Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO2) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO2 to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

/pdf/net-zero-emissions-energy-systems-2u8li57knv.pdf

Net-zero emissions energy systems

The global energy transition towards a carbon neutral society requires a profound transformation of electricity generation and consumption, as well as of electric power systems. Hydrogen has an important potential to accelerate the process of scaling up clean and renewable energy, however its integration in power systems remains little studied. This paper reviews the current progress and outlook of hydrogen technologies and their application in power systems for hydrogen production, re-electrification and storage. The characteristics of electrolysers and fuel cells are demonstrated with experimental data and the deployments of hydrogen for energy storage, power-to-gas, co- and tri-generation and transportation are investigated using examples from worldwide projects. The current techno-economic status of these technologies and applications is presented, in which cost, efficiency and durability are identified as the main critical aspects. This is also confirmed by the results of a statistical analysis of the literature. Finally, conclusions show that continuous efforts on performance improvements, scale ramp-up, technical prospects and political support are required to enable a cost-competitive hydrogen economy.

Hydrogen energy systems: A critical review of technologies, applications, trends and challenges

Models show that to avert dangerous levels of climate change, global carbon dioxide emissions must fall to zero later this century. Most of these emissions arise from energy use. Davis et al. review what it would take to achieve decarbonization of the energy system. Some parts of the energy system are particularly difficult to decarbonize, including aviation, long-distance transport, steel and cement production, and provision of a reliable electricity supply. Current technologies and pathways show promise, but integration of now-discrete energy sectors and industrial processes is vital to achieve minimal emissions. Net emissions of CO2 by human activities - including not only energy services and industrial production but also land use and agriculture - must approach zero in order to stabilize global mean temperature. Energy services such as light-duty transportation, heating, cooling, and lighting may be relatively straightforward to decarbonize by electrifying and generating electricity from variable renewable energy sources (such as wind and solar) and dispatchable ("on-demand") nonrenewable sources (including nuclear energy and fossil fuels with carbon capture and storage). However, other energy services essential to modern civilization entail emissions that are likely to be more difficult to fully eliminate. These difficult-to-decarbonize energy services include aviation, long-distance transport, and shipping; production of carbon-intensive structural materials such as steel and cement; and provision of a reliable electricity supply that meets varying demand. Moreover, demand for such services and products is projected to increase substantially over this century. The long-lived infrastructure built today, for better or worse, will shape the future. Here, we review the special challenges associated with an energy system that does not add any CO2 to the atmosphere (a net-zero emissions energy system). We discuss prominent technological opportunities and barriers for eliminating and/or managing emissions related to the difficult-to-decarbonize services; pitfalls in which near-term actions may make it more difficult or costly to achieve the net-zero emissions goal; and critical areas for research, development, demonstration, and deployment. It may take decades to research, develop, and deploy these new technologies. DOI Link: https://doi.org/10.1126/science.aas9793

Net-zero Emissions Energy Systems

Hydrogen technologies can play an important role in decarbonising our energy system in a variety of ways across the energy value chain. It is therefore critical to identify the strategic roles as well as the conditions under which hydrogen energy systems become attractive for the energy transition. In this paper, the authors present a techno-economic review of hydrogen energy systems including power-to-power, power-to-gas, hydrogen refuelling and stationary fuel cells. We focus on their optimal operation as flexible assets and we identify three actions that can foster their uptake beyond technological progress. First, we recommend optimal electricity supply with dedicated control strategies considering that electricity dominates the levelised cost of hydrogen production via electrolysis. Secondly, hydrogen can enable the further integration of traditionally independent sectors, namely electricity, heat and transport while contributing to decarbonise all. This position can also be advantageous for investors who sell heat and fuels as energy efficient products. Lastly, we examine a whole range of revenues from different products and applications which can be combined (i.e. benefit stacking) to match capital and operational expenditures. We discuss these roles in depth and we conclude that policy makers together with technology developers should elaborate smart strategies to reduce cost by scaling production, stimulate standardisation (e.g., similar to the PV industry) as well as develop new market structures and regulatory frameworks which allow hydrogen technologies to deliver multiple low carbon applications and products.

A review on the role, cost and value of hydrogen energy systems for deep decarbonisation

Reversible fuel cells based on both proton exchange membrane fuel cell and solid oxide fuel cell technologies have been proposed to address energy storage and conversion challenges and to provide versatile pathways for renewable fuels production. Both technologies suffer challenges associated with cost, durability, low round-trip efficiency and the need to separate H2O from the product fuel. Here, we present a reversible protonic ceramic electrochemical cell based on an yttrium and ytterbium co-doped barium cerate–zirconate electrolyte and a triple-conducting oxide air/steam (reversible) electrode that addresses many of these issues. Our reversible protonic ceramic electrochemical cell achieves a high Faradaic efficiency (90–98%) and can operate endothermically with a >97% overall electric-to-hydrogen energy conversion efficiency (based on the lower heating value of H2) at a current density of −1,000 mA cm−2. Even higher efficiencies are obtained for H2O electrolysis with co-fed CO2 to produce CO and CH4. We demonstrate a repeatable round-trip (electricity-to-hydrogen-to-electricity) efficiency of >75% and stable operation, with a degradation rate of <30 mV over 1,000 h. Reversible electrochemical cells can operate in both fuel cell and electrolysis modes to interconvert between chemical and electrical energy. Here, Duan et al. design a reversible protonic ceramic electrochemical cell that operates stably at 500–600 °C, with high Faradaic and round-trip efficiencies, by minimizing electronic leakage.

Highly efficient reversible protonic ceramic electrochemical cells for power generation and fuel production

Fuel Cell Fundamentals: O'Hayre/Fuel Cell Fundamentals

A membrane electrode assembly (MEA) (250) having a nano-tubular patterened structure and having solid (instead of porous) electrode layers (206,202) is provided. Increased mechanical strength is provided by the use of solid electrode layers (206,202). The electrode layers (206,202) are sufficiently thin to permit the flow of reactants to the ebctrolyte (204). The nano-tubular pattern includes multiple closed-end tubes and increase the reaction area to vulume ratio of the MEA (250). The nano-tubular pattern also serves to increase mechanical strength, especially in a preferred honey-comb like arrangement of the closed-end tubes. A catalyst (210,208) is preferably disposed on the anode and cathode surfaces (230,220) of the MEA (250), and is preferably in the form of separated catalyst islands in order to increase reaction area. MEAs (250) according to the invention can be fabricated by layer deposition on a patterned. Atomic layer deposition is a preferred deposition technique.

Nanotubular solid oxide fuel cell

A simple, inexpensive and highly efficient fuel cell has boundary structures made of a photo-sensitive material in combination with selective patterning. Printed circuit board (PCB) fabrication techniques combine boundary structures with two and three dimensional electrical flow path. Photo-sensitive material and PCB fabrication techniques are alternately or combined utilized for making micro-channel structures or micro stitch structures for substantially reducing dead zones of the diffusion layer while keeping fluid flow resistance to a minimum. The fuel cell assembly is free of mechanical clamping elements. Adhesives that may be conductively contaminated and/or fiber-reinforced provide mechanical and eventual electrical connections, and sealing within the assembly. Mechanically supporting backing layers are pre-fabricated with a natural bend defined in combination with the backing layers' elasticity to eliminate massive support plates and assist the adhesive bonding. Proton insulation between adjacent and electrically linked in-plane cell elements is provided by structural insulation within the central membrane.

Improved fuel cell

Chapter 9: PEMFC and SOFC Materials

Chapter 14: Environmental Impact of Fuel Cells

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