites deliver notable peak power densities with impressive voltage efficiencies, which further supports their efficacy in ORR catalysis and the broad implementation of this technology

**Keywords:** oxygen reduction reaction; metal-free catalysts; ball milling; anion-exchange membrane fuel cell; density functional theory; energy conversion; device fabrication

**1. Introduction**

The broad deployment of fuel cell technologies for green electrochemical energy generation depends, among other factors, on advances in the development of precious metal-free catalyst chemistries to replace the expensive and scarce state-of-the-art platinum-group metal (PGM) catalysts. Among the various types of fuel cells, anion-exchange membrane (AEM) fuel cells (AEMFCs) are very promising because they allow, from a thermodynamic stability viewpoint, the implementation of PGM-free catalysts thanks to their alkaline environment.[1,2] In contrast to the mainstream proton-exchange membrane fuel cell (PEMFC) technology, the equivalent pH of the AEMFC is high, due to the anion-conducting properties of the solid polymeric electrolyte. Owing to the activity and stability of numerous earth-abundant
metallic, metal-oxide, metal-nitrogen-carbon (M-N-C) and other non-metallic (metal-free) materials in high pH conditions, the alkaline medium of the AEM greatly expands the library of available PGM-free materials to be used suitable as catalysts, for either the oxygen reduction reaction (ORR)[³⁻⁶] or the hydrogen oxidation reaction (HOR).[⁷] ORR is the major contributor to cell overpotential losses and thus is the focus of investigation in this article [⁴²,⁴³]. ORR catalysis occurs via the direct 4e⁻ transfer pathway on PGMs such as Pt,[⁴⁴,⁴⁵] Pd,[⁴⁴,⁴⁶] and Au.[⁴⁷] Apart from these, PGM-free transition metals-nitrogen-carbon (M-N-C) also efficiently catalyze the ORR in an alkaline medium through direct 4e⁻ pathways.[³⁴,⁴⁸⁻⁵⁷] Another class of PGM-free catalysts investigated is metal-free doped-carbon-based catalysts for ORR, particularly in alkaline electrolytes. Initial findings by Gong *et al*. on vertically aligned N-doped-carbon nanotube arrays, demonstrated reasonable ORR performance even in the absence of metal in the carbon matrix.[⁵⁹] Following that pioneering work, the doping of carbon with several heteroatoms such as N,[⁵⁹⁻⁶³] P,[⁶⁴] halogens,[⁶⁵⁻⁶⁷] B,[⁶⁸⁻⁷⁰], S,[⁷¹⁻⁷⁴] and co-doped heteroatoms[⁶⁸,⁷⁵⁻⁷⁷] have been recently reported to improve the ORR activity as well.

Various theories were proposed in the literature to explain the improved ORR activity of metal-free heteroatom-doped carbon vs. pure carbon: (i) enhanced ORR performance by dopants itself,[⁸⁸⁻⁹⁰] (ii) the distorted electronic environment of the neighboring carbon caused by the difference in electronegativity between the heteroatoms and the carbon, which enables oxygen adsorption,[⁵⁹,⁹¹⁻⁹⁵] and (iii) modification of the electronic property (the electronic cloud) at the carbon surface, as determined by its work function.[⁹⁶,⁹⁷] Based on these recent theoretical and experimental findings, the carbon atoms adjacent to the doping heteroatom are likely the most active site for the ORR in metal-free doped carbons.

As compared to PGM-based catalysts, metal-free catalyst shows high tolerance to carbon monoxide and methanol. Their electrochemical stability may be expected to be better than for
metal-based cathodes, due to the lack of metal-related degradation mechanisms, such as metal leaching and Ostwald ripening effects and aggregation, metal migration through the AEM, and metal detachment from carbon support in alkaline environment. Besides their cost and stability advantages, catalysts based on carbon and non-metallic doping elements may significantly reduce the degradation of AEM and anion-exchange ionomers (AEIs) in the electrodes by limiting radical formation [103,104]. While Fenton reactions between transition metals and hydrogen peroxide are less severe in alkaline than in acidic medium, AEMs may still be degraded by the radicals generated by metal-based ORR catalysts [97,105,106].

Among the various types of reported methods for the synthesis of metal-free carbon catalysts, the solid-state synthesis of heteroatomsdoped carbon using graphite as a carbon precursor is efficient for upscaled production with good reproducibility and thus possesses high promise for their practical use. It is noteworthy to mention that bare graphite alone does not have appreciable ORR activity in alkaline electrolyte. Modified graphites are reported to show appreciable ORR activity mainly due to the addition of defects and doping. In this regard, Dai et al. synthesized edge-halogenated graphene nanoplatelets (GnPs) for the ORR in alkaline electrolytes by planetary ball milling.[66] In contrast to their electronegativity, they found that the ORR activity follows the trend of I>Br>Cl. They attributed this trend to the atomic sizes of heteroatoms and charge-transfer-induced weakening of the O–O bond. The same research group also doped N,[85] H,[86] carboxylic,[86] sulphonic acid,[86] and a carboxylic/sulphonic acid mixture[86] in graphite by the same planetary ball-milling technique. In this technique, the doping process is facilitated by the generation of carbon-free-radical during the ball milling.[66,73,85–87] While numerous studies have investigated metal-free doped carbons as ORR catalysts in alkaline media using a rotating ring-disk electrode (RRDE) setup, only very few studies have investigated their performance at the cathode of AEMFC devices until now.
There is also a lack of comparison between ex situ performance of the catalysts and in situ
catalyst performance in operando AEMFCs.

In this contribution, we have selected graphite as an ultra-low-cost precursor for carbon,
which is ~60,000× cheaper than platinum and suitable for the large-scale production of
practical fuel cell cathodes. Likewise, we chose a one-pot modified ball milling technique for
doping, to remove the contamination from the Fe-impurities by reducing the overall ball
milling time to 8 h. This technique allows for a quick, one-pot process, able to produce high
quantities of catalysts at ultra-low costs. Heteroatoms-doped (I, S, N, or B) graphite as metal-
free ORR catalysts were investigated and a set of materials and surface characterization
techniques were employed to understand the observed ORR activity. The ORR
activity/selectivity of these doped graphites were investigated in RRDE techniques and
among them N-graphite shows the highest activity. Density functional theory (DFT) studies
were carried out to shed light on the mechanistic aspects and observed experimental trends.
Stability and corrosion measurements were also performed to illustrate the robustness of
these doped graphites in alkaline electrolyte.

The catalyst-ionomer interaction was studied to further understand the intimate contact
between them, particularly for I- and S-graphite. Finally, we have tested them as a promising
metal-free cathode in AEMFCs.

## 2. Experimental

### 2.1 Materials

Graphite powder (Fisher Chemical), iodine (99.999% from Merck), melamine (Sigma Aldrich, 99%), sulfur (99.99%, Sigma Aldrich), and $H_3BO_3$ acid (Bio-Lab) were used as received for the catalyst synthesis. Acetone (Bio-Lab, Israel), potassium hydroxide pellets (99.9%, Sigma Aldrich), isopropyl alcohol (99.8 wt% assay, Gadot group, Israel), Nafion
ionomer® perfluorinated resin (10 wt% in water, Sigma Aldrich), CS₂ (≥99.9%, Sigma Aldrich), hydrochloric acid (Daejung Chemicals, Republic of Korea) and deionized water (18.2 MΩ cm) were used for the catalyst ink preparation, cleaning, and electrolyte preparation. The gas diffusion layer (GDL) was Toray Paper 060 - TGP-H-060 (5 wt% polytetrafluoroethylene (PTFE) wetproofing, Fuel Cell Store). Vulcan XC-72 (Cabot) was deposited onto the GDLs to create microporous layers. Platinum 40%, ruthenium 20% on carbon black (HiSPEC® 10000, Alfa Aesar) was used to prepare the anode catalyst layer and the Fumion® AEI was received from Fumatech BWT GmbH. High-purity hydrogen, oxygen and argon gases (99.999%, Maxima, Israel) were used for RRDE and AEMFC tests.

## 2.2 Synthesis of Heteroatom-doped Graphite

I-, S-, N- or B-doped graphite was synthesized by a simple and low-cost one-step ball milling of graphite in the presence of iodine, sulfur, melamine, or $H_3BO_3$, respectively. First, a stainless-steel jar (80 ml) was loaded with ca. 125 g of stainless-steel balls (diameter 5 mm, weight 0.5 g), graphite (2.0 g), and heteroatom precursors to achieve an equimolar ratio of dopants to graphite (Table S1), and the jar installed in the ball-milling setup (Fritsch Pulverisette 6, classic line). Then the jar was charged and discharged with ten cycles of argon at 1.0 bar pressure to remove the air. The mixture was then ball milled at 500 rpm for 16 consecutive cycles of 30 min each with a 10 min cool-down period after each cycle. After completion of the milling cycles, the mixture was exposed to air to terminate potential residual carbon free radicals formed during the doping process.[66] The catalyst material was then collected from the jar and Soxhlet extracted in acetone to remove $I_2$ and $H_3BO_3$, and in $CS_2$ to remove S. The N-graphite was washed five times with hot water to remove the excess potential organic impurities. After the initial cleaning, all the catalysts were thoroughly washed twice with 5% HCl to remove any metal impurities that may be present from the stainless steel balls and jar. The resultant product was dried in a vacuum oven for 10 h at 60
°C. Then the powder was collected, ground with mortar and pestle, and used for further characterizations and tests. In the case of I-graphite, the nominal weigh ratio of iodine to graphite was 1:1, which was selected to get a high uptake of iodine.[66] An approximate nominal molar ratio of dopants to graphite, ca. 1.4:1, was maintained for all other dopants (S-, N-, B-). The milled graphite with I₂, melamine, sulfur, or H₃BO₃ was denoted as I-graphite, N-graphite, S-graphite, and B-graphite, respectively.

## 2.3 Instrumentation and Methods

Scanning electron microscopy (SEM) was performed at the electron microscopy center (MIKA, Materials Engineering Department) at the Technion on a high-resolution SEM equipped with a Schottky field-emission electron gun and a Gemini electron-beam column design (Zeiss Ultra Plus, Germany). For high-resolution imaging, the microscope was operated at a low voltage of 4 kV. Images were acquired by a classical Evart-Thornley or a high-resolution In-the-Lens secondary electrons (SE) detectors, using a working distance of about 3.5–4.5 mm. No conductive coating was applied at specified working conditions. Advanced transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) analysis of materials studies was performed using an aberration-corrected FEI Titan 80-200 ChemiSTEM microscope equipped with a field emission gun (FEG) and operated at 200 kV. TEM images were collected using a CCD Gatan UltraScan camera, while STEM images were obtained using a high-angle annular dark-field (HAADF) detector. The chemical maps were acquired by Energy Dispersive Spectroscopy (EDS) using a Super-X detector system based on four windowless silicon drift detectors, which enhance acquisition efficiency and spatial resolution. Compositional analyses of samples were conducted in STEM mode and using the ESPRIT proprietary software. Phase contrast TEM images were processed *via* Digital Micrograph. An FEI Talos 200C, high-
resolution FEG Cryo-Transmission Electron Microscope was also used to record TEM images.

X-ray diffraction (XRD) analyses were performed using a Rigaku Smartlab diffractometer with CuKα₁ X-ray source (λ = 0.15406 nm). The XRD data were recorded at medium resolution parallel beam geometry at a tube current of 150 mA and a tube voltage of 45 kV, in θ/2θ scan mode with a scan rate of 4° min⁻¹ in 0.01° steps, in a range of diffraction angles from 10 to 60°. Phases were identified via matching results with the International Centre for Diffraction Data (ICDD) PDF4+ (2018) database. Raman spectra were recorded using a confocal micro-Raman LabRam HR instrument from Horiba Scientific in backscattering geometry with a 50× objective mounted on an Olympus optical microscope. The excitation line was provided by VIS 532 nm laser (Torus, 100 mW) and a Peltier cooled charge-couple device (1024 × 256 pixels) was used as the detector. The calibration was initially made using a silicon reference at 520 cm⁻¹, which gives a peak position resolution of less than 1 cm⁻¹. For each acquisition, an average of five spectra and an acquisition time of 30 s were used. The Attenuated Total Reflectance (ATR) spectroscopy analyses were performed to measure the chemical signature of powders using a Bruker (Tensor 27) Fourier Transform Infrared (FTIR) spectrometer equipped with a high-sensitivity LN-cooled MCT detector.

The N₂-physisorption measurements were conducted using 3 Flex Brunauer, Emmett, and Teller (BET) surface area analyzer from Micromeritics using high-purity N₂. X-ray photoelectron spectroscopy (XPS) measurements were performed under ultra-high vacuum conditions (2.5 × 10⁻¹⁰ Torr base pressure) using a 5600 Multi-Technique System (PHI, USA). The samples were irradiated with an Al Kα monochromated source (1486.6 eV), and the emitted electrons were analyzed by a spherical capacitor analyzer using a slit aperture of 0.8 mm. The samples were analyzed at the surface and near-surface region only, and they
were not charged during measurements. Curve-fitting was performed with the Gaussian-Lorentzian function by using a 5600 multi-technique system software.

