In the past decades, the surplus of atmospheric CO2 concentration has drawn tremendous political and scientific attention for its negative impacts, such as the greenhouse effect and ocean carbonation. To mitigate such CO2 issues, a combination of various strategies is required. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative to convert CO2 into carbon-based chemicals and fuels, and electricity generated from the renewable sources (solar and wind) could be employed to sustain this transformation. At the current moment, the technological viability of this process is still contingent on finding affordable and efficient catalysts. In this thesis, a family of catalyst materials composed of abundant elements, in particular, non-precious metals, nitrogen, and carbon, typically referred to as precious group metal (PGM)-free "M-N-C" catalysts, were synthesized and mechanistically investigated-both experimentally and computationally-as catalyst candidates for the CO2RR. MNC catalysts feature hemoglobin-like single-site metallated porphyrin moieties with great impact on the catalytic reactivity and selectivity of the CO2RR. Among our studied M-N-C catalysts, the Ni-functionalized one exhibits great efficiency for CO yielding at high potentials and current densities. In particular, employment of Ni-N-C-based gas diffusion electrodes (GDE) combined with micro flow cells, allowed high CO evolution that could exceed 80% faradaic efficiency at 250 mA cm-2 current density, outperforming the industry commonly used Ag benchmark. By coupling our experimental observation and density functional theory (DFT) simulation, the reaction path from CO2 to CO over this sort single site catalyst could be deduced. Unlike the Ni-N-C catalyst, the Fe-N-C shows selective CO production only at low potentials. Further, due to relatively strong interaction with CO*, it opens the chance for hydrocarbons formation, yet showing little selectivity. To understand the mechanism behind this kind of selectivity, we carried out a series of studies, discussing catalytic tests, in-operando spectroscopic analysis, and computational modeling. Towards material research, operando-XAFS measurements identified an unusual Fe-N3, possibly a Fe I-N3 state, which appears to enable CH4 evolution. Further mechanistic studies included the electrocatalytic reduction of CO and CH2O as possible reactive intermediates for CH4 production. By combining the experimental and computational results, we suggest a reaction network for CO2 reduction into a variety of carbon-based products over the Fe-N-C catalyst. This contributes to the overall mechanistic understanding of CO2RR over the M-N-C catalysts and delivers perspectives to evolve and design novel catalysts to produce hydrocarbons of high value.

We report novel structure–activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO2 reduction reaction (CO2RR) catalyzed by a series of porous iron–nitrogen–carbon (FeNC) catalysts.

Metal−nitrogen−carbon (MNC) catalysts represent a potential means of reducing cathode catalyst costs in low temperature fuel cell cathodes. Knowledge-based improvements have been hampered by the difficulty to deconvolute active site density and intrinsic turnover frequency. In the present work, MNC catalysts with a variety of secondary nitrogen precursors are addressed. CO chemisorption in combination with Mossbauer spectroscopy are utilized in order to unravel previously inaccessible relations between active site density, turnover frequency, and active site utilization. This analysis provides a more fundamental description and understanding of the origin of the catalytic reactivity; it also provides guidelines for further improvements. Secondary nitrogen precursors impact quantity, quality, dispersion, and utilization of active sites in distinct ways. Secondary nitrogen precursors with high nitrogen content and micropore etching capabilities are most effective in improving catalysts performance.

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.

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Iron based nitrogen doped carbon (FeNC) catalysts are synthesized by high-pressure pyrolysis of carbon and melamine with varying amounts of iron acetate in a closed, constant-volume reactor. The optimum nominal amount of Fe (1.2 wt%) in FeNC catalysts is established through oxygen reduction reaction (ORR) polarization. Since the quantity of iron used in FeNCs is very small, the amount of Fe retained in FeNC catalysts after leaching is determined by UV-VIS spectroscopy. As nitrogen is considered to be a component of active sites, the amount of bulk and surface nitrogen retention in FeNC catalysts are measured using elemental analysis and X-ray photoelectron spectroscopy, respectively. It is found that increasing nominal Fe content in FeNC catalysts leads to a decreased level of nitrogen retention. Thermogravimetric analysis demonstrates that increasing nominal Fe content leads to increased weight loss during pyrolysis, particularly at high temperatures. Catalysts are also prepared in the absence of iron source, and with iron removed by washing with hot aqua regia post-pyrolysis. FeNC catalysts prepared with no Fe show high retained nitrogen content but poor ORR activity, and aqua regia washed catalysts demonstrate similar activity to Fe-free catalysts, indicating that Fe is an active site component.

