Environmental, green and energy chemistry

Artificial photosynthesis

How light absorbers, catalysts and membranes can convert sunlight into hydrogen or carbon-based fuels, and why efficiency, stability and scale-up remain challenging.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: sunlight stores energy in chemical bonds

Natural photosynthesis uses light to build energy-rich molecules from carbon dioxide and water. Artificial photosynthesis borrows that strategy: a light absorber drives oxidation and reduction, ideally storing sunlight in hydrogen or carbon-based fuels.

Select a mode. Inspect the tandem photoelectrochemical device and trace water oxidation, membrane transport and fuel-forming reduction; then compare the two-absorber Z-scheme redox levels.

SchoolSchool level: overall reaction and energy

6\ce{CO2}+6\ce{H2O}\xrightarrow{h\nu}\ce{C6H12O6}+6\ce{O2}\qquad \Delta G^\circ\approx+2870\ \mathrm{kJ\ mol^{-1}}

The positive standard free-energy change means light energy must be supplied to build glucose. Artificial systems may instead split water into hydrogen and oxygen, or reduce captured carbon dioxide to products such as carbon monoxide, formate or hydrocarbons; each pathway has its own selectivity and energy cost.

2\ce{H2O}\rightarrow2\ce{H2}+\ce{O2}\qquad E^\circ=1.23\ \mathrm{V}\qquad \Delta G^\circ=237\ \mathrm{kJ\ mol^{-1}_{H2}}

Definition: Thermodynamic water-splitting voltage

The reversible voltage 1.23 V is the minimum free-energy requirement at standard conditions. Real devices require extra voltage for electrode overpotentials, ohmic loss, mass transport and imperfect light-to-charge conversion; the thermoneutral voltage based on enthalpy is about 1.48 V.

A semiconductor must absorb sunlight and provide conduction- and valence-band edges capable of driving the desired reduction and oxidation. The thermodynamic gap is at least 1.23 eV, but practical overpotentials and losses often motivate useful absorbers or tandem combinations spanning roughly 1.6–2.4 eV of usable photovoltage/energy, subject to band alignment and architecture.

Example: Estimating solar-to-hydrogen efficiency

A device operates at $J_{op}=10$ mA cm⁻² under $P_{in}=100$ mW cm⁻², with Faradaic efficiency $\eta_F=0.95$ . Calculate STH.

Solution

Using $\mathrm{STH}=J_{op}(1.23\ \mathrm V)\eta_F/P_{in}$ with consistent units gives $(0.010\times1.23\times0.95)/0.100=0.117$ , or 11.7%. This assumes unbiased operation and reports chemical energy using the reversible water-splitting voltage; an externally supplied bias must not be counted as sunlight.

UndergraduateUniversity: photoelectrochemical architectures

A photoelectrochemical (PEC) cell integrates semiconductor absorbers and catalytic interfaces in electrolyte, often with a membrane separating product gases. A tandem stacks absorbers to supply the required photovoltage; a Z-scheme uses two light-driven steps to preserve both strong oxidation and reduction power, analogous in logic—not materials—to two photosystems.

\eta_{\mathrm{STH}}=\frac{J_{op}(1.23\ \mathrm{V})\eta_F}{P_{in}}\qquad \Delta G=-nFE

The oxygen-evolving catalyst must mediate a demanding four-electron oxidation; the hydrogen-evolving catalyst combines protons and electrons. Cobalt-phosphate (Co-Pi) is a notable self-assembled oxygen-evolving catalyst reported by Kanan and Nocera. Molecular catalysts based on Ru or Ir can be highly active but raise cost, stability and critical-element concerns; earth-abundant catalysts are actively developed.

Example: Photon energy threshold

What wavelength corresponds to a photon energy of 1.23 eV, the reversible water-splitting energy per transferred electron?

Solution

Using $\lambda(\mathrm{nm})\approx1240/E(\mathrm{eV})$ , $\lambda\approx1240/1.23=1008$ nm. This is only the reversible thermodynamic threshold; practical systems need extra driving force and charge-transfer efficiency.

AdvancedAdvanced: biological inspiration and scaling

In oxygenic photosynthesis, the Mn4CaO5 oxygen-evolving complex advances through S-states to accumulate four oxidizing equivalents and release O2. Artificial catalysts seek analogous multi-electron chemistry while avoiding damaging intermediates. Natural photosynthesis is not simply a high-efficiency benchmark: it couples light capture, repair, carbon fixation and ecosystem constraints.

Photocatalytic sheets can make hydrogen and oxygen on particles, but separating gases safely and collecting dilute products at scale are major engineering barriers. High apparent quantum yield at one ultraviolet wavelength does not equal high solar-to-hydrogen efficiency under the full solar spectrum. Module stability, corrosion, catalyst replacement and lifecycle materials also govern practical performance.

Overall water splitting uses four electrons per O2; two H2 are formed for every O2, so STH energy accounting uses the 1.23 V reversible voltage and Faradaic efficiency. For CO2 fuels, report product-specific Faradaic efficiency and carbon balance as well as total current, because competing hydrogen evolution can dominate.

ResearchResearch frontier: integrated solar fuels

A landmark photocatalyst sheet based on Al-doped SrTiO3 with Rh/Cr2O3 and CoOOH cocatalysts reported an apparent quantum yield near 96% at 350–360 nm for overall water splitting; that wavelength-specific figure is not an STH efficiency. Subsequent panel-scale demonstrations emphasize that reactor geometry, gas handling and sunlight variability matter as much as active material.

The research frontier is system integration: durable absorbers, selective catalysts, low-resistance membranes, safe product separation and scalable manufacturing. Tandem perovskite–silicon absorbers can offer high photovoltage, but stability and encapsulation remain central. Hybrid bio-inorganic designs couple electrochemical catalysts to microbes for carbon fixation, while requiring careful accounting of solar input and product energy.

References

Electrochemical photolysis of water at a semiconductor electrode · A. Fujishima, K. Honda, 1972
In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+ · M. W. Kanan, D. G. Nocera, 2008
Photocatalytic water splitting with a quantum efficiency of almost unity · T. Takata, J. Jiang, Y. Sakata, M. Nakabayashi, N. Shibata, V. Nandal, K. Seki, T. Hisatomi, K. Domen, 2020
Water splitting–biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis · C. Liu, B. C. Colón, M. Ziesack, P. A. Silver, D. G. Nocera, 2016
A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting · O. Khaselev, J. A. Turner, 1998