The bulk composition of elements (mainly iron, sulfur, and boron) of the obtained catalyst powders was determined using a dual-view high-resolution inductively coupled plasma-optical emission spectrometer (ICP-OES) spectrometer (PlasmaQuant PQ9000, Analytik Jena, Germany). The weighed samples (ca. 50 mg) were extracted with boiling Aqua Regia for 2 h in a 100 mL Erlenmeyer flask (Pyrex). After cooling, the extracts were transferred to a polypropylene flask and filled up to 20 ml with deionized water. The samples were filtered (0.45 µm filter Millex) prior to ICP measurement.

## 2.4 Electrochemical Characterization

Electrochemical measurements of the as-synthesized heteroatom-doped graphite catalysts were conducted in a three-electrode RRDE setup in an electrochemical cell (a water-jacketed, five-compartment glass cell) at 25±0.2 °C using WaveDriver electrochemical workstation (Pine Instruments). The RRDE with a disk diameter of 5 mm coated with doped-graphite catalysts was used as a working electrode, graphite rod as a counter electrode (ca. 10 times larger than the working electrode), and Hg/HgO (4.24 M KOH) was used as a reference electrode. The electrodes were acquired from Pine Instruments. The potential of the reference electrode was calibrated and reported against a reversible hydrogen electrode (RHE) in the same electrolyte.

For the typical preparation of the catalyst ink, we followed the procedure reported in our previous works.[31,107] In brief, 10 mg of catalyst sample was dissolved in 1.0 ml of de-ionized water, 4.0 mL of isopropyl alcohol (IPA), and 9.52 µL of Nafion® ionomer solution (10 wt.% in water), the mixture was then sonicated for 30 min in a Grant XUBA3 ultrasonic bath (100 % intensity, 200 W) to obtain a uniform suspension. An amount of 15.0 µL of ink suspension was coated on the cleaned, polished RRDE to achieve a catalyst loading of ~153
µg cm⁻². The deposited catalysts were dried in air for ca. 1 h to form a mechanically stable catalyst layer. Before ORR measurements, cyclic voltammograms (CVs) were recorded in Ar-saturated 0.10 M aqueous KOH electrolyte solution until the CVs were stabilized. The specific capacitance was calculated from CVs using equation (1):[¹⁰⁸]

$$C = \frac{1}{2 \text{ m} \nu (V_a - V_b)} \int I \, dV \qquad (1)$$

where C is the specific capacitance in F g⁻¹, m is the catalyst mass in g, v is the scan rate in V s⁻¹, V_a and V_b are the initial and final potential in V, respectively, and I is the current in A.

Subsequently, the electrolyte solution was saturated with O₂ until a constant open-circuit potential (OCP) was obtained; then linear sweep voltammograms (LSVs) were recorded between 1.00 to 0.00 V vs. RHE at a potential scan rate of 2.00 mV s⁻¹ with 1600 rpm. The scan rate is sufficiently low so that the capacitive current can be minimized. The % HO₂⁻ was calculated using equation (2):[¹⁰⁹]

$$\% \text{ H}_2\text{O}_2 = \frac{200 * (I_R/N)}{I_D + (I_R/N)} \qquad (2)$$

where % H₂O₂⁻ is the peroxide percentage, I_R is the ring current, I_D is the disk current, and N is the collection efficiency of RRDE (25 %, as reported by the manufacturer).

The number of electrons transferred (n) in the ORR was calculated using equation (3).[⁵⁹]

$$n = \frac{4 I_D}{I_D + (I_R/N)} \qquad (3)$$
The kinetic current ($i_k$) was calculated from the measured current corrected for the mass transport using Koutecký–Levich equation (equation 4) at 1600 rpm.[110]

$$ \frac{1}{i} = \frac{1}{i_k} + \frac{1}{i_d} \quad (4) $$

where $i$ is the measured disk current, $i_k$ is the kinetic current, and $i_d$ is the diffusion-limited current of the ORR. The $i_k$ is then normalized either to the catalyst mass to obtain mass-activity (mA mg⁻¹).

To evaluate the stability of the catalysts in an environment relevant to fuel cells, accelerated stress tests (AST) cycles were performed in O₂-saturated 0.10 M aqueous KOH solution between 0.60 - 1.00 V vs. RHE for 5000 cycles, at a scan rate of 100 mV s⁻¹. ORR voltammograms after the AST were recorded in a fresh electrolyte solution.

## 2.5 Density Functional Theory (DFT) Calculations

### 2.5.1 Computational Details

We have investigated the structural and catalytic properties of doped graphene using the Vienna ab initio Simulation Package (VASP) DFT code.[111–114] The core electron interactions were treated with the Projected Augmented Wave method (PAW).[115] The exchange-correlation functional was treated using the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE). We have used the VASPsol[116] scheme for an implicit description of the water solvent.[116] The atomic forces and total energy calculations were minimized using the conjugated gradient method in order to obtain relaxed atomic positions until below $10^{-3}$ eV Å⁻¹. An estimated energy difference of $10^{-6}$ eV was set for the geometrical convergence with a plane wave cut-off energy of 550 eV to expand the valence electron wave functions. We have used a graphene 4 × 4 × 1 supercell and a 5 × 5 × 1 Monkhorst-Pack K-points scheme for the geometry optimization. The model used for the
calculations consisted of a 4 × 4 × 1 graphene nanosheet in which doping was performed by substitution/replacement of a single carbon atom. A vacuum region of 15 Å was employed in the z-direction for eliminating interactions between adjacent cells layers. The structures geometries were drawn and visualized with the VESTA [117] and p4vasp [118] packages.

### 2.5.2 ORR Modelling

In alkaline environment conditions, the ORR process can be described by the following set of reaction steps that occur at the electrode surface: [119,120]

$$O_2 (g) + H_2O (l) + e^- + * \rightarrow OOH* + OH^- \quad (5)$$

$$OOH* + e^- \rightarrow O* + OH^- \quad (6)$$

$$O* + H_2O (l) + e^- \rightarrow OH* + OH^- \quad (7)$$

$$OH* + e^- \rightarrow * + OH^- \quad (8)$$

Here, * refers to a surface of pristine and doped graphene. The terms (g) and (l) refer to the gas and liquid phases, respectively; where OOH*, O*, and OH* are the adsorbed intermediates on the electrode surface.

Following Nørskov et al. [121] we assume that for the standard hydrogen electrode, at 298 K, pH=0, and 1.00 bar of H₂ pressure the reaction $\frac{1}{2}H_2 \rightarrow H^+ + e^-$ is balanced. Under alkaline conditions we can assume that at the reversible hydrogen electrode (RHE) the following reaction is balanced: [122][123]

$$H_2O + e^- \rightarrow \frac{1}{2} H_2 + OH^- \quad (9)$$

In addition, when the pH is different than zero, we correct the difference in free energy, due to entropy, by equation (10): [124]

$$\Delta G_{\text{pH}} = 0.0592\text{pH} \quad (10)$$
Whenever we apply a potential U to the electrode, the electron e⁻ experiences a change in its free (Gibbs) energy of -eU. Finally, since DFT typically overestimates the O₂ binding energy, we calculate the energy of molecular oxygen according to equation (11):[119,121]

$$G(O_2) = G(H_2O) - 2G(H_2) + 4.92\text{eV} \quad (11)$$

G(O₂), G(H₂O), and G(H₂) are free (Gibbs) energies of O₂(g), H₂O(l), and H₂(g), respectively. For each step of the overall ORR, the reaction free (Gibbs) energy ΔG is calculated as the difference between the free energies of the product and reactant molecules adsorbed on the surface which is given by:[119,120]

$$\Delta G_1 = \Delta G_{OOH^*} - 4.92 \text{ eV} + \Delta G(\text{pH}) + \text{eU} \quad (12)$$

$$\Delta G_2 = \Delta G_O^* - \Delta G_{OOH^*} + \Delta G(\text{pH}) + \text{eU} \quad (13)$$

$$\Delta G_3 = \Delta G_{OH^*} - \Delta G_O^* + \Delta G(\text{pH}) + \text{eU} \quad (14)$$

$$\Delta G_4 = -\Delta G_{OH^*} + \Delta G(\text{pH}) + \text{eU} \quad (15)$$

The ΔGᵢ describes the change of free (Gibbs) energy in equations 12-15 (a derivation for ΔG₁ is shown in the **Supplementary note S4**). The adsorption free (Gibbs) energy changes of these intermediate species are calculated using equation (16).

$$\Delta G_{\text{ads}} = \Delta E_{\text{ads}}^{\text{DFT}} + \Delta ZPE - T\Delta S \quad (16)$$

where the ΔE<sub>ads</sub><sup>DFT</sup> values are calculated with reference to H₂O(g) and H₂(g) as follows:[119,120]

$$\Delta E_{OH^*} = E_{(OH^*)} - E_{(*)} - (E_{H2O} - 1/2E_{H2}) \quad (17)$$

$$\Delta E_{OOH^*} = E_{(OOH^*)} - E_{(*)} - (2E_{H2O} - 3/2E_{H2}) \quad (18)$$

$$\Delta E_O^* = E_{(O^*)} - E_{(*)} - (E_{H2O} - E_{H2}) \quad (19)$$

The E<sub>H2O</sub> and E<sub>H2</sub> values are the DFT energies of H₂O and H₂ molecules in the gas phase, ΔZPE and TΔS are the zero-point energy difference and the free (Gibbs) energy change due
to entropy change, and T is the temperature (298.15 K in present work). The ΔZPE and TΔS values of OOH*, O*, and OH* species are taken from Nørskov et al. [121]

Based on the computational normal hydrogen electrode (NHE) model,[125] the limiting potential (U<sub>lim</sub>) can be obtained from equation (20) by determining the highest free (Gibbs) energy increase of all reaction steps:[120]

$$U_{\text{lim}} = -\max\{\Delta G_{1}, \Delta G_{2}, \Delta G_{3}, \Delta G_{4}\}/e \quad (20)$$

The limiting overpotential can be calculated as the absolute deviation between the calculated limiting potential and the 4e⁻ ORR equilibrium potential from equation (21) as below:[120]

$$\eta_{\text{lim}} = 1.23 - U_{\text{lim}} = 1.23 + \max\{\Delta G_{1}, \Delta G_{2}, \Delta G_{3}, \Delta G_{4}\}/e \quad (21)$$

Lower η<sub>lim</sub> represents a better ORR performance. The reaction step with the greatest increase in free (Gibbs) energy determines U<sub>lim</sub> and η<sub>lim</sub>, and we refer in the following to this step as the limiting step.

## 2.6 Anion-Exchange Membrane Fuel Cell Testing

The anode and cathode gas diffusion electrodes (GDEs) were prepared by the method described elsewhere.[14,31,38,57,126,127] Microporous layers consisting of carbon black were first added to the gas diffusion layers (GDLs) prior to depositing the respective catalyst layers. PtRu/C with a loading of 0.7 mg<sub>PtRu</sub>cm<sup>-2</sup> was used as a common anode catalyst and combined with the various doped-graphites as cathode catalysts (0.7 mg<sub>doped-graphite</sub>cm<sup>-2</sup>) to make a total of five membrane electrode assemblies. The ionomer-to-catalyst weight ratio was maintained at 20:80 and 30:70, for anode and cathode, respectively. The anode and cathode GDEs and radiation-grafted ETFE-based benzyltrimethylammonium-(BTMA)-AEM (55 ± 5 µm hydrated thickness and ion exchange capacity of 2.22 mmol g<sup>-1</sup>),[128] were immersed in
separate baths of aqueous 1 M KOH for 1 h. The aqueous KOH solution was changed every 20 min, thus three times, to ensure the complete exchange of Cl⁻ ions.

