| Energy sustainability is perhaps the greatest ongoing challenge facing humanity. Our need to replace fossil fuel-based sources of energy with environmentally friendly, secure, and renewable alternatives continues to grow, and although this fact has been long realized by the scientific community and beyond, no present-day solution effectively competes with fossil fuels from an economic or performance standpoint. Although several renewable energy technologies, including photovoltaic solar cells and fuel cell systems, can efficiently supply usable power with little or no environmental impact, they often suffer from high costs due to the expensive raw materials and complex processing steps required to produce high performance devices. These costs ultimately limit the scalability of such technologies and, consequently, their potential to address our looming energy concerns. However, the viability of many renewable energy technologies---particularly those rooted in electrochemistry---could be substantially increased by replacing expensive and scarce materials (such as noble metals) with low-cost, earth-abundant alternatives that exhibit comparable performance. The work collected here primarily focuses on identifying and developing such alternative electrocatalysts, generally within the family of transition metal chalcogenides, and assessing their utility in electrochemical energy conversion applications. Chapter 1 reviews both electrochemical energy conversion and alternative earth-abundant electrocatalyst materials, motivating their investigation and outlining the key challenges yet to be overcome. In Chapter 2, metallic cobalt pyrite (cobalt disulfide, CoS 2) is introduced as a new earth-abundant electrocatalyst candidate material capable of boosting the performance of quantum dot-sensitized solar cells while simultaneously eliminating their reliance on precious platinum-based electrodes. Chapter 3 further builds upon Chapter 2 by establishing the high intrinsic electrocatalytic activity of CoS2 toward the hydrogen evolution reaction. Here, micro- and nanostructuring strategies are also demonstrated to synergistically enhance the electrocatalytic performance and stability of CoS2. Chapter 4 broadens the family of pyrite-phase electrocatalysts by showing that other earth-abundant transition metal disulfides exhibit electrocatalytic activity toward both polysulfide reduction and the hydrogen evolution reaction. Collectively, this work represents substantial progress toward the development of earth-abundant transition metal chalcogenide electrocatalysts for renewable energy applications, with the expectation that the lessons learned here should translate to other materials systems. |
