Ashwin Ramasubramaniam, Chair, Mechanical & Industrial Engineering
Friederike Jentoft, Member, Chemical Engineering
Bret Jackson, Outside Member, Chemistry
We derive most chemicals and fuels from fossil fuels. However, fossil fuel reserves are finite, and their use aggravates the problems of environmental pollution and climate change. Electrochemical reactions, when combined with renewable energy sources, offer renewable fossil-free routes for chemicals and fuels production. Thus, electrochemical reactions are crucial to reduce our dependence on fossil fuels. Hydrogen (H2) and ammonia (NH3) are two of the most important raw materials for the chemical industry and are typically produced by fossil fuel-based, energy-intensive processes. It is possible though to produce H2 and NH3 electrochemically via the hydrogen evolution reaction (HER) and the N2 reduction reaction (NRR), respectively, using appropriate electrocatalysts. However, the limited efficiency and selectivity of several electrocatalysts or, in some instances, the high cost of suitable electrocatalysts limits the large-scale adoption of these electrochemical processes. Thus, in this dissertation, we employ first-principles density functional theory (DFT) calculations to study and design low-cost efficient electrocatalysts for HER and NRR.
HER is most efficiently catalyzed by platinum (Pt), which is expensive. To reduce Pt loading and the catalyst cost, first, we investigate core-shell nanoparticles of inexpensive tungsten-carbide (WC) and Pt (WC@Pt). Recent experiments show the formation of two types of stable WC@Pt nanoparticles, one with the α-WC (a room-temperature phase) and the other with the β-WC (a high-temperature phase) core. Using density functional theory (DFT) calculations, we compare the suitability of α-WC and β-WC phases as support materials (cores) for 1-2 atom-thick Pt overlayers (shells). We dope WC with titanium (Ti) and examine its effects on the thermodynamic stability and HER activity of WC and WC@Pt nanoparticles. Overall, we show that β-WC is more suited for Pt overlayers than α-WC, and that moderate Ti doping of WC is an effective approach to stabilize β-WC and β-WC@Pt nanoparticles without significantly affecting the HER activity of Pt overlayers.
Thereafter, we investigate an inexpensive electrocatalyst, semiconducting molybdenum diselenide (MoSe2) for HER. MoSe2 is a promising HER catalyst because of its highly active edges. Yet, the overall activity of the material is limited, as most of its electrochemical surface area—the basal plane—is inert towards HER. To activate the MoSe2 basal plane, we examine the effect of doping MoSe2 with select electron-rich transition metals (Mn, Fe, Co, and Ni). Our DFT studies show that all of these chosen transition-metal (TM) dopants improve H adsorption (or HER) thermodynamics on the MoSe2 basal plane. We also find that all selected TM dopants promote the formation of HER active Se-vacancy sites in the basal plane and edges. In short, our studies demonstrate that TM-doping of MoSe2 is an effective strategy to generate new HER active sites in the basal plane and edges, and further activate MoSe2 for HER.
Finally, we investigate a low-cost catalyst, molybdenum disulfide (MoS2), for NRR. MoS2 possesses a limited number of active edges and displays low activity and selectivity for NRR. To overcome these challenges, we dope MoS2 with iron (Fe). From first-principles thermodynamics calculations, we show that the formation of Fe-doped edges is energetically preferred over undoped edges of MoS2, leading to Fe-rich edges in the doped material. Furthermore, we show that catalytically active sulfur vacancies are more readily formed at Fe-doped edges than at undoped edges of MoS2. We show that such defect-rich Fe-doped S edges can catalyze NRR at moderate cathodic potentials and we also propose a new mechanism, the “H-mediated enzymatic pathway”, for NRR at these sites. Our studies, therefore, show that Fe doping of MoS2 is a potentially viable strategy for producing inexpensive, active, and selective NRR electrocatalysts.