For the cell assembly and testing, 152.4 µm PTFE gaskets were used on the anode sides coupled with 127 µm PTFE gaskets on the cathode sides. A pinch of ca. 35% was achieved for each membrane electrode assembly, assembled *in situ* (not hot-pressed) between two 5.0 cm² single serpentine flow fields in Scribner Associates fuel cell testing hardware. The eight bolts of the cell assembly were tightened in a cross pattern at room temperature to 4.5 Nm torque. The cells were then activated by flowing N₂ at 1000 mL min⁻¹ for 1 h while heating up the cell from room temperature to 60 °C. After 1 h, humidified O₂ at the cathode (1000 mL min⁻¹, 2.0 bar back pressure, dew point 59 °C) and humidified H₂ (1000 mL min⁻¹, no backpressure, dew point 55 °C) at the anode were supplied. Following open circuit voltage (OCV) stabilization, the voltage was stepped down, in a 0.1 V increments, until arriving at 0.4 V while also adjusting the dew points to maximize the resultant current density. The same experimental conditions were employed in each fuel cell test for all the doped-C catalysts. All the reported polarization curves were obtained galvanostatically by point-by-point collection from high to low current at a 50 mA step size with 10 s at each step.
### 3. Results and Discussion

SEM images of the pristine graphite, milled graphite, and doped-graphite materials are shown in **Figures 1a-f**, respectively. After the milling process, the bulk graphite was fragmented into particles a few microns in size, which was further decreased in the presence of dopants (**Figures 1c-f**). These results suggest that sufficient mechanochemical energy was generated during the ball milling process and capable of breaking the C-C bonds, thus leading to delamination in presence of dopants. The TEM images of the B-, N-, S-, and I-graphites show a sheet-like structure (**Supplementary Figure S1**), which further reveals the delamination of the graphitic layer into a few layers of graphene. The corresponding XRD patterns are shown in **Figure 1g**. Graphite shows a predominant peak at a 2 theta angle of 26.5° due to the (002) lattice reflection. A significant decrease in the XRD peak intensity at 26.5° is observed for all the milled graphites. Their interplanar spacing (d) and the crystallite size estimated from the (002) graphite reflection are given in **Table 1**.

The ranking of materials according to their lateral graphite crystallite size follows the trend I-graphite << S-graphite < N-graphite < B-graphite < milled-graphite < pristine-graphite. The crystallite size of the doped-graphite decreases relative to the pristine graphite, and it is reduced up to a factor of 1/16 in the case of the I-graphite. The reduction in crystal size from pristine (or milled) graphite to the milled doped-graphite strongly correlates to the atomic number of the doping atoms. This is supported by the finding of Jeon *et al*. where they observed a similar trend.[66] For instance, iodine, which has the highest atomic number, resulted in the smallest crystal size as estimated from the XRD. At the same time, the lattice parameter in the case of the doped-graphite is similar to pristine graphite.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Secondary electron SEM images of the pristine graphite (a), milled graphite (b), B-graphite (c), N-graphite (d), S-graphite (e), and I-graphite (f). XRD patterns (g) and Raman spectra (h) of the pristine graphite (i), milled graphite (ii), B-graphite (iii), N-graphite (iv), S-graphite (v), and I-graphite (vi).</figcaption></figure>

<table><caption>**Table 1.** The crystallite size, interplanar spacing (d) from the XRD (002) reflection, and $I_D/I_G$ ratio of doped-graphite catalysts.</caption><thead><tr><th>Samples</th><th>Crystallite size<br>(nm)</th><th>d<br>(Å)</th><th>I<sub>D</sub>/I<sub>G</sub></th></tr></thead><tbody><tr><td>I-graphite</td><td>1.7</td><td>3.42</td><td>1.75</td></tr><tr><td>S-graphite</td><td>9.2</td><td>3.38</td><td>0.95</td></tr><tr><td>N-graphite</td><td>14.0</td><td>3.38</td><td>1.13</td></tr><tr><td>B-graphite</td><td>18.8</td><td>3.38</td><td>0.36</td></tr><tr><td>milled-graphite</td><td>25.4</td><td>3.38</td><td>0.38</td></tr><tr><td>pristine-graphite</td><td>27.8</td><td>3.37</td><td>0.30</td></tr></tbody></table>
These results suggest that the graphite has delaminated into a few layers of graphene-sheets (Figure S1). In addition, the decrease in the XRD peak intensity after milling may suggest a significant exfoliation of the graphitic layer, which is highest in the case of I-graphite, similar to that reported by Jeon *et al.*[66] However, these powders have very different crystal sizes. Given that their compaction is different and may influence the peak intensity of the (002) graphitic lattice reflection, it is therefore, it is important to compare the crystal size rather than the peak intensity. The corresponding Raman spectra of the doped-graphite catalysts are shown in **Figure 1h**. A characteristic band at 1345 and 1580 cm⁻¹ is observed for all the samples, noted as D and G peaks, respectively. The D band is assigned to the disorder in the sp² hybridized carbon, while the G band corresponds to the E₂g phonon vibration of the sp² hybridized carbon.[129] Interestingly, after milling, all the doped-graphite samples show an increase in the D-band intensity indicating the defects are introduced in the material. The intensity ratio of peaks D and G (I_D/I_G ratio) indicates that defects are present in the graphitic structure, and the following I_D/I_G trend is observed (**Table 1**): pristine graphite ~ B-graphite < milled graphite < S-graphite < N-graphite < I-graphite. Except for the B-graphite, there is an inverse relationship between the graphite crystal size estimated from XRD and the I_D/I_G ratio. The slightly higher I_D/I_G ratio of the N-graphite compared to the S-graphite is anticipated mainly due to the ca. 6-fold higher nitrogen content as compared to the S content (see below, in the XPS results discussion).

TEM/STEM images of the I- and S-graphite catalysts are shown in **Figures 2 and 3**, respectively. The I- and S-graphite samples were chosen as they show a better mass-thickness contrast between the dopant and carbon atoms in HAADF STEM. The TEM images show a distribution of particle sizes ranging between 0.3 µm to 1.5 µm (**Figure 2a**), as well as the stacking of a few graphene sheets (**Figure 2b**). A close examination of the exfoliated layers (edge-on) reveals a thickness of ca. 5 nm (**Figures 2c, d**). The graphene sheets shown in
**Figure 2d** are crystalline in nature, as confirmed by the fast Fourier transform (FFT) pattern (insets in **Figures 2c,d**). The HAADF STEM images and the STEM-EDS maps were recorded to show the iodine distribution in the I-graphite (**Figures 2e–l**). The maps are recorded at various magnifications to better show the atomic dispersion of the iodine in the structures.

<figure><img src="image_3.png" /></figure>

**Figure 2.** TEM and HAADF-STEM images, as well as STEM-EDS elemental mappings of the I-graphite: Bright-field TEM image (a-b), Phase contrast TEM images (c-d), HAADF STEM image (e), Overlay EDS map of carbon and iodine with HAADF image (f), EDS map of carbon (g), EDS map of iodine (h), higher magnifications (i-l) of regions (e-h). The insets in (c) and (d) show the FFT patterns obtained from the phase-contrast images.

Similar images of the S-graphite are shown in **Figure 3**. The HAADF STEM images and the STEM-EDS maps (**Figures 3e–l**) show the distribution of C and S in the S-graphite. The EDS maps of **Figure 3** show a homogenous dispersion of S. The difference in phase contrast is caused by the difference in thickness of the grains.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** TEM and HAADF-STEM images, as well as STEM-EDS elemental mappings of the S-graphite: Bright-field TEM image (a-b), Phase contrast TEM images (c-d), HAADF STEM image (e), Overlay EDS map of carbon and sulfur with HAADF image (f), EDS map of carbon (g), EDS maps of sulfur (h), higher magnifications (i-l) of regions (e-h).</figcaption></figure>

$N_2$ physisorption measurement of the doped graphite catalysts is performed to analyze the isotherm type, pore size distribution, and the BET surface area (Figure 4 and Table S2). I-graphite clearly shows a much higher microporous volume compared to the other three doped graphites, due to the vertical rise of its isotherm at low $P/P_o$ (Figure 4a). In contrast, the shape of the isotherms at intermediate $P/P_o$ values of 0.2-0.8 identifies a smaller mesopore volume for the I-graphite vs. others. This is quantitatively confirmed in the pore size distribution (PSD) curve (Figure 4b). All the doped-graphites show a small hysteresis during $N_2$ desorption, which closes below 0.45 ($P/P_o$) for S-, N- and B-graphites. The strongest hysteresis was observed with S-graphite, which is typical of macroporous agglomerates.[130]
The porosity at low P/Po increases with an increase in the atomic weight of the dopants and it is highest with I-graphite.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** The N₂ adsorption-desorption isotherms (a) and corresponding pore size distributions (PSDs) of the N-, I-, S-, B-doped graphites, and milled graphite (b).</figcaption></figure>

The pore size distribution reveals both micro- and meso-porous structures (Figure 4b). The BET surface area is in the order of I-graphite > S-graphite > B-graphite > N-graphite > pristine graphite ~ milled-graphite (Table S2). Pristine graphite has a BET surface area of
only 28 m² g⁻¹. The highest BET surface area of 331 m² g⁻¹ is obtained with the I-graphite, with a microporous surface area of 119 m² g⁻¹, while the S-graphite shows the highest pore volume at the highest P/P₀ (Figure 4b). The increased BET surface area of the doped graphite by milling versus the pristine graphite suggests the creation of pores, either between graphitic crystallites and/or internal pores/defects inside the graphene sheets. It is interesting to look at the correlation between the types of pores and the crystal size of the graphitic crystallites measured by XRD. The size of the space between graphite crystals is expectedly commensurate with the lateral size of these crystallites. This is obvious for I-graphite that there is a correlation between its small average lateral crystal size (1.7 nm, Table 1) and the average size of the micropores (peak at 5 Å in the PSD, Figure 4b), while the three other materials are dominated with mesopores of 3-10 nm width, corresponding with their higher average lateral graphite crystal size (9-19 nm). The average number of stacked graphene monolayers can be estimated from the measured BET areas and using the theoretical value of 2630 m² g⁻¹ as the surface area of monolayer graphene,[131] (neglecting surface area created by defects within graphene sheets and neglecting surface area created by the edges of graphite crystallites), yielding values of 8, 20, 89, 30, and 95 stacked graphene sheets for the I-graphite, S-graphite, N-graphite, B-graphite, and pristine-graphite, respectively. Thus, ball milling in the presence of a heteratom doping precursor can be efficiently utilized to simultaneously produce multi-layer structured graphene from graphite. Milling the pristine graphite in the absence of a doping precursor did not result in an enhancement to the BET area (Table S2), indicating the essential role of the doping precursor also in delaminating the graphite and reducing the lateral crystal size.

The elemental composition of the doped-graphite structures was investigated by XPS analysis. The XPS survey spectra of the B-, N-, S-, and I-doped graphite materials are shown in Figure S2. In all doped-graphite samples, a predominant peak of C1s (284.0 eV) and a
minor peak of O1s (532.6 eV) are observed. The small O1s peak can be due to the formation
of oxygen groups from highly reactive C groups upon air exposure after ball-milling. The
highest oxygen elemental content is found with the I-graphite, reaching 3.2 at% (Table S3),
suggesting at first sight that the I-graphite may be more prone to oxidation in air than other
doped-graphite catalysts. To compare the oxidation of the carbon surface among samples
with vastly different BET areas, we think however that a more correct descriptor is the
oxygen content normalized by the BET area. After such normalization, the I-graphite appears
in fact to have the lowest oxidation vs. other doped-graphite catalysts, which may be
connected to its higher microporosity (Table S3). The XPS signals of B, N, S, and I observed
in the survey spectra prove the presence of dopants in the graphite structure and their
estimated atomic composition is given in Table S3. The atomic content of the dopant in B-,
N-, S-, and I-graphite is 0.05, 13.45, 2.17, and 2.09 %, respectively. The atomic content of I
in I-graphite is very close to that obtained from the bulk EDS (Table S3), in line with the
high BET area of this material and the recognized fact that XPS is truly not a surface
sensitive technique for high surface area carbons due to the escape depth of photoelectrons
through carbon. The highest doping with this doping method was observed with N-graphite,
at 13.45 at% of N. This value is comparable to the work by Xue *et al.*, where they found 11.4
at% of N after functionalization via ball milling for 48 h.[132] To verify that the high N-
content primarily originates from N-groups covalently bound to the carbon surface and not
from unreacted melamine that was used as an N-precursor, we recorded the FTIR-ATR
spectra of melamine and N-graphite that are shown in **Figure S3**. The peaks observed on
melamine at ~3417, 3347, 3329, and 3129 cm⁻¹ are from –NH₂ stretching vibrations, and the
peaks at ~1650, 1535, and 812 cm⁻¹ are from the triazine ring of melamine.[133] All these
peaks are almost absent in the N-doped graphite, suggesting there are no melamine traces
present indicating that the experimentally determined N-content is from the sample. B is not
seen in the quick survey spectrum of B-graphite (Figure S2), due to the small amount of B present, estimated to 0.05 at % from narrow scan spectra (Figure S4).

To understand the different bonding sites of the chemical species, the high-resolution XPS spectra for C1s, N1s, S2p, and I3d were analyzed and fitted for the N-, S-, I-graphite (Figure 5). For B-graphite, the XPS spectra of B1s and C1s are shown in Figure S4 and Figure S5, respectively. The high-resolution C1s and O1s spectra were also measured for the pristine and milled graphites (Figure S6). In addition, O1s spectra were measured for all the doped graphite catalysts, showing only a broad peak at 532.6 eV (Figure S7).

