Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO₂emissions in the atmosphere demands that holding atmospheric CO₂levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

Nanoparticles of nickel phosphide (Ni₂P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni₂P nanoparticles were hollow and faceted to expose a high density of the Ni₂P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni₂P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H₂(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media.

At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable step-change approaches to cost-effective, globally scalable systems for solar energy use.

Path to zero carbon emissions: 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. Daviset 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.Science, this issue p.eaas9793

Relying more on the Sun: Improved technologies for harnessing solar energy are not limited to creating more efficient solar cells. The associated hardware of delivering power from solar cells to homes and businesses, and storing this intermittent resource on the grid, offer R&D opportunities. Lewis reviews the status of these areas, as well as solar thermal and solar fuels approaches for harnessing solar energy. Science , this issue p. 10.1126/science.aad1920

Keeping semiconductors safe from harm: Solar cells harvest the energy of sunlight to create electricity, but electricity is hard to store. Solar cells could also be used to make hydrogen from water, which can be stored as a fuel. Separating water into hydrogen and oxygen, however, presents challenges, especially if this is done directly by illuminating the anode that oxides water. Under the acidic or alkaline conditions needed for practical devices, semiconducting anode materials corrode during operation. Hu et al. now show that amorphous titanium dioxide coatings can protect semiconductors from alkaline corrosion while still allowing light through. Science , this issue p. 1005