Newsletter Issue # 44
Direct Solar Hydrogen Production with Breakthrough Efficiency
After sustained efforts into the improvement of performance and cost competitiveness, photovoltaic cells have become commercially viable and are seeing a rampant large-scale deployment. Achieving global renewable energy transition further relies on addressing the intermittency of renewables through the development of transportable energy storage means. An elegant and potentially economical route to storing solar power is to convert the energy from sunlight directly into hydrogen, analogous to the photosynthesis process exploited by nature.
There are significant cost benefits to be achieved through the use of direct solar-to-hydrogen approach as it avoids the need for added power and network infrastructure when hydrogen is instead produced using an electrolyser. The direct conversion of solar energy into hydrogen can achieve a higher overall efficiency for the total process by avoiding the need to convert solar power from DC to AC power and back again, in addition to avoiding power transmission loses. In a recently published article in Advanced Energy Materials journal, the ANU team led by Dr. Siva Karuturi developed perovskite photovoltaic coupled Si photoelectrode to achieve an unprecedented solar-to-hydrogen (STH) efficiency of over 17%. This represents the highest efficiency ever achieved using low-cost semiconductors.
Figure 1. Illustration of the perovskite/Si tandem water splitting device for direct solar hydrogen production
The STH efficiency of a solar water splitting cell is determined by the spectral region in which the semiconductor absorbs the light, which is in turn determined by its bandgap. A single semiconductor based cell requires a semiconductor with a bandgap >2.0 eV for spontaneous water spiting, and thus presents low theoretical efficiencies as it can only harvest a small fraction of solar power. To address this, dual-semiconductor system can be used to achieve higher efficiency for water splitting. Because the energy required to drive the water splitting reaction is drawn from two photons, semiconductors with smaller bandgaps can harvest a larger fraction of the solar power reaching theoretical STH efficiencies as high as 25%.
Figure 2. (a) Illustration of the Si photocathode developed by the ANU team. (b) Overlay of current-voltage curves of the perovskite cell, the Si photocathode behind the perovskite cell, and a dimensionally stable anode (DSA).
A buried p-n junction Si photocathode (Figure 2a) was developed to minimize photovoltage losses from poor band energetics and a decoupling strategy separating light harvesting with reactive interfaces was adopted to mitigate photocurrent losses from parasitic light absorption. The Si photocathode exhibited a current density of 39.7 mA cm-2 at 0 V versus RHE, an onset potential of 590 mV and photostability of over 3 days, and a remarkable 14.1% half-cell efficiency.
Despite this impressive performance, the photovoltage generated by the Si photocathode is still less than half the value required for spontaneous solar water splitting without external electricity input. To address this issue, perovskite cell of an appropriate bandgap (≈1.75 eV) was custom-designed as a front absorber. The resulting perovskite/Si dual-absorber cell enabled spontaneous solar water splitting with an unprecedented over 17% STH efficiency (Figure 2b).
Modelling and analysis of the dual-absorber stand-alone PEC system show that there are opportunities for further optimization of the materials and performance, with a potential to accomplish an STH efficiency of over 20% using all low-cost materials. The ANU team is currently working towards achieving this goal. This research is supported by funding from the Australian Renewable Energy Agency. Access to fabrication facilities was enabled by the Australian National Fabrication Facility (ANFF), ACT node.
CECS, ANU