The C1s XPS spectrum is deconvoluted into seven peaks corresponding to graphite, hydrocarbon (C–H) contamination, C–O, C=O, C–O–C=O, HO–C=O, and π-π* transitions (Figures 5a,c,e).[134] Among them, the highest contribution to the C1s spectrum comes from the graphite and hydrocarbon contamination for all the samples and their relative percentages are given in Table S4. The predominant graphitic peak of C1s spectra is negatively shifted (to lower binding energy values), especially in the case of N-, S-, and I- graphite catalysts, as compared to that of the pristine and milled graphite (Table S4). This shift is assigned to the presence of the heteroatoms in the graphitic carbon structure (Figure S6). The C–N and C–S bond formations with the N-, S-, and I-graphite catalysts, respectively, were observed from the C1s spectra (Figures 5a,c). The high-resolution fitted elemental XPS spectra of the N1s, S2p, and I3d bands are shown in Figures 5b, 5d, and 5f, respectively. The fitting results of N1s, S2p, and I3d narrow scan regions are given in Table S5.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** High resolution experimental and fitted XPS spectra of the N-graphite for C1s (a) and N1s (b); spectra of the S-graphite for C1s (c) and S2p (d); and spectra of the I-graphite for C1s (e) and I3d (f).</figcaption></figure>

The C–N bond formation is confirmed by the N1s spectrum of the N-graphite. The N1s spectrum is deconvoluted into four peaks located at 398.65, 399.50, 400.75, and 402.20 eV,
which are assigned to pyridinic N,[135] pyrrolic N,[135] -NHC=O[135]/C-NH-C, and -N(CH₃)³⁺ (quaternary N),[134] respectively (Figure 5b). The summary of the fitted parameters are listed in Table S5.

Similarly, S2p spectrum is deconvoluted into four different binding configurations that correspond to R-SH, C-S-C, S-S, and C-SO₂-C (Figure 5d).[136,137] The major contribution is mainly from the C-S-C bond, representing 72 % of the sulfur. The S-S bond (ca 11 % of the sulfur) is likely due to unreacted sulfur, which was not removed by Soxhlet extraction in CS₂. The I3d spectrum is split into two regions (I3d₅/₂ and I3d₃/₂) due to spin-orbit coupling. The I3d₅/₂ peak is deconvoluted into three different peaks at 618.70, 620.70 and 622.30 eV (Figure 5f), primarily attributed to different C-I bonding configurations. These results suggest no C-I⁺ bond formation at 619.9 eV, in contrast to the work reported by Jeon *et al.*[66]

Following the chemical characterization of the doped graphite catalysts, the electrochemical measurement of the doped graphite is performed to understand the effect of the dopant onto the ORR performance. **Figures 6a,b** show overlaid CV profiles and mass-specific capacitance vs. BET surface area graphs of the pristine and heteroatom-doped graphite materials. The CVs combined with the known loadings were used to determine the average specific capacitances in the potential range 0-1.0 V vs. RHE. The doped graphite catalysts show an improved mass-specific capacitance as compared to the pristine graphite (Figure 6a). For the I-, S-, N-, B-, and milled-graphite, the mass-specific capacitance is 16.2, 6.9, 6.1, 4.2, and 2.2 times higher, respectively, than that of the pristine graphite. This is attributed to the delamination of the graphitic structure with doping, leading to increased BET area (Figure 6b). For N-graphite, the BET is almost the same as pristine graphite, so the delamination does not occur and the increase in capacitance seems to be only due to the doping effect.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** CV profiles of the N-graphite, I-graphite, S-graphite, and B-graphite, pristine graphite, and milled graphite (a) in nitrogen-saturated 0.10 M aqueous KOH solution acquired at a potential scan rate of 50.0 mV s⁻¹. Specific capacitance vs. BET surface area graph (b).</figcaption></figure>

Next, the doped and undoped graphite materials were characterized in a RRDE using a three-electrode configuration, to evaluate their alkaline ORR performance. The registered ORR polarization curves of the doped-graphite materials are shown in **Figure 7a**, and mass-transport corrected Tafel plots are presented in **Figure 7b**. After doping with the heteroatoms, the graphite materials show a clear improvement in the ORR performance relative to the pristine- and milled-graphite, indicating that these materials can be excellent catalysts towards alkaline ORR. For instance, ORR onset potentials of 0.84, 0.83, 0.87, 0.85, and 0.78 V are observed for I-, S-, N-, B-graphite, and milled-graphite, respectively, which is higher than for pristine graphite (0.76 V). A positive shift in ORR potential was observed with N-, I-, B-, S-graphite, and milled graphite, and it is 150, 104, 94, 75, and 30 mV @-1 mA cm⁻², respectively, compared to pristine graphite. It is important to note at this point that we ruled out the possibility of iron contamination on the doped-graphite catalysts during the milling
process. In order to confirm this, we carried out chemical analyses of the catalysts after the milling process, and no significant traces of iron were found (see **Table S6** and **Supplementary Note 1**).

To further quantify the effect of doping, the ORR polarization curves were corrected for diffusion-limitation (Tafel plot) to extract the kinetic parameters. From **Figure 7b** it can be seen that all the heteroatom-doped graphite catalysts show significantly higher ORR mass activity as compared to that measured with pristine graphite. At 0.75 V, we observed 9.1, 50.4, 6.1, 14.2 times higher mass activity for B-graphite, N-graphite, S-graphite, and I-graphite, respectively, than with pristine graphite (**Figure 7c**). This high ORR performance is attributed to a combination of increased BET and doping; except for N-graphite, where BET is virtually unchanged.

It is reported that pyridinic and graphitic N contribute ca. 50 and 30 %, respectively, to the active N sites for ORR.[80] In this study, both pyridinic and graphitic N contribute up to 20% of the overall N active sites, which are responsible for the electrocatalytic activity towards the ORR. The onset potential, the half-wave potentials, and the stability of the doped graphite in comparison to all the relevant literature data are given in **Table S7**. The ORR onset potential and diffusion-limited currents of these doped-graphite catalysts are higher than the catalysts produced by a ball milling method.[66,73,85] For instance, Jeon *et al.*[85] reported N-GnPs synthesized by ball-milling showed the onset potential of 0.77 V vs. RHE and limiting current density of -1.7 mA cm⁻² (at the same rotation rate used in this work), which is lower than any of the metal-free graphite reported in this study.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** RRDE ORR polarization curves and mass-transport corrected Tafel plots (a,b) of the doped-graphite catalysts in O₂ saturated 0.10 M KOH aqueous solution acquired at a potential scan rate of 2.00 mV s⁻¹ with a rotational speed of 1600 rpm. Mass-transport corrected mass activity (mA mg⁻¹) at 0.75 V (c). Percentage of peroxide detected at the ring (d). The data is not corrected for the ohmic loss. For the control experiment, the ORR polarization curves of pristine, and milled graphite were recorded at 5.00 mV s⁻¹ by virtue of their smaller BET surface area. The catalysts loading is 0.15 mg cm⁻² on the electrode.</figcaption></figure>

The percentage of peroxide formed and the corresponding number of transferred electrons (n) are analyzed for the heteroatom-doped graphite catalysts as a function of potential and are shown in **Figure 7d** and **Figure S8**, respectively. The trend of peroxide percentage is bare GC > milled graphite > B-graphite ~ I-graphite > pristine graphite > N-graphite > S-graphite. A relatively low peroxide generation is obtained with S- and N-doped graphite catalysts, with n = 3.5-3.6 electrons transferred, suggesting their effectiveness in catalyzing the four-electron ORR. Given that these catalysts (and their preparation method) are not optimized, these values are higher[71,73,86] than those reported in the literature for metal-free ORR catalysts.
Further optimization will improve the catalyst performance even more, while keeping the advantage of ultra-low cost, and a simple and rapid one-step fabrication process.

We analyze the effect of loading on the ORR performance. For this, we focus on the N-doped graphite, as this catalyst provides good electrocatalytic properties. The ORR polarization curves and the corresponding peroxide current of the N-doped graphite material were acquired for two different loadings (Figure 8). With the increase of catalyst loading from 0.153 to 0.459 mg cm⁻², ~2 times increase in mass activity @0.75 V is observed, and, similarly, the kinetic current at 0.80 V increases from 0.45 to as high as 1.00 mA cm⁻² (Figure 8a). This increase in performance can be explained based on the high active sites, in line with other reports.[138,139] A decrease in peroxide generation (and an increase in the number of electrons transferred n) is also seen with an increase in loading (Figure 8b). Similar behavior was also reported by Bonakdarpour et al. for Fe-N-C catalysts, suggesting that the ORR proceeds through H₂O₂ intermediates.[139] The peroxide currents for both loadings of the N-doped graphite are comparable in the potential range of 1–0.70 V. Below 0.70 V lower peroxide currents are observed at higher loadings. The high loading increases the probability for peroxide produced deep inside the layer to further be reduced before escaping into the electrolyte. The strong trend in the peroxide formation can be explained based on a dual-path (2+2 electron) ORR mechanism, and the evolution of peroxide features shed light on the ORR pathway of these catalysts.[140] The oxygen reduction may proceed through the series of reaction pathways (equations 22-24) via the inner electron transfer mechanism, including HO₂,ads reaction intermediates.[3]

$$O_2 \rightarrow O_{2,ads} \qquad (22)$$

$$O_{2,ads} + H_2O + 2e^- \rightarrow (HO_2^-)_{ads} + OH^- \qquad (23)$$

$$(HO_2^-)_{ads} + H_2O + 2e^- \rightarrow 3OH^- \qquad (24)$$
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Tafel plots of the N-graphite with different catalyst loadings (a) and the corresponding ring current (b). The data were acquired in O₂-saturated 0.10 M aqueous KOH at a potential scan rate of 2.00 mV s⁻¹ with 1600 rpm. The inset (b) shows the corresponding H₂O₂ %. *j*ₖ is normalized with respect to the geometric surface area of the electrode.</figcaption></figure>

However, the possibility of the outer electron transfer mechanism cannot be excluded in the case of these metal-free ORR catalysts.[³] In brief, in the range of 1.00–0.80 V, negligible peroxide is detected, as expected, as thermodynamically speaking no peroxide formation occurs in this range. In the potential range of 0.80–0.60 V, a gradual increase in peroxide formation with the decrease in potential is mainly attributed to the series of reaction pathways involving electrochemical reduction of O₂,ads to HO₂,ads (equation 23), and this transformation is driven by the potential-activated step. In this potential range, the ORR predominantly occurs through the HO₂⁻ mediated pathway. Below 0.60 V, a decrease in the peroxide generation is noticed, mainly due to the gradual change in the reaction pathway to the direct 4 electron pathways to water (equation 24). Thus, the ORR proceeds through 2+2 electron pathways; first, O₂ is reduced to peroxide intermediate and then to hydroxide ion.

**Figure 9** shows the ORR polarization curves (Figure 9 a,d,g,j), Tafel plots (Figure 9b,e,h,k), and corresponding % H₂O₂ produced (Figure 9c,f,i,l) of the doped-graphite catalysts, before and after 5,000 accelerated stress test (AST) load cycles. The negative shift in the potentials at -1.00 mA cm⁻² is only 20, 10, 5, and 20 mV for the B-, N-, S-, and I-
graphite, respectively, pointing to their remarkable stability in alkaline medium. The corresponding relative decrease in the mass activity ($i_{k,m}$) of the doped graphite catalysts at 0.75 V is 37, 29, 11, and 20 %. The stability results of the doped graphite catalysts are comparable to the high stability of Fe-N-C catalysts reported by Santori *et al.*,[56] and significantly more stable than Pt/C catalyst, reported by Lafforgue *et al.*,[100] highlighting the advantage of the metal-free heteroatom-doped graphite catalysts for AEMFC applications.

The corresponding peroxide percentage values shown in **Figures 9c,f,i,l** reveal an increase in the peroxide formed after AST cycles for all four materials, but more for S-graphite and to a lesser extent N-graphite, than for I- and B-graphite. Therefore, the excellent stability of these catalysts during the load-cycling AST in $O_2$-saturated electrolyte is very promising for their eventual implementation at the cathode of highly durable AEMFCs.

<figure><img src="image_10.png" /><figcaption>**Figure 9.** ORR polarizations (a,d,g,j), corresponding Tafel (b,e,h,k) and %$H_2O_2$ (c,f,i,l) plots before and after 5000 load cycles of the accelerated stress test (AST) for doped-graphite: the</figcaption></figure>
I-graphite (a,b,c), S-graphite (d,e,f), N-graphite (g,h,i) and B-graphite (j,k,l) with 1600 rpm in O₂-saturated 0.10 M aqueous KOH acquired at a potential scan rate of 2.00 mV s⁻¹. The load cycles were performed in O₂-saturated 0.1 M KOH solution from 0.60 – 1.0 V vs. RHE at scan rate of 100.0 V s⁻¹ in the anodic direction.

