Biomass conversion is one of the most affordable routes to generate sustainable H₂, but this process requires the demanding chemical transformation of lignocellulose. Lignocellulose is the main constituent of biomass and can be cultivated worldwide, even on unfertilized, marginal land. Lignocellulose conversion to H₂ has predominantly been realized through gasification, which uses high temperatures (>750 ˚C) to decompose its organic structure and release H₂, alongside other gases, such as CO, CO₂ and CH₄. In the interest of increasing the selectivity and efficiency of this conversion, it is possible to replace the thermal input with sunlight. Solar light offers an essentially inexhaustible source of globally available energy, and therefore the photoreforming of biomass-derived compounds is a fast-growing field of research.

Photoreforming requires a photocatalyst able to generate holes to oxidize lignocellulose and use the resultant electrons to reduce aqueous protons to H₂. Lignocellulose therefore adopts the role of a hole scavenger, providing a continuous supply of electrons for fuel production. Thus far, this field has focused on H₂ evolution from substrates that could be derived from lignocellulose, such as methanol, glycerol or glucose, but lignocellulose refining is expensive and inefficient, usually requiring acid hydrolysis, enzymatic hydrolysis or pyrolysis to produce more manageable substrates. Viable H₂ production systems should therefore reform lignocellulose directly, to compete with thermochemical processes. This is challenging at ambient temperatures, as the structure of lignocellulose has evolved to prevent its consumption by microbial and animal life. … Examples of the direct photoreformation of lignocellulose or even purified cellulose to H₂ are consequently rare, and until now required a ultraviolet-light-absorbing TiO₂ architecture loaded with expensive, non-scalable noble-metal catalysts, such as Pt and RuO₂.

To address these issues, the Cambridge team developed a photocatalytic system based on semiconducting CdS quantum dots (QDs) able to photoreform cellulose, hemicellulose and lignin into H₂ at room temperature. CdS is an inexpensive, visible-light-absorbing photocatalyst with a bulk electronic bandgap of around 2.4 eV.

The catalytic nanoparticles were added to alkaline water in which the biomass was suspended. This was then placed in front of a light in the lab which mimicked solar light.

Photoreforming of lignocellulose to H2 on CdS/CdOx. a, Lignocellulose exists as microfibrils in plant cell walls and consists of cellulose surrounded by the less crystalline polymers hemicellulose and lignin. b, These components can be photoreformed into H2 using semiconducting CdS coated with CdOx. Light absorption by CdS generates electrons and holes, which travel to the CdOx surface and undertake proton reduction and lignocellulose oxidation, respectively. c, This combination creates a highly robust photocatalyst able to generate H2 from crude sources of lignocellulose when suspended in alkaline solution and irradiated with sunlight. Wakerley et al. Click to enlarge.

The nanoparticle is able to absorb energy from solar light and use it to oxidize the biomass, providing electrons for the reduction of aqueous protons to generate H₂.

The team achieved high rates of H₂ evolution, without the photocorrosion of CdS or noble-metal co-catalysts, through use of highly basic conditions that synergistically enabled formation of robust CdS/CdO_x and dissolution of lignocellulose.

The oxidation reaction generates CO₂ that is sequestered as carbonate, resulting in an overall negative CO₂ balance in the atmosphere when taking into account the CO₂ fixation required for biomass growth.

Noting that the system’s tolerance to a range of substrates is “particularly appealing” for H₂ generation without need for lignocellulose processing, the researchers are now focusing on replacing the Cd with a more environmentally benign metal.

The technology was developed in the Christian Doppler Laboratory for Sustainable SynGas Chemistry at the University of Cambridge. The head of the laboratory, Dr. Erwin Reisner, observed:

Our sunlight-powered technology is exciting as it enables the production of clean hydrogen from unprocessed biomass under ambient conditions. We see it as a new and viable alternative to high temperature gasification and other renewable means of hydrogen production. Future development can be envisioned at any scale, from small scale devices for off-grid applications to industrial-scale plants, and we are currently exploring a range of potential commercial options.

With the help of Cambridge Enterprise, the commercialization arm of the University, a UK patent has been granted and talks are under way with a potential commercial partner.

Resources

• David W. Wakerley, Moritz F. Kuehnel, Katherine L. Orchard, Khoa H. Ly, Timothy E. Rosser & Erwin Reisner (2017) “Solar-driven reforming of lignocellulose to H₂ with a CdS/CdO_x photocatalyst” Nature Energy 2, Article number: 17021 doi: 10.1038/nenergy.2017.21