Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe–N–C catalysts represent the most promising alternative to precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe–N4 sites, and high active site densities have been out of reach (dilemma of Fe–N–C catalysts). We herein identify a paradigm change in the synthesis of Fe–N–C catalysts arising from the developments of other M–N–C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M–N–C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe–N–C catalysts. The selective formation of tetrapyrrolic Zn–N4 sites in a tailor-made Zn–N–C material is utilized as an active-site imprint for the preparation of a corresponding Fe–N–C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe–N–C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe–N4 sites. The density of tetrapyrrolic Fe–N4 sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe–N–C fuel cell catalysts.

sales of light electric vehicles (battery electric vehicles and plug-in hybrid electric vehicles) are politically pushed and increasing exponentially. [1] Vehicles based on proton-exchange-membrane fuel cells (PEMFCs) have the potential to surpass the limitations of battery-based ones, especially, regarding the driving range. Unfortunately, the mass commercialization of this technology is hampered by the limited availability and high cost of Pt, which is required to speed up the anodic and cathodic reactions happening in a PEMFC. [2,3] Since ≈4 times more Pt is required on the cathode than at the anode side, the development of cathode materials containing low Pt amount is a promising way to reduce costs. Unfortunately, low-Pt cathode materials suffer from other limitations, such as losses due to mass transport; [4] also, when the Pt content is reduced below 100 µg cm −2 , the cost of other components rises. [3] For these reasons, completely replacing the Pt at the cathode side is a reasonable and promising way to go.In the last 10 years, platinum-group-metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) have drawn the attention of many research groups all over the world. Since it has been shown that bioinspired metal-nitrogen-doped-carbon (M-N-C, M = Fe, Co) catalysts could meet the requirements for Atomically dispersed Fe-N-C catalysts are considered the most promising precious-metal-free alternative to state-of-the-art Pt-based oxygen reduction electrocatalysts for proton-exchange membrane fuel cells. The exceptional progress in the field of research in the last ≈30 years is currently limited by the moderate active site density that can be obtained. Behind this stands the dilemma of metastability of the desired FeN 4 sites at the high temperatures that are believed to be a requirement for their formation. It is herein shown that Zn 2+ ions can be utilized in the novel concept of active-site imprinting based on a pyrolytic template ion reaction throughout the formation of nitrogen-doped carbons. As obtained atomically dispersed Zn-N-Cs comprising ZnN 4 sites as well as metal-free N 4 sites can be utilized for the coordination of Fe 2+ and Fe 3+ ions to form atomically dispersed Fe-N-C with Fe loadings as high as 3.12 wt%. The Fe-N-Cs are active electocatalysts for the oxygen reduction reaction in acidic media with an onset potential of E 0 = 0.85 V versus RHE in 0.1 m HClO 4 . Identical location atomic resolution transmission electron microscopy imaging, as well as in situ electrochemical flow cell coupled to inductively coupled plasma mass spectrometry measurements, is employed to directly prove the concept of the active-site imprinting approach.

Platinum-group-metal-free (PGM-free) catalysts are currently considered as potential oxygen-reduction-reaction (ORR) catalysts to replace costly and supply-limited platinum at the cathode side of proton exchange membrane fuel cells (PEMFCs). Extensive research efforts have led to substantial progress with regards to the ORR activity of PGM-free ORR catalysts, but there is uncertainty about the dependence of the mass activity on the catalyst loading. In this study, the effect of catalyst loading on the mass activity is investigated by means of rotating disk electrode measurements as well as single cell PEMFC tests using a commercial PGM-free ORR catalyst. Single cell tests with a wide range of loadings (0.4–4.0 mgcat cm−2 MEA) are compared to rotating disk electrode measurements with low loadings of 40–600 μgcat cm−2 disk. In contrast to indications in the literature that the ORR activity depends on catalyst loading, our results reveal an independence of the ORR mass activity from the catalysts loading in both RDE and PEMFC tests, if corrections for the voltage losses in H2/O2 single cell tests are considered. Moreover, no clear relation of the stability to the catalyst loading was found in H2/O2 PEMFCs.

In this contribution, we demonstrate the presence of high-spin Fe 3+ in Fe-substituted ZrO 2 (Fe x Zr 1−x O 2−δ), as deduced from X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and 57 Fe Mössbauer spectroscopy measurements. The activity of this carbon-supported Fe x Zr 1−x O 2−δ catalyst toward the oxygen reduction reaction (ORR) was examined by both rotating (ring) disk electrode (R(R)DE) method and single-cell proton exchange membrane fuel cells (PEMFCs). DFT calculations suggest that the much higher ORR mass activity of Fe x Zr 1−x O 2−δ compared to Fe-free ZrO 2 is due to the enhanced formation of oxygen vacancies: their formation is favored after Zr 4+ substitution with Fe 3+ and the oxygen vacancies create potential adsorption sites, which act as active centers for the ORR. H 2 O and/or H 2 O 2 production observed in RRDE measurements for the Fe 0.07 Zr 0.93 O 1.97 is also in agreement with the most likely reaction paths from DFT calculations. In addition, Tafel and Arrhenius analyses are performed on Fe 0.07 Zr 0.93 O 1.97 using both RRDE and PEMFC data at various temperatures.

MÀ NÀ C electrocatalysts are considered pivotal to replace expensive precious group metal-based materials in electrocatalytic conversions. However, their development is hampered by the limited availability of methods for the evaluation of the intrinsic activity of different active sites, like pyrrolic FeN 4 sites within FeÀ NÀ Cs. Currently, new synthetic procedures based on active-site imprinting followed by an ion exchange reaction, e.g. Zn-to-Fe, are producing single-site MÀ NÀ Cs with outstanding activity. Based on the same replacement principle, we employed a conservative iron extraction to partially remove the Fe ions from the N 4 cavities in FeÀ NÀ Cs. Having catalysts with the same morphological properties and Fe ligation that differ solely in Fe content allows for the facile determination of the decrease in density of active sites and their turnover frequency. In this way, insight into the specific activity of MÀ NÀ Cs is obtained and for single-site catalysts the intrinsic activity of the site is accessible. This new approach surpasses limitations of methods that rely on probe molecules and, together with those techniques, offers a novel tool to unfold the complexity of FeÀ NÀ C catalyst and MÀ NÀ Cs in general.