The stability of electrode materials is very important and determines their applicability in real life devises. Consequently, we also investigated in liquid electrolyte the sensitivity of these doped-graphite catalysts to electrochemical carbon corrosion.[141] This is only done as an example for one sample. The I-graphite has been chosen for these measurements due to its highest BET surface area and expected proneness to the highest degradation to carbon corrosion. The carbon corrosion is an important degradation mechanism reported for PEMFCs,[142–145] resulting from cathode potential excursions generated by repeated start-up/shut-down cycle.

We used a protocol originally developed by Jerkiewicz group[146–148] for bulk and nanoparticles of Pt in acidic environment. The PDP measurements are used to access the corrosion properties in both anodic and cathodic directions at a low potential scan rate of 0.10 mV s⁻¹.[147] Both anodic and cathodic corrosion plots of the I-graphite in N₂ and O₂ environments, in 0.10 M aqueous KOH electrolyte, are shown in **Figure S9**. The corresponding CV and ORR polarizations recorded before and after PDP measurements are shown in **Figure S10**. The CV profiles and ORR polarization curves suggest stable catalyst performance in both anodic and cathodic PDP measurements. The corrosion potential (Ecorr) and corrosion current density (jcorr) in each condition are deduced from the logarithmic plot and listed in **Table S8**. The Ecorr was observed to be similar in both N₂ and O₂ environments, but corrosion behavior is noticed to depend on the direction of the potential scan, thus on the state of the I-graphite prior to the corrosion test. The higher jcorr and lower Ecorr values were observed in the anodic scans as compared to the cathodic scans with a lower corrosion rate irrespective of the gaseous environment, thus suggesting that the I-graphite is prone to
comparatively higher corrosion rate in anodic scan most likely the chemical state of the electrode is different. Therefore, these findings show the stability of I-graphite towards carbon corrosion.

To further increase the understanding of the improved performance and the alkaline ORR mechanism and intermediates, thorough DFT calculations were performed. **Figure 10** shows the optimized energy structures of N-, S-, and B-doped graphene, in which one C atom is replaced by N, S, or B atom. Further explanation is given in **Supplementary Note 2** and shown in **Figure S11**. The adsorption of the ORR process intermediates, namely OOH, O, and OH, on the pristine and doped graphenes was then estimated. To obtain the lowest energy structure, we placed the oxygenated species in numerous different locations and orientations on the doped and pure graphene and performed geometrical relaxation.

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Top and side view of the optimized structure of N- (a), B- (b), and S-doping (c) on a 4 × 4 × 1 supercell of graphene. Carbon atoms are shown in brown color, nitrogen in grey, boron in green, and sulfur in yellow.</figcaption></figure>

We then re-calculated the energy of all the selected structures with an implicit solvent effect using the VASPSol model for water. In the S-doped and B-doped graphene, we noticed that the lowest energy configuration of the OOH adsorption led to an immediate dissociation of the molecule (see **Figure S11** and related discussion in **Supplementary Note 2** for more details) to O and OH which are adsorbed separately at the surface. We, therefore, considered an alternative adsorption geometry, adopted from the N-doped case, and then relaxed and re-
calculated with the VASPSol implicit solvent water model. This configuration, although
higher in energy, led to an OOH species that is adsorbed at the surface without an immediate
molecular dissociation. Choosing an alternate adsorption geometry for the OOH, also affects
the O, and OH configurations, creating an alternate path. A top-view of the lowest energy
structures for the pristine graphene and the N-doped graphene is shown in **Figure 11**. For the
B-doped and the S-doped graphene, we show a top-view of the alternate adsorption
geometries, where a full description of both the lowest energy geometry and the alternate
geometry is found in **Figure S11**. From **Figure 11**, it is evident that in the case of pristine
graphene, the adsorption of OH and OOH species is found to be in the hollow site, while the
O atom is adsorbed in the bridge site. For the N-doped graphene, all species are adsorbed on
top of one of the near carbons. For the S-doped graphene alternate structure, all species are
adsorbed at what can be considered to be a bridge site. For the B-doped graphene alternate
structures, the OH and OOH are adsorbed on top of one of the near carbons, while the O atom
is adsorbed in a bridge site. In **Table S9**, we show the adsorption energy values of all the
species on pristine and doped graphene, which are used for the adsorption free energy
changes of intermediate species as given in **Table S10**. Our calculated formation energies
relative to graphene and also to graphene with a vacancy agree with previous works.[149–151]
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Top-view of the optimized ground state energy structure (for the pristine and N-doped) and alternative structures (for S- and B-doped) for the various ORR-involved species adsorbed on the nanosheet (side view can be seen in **Figure S12**). Hereafter, brown, red, grey, yellow, green, and white spheres represent C, O, N, S, B, and H atoms, respectively.</figcaption></figure>

Next, we calculated the changes in free energy according to equations 12-15 (section 2.5.2) for both the lowest energy structures and the alternative adsorption geometries (a full discussion of the alternative sites is found in **Supplementary Note 2** and **Table S11**). **Figure 12** summarizes the results for the changes in the Gibbs free energy for all geometries. **Figure 12a** shows the changes in Gibbs free energy without solvent effects, while **Figure 12b** shows the same quantities with the solvent effect as simulated by the implicit water model of VASPSol. While the solvent has some effect on the results, it does not change the main quantitative conclusions. As is evident, if we examine the lowest energy structures (solid
lines), we would get that the B-doped graphene is the most reactive (has the lowest energy barriers), then N-doped, then pristine, and finally S-doped.

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Schematic Gibbs free energy diagrams of ORR for pristine and doped graphene by VASP (a) and VASPSol (b) implicit water solvent model calculations. The solid lines show the lowest energy structures, N-doped in blue, B-doped in magenta, S-doped is shown in black, and pristine graphene in grey. The dashed lines show alternative adsorption sites for the S-doped and B-doped cases.</figcaption></figure>

Another observation for the lowest energy structures is that for the N-doped and for the
pristine graphene, the rate-determining step is the first one (**equation 5**), while for the S-
doped and B-doped it is the third step (**equation 7**). If we now examine the alternative
adsorption sites, we get that the trend of ORR is N-graphene > S-graphene > B-graphene >
pristine graphene. With the alternative adsorption sites, it is evident that for the N-doped, B-
doped, and pristine graphene, the first step (**equation 5**) is the rate-limiting step, while for the
S-doped graphene, the third step (**equation 7**) is the rate-limiting step. Zhang *et al.* reported
similar values for the reaction steps we found for the N-doped graphene (**Table S10**).[119]
They showed that based on scaling relationships, the B-doped graphene should behave
similarly (**Table S10**). We show in **Supplementary Note 2** that the alternate path obeys the
scaling relationships for ΔE<sub>O</sub>, ΔE<sub>OH</sub>, ΔE<sub>OOH</sub> better than the lowest energy path (which leads to
an immediate dissociation of the OOH on the B- and S-doped graphene). According to the
definition and **equations 20** and **21**, U<sub>lim</sub> and η<sub>lim</sub> were found to be very low and high,
respectively, for the N-doped graphene, which strongly indicates that the N-doped graphene
catalyst's ORR performance is better than pristine graphene.

Thus, based on DFT investigations, we have correlated the experimentally observed trend in
the observed ORR activity of doped-graphite (Figure 7) and they are in close agreement with
the experimental trend observed with N-, S-, and B-graphites after considering the alternative
path for the S- and B-graphites. A more detailed simulation that includes kinetic effects is
probably needed for the case of the B- and S-doped graphenes. Finally, we have not taken
into account possible edge effects of the reactions as we considered only the basal plane in
the simulations. In addition, since the simulation was conducted on monolayer graphene, we
anticipate some minor additional effects for multilayer graphite.

Both experimental electrochemical characterization and DFT calculations confirmed the good
ORR performance and stability of the heteroatom-doped graphite catalysts, suggesting that
these materials may be very promising for practical AEMFC applications. We next use these
materials and evaluate them as cathode ORR catalysts *in operando* AEMFCs. Membrane-
electrode assemblies for AEMFC tests were prepared using the different doped graphite
cathode catalysts developed in this study. The distribution of the ionomer and catalyst in a
few catalyst layer samples (I- and S-graphites) was analyzed using TEM/STEM (Figure 13).

In the case of I-graphite, we see a clear exfoliation of the graphite and the formation of
flexible graphene sheets surrounding the ionomer grains with a size of ~50 nm (Figures 13 a-
d). However, in the case of S-graphite, the morphology of the graphite looks the same for the
S-graphite without ionomer (Figure 3) and as in the catalyst layer with ionomer (Figure 13e-
h), respectively.
<figure><img src="image_14.png" /><figcaption>**Figure 13.** The presence of the ionomer in the I-and S-graphites: ionomer mixed with the I-graphite (a-d) and ionomer mixed with the S-graphite (e-h).</figcaption></figure>
The flexible graphene layers are not observed (Figures 13 e-h) in agreement with the BET data which demonstrate the BET surface area of the S-graphite is significantly lower than that of I-graphite (ca. 3 times lower). In the case of S-graphite, the ionomer was only deposited on the top surface (Figures 13e-h). As shown by the TEM images, the ionomer is distributed uniformly with the catalysts and exhibits minor agglomeration, thus creating the ionic pathways through the catalyst layers. The crystalline graphitic phase (Figure 13) is attributed to the ordered carbon support for the catalyst, unlike the ionomeric phase, which is amorphous, with a particle size of ca. 35 nm (Figure S13). It is known that the distribution of the dispersed ionomer at the nanoscale level influences the service life of PEMFC;[152] however, a very limited number of studies have been devoted to investigating this for AEMFCs, and, thus, a more in-depth investigation of electrode structure is needed for this technology.

The as-synthesized heteroatoms (I, N, S, and B)-doped graphite were successfully tested in *operando* AEMFCs as an active metal-free cathode catalyst. In these tests, the anode electrode, membrane, and GDLs were always kept the same, so we can attribute all the changes only to the different cathode catalysts used in the cell. The performance of the AEMFCs prepared using the different doped-graphite catalysts as a cathode is shown in **Figure 14a,b**. The performance after iR-correction is shown in **Figure S14** and listed in **Table S12**. For comparison, the analogous data of the (undoped) milled graphite are also included in the graphs and table. As can be seen, all the metal-free heteroatom-doped graphite catalysts show surprisingly high performance in *operando* AEMFCs. Peak power densities of 361, 325, 302, and 194 mW cm⁻² and highest current densities of 930, 870, 740, and 460 mA cm⁻² were achieved with N-doped, I-doped, S-doped, and B-doped graphite catalysts, respectively.
The general AEMFC performance of the doped-graphite cathode catalysts (Figure 14a,b) follows the order N-graphite > I-graphite > S-graphite > B-graphite > milled graphite. This order is consistent with the ORR performance measured *ex situ* by RRDE (Figure 7a,c). Quantitatively however, a larger difference in AEMFC performance between N-graphite, I-graphite and S-graphite could have been expected based on RRDE results, but is not seen, n particular at low current densities where the AEMFC polarization curves overlap. This may be due to different temperature-dependencies of the ORR kinetics for the different materials (RRDE performed at 25 °C and AEMFC at 60 °C), or other complex effects due to the different environments in AEMFC and RRDE setup.

