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Half-cell gas diffusion electrode (GDE) setups have recently been shown to allow characterization of oxygen reduction reaction (ORR) catalysts at fuel cell relevant current densities and potentials while offering the advantage of fast screening and requiring only minimal sample usage. Most recently, publications suggesting best practices for GDE half-cell characterization have highlighted key challenges in running GDE experiments and have presented measurement protocols that allow excellent Inter-lab comparability. This step can be seen as the cornerstone for broad-based utilization of GDE half-cells. However, what is still missing, is an understanding of what limits the maximum achievable current density in GDE measurements. In this study we highlight the influence of various setup parameters (electrode area, electrolyte concentration, as well as size and position of the counter electrode) on the maximum achievable current density in an automated GDE setup. Furthermore, we present the metrics that need to be considered when adapting the electrochemical protocol for recording ORR polarization curves at higher current densities. The findings observed in this study are not limited to testing the ORR, but can be transferred to any other electrocatalytic reaction tested in a half-cell GDE at high current densities.

The oxygen reduction reaction (ORR) is pivotal for optimizing the energy output of fuel cells and metal-air batteries; however, their commercialization is precluded by the intolerable cost, instability, earth-scarcity of Pt that remains a prestigious catalyst for ORR. Porous ternary Pt-based branched nanostructures (PTB-Pt) with unique catalytic properties, low-density structural stability, multiple electronic effects, high surface area, and maximized atomic utilization can maximize the ORR catalytic activity at low Pt content. This review provides broad and balanced insights on the substantial progress on the rational design of porous PTB-Pt for ORR and their fundamental mechanisms besides the current challenges and perspectives on designing ideal PTB-Pt catalysts for efficient ORR and guidance for future development of real-device.

[Display omitted] • Poisoning effect of benzene-containing hydrocarbon-based ionomers on Pt catalysts. • Two types of orientation-dependent adsorption of benzenesulfonic acid anion for Pt. • Changes in PZTC of Pt surface according to the sequence of electrolyte replacement. • Combination of various electrochemical analyses and XPS/TPD measurements. • Time-dependent anion adsorption affecting the long-term stability of fuel cell electrodes. Membrane electrode assembly (MEA) is an essential component that mainly determines the performance of proton exchange membrane fuel cells (PEMFCs). In the electrodes of the MEA, ionomers are mixed with Pt-based nanoparticles to transfer protons to the Pt surface where electrochemical reactions take place. However, Pt catalysts suffer from the poisoning effect of ionomers with proton-conducting groups such as sulfonic acid (SA), resulting in performance deterioration. In particular, for emerging hydrocarbon-based ionomers, the benzene group, which is commonly used to improve their chemical structure, accelerates the poisoning effect. Herein, we experimentally simulate the adsorption of SA and benzenesulfonic acid (BSA) on Pt nanoparticles, revealing the orientation-dependent poisoning effect of BSA on the Pt surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • ORR reaction mechanism in CW-MFCs occurred through the 4-electrons pathway. • Maximum power density was produced with 1 mg/cm2 of G/TiO 2 in CW-MFCs. • Photosynthetic compounds were used as substrate for the anodic biofilm in CW-MFCs. Constructed wetlands-microbial fuel cells (CW-MFC) are an innovative technology used for simultaneous bioelectricity generation and wastewater treatment. This is possible due to the installation of macrophytes in an electrode configuration, in which electroactive microorganisms use organic substrates as biofuel. One way to improve the electrochemical performance of CW-MFCs is through the impregnation of cathodic electrocatalysts. Therefore, in this study the bioelectricity production capacity of CW-MFCs was evaluated from the oxygen reduction reaction (ORR). For this study, the concentrations 0 (CW-MFC1), 0.5 (CW-MFC2), and 1 mg/cm2 (CW-MFC3) of graphene/titanium dioxide (G/TiO 2) as electrocatalyst on the cathodes were evaluated. Using the Koutecky-Levich analysis, it was determined that the ORR transfer mechanism arises via a 4-electron pathway. The electrokinetic parameters of Tafel slope, charge transfer coefficient, and exchange current density determined the efficiency of the ORR, registering 92 mV/dec, 0.93 (α), and 2.30 x10-3 mA/cm2, respectively for CW-MFC3. The highest electrochemical performance was obtained at a concentration of 1 mg/cm2 (CW-MFC3) of G/TiO 2 , generating 144 mW/m2 of power density, 157 Ω of internal resistance, −150 mV of anodic potential, and 383 mV of cathodic potential. The surface modification carried out on the cathodes resulted in a catalytic increase in the ORR. [ABSTRACT FROM AUTHOR]

