The prospect of sustainably generating fuels from sunlight via artificial photosynthesis has inspired scientists for decades. Over time, intensive research devoted to materials for catalysis and light harvesting provided a set of components individually optimized for specific steps of sunlight-to-fuel conversion, as well as a conceptual basis for understanding conversion mechanisms. These efforts also revealed critical gaps in properties of existing materials, in models of non-ideal function, and especially in our understanding of concerted processes occurring in integrated photosystems. Within this context, the Joint Center for Artificial Photosynthesis (JCAP) was established with the mission ‘to demonstrate a scalable, manufacturable solar-fuels generator using Earth-abundant elements, that, with no wires, robustly produces fuel from the sun ten times more efficiently than (current) crops’. Intrinsic to this mission was the recognition that integration of components into functional systems introduces new scientific challenges that must be addressed to advance along the path to a technology. The drive towards integration forces the researcher to consider how components are physically, electronically, and chemically coupled over multiple length, time, and energy scales. Such coupling places dramatic restrictions on material synthesis processes, electrolyte environments, and system geometries. However, these considerations also provide a framework for defining research priorities and for conceiving practical device architectures.
This text describes recent developments in the field of solar water splitting that encompass major work at JCAP and its scientific environment. The Center was operated as an Energy Innovation Hub, a novel concept at that time, initiated by the former US Secretary of Energy Steven Chu. He modeled the hubs after Bell Laboratories, also known as ‘The Idea Factory,’ where fundamental and applied science, extending to devices and systems, were integrated into a coherent research and development effort. This model led to the ‘one-roof’ concept of the Energy Innovation Hubs of the US Department of Energy. The hubs were considered smaller Bell Laboratories—affine units. The Hub concept entails the participation of researchers and engineers from various scientific disciplines in a proactive approach to conducting research and in managing activities towards applications. From the beginning, the focus was set on well-funded, accelerated development achieved by highly integrated research and development teams that could advance progress more rapidly than researchers working separately. In this way, the Energy Innovation Hubs were conceptualized and modeled after the way industrial laboratories operated during their most productive periods.
When the Hub concept was launched, the production of fuels from sunlight was considered as one of the grand challenges where interdisciplinary work would particularly benefit accelerated development. In 2010, the activity for artificial photosynthesis was started in a joint effort that involved the California Institute of Technology, the Lawrence Berkeley National Laboratory and the Stanford Linear Accelerator as major partners, including also the Universities of California at Irvine and San Diego under the leadership of Prof. Nathan Lewis, the Founding Director.
This book comprises the outcome of the first 5-year period of the Solar Fuels Hub and, also, puts the work into an international context by inclusion of chapters from renowned researchers in the field. In the spirt of the Hub model of scientific activity inspired by industrial-style research, we include and emphasize the prospective end-use application and its role in the development of integrated solar water splitting devices. Research successes are described alongside those that ultimately failed or were deemed impractical, especially at the device level. The aim is to provide a record that is useful to the practitioner by presenting not just major advancements, but also the path towards their realization.
This book is divided into five sections comprising a total of thirteen chapters.
The first section, Introduction and System Considerations, begins with a basic introduction by Lewerenz and Sharp to key concepts in photovoltaic, electrochemical, and photoelectrochemical energy conversion processes and materials. This first chapter provides a basis for understanding the more in-depth chapters within the book. The following chapter, by Greenblatt, describes a complete life cycle assessment of a prospective 1 GW solar hydrogen plant. The outcome of this assessment motivates research into robust and high efficiency solar water splitting devices and provides context to targeted research—from materials to devices—described in the remainder of the book.
The second section focuses on Electrocatalysis and spans from materials and mechanisms to advanced methods of characterization. The chapter by Bell provides a detailed account of how the understanding of the function of water oxidation catalysts, particularly transition metal oxides and related compounds, has evolved and deepened in recent years. Much of this improved understanding has been enabled by advances in characterization and the subsequent chapter by Friebel describes state-of-the-art x-ray and electron spectroscopic methods that are used to probe catalysts, increasingly under realistic operating conditions. The section on electrocatalysis concludes with a chapter by McCrory et al., which presents the importance of standardized benchmarking methods and protocols for assessing activities and stabilities of both oxygen and hydrogen evolution catalysts in reactive environments that are relevant to integrated solar water splitting devices.
The third section is dedicated to Semiconductor Light Absorbers, which are the engines of any solar fuel device. The chapter by Ager explores heterojunction concepts and configurations upon which integrated devices are based. A particular emphasis of this chapter is on the role solid–solid junctions in semiconductor stabilization, which represents one of the most significant breakthroughs in solar water splitting systems over the last decade and enables highly efficient and stable system constructions. In the next chapter, Osterloh provides a comprehensive overview of particle-based approaches to photochemical energy conversion. Such approaches offer significant opportunity for scalability compared to thin film architectures, but present another set of fascinating challenges in terms of understanding photochemical mechanisms, engineering materials, and integrating into systems. The section on semiconductors concludes with a chapter by Toma and co-workers that describes ubiquitous (photo)chemical instabilities of semiconductor light absorbers, how their mechanisms are determined, and approaches for developing robust photoelectrodes.
The fourth section, entitled New Materials and Components, provides two case examples of approaches to new materials discovery and component integration. In the chapter by Gregoire et al., the modern approach to discovery of new materials via high throughput experimentation is presented. The complete pipeline, including material synthesis, screening, characterization, and data management is discussed and examples of how this pipeline has been implemented for discovery of electrocatalysts and photoelectrodes is provided. The next chapter, by Miller and Houle, turns towards the challenge of integrating membranes, which are necessary for creating operationally safe devices by separating gaseous hydrogen and oxygen products while simultaneously allowing ionic transport between the anolyte and catholyte. The membrane properties specific to integrated solar fuel generators are discussed and opportunities for engineering new materials for this application are presented.
The fifth and final section is devoted to Devices and Modelling and represents the heart of this book. The chapter by Xiang and colleagues describes the scientific and engineering efforts at JCAP devoted to prototype development and integrated device realization, with a true emphasis on the process. Descriptions of the challenges of moving from isolated components to integrated devices provide important insights that can aid those seeking to develop solar fuels generators. The next chapter, by May, Döscher, and Turner, describes the path to modern high efficiency integrated solar water splitting devices, as well as important considerations for accurate assessment of key performance indicators. Finally, the chapter by Singh, Haussener, and Weber, develops a continuum scale model of the function of integrated solar water splitting devices. As integrated systems are experimentally realized, optimization of device design, specification of material and component properties, and exploration of novel architectures are all greatly advanced by computational models.
Ian Sharp, Joachim Lewerenz, and Harry Atwater
Munich and Pasadena