Anna Hankin

From millimetres to metres: the critical role of current density distributions in photo-electrochemical reactor design

0.1 × 0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen. Hitherto, photo-electrochemical research focussed largely on the fabrication, properties and behaviour of photo-electrodes, whereas both experimental and modelling results reported here address reactor scale-up issues of minimising inhomogeneities in spatial distributions of potentials, current densities and the resultant hydrogen evolution rates. Such information is essential for optimising the design and photon energy-to-hydrogen conversion efficiencies of photo-electrochemical reactors to progress their industrial deployment. The 2D and 3D reactor models presented here are coupled with a modified micro-kinetic model of oxygen evolution on hematite thin films both in the dark and when illuminated. For the first time, such a model is applied to a scaled-up photo-electrochemical reactor and validated against experimental data.

Electrochemical recovery of nickel from nickel sulfamate plating effluents

Nickel sulfamate solutions are widely used for industrial nickel plating, when electrodeposits with low stress are required. Partial decomposition of sulfamate with decreasing pH below ca. 2.5 degrades the properties of nickel electrodeposits, decreases the charge yield and results in spent solutions, from which nickel must be recovered before they could be discharged to sewers. Results are reported of charge yields for nickel recovery from an industrial sulfamate effluent, using an electrochemical reactor operated at constant current in batch-recycle mode and incorporating a nickel mesh cathode, a Ti/Ta2O5–IrO2 mesh anode and a cation-permeable membrane to prevent anodic oxidation of sulfamate. A micro-kinetic model was developed, treating the processes of nickel(II) and proton reduction in sulfamate solutions as two multi-step reactions involving adsorbed intermediates, NiadsI and Hads, respectively. The unknown kinetic parameters were obtained using gPROMS software by iterative fitting of the model to experimental data obtained over a range of nickel(II) concentrations and bulk solution pH, enabling evaluation of nickel(II) reduction charge yields as a function of nickel(II) concentration, bulk pH and electrode potential. A model combining the micro-kinetic equations with mass and charge balances on the reactor was used to determine the control parameters for electrochemical recovery of elemental metal from nickel(II) in batch-recycle mode. It was determined experimentally that a decrease in catholyte pH to values below ca. 2.5 resulted in a decrease in nickel(II) reduction charge yields to values below 0.9. The decrease in catholyte pH, caused by the flux of protons from the anolyte where they were generated via anodic oxygen evolution, was obviated by continuous addition of NaOH at a rate determined by the model, permitting nickel(II) recovery with an average charge yield of 0.94.

En route to a unified model for photo-electrochemical reactor optimisation. I - Photocurrent and H2 yield predictions

A semi-empirical model was developed for the prediction of photocurrent densities and implemented to predict the performance of a photo-electrochemical reactor for water splitting in alkaline solutions, using SnIV-doped α-Fe2O3 photo-anodes produced by spray pyrolysis. Photo-anodes annealed at different temperatures were characterised using photo-electrochemical impedance spectroscopy, cyclic voltammetry in the presence and absence of a hole scavenger and also the open circuit potential under high intensity illumination. Mott–Schottky analysis was used cautiously to estimate the charge carrier concentration and the flat band potential. In addition to overpotential/current distribution and ohmic potential losses, the model also accounts for absorbed photon flux, surface and bulk electron–hole recombination rates, gas desorption, bubble formation and (H2–O2) cross-over losses. This allows the model to estimate the total yield of hydrogen, charge and gas collection efficiencies. A methodology is presented here in order to evaluate the parameters required to assess the performance of a photo-electrochemical reactor in 1D and 2D geometries. The importance of taking into account bubble generation and gas desorption is discussed, together with the difficulties of measuring charge carrier concentration and electron–hole recombination in the bulk of the semiconductor, which are of major importance in the prediction of photocurrent densities.

Introducing novel light management to design a hybrid high concentration photovoltaic/water splitting system

We present a novel way to utilize high-concentration photovoltaic (HCPV) radiative losses and diffuse light, otherwise unused in conventional HCPV systems, to power an Imperial College designed photoelectrochemical reactor (PECR) producing H2 fuel through water splitting. A high efficiency photovoltaic (HEPV) is embedded inside a Luminescent Solar Concentrator (LSC). Edge emission from the radiative recombination loss mechanism in the HEPV is guided within the LSC to the PECR photocathode, whilst the LSC emitted light is guided to the photoanode. The photon streams can be independently optimised in intensity and wavelength. We demonstrate how photon streams with balanced intensity can be achieved.