• Iodine and Fe-N x were simultaneously introduced into carbon (FeNI-CNS) by a solution plasma process. • Iodine dopants contribute to the oxygen reduction reaction rate with a higher current density. • A synergistic effect further enhances the catalytic activity of the FeNI-CNS. • FeNI-CNS exhibits better durability than commercial Pt/C catalysts. The oxygen reduction reaction (ORR) plays a critical role at the cathode of fuel cells and metal–air batteries. In this study, we report the one-pot synthesis of iodine- and Fe-N x impregnated carbon nanospheres (FeNI-CNS) using a solution plasma process and their effect on electrocatalytic activity. FeNI-CNS exhibits a fast four-electron transfer reaction with a high current density and superior durability in both alkaline and acidic media. The excellent catalytic performance is attributed to the synergetic effect between the iodine and Fe-N x sites, which provide a higher reaction rate and faster ORR, respectively. We believe that these findings represent a new approach for enhancing ORR catalytic performance and catalyst synthesis. [ABSTRACT FROM AUTHOR]

• GDE measurements for evaluation of oxygen reduction reaction catalysts. • Avoiding current limitations in high-current density testing. • Guidelines for design of gas diffusion electrode half-cells. • Best practices for iR correction in high-current density testing. • Comparison of GDE and MEA. Half-cell gas diffusion electrode (GDE) setups have recently been shown to allow characterization of oxygen reduction reaction (ORR) catalysts at fuel cell relevant current densities and potentials while offering the advantage of fast screening and requiring only minimal sample usage. Most recently, publications suggesting best practices for GDE half-cell characterization have highlighted key challenges in running GDE experiments and have presented measurement protocols that allow excellent Inter-lab comparability. This step can be seen as the cornerstone for broad-based utilization of GDE half-cells. However, what is still missing, is an understanding of what limits the maximum achievable current density in GDE measurements. In this study we highlight the influence of various setup parameters (electrode area, electrolyte concentration, as well as size and position of the counter electrode) on the maximum achievable current density in an automated GDE setup. Furthermore, we present the metrics that need to be considered when adapting the electrochemical protocol for recording ORR polarization curves at higher current densities. The findings observed in this study are not limited to testing the ORR, but can be transferred to any other electrocatalytic reaction tested in a half-cell GDE at high current densities. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highlighting the rational design of ternary Pt-based branched nanostructures (PTB-Pt) • Discussion the fabrication methods of PTB-Pt and their mechanisms. • Emphasizing the effect of shape and composition of PTB-Pt on oxygen reduction (ORR) activity. • Identifying the current limitations and perspectives direction on design PTB-Pt for ORR. The oxygen reduction reaction (ORR) is pivotal for optimizing the energy output of fuel cells and metal-air batteries; however, their commercialization is precluded by the intolerable cost, instability, earth-scarcity of Pt that remains a prestigious catalyst for ORR. Porous ternary Pt-based branched nanostructures (PTB-Pt) with unique catalytic properties, low-density structural stability, multiple electronic effects, high surface area, and maximized atomic utilization can maximize the ORR catalytic activity at low Pt content. This review provides broad and balanced insights on the substantial progress on the rational design of porous PTB-Pt for ORR and their fundamental mechanisms besides the current challenges and perspectives on designing ideal PTB-Pt catalysts for efficient ORR and guidance for future development of real-device. [ABSTRACT FROM AUTHOR]

