Stringent global policies regarding CO2 emissions presently being implemented have coerced the development of alternative renewable and environmentally friendly energy systems worldwide. To date, scientists have explored solar, wind, hydroelectric, tidal wave energy, hydrogen -among other forms of green energy. Among these, hydrogen is the most prospective as it can be used to store other forms of energy. Normally, hydrogen is made from water where 3 methods have been widely investigated, namely: photocatalysis (PC), photoelectrochemical water splitting (PEC) and an integrated photovoltaic electrolysis (PV electrolysis) system. Focusing on the PV-electrolysis systems, power matching between a PV-cell and an electrolyzer is crucial for maximal PV-cell efficiency. Noteworthy research has shown that when the voltage at maximum power (Vmpp) of the PV-cell is much higher than the reversible water splitting potential, the short circuit current (Isc) becomes a limiting factor. Under such conditions, electrolyte conduction affects the rate of reaction while temperature has marginal effect. For instance, the PV-electrolysis under one Sun illumination and low pH requires operating voltage close to 1.7V.
Overall, a focus of PV-electrolysis coupled with a thorough review of published literature reveals that precise and careful design of directly coupled system needs to be known in order not to offset the saved cost in the absence of power electronics. In this view, researchers from SABIC-Corporate Research and Development (CRD) at King Abdullah University for Science and Technology (KAUST) Dr. Muhammad Amtiaz Nadeem and Professor Hicham Idriss (who is also Prof. (Hon.) at the Department of Chemistry, University College London) investigated the maximum possible solar-to hydrogen conversion number (STH) that can be obtained via changing the pH, low light intensity, and Sun/PV-cell angles. Their investigation was carried out using a direct PV electrolysis system. Their work is currently published in the research journal, Journal of Power Sources.
In their approach, a triple junction (3J) solar cell having 33% efficiency at one Sun condition was integrated with an electrolyzer containing Pt wire electrodes to study the effect of pH, temperature and low flux solar light on H2 production from water. The research pair conducted measurements both indoors, under 100–500mWcm -2 insolation, and outdoors.
Based on their approach, the obtained results indicated that electrolyte conduction affects the rate of reaction mostly due to the large voltage at maximum power (Vmpp) of the PV-cell. Further, the electrochemical impedance measurements indicated that the exchange current, ji, depends largely on the electrolyte resistance, r (ji ¼ (Vapp-Veq)/r); where Vapp is applied voltage provided from the cell and Veq is the equilibrium potential for water electrolysis (1.23 V). In a follow up work, the team led by Idriss put together a demonstration of the same set up at 750 suns and obtained a an average STH of 18.7% producing about 1 L of Hydrogen per minute and per square meter of land area (Solar Energy 205, 461-464 (2020)).
In summary, the study critically investigated the effect of various parameters on water splitting using PV-electrolysis system. The study found that the high cell voltage (2.5 V) of the PV cell compared to operating voltage (1.7 V) made the current at maximum power (Impp) the limiting factor. Also, the authors showed the maximum STH outdoor to be about 16% under acidic conditions. In a statement to Advances in Engineering, Professor Idriss indicated the following. We now know how to make hydrogen from water using sun light at an overall efficiency approaching 30%. The cost which is higher than that obtained from natural gas requires government involvement to lift the technology up. The crucial point, or one may say priority, is the need to make energy vectors (such as hydrogen) without producing CO2 in the race to halt the continuously rising temperature on earth and this too comes at a high cost probably higher than what humanity can afford.
M.A. Nadeem, H. Idriss. Effect of pH, temperature, and low light flux on the performance (16% STH) of coupled triple junction solar cell to water electrolysis. Journal of Power Sources, issue 459 (2020) 228074.