Accurate Evaluation of Active-Site Density (SD) and Turnover Frequency (TOF) of PGM-Free Metal–Nitrogen-Doped Carbon (MNC) Electrocatalysts using CO Cryo Adsorption

Abstract

The number of catalytically active sites (site density, SD) and the catalytic turnover frequency (TOF) are critical for meaningful comparisons between catalytic materials and their rational improvement. SD and TOF numbers have remained elusive for PGM-free, metal/nitrogen-doped porous carbon electrocatalysts (MNC), in particular, FeNC materials that are now intensively investigated and widely utilized to catalyze the oxygen reduction reaction (ORR) in fuel cell cathodes. Here, we apply CO cryo sorption and desorption to evaluate SD and TOF numbers of a state-of-art FeNC ORR electrocatalyst with atomically dispersed coordinative FeN x (x ≤ 4) sites in acid and alkaline conditions. More specifically, we study the impact of thermal pretreatment conditions prior to assessing the number of sorption-active FeN x sites. We show that the pretreatment temperature sensitively affects the CO sorption uptake through a progressive thermal removal of airborne adsorbates, which, in turn, controls the resulting catalytic SD numbers. We correlate CO uptake with CO desorption and analyze the observed temperature-programmed desorption characteristics. The CO uptakes increased from 45 nmol·mg–1 catalyst at 300 °C cleaning to 65 nmol·mg–1 catalyst at 600 °C cleaning, where it leveled off. These values were converted into apparent SD values of 2.7 × 1019 to 3.8 × 1019 surface sites per gram catalyst. Because of similar ORR activity of the pristine catalyst and of the sample after 600 °C cleaning step, we conclude that the nature and number of surface FeN x sites remained largely unaffected up to 600 °C and that cleaning to less than 600 °C was insufficient to free the sites from previously adsorbed species, completely or partially impeding CO adsorption. Cleaning beyond that temperature, however, led to undesired chemical modifications of the FeN x moieties, resulting in higher TOF. In all, this study identifies and recommends a practical and useful protocol for the accurate evaluation of catalytic SD and TOF parameters of PGM-free ORR electrocatalyst, which enables a more rational future catalyst development and improvement.

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

“…Pulse chemisorption at À80 1C (dry ice and acetone) consisted of 10 min line flushing (helium, 20 cm 3 min À1 ), followed by six consecutive CO pulses injected by the automated sample loop (helium as a carrier gas, 20 cm 3 min À1 , loop volume was determined to be 0.341 mL) in intervals of 25 min. 68,85 Prior to TPD analysis, three consecutive CO pulses are performed to ensure the saturation of the active centres with CO. Thereafter TPD (À80 1C to 600 1C, 10 1C min À1 , hold time 10 min, He as carrier, 20 cm 3 min À1 ) with a consecutive cooling to 30 1C (201 min À1 ) were performed.…”

“…84 The ex situ, lowtemperature CO cryo pulse chemisorption/desorption technique featured good specificity to Fe sites and resulted in reproducible SD values for different single metal active sites, in particular for Fe-N-C materials. 68,85,86 The technique relies on rapid adsorption rates and strong binding at À80 1C between CO molecules and atomically dispersed single Fe-N x sites embedded in a carbon framework. Possible pitfalls of this technique include overestimation, because it is not possible to show that ORR is blocked by CO and due to the possibility of single sites to bind more than one CO molecule.…”

“…A standardized thermal pretreatment protocol of Fe-N-C now ensures reproducible CO uptake values on oxygen-free Fe(II)N x sites. 85,86 Second, a complementary in situ electrochemical nitrite adsorption/NO electrostripping technique was put forward by Kucernak's group. 77 The method relies on the very specific and strong interaction of Fe-N x sites with nitrite anions resulting in NO adsorption, followed by electrochemical reductive stripping of NO into ammonia.…”

Homogenous Meets Heterogenous and Electro‐Catalysis: Iron‐Nitrogen Molecular Complexes within Carbon Materials for Catalytic Applications

“…However, the experimental details need to be taken into consideration when comparing the catalysts performance, as different technical parameters vary in the literature (i. e. reference electrode selection, mass loading of electrocatalyst, data correction prior to extracting kinetic currents, etc. ) Practical and useful protocols are under investigation for quantifying the active sites and accurate evaluate the catalytic performance of the catalysts, such as the kinetic isotope effect, site density, and turnover frequency, which were found to play significant role in determining the ORR performance of Fe/N@C catalyst in fuel cells …”

Oxygen Reduction Reaction on Metal and Nitrogen–Doped Carbon Electrocatalysts in the Presence of Sodium Borohydride

“…These last two materials fall into the category of metal-nitrogen-carbon (M-N-C) catalysts, which are now under tremendous focus to replace Pt-based ones for PEMFC applications (see e.g. [45][46][47]). Because these classes of material are commonly-admitted to be tolerant to fuel species, and some of them have indeed shown tolerance to borohydride [38,44], the present contribution explores two families of M-N-C compounds, based on cobalt and iron metal centers.…”