The carbon footprint of hydrogen production is of utmost importance for its use as a sustainable fuel. The plurality of production processes and associated energy sources is further complicated by the plurality of raw materials. Figure 2.1 lists examples of production processes along with their associated raw materials and energy sources. For the raw materials, the key attribute is the ability to close the material cycle, that is, to regenerate the starting material without emissions discharge to the environment. For the energy sources, the key attribute is renewable or nonrenewable. A most cited example of a production process theoretically approaching zero carbon footprint is the electrolysis of water (Chapter 7) driven by solar, wind, hydropower, or any other renewable electricity source. Competing production processes that also use water as a raw material and a renewable energy resource are solar thermochemical cycles (Chapter 12) and solar photosynthesis (Chapter  13). Thermochemical cycles may also be driven by nuclear process heat (Chapter 8). An example of a production process with a relatively large carbon footprint is the reforming of natural gas driven by heat from fossil fuel combustion (Chapter 3), which presently is the most practiced manufacturing process at an industrial scale because of its economic feasibility. Given the future importance of solid carbonaceous feedstocks such as coal, coke, biomass,

bitumen, and carbon-containing wastes, gasication technologies are developing rapidly. An example of this category of thermochemical processes with a low carbon footprint is the gasication of biomass driven by heat from biomass combustion (Chapter 5) or from concentrated solar energy (Chapter 12). Alternatively, when coal is the raw material and coal combustion the source of process heat, the large carbon footprint may be mitigated by implementing CO2 capture technologies (Chapter 4). Ultimately, the carbon footprint of hydrogen production is determined via a life cycle assessment (Chapter 14), which evaluates the environmental burdens associated with the production process by identifying and quantifying the energy and materials used and wastes released to the environment and accounts for the entire life cycle of the process encompassing extraction and processing of raw materials, manufacturing, transportation, distribution, recycling, and nal disposal.