**Figure 14c** and **Table S13** summarize the main parameters for AEMFC performance obtained with the different heteroatom-doped graphite catalysts and their comparison with the data available of the metal-free cathode in the literature. As can be seen, the non-optimized AEMFCs made with metal-free heteroatom-doped graphite catalysts exhibited higher performance than most of the AEMFC results obtained with metal-free cathode catalysts.[61,62,153–155] In the case of S-doped graphite, the catalyst exhibited two orders of magnitude higher AEMFC performance than that previously reported in the literature.[71] For B- and I-doped graphite catalysts, these results are the first data of AEMFC performance ever published with these catalysts.
<figure><img src="image_15.png" /><figcaption>**Figure 14.** Performance of AEMFCs made with heteroatom-doped graphite cathode catalysts as compared to the undoped milled-graphite: polarization curves (a) and power density curves as a function of current density (b). Cathode catalyst loading of 0.70 $mg_{doped-graphite}cm^{-2}$ (96% RH with 2.0 bar back pressure) and anode catalyst loading of 0.70 $mg_{PtRu}cm^{-2}$ (79% RH with no backpressure). Cell temperature 60 °C, $H_2$ and $O_2$ flow rates are 1000 ml min$^{-1}$. Comparison of the AEMFC performance of the metal-free cathode: N-doped CNTs,[156] N-doped graphene,[60] N-CNTs,[157] N-doped nanoporous graphene,[158] N-doped single-walled carbon nanohorn,[159] Amorphous N-doped carbon film,[61] N-doped hierarchical carbon,[155] N-doped carbon,[14] N-doped carbonaceous material,[62] N-P doped mesoporous carbon,[160] N-S doped mesoporous carbon,[76] and S-doped graphene oxide (c).[71]</figcaption></figure>

The heteroatom-doped graphite catalysts were further tested in fuel cell mode under a constant load of 200 $mA cm^{-2}$, to evaluate their initial stability in operando AEMFCs. The results are shown in **Figure S15**. As can be seen during the time of the test, the doped graphite catalysts demonstrated excellent initial stability under real device operation. This is
consistent with the good stability results of the doped graphite catalysts in the AST cycles (Figure 9). The initial durability of the N-graphite suggests no significant degradation (30 mV h⁻¹) measured at 200 mA cm⁻² constant current density load. It is worthwhile mentioning that, in the literature, only two studies reported AEMFC durability with N-doped carbon cathode by Rao *et al* (no measurable degradation at 20 mA cm⁻²)[¹⁵⁷] and Douglin *et al* (14 mV h⁻¹ @ 500 mA cm⁻²).[¹⁴]

The high *ex situ* RRDE and *in operando* cell performance and high stability of these heteroatom-doped graphite catalysts are achieved in a simple, one-step catalyst synthesis, using an ultra-low-cost ball milling process that can be easily reproduced and scaled up. Moreover, these catalysts use ultra-low-cost graphite raw materials, which are six orders of magnitude cheaper than PGMs (30,000 vs. 0.5 $ kg⁻¹) (Table S14). We further calculated the cost in $cathode catalyst per kW output power, to obtain an estimated cost of PGM-based and metal-free cathodes in the literature compared with this work. The calculated cost (dollar per power output) for N-graphite reported in this work is 2.3 × 10⁻² $cathode kW⁻¹ which is three orders of magnitude lower than the state-of-the-art PGM-based AEMFC reported (Figure 15).[¹⁶¹] We compared the cost of the two best N-graphites, i.e., N-doped carbon[¹⁴], and N-doped carbonaceous material[⁶²] to the N-graphite in this study, as we see our N-doped-graphite showed the lowest cost and this is mainly due to the inexpensive raw materials used for the synthesis. It is worth mentioning that we have included only the cost of nitrogen precursor but not the processing cost for this comparison.
<figure><img src="image_16.png" /><figcaption>**Figure 15.** Cost comparison of PGM cathode catalysts reported in literature PdAg[162], Pt/C[161], N-doped carbon[14], and N-doped carbonaceous material[62] to this work on N-graphite. It is worthwhile mentioning that, for two N-doped materials from the literature we have used the cost of carbon as shown in Table S14.</figcaption></figure>

**4. Conclusions**

Metal-free I-, S-, N-, and B-doped graphites are synthesized at room temperature in a single-step, ultra-low-cost ball milling of graphite in the presence of dopant salts. The ORR performance of the doped-graphite catalysts is evaluated in both *ex situ* RRDE and *in operando* AEMFCs. The extent of the doping is verified by XPS, TEM, HAADF STEM, and STEM-EDS analyses. The physicochemical characterization reveals that significant defects are created during the ball milling in the presence of dopants. The increase in the double-layer capacitance and a considerable decrease in XRD crystal size suggest that delamination of the graphitic layers occurs during the ball milling process in the presence of the dopants. All the doped-graphite samples exhibited substantially enhanced ORR activity, as compared
to the (undoped) graphite. The mass normalized ORR activity derived from the Tafel plot is 50.4, 14.2, 9.1, and 6.1 times higher mass activity for N-, I-, B-, and S-graphite compared to pristine graphite, respectively. The measurement of peroxide generation and the effect of catalyst loading of the catalyst on the activity shed light on the ORR mechanism and product selectivity. The AST reveals the excellent stability of the N-graphite catalyst (after 5000 AST cycles, a shift of 5 mV). We have also demonstrated electrochemical carbon corrosion using I-graphite as an example for the first time showing promising stability.

Using DFT calculations, we investigated the mechanism of the ORR catalyzed by doped graphene. The lowest energy structures for the adsorption of O, OH, and OOH, lead to an activity estimation that is different than that of the experiment – finding B-doped graphene as the most reactive, then the N-doped, then pristine graphene, and finally the S-doped graphene. However, when we examined an alternative path for the OOH adsorption on the B-, and S-doped graphenes, we found an activity order closer to the experiment, that is the N-doped being the most reactive, then the B-doped, then the S-doped, and finally the pristine graphene. The highest mass activity of the N-graphite is attributed to the higher intrinsic activity also supported by the DFT calculations. Therefore, the experimental results are in close agreement with that calculated from DFT.

The prepared metal-free doped-graphite catalysts were also successfully employed as an AEMFC cathode. High peak power densities of 361, 325, 302, and 194 mW cm⁻² were obtained with the N-, I-, S-, and B-graphites, respectively, and they are found to be substantially higher than the performance of the milled-graphite. The improved AEMFC performance is due to the enhanced alkaline ORR activity achieved by the doped graphite catalysts. Analysis of the catalyst layers carried out by TEM/STEM suggests the presence of an intimate mixture of ionomer with the catalysts, thus facilitating the reaction at the triple-phase boundary. The I- and B-doped graphite catalysts show the highest first ever reported.
For S-graphite, we reported two orders of magnitude higher AEMFC performance. Most importantly, we reached a significantly high specific peak power of 515.7 W g⁻¹ N-graphite which is 4× or higher than most of the metal-free catalysts reported so far in the literature. The calculated power output per raw material cost is 2.3 × 10⁻² $cathode catalyst kW⁻¹ with N-graphite, which is three orders of magnitude lower than the state-of-the-art AEMFC reported based on Pt cathode (5.1 $/kW).[161] These results provide a strong correlation between the theory-guided comprehensive study on ORR and fuel cell operation on metal-free carbon catalysts and inspire the development of AEMFC cathode work in the future. Our metal-free catalysts show a significantly higher AEMFC performance than most of the metal-free carbon cathodes reported in the literature, demonstrating their potential. Consequently, this study encourages the development of the ultra-low-cost, highly active, stable, and scalable production of fuel cell cathode materials that can significantly reduce the cost of AEMFCs and contribute to future commercialization and broad implementation of this technology.

## Acknowledgments

This work was partially funded by the Nancy & Stephen Grand Technion Energy Program (GTEP); by the French-Israeli Joint Research Project (PRC 2019–2021; MOST and CNRS) [PRC2347] (MOST grant No.3-15578); by the Israeli Science Foundation, grant number 169/22; and by The Israeli Smart Transportation Research Center (ISTRC), grant No. 2070512. PF would like to acknowledge the support by FCT, through IDMEC, under LAETA, project UIDB/50022/2020. J.C.D. personally wishes to thank Mr. and Mrs. Bela B. Nevai as well as The Israeli Smart Transportation Research Center for their generous financial support in the forms of the Clara and Bela B. Nevai Scholarship and ISTRC Scholarship, respectively. R.K.S. would also like to thank Vellore Institute of Technology, Vellore, India for providing “VIT SEED Grant-(SG20220086)”.
References

[1] D. R. Dekel, *J. Power Sources* **2018**, *375*, 158.

[2] S. Gottesfeld, D. R. Dekel, M. Page, C. Bae, Y. Yan, P. Zelenay, Y. S. Kim, *J. Power Sources* **2018**, *375*, 170.

[3] N. Ramaswamy, S. Mukerjee, *Adv. Phys. Chem.* **2012**, *2012*, 1.

[4] X. Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, T. S. A. Hor, Y. Zong, Z. Liu, *ACS Catal.* **2015**, *5*, 4643.

[5] Y. Yang, C. R. Peltier, R. Zeng, R. Schimmenti, Q. Li, X. Huang, Z. Yan, G. Potsi, R. Selhorst, X. Lu, W. Xu, M. Tader, A. V. Soudackov, H. Zhang, M. Krumov, E. Murray, P. Xu, J. Hitt, L. Xu, H.-Y. Ko, B. G. Ernst, C. Bundschu, A. Luo, D. Markovich, M. Hu, C. He, H. Wang, J. Fang, R. A. DiStasio, L. F. Kourkoutis, A. Singer, K. J. T. Noonan, L. Xiao, L. Zhuang, B. S. Pivovar, P. Zelenay, E. Herrero, J. M. Feliu, J. Suntivich, E. P. Giannelis, S. Hammes-Schiffer, T. Arias, M. Mavrikakis, T. E. Mallouk, J. D. Brock, D. A. Muller, F. J. DiSalvo, G. W. Coates, H. D. Abruña, *Chem. Rev.* **2022**.

[6] M. M. Hossen, M. S. Hasan, M. R. I. Sardar, J. bin Haider, Mottakin, K. Tammeveski, P. Atanassov, *Appl. Catal. B Environ.* **2023**, *325*, 121733.

[7] E. S. Davydova, S. Mukerjee, F. Jaouen, D. R. Dekel, *ACS Catal.* **2018**, *8*, 6665.

[8] G. Huang, M. Mandal, X. Peng, A. C. Yang-Neyerlin, B. S. Pivovar, W. E. Mustain, P. A. Kohl, *J. Electrochem. Soc.* **2019**, *166*, F637.

[9] N. Ziv, W. E. Mustain, D. R. Dekel, *ChemSusChem* **2018**, *11*, 1136.

[10] J. C. Douglin, J. A. Zamora Zeledón, M. E. Kreider, R. K. Singh, M. B. Stevens, T. F. Jaramillo, D. R. Dekel, *Nat. Energy* **2023**, *8*, 1262.

[11] D. R. Dekel, M. Amar, S. Willdorf, M. Kosa, S. Dhara, C. E. Diesendruck, *Chem. Mater.* **2017**, *29*, 4425.

[12] J. Fan, S. Willdorf-Cohen, E. M. Schibli, Z. Paula, W. Li, T. J. G. Skalski, A. T. Sergeenko, A. Hohenadel, B. J. Frisken, E. Magliocca, W. E. Mustain, C. E. Diesendruck, D. R. Dekel, S. Holdcroft, *Nat. Commun.* **2019**, *10*, 2306.

[13] J. Wang, Y. Zhao, B. P. Setzler, S. Rojas-Carbonell, C. Ben Yehuda, A. Amel, M. Page, L. Wang, K. Hu, L. Shi, S. Gottesfeld, B. Xu, Y. Yan, *Nat. Energy* **2019**, *4*, 392.

[14] J. C. Douglin, R. K. Singh, S. Haj-Bsoul, S. Li, J. Biemolt, N. Yan, J. R. Varcoe, G. Rothenberg, D. R. Dekel, *Chem. Eng. J. Adv.* **2021**, *8*, 100153.

[15] X. Liu, N. Xie, J. Xue, M. Li, C. Zheng, J. Zhang, Y. Qin, Y. Yin, D. R. Dekel, M. D.
Guiver, *Nat. Energy* **2022**, *7*, 329.

[16] S. Haj-Bsoul, J. R. Varcoe, D. R. Dekel, *J. Electroanal. Chem.* **2022**, *908*, 116112.

[17] K. Yassin, I. G. Rasin, S. Brandon, D. R. Dekel, *J. Memb. Sci.* **2024**, *690*, 122164.

[18] T. Zelovich, D. R. Dekel, M. E. Tuckerman, *J. Phys. Chem. Lett.* **2024**, *15*, 408.

[19] T. J. Omasta, A. M. Park, J. M. LaManna, Y. Zhang, X. Peng, L. Wang, D. L. Jacobson, J. R. Varcoe, D. S. Hussey, B. S. Pivovar, W. E. Mustain, *Energy Environ. Sci.* **2018**, *11*, 551.

[20] C. E. Diesendruck, D. R. Dekel, *Curr. Opin. Electrochem.* **2018**, *9*, 173.

[21] X. Peng, D. Kulkarni, Y. Huang, T. J. Omasta, B. Ng, Y. Zheng, L. Wang, J. M. LaManna, D. S. Hussey, J. R. Varcoe, I. V. Zenyuk, W. E. Mustain, *Nat. Commun.* **2020**, *11*, 3561.

[22] N. Ul Hassan, M. Mandal, G. Huang, H. A. Firouzjaie, P. A. Kohl, W. E. Mustain, *Adv. Energy Mater.* **2020**, *n/a*, 2001986.

[23] J. C. Douglin, R. K. Singh, E. R. Hamo, M. B. Hassine, P. J. Ferreira, B. A. Rosen, H. A. Miller, G. Rothenberg, D. R. Dekel, *J. Solid State Electrochem.* **2022**.

[24] Y. Xue, L. Shi, X. Liu, J. Fang, X. Wang, B. P. Setzler, W. Zhu, Y. Yan, Z. Zhuang, *Nat. Commun.* **2020**, *11*, 5651.

[25] J. C. Douglin, A. Sekar, R. K. Singh, Z. Chen, J. Li, D. R. Dekel, *Small* **2023**, *n/a*, 2307497.

[26] R. Wang, D. Li, S. Maurya, Y. S. Kim, Y. Wu, Y. Liu, D. Strmcnik, N. M. Markovic, V. R. Stamenkovic, *Nanoscale Horiz.* **2020**, *5*, 316.

