High surface area electrocatalysts, which are uniformly embedded within a composite layer that makes up the anode and cathode (See Chapter 1), are used to accelerate the reaction kinetics of ORR/OER. The breadth of research devoted to oxygen catalysis using a wide range
of systematic, experimental, and theoretical approaches, has led to the discovery of several novel active catalyst materials. Currently, platinum (ORR) or iridium (OER) are among the standard cat- alysts in commercial proton-exchange membrane fuel cells (PEMFCs) and electrolyzers, respec- tively. However, these materials are among the rarest and most expensive noble metals, incurring a large fraction of the overall manufacturing cost, and severely limiting the production through- put. In addition to several studies that focus on reducing the material loading of Pt and Ir, many researchers have sought to eventually replace them with less expensive and more abundant alter- native (non-Pt and non-Ir) catalysts, without compromising the overall catalytic activity. Although many have reported a wide range of catalyst systems that showcase very promising results, there has been limited success in implementing them commercially, thereby replacing the current noble metal standards.
The search for alternative oxygen catalysts is the primary motivation behind this research. Thus, this chapter will convey the massive scope of oxygen catalysis, generalizing previous studies on notable catalyst systems, particularly metal oxides for alkaline fuel cells and electrolyzers. With regards to this research, two key oxide systems will be discussed in Chapters 5 and 6: manganese- doped ruthenium oxide (MnxRu1-xO2-y) and pyrochlores ((Bi,Pb)2(Ir,Ru)2-x(Bi,Pb)xO7-y). Both sys-
tems were evaluated as bifunctional catalysts using combinatorial high-throughput techniques de- rived from methodologies established by previous graduate students within the van Dover group, as well as prolific electrocatalyst researchers (e.g. Goodenough, Stamenkovic, Abru˜na, Gasteiger, etc.).[182, 242, 74]
2.2.1
The Role of Oxygen Catalysts
The performance bottleneck from sluggish ORR and OER is induced by the high kinetic activation thresholds for oxygen redox (See Appendix I summary on Transition State Theory). Cat- alytically active materials contain reactive surface sites that unlock faster reaction pathways involv-
ing heterogeneously formed intermediates (Figure S.2).[94] These pathways are generally charac- terized by a reaction sequence of steps consisting of molecular and ionic adsorption/desorption; protonation; production of oxygen intermediates, and oxygen bond-breaking.[84, 170, 215, 131] However, the predicted reaction sequence and rate-limiting step(s) are highly dependent on the catalyst material and environment, often becoming a major source of discord amongst electocata- lyst researchers. But as in-situ and ex-situ electrochemical and characterization techniques become increasingly more sophisticated, and when combined with the latest theoretical models, the conflu- ence of these methods help further expound the factors that drive specific reaction mechanisms in relation to key operating conditions (e.g. pH, temperature, electrolyte, etc.) and catalyst material properties (i.e. electronic, surface chemistry, structure, surface morphology, etc.).
2.2.2
Oxygen Reduction
The most prolific class of catalysts for oxygen reduction, as well as hydrogen oxida- tion/evolution is platinum (Pt) and platinum alloys (Pt-M, M=Co, Ir, Cr, Ni, Cu, etc.).[94, 70, 251, 160, 159, 61] In addition to consistently exhibiting the highest catalytic activities for ORR, Pt is chemically inert, and highly robust against corrosive environments. Consequently, applications such as proton-exchange membrane fuel cells (PEMFCs), which operate under highly acidic condi- tions, employ Pt-based materials as the standard catalysts for both the anode (hydrogen oxidation) and cathode (oxygen reduction). However, Pt is one of, if not the most expensive precious metal in the market due to its wide ranging application value and natural scarcity. Thus, the incorporation of Pt constitutes a significant fraction of the overall production cost of PEMFCs, ultimately stifling large-scale manufacturing and any subsequent long term growth.