An electrode-scale, transport model for a proton-exchange-membrane fuel cell (PEMFC) cathode is presented. The model describes the performance of non-precious metal catalysts for the oxygen reduction reaction in a fuel cell context. Because of its relatively high thickness, emphasis is placed on phenomena occurring in the cathode layer. Water flooding is studied in terms of its impact on gas-phase transport and on electrochemically accessible surface area (ECSA). Although cathode performance in both air and oxygen are susceptible to ECSA loss, gas diffusion limitations at high current density in air are more significant. In oxygen, catalyst utilization at high current density is primarily limited by conductivity. For this reason, air fuel cell data is recommended over oxygen data for characterizing catalyst performance. Due to both ohmic and mass transport limitations, increased loading of low-cost catalysts does not necessarily lead to higher performance. Therefore, careful optimization of catalyst layer thickness is required. The high environmental costs of current energy systems drives a search for commercializable energy technologies with low carbon footprints. Proton exchange membrane fuel cells present one energy option for transportation applications. A significant impediment to commercialization has been the cost and availability of catalysts for the oxygen reduction reaction (ORR) due to prevalent use of platinum group metals (PGM). Metal-Nitrogen-Carbon (MNC) catalysts are a potential solution to the cost and availability challenges that come with using PGMs. MNC catalysts are synthesized by mixing metal, carbon, and nitrogen precursors followed by one or more pyrolysis steps at 700-900 C.1-8 Such MNC catalysts are generally washed in acid to remove excess metal precursors.1-5 These catalysts generally involve lower volume-specific activity than platinum, which results in thicker electrodes. It has been previously proposed that a low-cost catalyst may be allowed to have 10-fold lower activity than platinum, as long as the catalyst layer was 10-fold thicker.9 This proposal was based on the assumption that transport loses would not be significant in this thickness regime. The purpose of this paper is to understand quantitatively how the thickness of these electrodes will impact membrane electrode assembly (MEA) performance for MNC catalyst.A number of models have addressed electrode scale transport issues.10-16 These models generally concentrate on water flooding in the gas diffusion layer (GDL) and consider a relatively thin catalyst layer. Here, gas and liquid transport in the catalyst layer are considered in a way that is similar to treatment of gas diffusion layers in previous models. We also treat the GDL consistently with previous models.Multiple works have addressed gas diffusion in the cathode catalyst layers, 17-23 with a subset considering flooding impacts on gas diffusion. 17,19,21,23 The works that do consider flooding are primarily concerned with thin, precious-metal catalyzed layers of ∼10 ...

Porous carbon materials with varying structural and compositional properties were studied for their impact on the nitrogen content and activity of metal-nitrogen-carbon (MNC) oxygen reduction catalysts prepared using high-pressure pyrolysis. The carbon materials and resulting catalysts were characterized morphologically using nitrogen physisorption, coupled with non-local density functional theory (NLDFT) analysis to calculate pore size distributions. Graphiticity was assessed via X-ray Diffraction (XRD), bulk nitrogen content was observed using CHN combustion analysis and iron content by Inductively Coupled Plasma (ICP). The catalysts were characterized electrochemically using rotating ring-disk measurements. The results indicate that substrates adsorbing the most nitrogen and iron show the highest activity. Furthermore, a relationship found between mesoporosity and nitrogen adsorption indicate the importance of transport of precursors to potential active sites.The cost of precious metals has driven the search for lower-cost alternatives to catalyze the oxygen reduction reaction (ORR) and accelerate commercialization of low-temperature fuel cells. An important class of non-precious metal catalysts for oxygen reduction is pyrolyzed metal/nitrogen/carbon (MNC) compounds. 1-3 These compounds involve the association of metal atoms with nitrogen moieties immobilized in a conductive carbon matrix. Recent research has lead to increased understanding of key aspects of catalyst activation, function, and stability. However, the nature of the active site is not yet clear because of the complexities introduced by pyrolysis on high surface-area supports. Such lack of understanding has hindered the engineering and implementation of these catalysts in fuel cell applications.Dodelet et al. suggest that MNC catalytic sites occur where metal atoms are bridge-bonded to nitrogen sites and span micropores of width less than 2 nm. 4,5 Lefevre et al. also hypothesize that high temperature ammonia etching of micropores during pyrolysis creates potential catalytic site hosts. 6 Although microporosity can have a significant impact on activity, species transport to reactive sites occurs most efficiently through mesopores. This suggests that, for geometrically complex materials, mesoporosity will have a significant impact on the apparent catalyst activity. 7-9