[27] H. Wang, H. D. Abruña, *J. Am. Chem. Soc.* **2017**, *139*, 6807.

[28] Y. Wang, G. Wang, G. Li, B. Huang, J. Pan, Q. Liu, J. Han, L. Xiao, J. Lu, L. Zhuang, *Energy Environ. Sci.* **2015**, *8*, 177.

[29] Z. Zhuang, S. A. Giles, J. Zheng, G. R. Jenness, S. Caratzoulas, D. G. Vlachos, Y. Yan, *Nat. Commun.* **2016**, *7*, 10141.

[30] H. A. Miller, A. Lavacchi, F. Vizza, M. Marelli, F. Di Benedetto, F. D'Acapito, Y. Paska, M. Page, D. R. Dekel, *Angew. Chemie Int. Ed.* **2016**, *55*, 6004.

[31] R. K. Singh, E. S. Davydova, J. Douglin, A. O. Godoy, H. Tan, M. Bellini, B. J. Allen, J. Jankovic, H. A. Miller, A. C. Alba-Rubio, D. R. Dekel, *Adv. Funct. Mater.* **2020**, *n/a*, 2002087.

[32] Y. Zhou, Z. Xie, J. Jiang, J. Wang, X. Song, Q. He, W. Ding, Z. Wei, *Nat. Catal.* **2020**, *3*, 454.

[33] M. V. Pagliaro, C. Wen, B. Sa, B. Liu, M. Bellini, F. Bartoli, S. Sahoo, R. K. Singh, S.
P. Alpay, H. A. Miller, D. R. Dekel, ACS Catal. **2022**, *10894*.

[34] J. Xu, P. Gao, T. S. Zhao, *Energy Environ. Sci.* **2012**, *5*, 5333.

[35] P. G. Santori, F. D. Speck, S. Cherevko, H. A. Firouzjaie, X. Peng, W. E. Mustain, F. Jaouen, *J. Electrochem. Soc.* **2020**, *167*, 134505.

[36] X. Peng, T. J. Omasta, E. Magliocca, L. Wang, J. R. Varcoe, W. E. Mustain, *Angew. Chemie* **2019**, *131*, 1058.

[37] H. Adabi, P. G. Santori, A. Shakouri, X. Peng, K. Yassin, I. G. Rasin, S. Brandon, D. R. Dekel, N. U. Hassan, M.-T. Sougrati, A. Zitolo, J. R. Varcoe, J. R. Regalbuto, F. Jaouen, W. E. Mustain, *Mater. Today Adv.* **2021**, *12*, 100179.

[38] W. Zhu, Y. Pei, J. C. Douglin, J. Zhang, H. Zhao, J. Xue, Q. Wang, R. Li, Y. Qin, Y. Yin, D. R. Dekel, M. D. Guiver, *Appl. Catal. B Environ.* **2021**, *299*, 120656.

[39] Y. Kumar, E. Kibena-Põldsepp, J. Kozlova, M. Rähn, A. Treshchalov, A. Kikas, V. Kisand, J. Aruväli, A. Tamm, J. C. Douglin, S. J. Folkman, I. Gelmetti, F. A. Garcés-Pineda, J. R. Galán-Mascarós, D. R. Dekel, K. Tammeveski, *ACS Appl. Mater. Interfaces* **2021**.

[40] K. Kisand, A. Sarapuu, J. C. Douglin, A. Kikas, A. Treshchalov, M. Käärik, H.-M. Piirsoo, P. Paiste, J. Aruväli, J. Leis, V. Kisand, A. Tamm, D. R. Dekel, K. Tammeveski, *ACS Catal.* **2022**, *12*, 14050.

[41] D. R. Dekel, *ECS Trans.* **2013**, *50*, 2051.

[42] K. Yassin, I. G. Rasin, S. Brandon, D. R. Dekel, *J. Power Sources* **2022**, *524*, 231083.

[43] J. C. Douglin, K. V. Sankar, A. L. G. Biancolli, E. I. Santiago, Y. Tsur, D. R. Dekel, *ChemSusChem* n/a, e202301080.

[44] S. M. Alia, K. Duong, T. Liu, K. Jensen, Y. Yan, *ChemSusChem* **2014**, *7*, 1739.

[45] F. H. B. Lima, J. Zhang, M. H. Shao, K. Sasaki, M. B. Vukmirovic, E. A. Ticianelli, R. R. Adzic, *J. Phys. Chem. C* **2007**, *111*, 404.

[46] Y. Yang, W. Xiao, X. Feng, Y. Xiong, M. Gong, T. Shen, Y. Lu, H. D. Abruña, D. Wang, *ACS Nano* **2019**, *13*, 5968.

[47] J. Staszak-Jirkovský, R. Subbaraman, D. Strmcnik, K. L. Harrison, C. E. Diesendruck, R. Assary, O. Frank, L. Kobr, G. K. H. Wiberg, B. Genorio, J. G. Connell, P. P. Lopes, V. R. Stamenkovic, L. Curtiss, J. S. Moore, K. R. Zavadil, N. M. Markovic, *ACS Catal.* **2015**, *5*, 6600.

[48] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, *Nat. Mater.* **2011**, *10*, 780.

[49] J. Biemolt, J. C. Douglin, R. K. Singh, E. S. Davydova, N. Yan, G. Rothenberg, D. R.
Dekel, *Energy Technol.* **2021**, *n/a*, 2000909.

[50] Y. Xiong, Y. Yang, F. J. DiSalvo, H. D. Abruña, *J. Am. Chem. Soc.* **2019**, *141*, 10744.

[51] Y. Yang, H. Peng, Y. Xiong, Q. Li, J. Lu, L. Xiao, F. J. DiSalvo, L. Zhuang, H. D. Abruña, *ACS Energy Lett.* **2019**, *4*, 1251.

[52] Y. Wang, Y. Yang, S. Jia, X. Wang, K. Lyu, Y. Peng, H. Zheng, X. Wei, H. Ren, L. Xiao, J. Wang, D. A. Muller, H. D. Abruña, B. J. Hwang, J. Lu, L. Zhuang, *Nat. Commun.* **2019**, *10*, 1506.

[53] Y. Xiong, Y. Yang, X. Feng, F. J. DiSalvo, H. D. Abruña, *J. Am. Chem. Soc.* **2019**, *141*, 4412.

[54] Y. Yang, Y. Wang, Y. Xiong, X. Huang, L. Shen, R. Huang, H. Wang, J. P. Pastore, S.-H. Yu, L. Xiao, J. D. Brock, L. Zhuang, H. D. Abruña, *J. Am. Chem. Soc.* **2019**, *141*, 1463.

[55] H. T. Chung, J. H. Won, P. Zelenay, *Nat. Commun.* **2013**, *4*, 1922.

[56] P. G. Santori, F. D. Speck, J. Li, A. Zitolo, Q. Jia, S. Mukerjee, S. Cherevko, F. Jaouen, *J. Electrochem. Soc.* **2019**, *166*, F3311.

[57] N. Zion, J. C. Douglin, D. A. Cullen, P. Zelenay, D. R. Dekel, L. Elbaz, *Adv. Funct. Mater.* **2021**, *31*, 1.

[58] Y. Sun, L. Silvioli, N. R. Sahraie, W. Ju, J. Li, A. Zitolo, S. Li, A. Bagger, L. Arnarson, X. Wang, T. Moeller, D. Bernsmeier, J. Rossmeisl, F. Jaouen, P. Strasser, *J. Am. Chem. Soc.* **2019**, *141*, 12372.

[59] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, *Science* (80-.), **2009**, *323*, 760 LP.

[60] M. P. Kumar, M. M. Raju, A. Arunchander, S. Selvaraj, G. Kalita, T. N. Narayanan, A. K. Sahu, D. K. Pattanayak, *J. Electrochem. Soc.* **2016**, *163*, F848.

[61] J. Chen, X. Wang, X. Cui, G. Yang, W. Zheng, *Chem. Commun.* **2014**, 50, 557.

[62] Y. Lu, L. Wang, K. Preuß, M. Qiao, M.-M. Titirici, J. Varcoe, Q. Cai, *J. Power Sources* **2017**, *372*, 82.

[63] H. Jiang, J. Gu, X. Zheng, M. Liu, X. Qiu, L. Wang, W. Li, Z. Chen, X. Ji, J. Li, *Energy Environ. Sci.* **2019**, *12*, 322.

[64] C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, *Adv. Mater.* **2013**, *25*, 4932.

[65] X. Sun, Y. Zhang, P. Song, J. Pan, L. Zhuang, W. Xu, W. Xing, *Acs Catal.* **2013**, *3*, 1726.

[66] I. Y. Jeon, H. J. Choi, M. Choi, J. M. Seo, S. M. Jung, M. J. Kim, S. Zhang, L. Zhang, Z. Xia, L. Dai, N. Park, J. B. Baek, *Sci. Rep.* **2013**, *3*, 1.

[67] Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu, S. Huang, *Chem. Commun.* **2012**, 48, 1027.
[68] Y. Zheng, Y. Jiao, L. Ge, M. Jaroniec, S. Z. Qiao, *Angew. Chemie* **2013**, *125*, 3192.

[69] Z.-H. Sheng, H.-L. Gao, W.-J. Bao, F.-B. Wang, X.-H. Xia, *J. Mater. Chem.* **2012**, *22*, 390.

[70] L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu, J. Ma, Y. Ma, Z. Hu, *Angew. Chemie Int. Ed.* **2011**, *50*, 7132.

[71] M. Klingele, C. Pham, K. R. Vuyyuru, B. Britton, S. Holdcroft, A. Fischer, S. Thiele, *Electrochem. commun.* **2017**, *77*, 71.

[72] J. Liang, Y. Jiao, M. Jaroniec, S. Z. Qiao, *Angew. Chemie - Int. Ed.* **2012**, *51*, 11496.

[73] I.-Y. Jeon, S. Zhang, L. Zhang, H.-J. Choi, J.-M. Seo, Z. Xia, L. Dai, J.-B. Baek, *Adv. Mater.* **2013**, *25*, 6138.

[74] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, *ACS Nano* **2012**, *6*, 205.

[75] Y. Xue, D. Yu, L. Dai, R. Wang, D. Li, A. Roy, F. Lu, H. Chen, Y. Liu, J. Qu, *Phys. Chem. Chem. Phys.* **2013**, *15*, 12220.

[76] N. Xu, T. Zhu, J. Qiao, F. Zhang, Z. Chen, *Int. J. Hydrogen Energy* **2016**, *41*, 9159.

[77] Y. Zhan, J. Huang, Z. Lin, X. Yu, D. Zeng, X. Zhang, F. Xie, W. Zhang, J. Chen, H. Meng, *Carbon N. Y.* **2015**, *95*, 930.

[78] L. Yang, J. Shui, L. Du, Y. Shao, J. Liu, L. Dai, Z. Hu, *Adv. Mater.* **2019**, 1804799, 1804799.

[79] Q. Li, R. Cao, J. Cho, G. Wu, *Adv. Energy Mater.* **2014**, *4*, 1301415.

[80] A. B. Jorge, R. Jervis, A. P. Periasamy, M. Qiao, J. Feng, L. N. Tran, M.-M. Titirici, *Adv. Energy Mater.* **2019**, *n/a*, 1902494.

[81] Y. Li, W. Zhou, H. Wang, L. Xie, Y. Liang, F. Wei, J. C. Idrobo, S. J. Pennycook, H. Dai, *Nat. Nanotechnol.* **2012**, *7*, 394.

[82] X. Yan, Y. Jia, X. Yao, *Chem. Soc. Rev.* **2018**, *47*, 7628.

[83] X. Wang, G. Sun, P. Routh, D. H. Kim, W. Huang, P. Chen, *Chem. Soc. Rev.* **2014**, *43*, 7067.

[84] Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu, S. Huang, *Chem. Commun.* **2012**, *48*, 1027.

[85] I.-Y. Jeon, H.-J. Choi, M. J. Ju, I. T. Choi, K. Lim, J. Ko, H. K. Kim, J. C. Kim, J.-J. Lee, D. Shin, S.-M. Jung, J.-M. Seo, M.-J. Kim, N. Park, L. Dai, J.-B. Baek, *Sci. Rep.* **2013**, *3*, 2260.

[86] I.-Y. Jeon, H.-J. Choi, S.-M. Jung, J.-M. Seo, M.-J. Kim, L. Dai, J.-B. Baek, *J. Am. Chem. Soc.* **2013**, *135*, 1386.

[87] H. Huang, K. Ming, Y. Fang, H. Zhao, X. Wang, J. Chen, J. Guo, J. Zhang, *R. Soc.*
Open Sci. **2018**, *5*, 180925.

[88] L. Qu, Y. Liu, J. Baek, L. Dai, **2010**, *2*, 1.

[89] R. Liu, D. Wu, X. Feng, K. Müllen, *Angew. Chemie Int. Ed.* **2010**, *49*, 2565.