Despite the need for less-expensive alternative catalyst materials, Pt continues to be the most heavily studied catalyst, establishing an comprehensive network of research focused on different aspects to engineering Pt particles, including surface structure[231, 148, 147],
alloying[70, 160, 159, 230, 252], and particle morphology[147, 282, 165], all of which converge towards optimizing catalytic activity while minimizing volumetric and/or mass Pt loading. Promi- nent studies related to Pt ORR catalysis have attempted to reconcile theoretical models built on molecular simulations and density functional theory (DFT) with systematic ex-situ and in-situ ex- perimentation. For instance, Stemankovic et al. among several other researchers modeled ORR selectivity and activity on various faces of Pt (e.g. (111), (110), and (100)), reporting the (111) face as the most active.[230, 229] Other consequential studies have used kinetic and thermodynamic models to identify key descriptors that govern ORR activity.[94, 160, 159, 230, 84] These mod- els correlate with the Sabatier principle, which identifies an optimizal binding energy of oxygen and hydroxyl groups on the catalyst surface for maximum catalytic ORR activity.[230, 84, 229] The binding energy descriptor is governed by the electronic properties of the catalyst, particu- larly the interactions between the metallic d-band center with the molecular orbitals of the oxygen adsorbates. When the binding energy is too high, the Pt surface becomes poisoned by the accu- mulation of bound oxygen intermediates due to a low rate of oxide dissociation; conversely, when the binding energy is too low, partially reduced intermediates desorb, producing partially reduced byproducts (i.e. H2O2) that can have adverse effects on the surrounding components.[84, 131]
(a) (b)
Figure 2.3: (a) Volcano plot of metal catalysts. (b) Volcano plot of Pt-metal alloys at the (111) plane.[23]
Figure 2.4: (a) Potential vs. atomic number volcano plot relative to the theoretcial potentials for the reaction of hydrogen and oxygen.[23]
Consequently, the catalytic activity with respect to the binding energy descriptor estab- lishes a ”volcano” trend (Figures S.3a), resolving an activity peak. This model has led to sev- eral other studies that focused on adjusting the oxygen binding energy (tuning the d-band center) through alloying-induced strain effects (Figure S.3b) using a number of different noble and tran- sition metals. Alloying brings the additional benefit of reduced cost by reducing Pt-loading when alloyed with cheaper metals. Alloying has also been used to engineer the nanostructure of Pt- based catalysts through the design of core-shell bimetallic Pt particles, composed of a monolayer shell of Pt and a non-Pt core made of precious and/or transition metals (Ir, Pd, Au, Ni FeCu, etc.).[112, 274, 209, 198, 271, 235] These approaches optimize Pt use, while using the nanostruc- tured core as a template for maximizing the preferential orientation of the highly active (111) face. While a more in depth summary of Pt catalysts is warranted, it is well beyond the necessary scope of this thesis. Thus, I encourage the reader to explore this massive sphere independently.
2.2.3
Oxygen Evolution
In addition to proliferating energy conversion applications such as fuel cells, it is critical to establish an infrastructure that can facilitate large-scale generation and storage of hydrogen fuel.
Efficient electrochemical systems for generating hydrogen fuel through water oxidation have been highly sought, most prominently through electrical electrolysis and systems that facilitate solar- to-fuel conversion. However, current electrolysis systems suffer from low efficiencies and current outputs imposed by the large kinetic threshold of oxidizing water to generate oxygen. Efforts to optimize OER efficiencies have culminated into several studies on bulk and molecular catalyst systems for photo and electrical electrolysis.[23, 43, 143, 116, 261]
In the same fashion as PEMFCs, the harsh acidic environment in proton-exchange mem- brane electrolyzers severely limits the availability of compatible catalysts to a select group of noble metals (e.g. Pt, Rh, Ir, Au).[23] Iridium (Ir) and IrO2 are considered the benchmark catalysts for
PEM electrolyzers due to their high stability and strong catalytic activity.[42, 32, 31, 100, 59] How- ever, Ir is among the rarest and most expensive noble metals within the earths crust (less abundant than both Pt and Au). Accordingly, ongoing research has focused on minimizing Ir loading by mixing with less expensive elements.[168, 169, 278] Ruthenium (Ru) and RuO2 are another set
of highly active catalysts that exhibit even stronger OER activities than Ir/IrO2.[119, 31] Despite
this, Ru and RuO2 are susceptible to corrosion in acidic media, resulting in the formation of RuO4
compounds, which can subsequently react further to form various other undesired complexes with the surrounding reactants.[23, 119, 43, 30] Regardless, many have sought to better stabilize RuO2,
mostly through mixing.[6, 154, 142]