[Display omitted] • Method for synthesizing metal-free electrocatalyst with highly ordered nanochannels. • Thermal treatment in NH 3 atmosphere for activation of catalyst surface toward oxygen reduction. • Improved performance after stability test owing to ordered nanostructure with activated surface. • Verification of variations in N-containing species during oxygen reduction reaction. Lowering the cost of electrocatalysts for the oxygen reduction reaction (ORR) is indispensable for the wider commercialization of fuel cells. Therefore, the development of non-Pt, especially metal-free, carbon-based electrocatalysts, has been encouraged as an alternative to Pt. Here, we report active N-doped nanocarbon electrocatalysts in ORR activated by thermal treatment of pristine carbon precursors with highly aligned mesopore channels under an NH 3 atmosphere. This NH 3 -activation process adopts N-containing carbon sites, leading to enhanced ORR performance in an alkaline electrolyte with negligible formation of peroxide species. The stability of the activated carbon electrocatalysts was confirmed by performing 40,000 rounds of potential cycles in the accelerated stress test (AST). The variations in the voltammograms during AST were analyzed using X-ray photoelectron spectroscopy. The correlation between electrochemical behavior and the N 1 s spectrum corroborates the contribution of N-related sites toward voltammetric responses in ORR. [ABSTRACT FROM AUTHOR]

• Pt and PtNi NPs on metal oxides evaluated as bifunctional ORR/OER catalysts. • Ni addition reduces the catalysts cost while increasing its ORR/OER activity. • Use of Mn 2 O 3 -NiO bimetal oxide support enhances the catalyst stability. Three different metal oxides based on Mn 2 O 3 with TiO 2 or NiO were synthesised. Pt or PtNi nanoparticles were anchored on each support, creating a set of nine samples that were tested for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). PtNi/Mn 2 O 3 -NiO showed the most promising results for ORR as evidenced by the lowest Tafel slope, the highest diffusion-limited current density and number of electrons exchanged, along with the highest stability. The best performance of PtNi/Mn 2 O 3 -NiO reflects its highest electrochemical surface area and the lowest charge-transfer resistance. Furthermore, this catalyst showed high activity for the OER as evidenced by the low Tafel slope and high current density at an overpotential of 400 mV. The present study indicated different active sites for the two reactions, i.e., PtNi NPs for the ORR and NiO for the OER. [ABSTRACT FROM AUTHOR]

[Display omitted] • Potential pulses prior to the ORR reactivate catalytic sites which were oxidized at high anodic potentials. • Optimisation of the pulse potential allows to stabilize ORR activity. • RRDE measurements demonstrate a change in the ORR mechanism after applying high anodic potentials. The stability of electrocatalysts is of great importance to ensure their applicability, but stability is generally only considered for catalysts polarised to a constant potential or current density. This excludes stability evaluation under start/stop conditions in a fuel cell or in reversible batteries in which the catalyst is alternately polarised to high opposite potentials. For example, the poor cyclability of metal-air batteries is mainly due to the decrease in the oxygen reduction activity of electrocatalysts during the high applied potentials for the oxygen evolution reaction during battery charging. To investigate and at least partially mitigate the loss of electrocatalytic activity for the oxygen reduction reaction, we employed reductive pulses with the aim of restoring the catalytic activity of the active sites for the oxygen reduction reaction. Optimisation of the reductive pulse parameters makes it possible to substantially prolong the oxygen reduction activity of a Fe-N x -doped carbon-based oxygen reduction electrocatalyst. [ABSTRACT FROM AUTHOR]

• Method for rapid and comparable catalyst layer evaluation developed. • Successful benchmark experiments with commercial Fe-N-C catalyst and alkaline ionomer. • Optimized cathode performance after pre-conditioning for 48 h in 1 M KOH. • Fe-N-C catalyst layers show very high stability after 5000 cycles between 0.6 and 1.0 V vs. RHE. Anion exchange membrane fuel cells (AEMFC) are a promising technology to allow the application of non-precious metal catalysts. While many of such catalysts have been identified in numerous recent fundamental research studies, reports evaluating these catalysts in realistic AEMFC catalyst layers together with stability assessments are rare. In the present work we show that fast and reliable evaluation and optimization of Fe-N-C-based oxygen reduction reaction (ORR) catalyst layers can be achieved using a gas diffusion electrode (GDE) half-cell approach. To set a benchmark in such measurements, a commercial Pajarito Powder Fe-N-C catalyst and commercial AemionTM ionomer are used. It is demonstrated that the ORR performance can be increased significantly by fine-tuning of the ionomer activation time. Furthermore, the optimized Fe-N-C-based catalyst layer shows very high stability with no observable performance deterioration after 5000 cycles in the 0.6–1.0 V vs. RHE potential window. [ABSTRACT FROM AUTHOR]