[90] D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye, S. Knights, *Energy Environ. Sci.* **2011**, *4*, 760.

[91] Y. Jia, L. Zhang, A. Du, G. Gao, J. Chen, X. Yan, C. L. Brown, X. Yao, *Adv. Mater.* **2016**, *28*, 9532.

[92] L. Zhang, Q. Xu, J. Niu, Z. Xia, *Phys. Chem. Chem. Phys.* **2015**, *17*, 16733.

[93] Y. Jiang, L. Yang, T. Sun, J. Zhao, Z. Lyu, O. Zhuo, X. Wang, Q. Wu, J. Ma, Z. Hu, *ACS Catal.* **2015**, *5*, 6707.

[94] C. Tang, H.-F. Wang, X. Chen, B.-Q. Li, T.-Z. Hou, B. Zhang, Q. Zhang, M.-M. Titirici, F. Wei, *Adv. Mater.* **2016**, *28*, 6845.

[95] X. Yan, Y. Jia, X. Yao, *Chem. Soc. Rev.* **2018**, *47*, 7628.

[96] J. Y. Cheon, J. H. Kim, J. H. Kim, K. C. Goddeti, J. Y. Park, S. H. Joo, *J. Am. Chem. Soc.* **2014**, *136*, 8875.

[97] C. H. Choi, H.-K. Lim, M. W. Chung, G. Chon, N. Ranjbar Sahraie, A. Altin, M.-T. Sougrati, L. Stievano, H. S. Oh, E. S. Park, F. Luo, P. Strasser, G. Dražić, K. J. J. Mayrhofer, H. Kim, F. Jaouen, *Energy Environ. Sci.* **2018**, *11*, 3176.

[98] J. Kibsgaard, Y. Gorlin, Z. Chen, T. F. Jaramillo, *J. Am. Chem. Soc.* **2012**, *134*, 7758.

[99] P. J. Ferreira, G. J. la O', Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H. A. Gasteiger, *J. Electrochem. Soc.* **2005**, *152*, A2256.

[100] C. Lafforgue, M. Chatenet, L. Dubau, D. R. Dekel, *ACS Catal.* **2018**, *8*, 1278.

[101] A. Zadick, L. Dubau, N. Sergent, G. Berthomé, M. Chatenet, *ACS Catal.* **2015**, *5*, 4819.

[102] Z. Wang, E. Tada, A. Nishikata, *J. Electrochem. Soc.* **2016**, *163*, C853.

[103] T. de Wild, T. Nemeth, T. M. Nolte, T. J. Schmidt, T. Nauser, L. Gubler, *J. Electrochem. Soc.* **2021**, *168*, 54514.

[104] M. Chatenet, B. G. Pollet, D. R. Dekel, F. Dionigi, J. Deseure, P. Millet, R. D. Braatz, M. Z. Bazant, M. Eikerling, I. Staffell, *Chem. Soc. Rev.* **2022**.

[105] S. Wierzbicki, J. C. Douglin, A. Kostuch, D. R. Dekel, K. Kruczała, *J. Phys. Chem. Lett.* **2020**, *11*, 7630.

[106] S. Wierzbicki, J. C. Douglin, R. K. Singh, D. R. Dekel, K. Kruczała, *ACS Catal.* **2023**, *13*, 2744.

[107] R. K. Singh, J. C. Douglin, L. Jiang, K. Yassin, S. Brandon, D. R. Dekel, *CoOx-
[108] T. Kar, R. Devivaraprasad, R. K. Singh, B. Bera, M. Neergat, *RSC Adv.* **2014**, *4*, 57781.

[109] R. K. Singh, R. Rahul, M. Neergat, *Phys. Chem. Chem. Phys.* **2013**, *15*.

[110] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, *Chem. Rev.* **2016**, *116*, 3594.

[111] G. Kresse, J. Hafner, *Phys. Rev. B* **1993**, *47*, 558.

[112] G. Kresse, J. Furthmüller, *Comput. Mater. Sci.* **1996**, *6*, 15.

[113] G. Kresse, J. Hafner, *Phys. Rev. B* **1994**, *49*, 14251.

[114] G. Kresse, J. Furthmüller, *Phys. Rev. B* **1996**, *54*, 11169.

[115] P. E. Blöchl, *Phys. Rev. B* **1994**, *50*, 17953.

[116] K. Mathew, R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, R. G. Hennig, *J. Chem. Phys.* **2014**, *140*, 84106.

[117] K. Momma, F. Izumi, *J. Appl. Crystallogr.* **2011**, *44*, 1272.

[118] A. M. Ganose, A. J. Jackson, D. O. Scanlon, *J. Open Source Softw.* **2018**, *3*, 717.

[119] X. Zhang, Z. Xia, H. Li, S. Yu, S. Wang, G. Sun, *RSC Adv.* **2019**, *9*, 7086.

[120] Q. Deng, J. Zhao, T. Wu, G. Chen, H. A. Hansen, T. Vegge, *J. Catal.* **2019**, *370*, 378.

[121] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jónsson, *J. Phys. Chem. B* **2004**, *108*, 17886.

[122] G. Jerkiewicz, *ACS Catal.* **2020**, *10*, 8409.

[123] Q. Liang, G. Brocks, A. Bieberle-Hütter, *J. Phys. Energy* **2021**, *3*, 26001.

[124] G. Fazio, L. Ferrighi, C. Di Valentin, *J. Catal.* **2014**, *318*, 203.

[125] C. A. Campos-Roldán, L. Calvillo, G. Granozzi, N. Alonso-Vante, *J. Electroanal. Chem.* **2020**, *857*, 113449.

[126] T. Huang, G. He, J. Xue, O. Otoo, X. He, H. Jiang, J. Zhang, Y. Yin, Z. Jiang, J. C. Douglin, D. R. Dekel, M. D. Guiver, *J. Memb. Sci.* **2020**, *597*, 117769.

[127] J. Lilloja, E. Kibena-Põldsepp, A. Sarapuu, J. C. Douglin, M. Käärik, J. Kozlova, P. Paiste, A. Kikas, J. Aruväli, J. Leis, V. Sammelselg, D. R. Dekel, K. Tammeveski, *ACS Catal.* **2021**, *11*, 1920.

[128] L. Wang, E. Magliocca, E. L. Cunningham, W. E. Mustain, S. D. Poynton, R. Escudero-Cid, M. M. Nasef, J. Ponce-González, R. Bance-Souahli, R. C. T. Slade, D. K. Whelligan, J. R. Varcoe, *Green Chem.* **2017**, *19*, 831.

[129] D. B. Shinde, J. Debgupta, A. Kushwaha, M. Aslam, V. K. Pillai, *J. Am. Chem. Soc.* **2011**, *133*, 4168.
[130] L. Osmieri, R. Escudero-Cid, M. Armandi, A. H. A. Monteverde Videla, J. L. García Fierro, P. Ocón, S. Specchia, *Appl. Catal. B Environ.* **2017**, *205*, 637.

[131] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, *Nano Lett.* **2008**, *8*, 3498.

[132] Y. Xue, H. Chen, J. Qu, L. Dai, *2D Mater.* **2015**, *2*, 044001.

[133] H. Zhu, S. Xu, *RSC Adv.* **2018**, *8*, 17879.

[134] G. Beamson, D. Briggs, *High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database*, Wiley, **1992**.

[135] N. Graf, E. Yegen, T. Gross, A. Lippitz, W. Weigel, S. Krakert, A. Terfort, W. E. S. Unger, *Surf. Sci.* **2009**, *603*, 2849.

[136] Z. Lu, X. Xu, Y. Chen, X. Wang, L. Sun, K. Zhuo, *Green Energy Environ.* **2019**.

[137] X. Zhang, P. Yan, R. Zhang, K. Liu, Y. Liu, T. Liu, X. Wang, *J. Mater. Chem. A* **2016**, *4*, 19053.

[138] P. Madkikar, D. Menga, G. S. Harzer, T. Mittermeier, A. Siebel, F. E. Wagner, M. Merz, S. Schuppler, P. Nagel, A. B. Muñoz-García, M. Pavone, H. A. Gasteiger, M. Piana, *J. Electrochem. Soc.* **2019**, *166*, F3032.

[139] A. Bonakdarpour, M. Lefevre, R. Yang, F. Jaouen, T. Dahn, J.-P. Dodelet, J. R. Dahn, *Electrochem. Solid-State Lett.* **2008**, *11*, B105.

[140] P. S. Ruvinskiy, A. Bonnefont, C. Pham-Huu, E. R. Savinova, *Langmuir* **2011**, *27*, 9018.

[141] Y. Yi, G. Weinberg, M. Prenzel, M. Greiner, S. Heumann, S. Becker, R. Schlögl, *Catal. Today* **2017**, *295*, 32.

[142] H. Tang, Z. Qi, M. Ramani, J. F. Elter, *J. Power Sources* **2006**, *158*, 1306.

[143] C. A. Reiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. D. Yang, M. L. Perry, T. D. Jarvi, *Electrochem. Solid-State Lett.* **2005**, *8*, A273.

[144] N. Macauley, D. D. Papadias, J. Fairweather, D. Spernjak, D. Langlois, R. Ahluwalia, K. L. More, R. Mukundan, R. L. Borup, *J. Electrochem. Soc.* **2018**, *165*, F3148.

[145] R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima, N. Iwashita, *Chem. Rev.* **2007**, *107*, 3904.

[146] S. Tahmasebi, S. Jahangiri, N. Mosey, G. Jerkiewicz, S. Baranton, C. Coutanceau, Y. Furuya, A. Ohma, *ACS Appl. Energy Mater.* **2019**, *2*, 7019.

[147] S. Tahmasebi, M. A. Hossain, G. Jerkiewicz, *Electrocatalysis* **2018**, *9*, 172.
[148] S. Tahmasebi, G. Jerkiewicz, S. Baranton, C. Coutanceau, Y. Furuya, A. Ohma, *J. Phys. Chem. C* **2018**, *122*, 11765.

[149] I. A. Pašti, A. Jovanović, A. S. Dobrota, S. V. Mentus, B. Johansson, N. V. Skorodumova, *Phys. Chem. Chem. Phys.* **2018**, *20*, 858.

[150] M. Ali, X. Pi, Y. Liu, D. Yang, *AIP Adv.* **2017**, *7*.

[151] S. T. Skowron, I. V. Lebedeva, A. M. Popov, E. Bichoutskaia, *Chem. Soc. Rev.* **2015**, *44*, 3143.

[152] C.-Y. Ahn, J. Ahn, S. Y. Kang, O.-H. Kim, D. W. Lee, J. H. Lee, J. G. Shim, C. H. Lee, Y.-H. Cho, Y.-E. Sung, *Sci. Adv.* **2020**, *6*.

[153] O.-H. Kim, Y.-H. Cho, D. Y. Chung, M. J. Kim, J. M. Yoo, J. E. Park, H. Choe, Y.-E. Sung, *Sci. Rep.* **2015**, *5*, 8376.

[154] Y. J. Sa, C. Park, H. Y. Jeong, S.-H. Park, Z. Lee, K. T. Kim, G.-G. Park, S. H. Joo, *Angew. Chemie Int. Ed.* **2014**, *53*, 4102.

[155] J. W. F. To, J. W. D. Ng, S. Siahrostami, A. L. Koh, Y. Lee, Z. Chen, K. D. Fong, S. Chen, J. He, W.-G. Bae, J. Wilcox, H. Y. Jeong, K. Kim, F. Studt, J. K. Nørskov, T. F. Jaramillo, Z. Bao, *Nano Res.* **2017**, *10*, 1163.

[156] D. C. Higgins, Z. Chen, *ECS Trans.* **2010**, *28*, 63.

[157] C. Venkateswara Rao, Y. Ishikawa, *J. Phys. Chem. C* **2012**, *116*, 4340.

[158] T. Palaniselvam, M. O. Valappil, R. Illathvalappil, S. Kurungot, *Energy Environ. Sci.* **2014**, *7*, 1059.

[159] S. M. Unni, S. N. Bhange, R. Illathvalappil, N. Mutneja, K. R. Patil, S. Kurungot, *Small* **2015**, *11*, 352.

[160] S. Lee, M. Choun, Y. Ye, J. Lee, Y. Mun, E. Kang, J. Hwang, Y.-H. Lee, C.-H. Shin, S.-H. Moon, S.-K. Kim, E. Lee, J. Lee, *Angew. Chemie Int. Ed.* **2015**, *54*, 9230.

[161] M. Mandal, G. Huang, N. U. Hassan, X. Peng, T. Gu, A. H. Brooks-Starks, B. Bahar, W. E. Mustain, P. A. Kohl, *J. Electrochem. Soc.* **2019**, *167*, 54501.

[162] Z. Tatus-Portnoy, A. Kitayev, T. V. Vineesh, E. Tal-Gutelmacher, M. Page, D. Zitoun, *Chem. Commun.* **2020**, *56*, 5669.