Investigation of Iron Oxide Morphology in a Cyclic Redox Water Splitting Process for Hydrogen Generation

Michael M Bobek,David W Hahn,Richard C Stehle

doi:10.3390/ma5102003

Abstract

A solar fuels generation research program is focused on hydrogen production by means of reactive metal water splitting in a cyclic iron-based redox process. Iron-based oxides are explored as an intermediary reactive material to dissociate water molecules at significantly reduced thermal energies. With a goal of studying the resulting oxide chemistry and morphology, chemical assistance via CO is used to complete the redox cycle. In order to exploit the unique characteristics of highly reactive materials at the solar reactor scale, a monolithic laboratory scale reactor has been designed to explore the redox cycle at temperatures ranging from 675 to 875 K. Using high resolution scanning electron microscope (SEM) and electron dispersive X-ray spectroscopy (EDS), the oxide morphology and the oxide state are quantified, including spatial distributions. These images show the change of the oxide layers directly after oxidation and after reduction. The findings show a significant non-stoichiometric O/Fe gradient in the atomic ratio following oxidation, which is consistent with a previous kinetics model, and a relatively constant, non-stoichiometric O/Fe atomic ratio following reduction.

Highlights

With the vast potential energy provided daily by the sun, roughly 5.6 × 1024 J/year, compared to the average energy of the world usage of 5 × 1020 J/year, many different avenues are being pursued in Materials 2012, 5 order to harness this untapped resource
Representative scanning electron microscope (SEM) images of the oxide layer films for two cycles terminated on oxidation are shown in Figure 3 for reactor temperatures of 685 and 765 K
Given the equilibrium thermodynamics of the various iron oxide states [9,15,22,23], combined with the physical morphology and stability of the oxide layers, the ability for total stoichiometric reduction will be difficult to achieve in any practical reactor with high surface-to-volume ratios and rapid cycling times

Summary

Introduction

With the vast potential energy provided daily by the sun, roughly 5.6 × 1024 J/year, compared to the average energy of the world usage of 5 × 1020 J/year, many different avenues are being pursued in Materials 2012, 5 order to harness this untapped resource. Using concentrated sunlight for thermal processes is good on multiple scales from HVAC to power plant applications [1], but all solar power harnessing methods suffer from the potential disconnect of the sun’s availability and our societies energy usage This leads to the importance of storing the solar-derived energy; for PV the direct conversion into electricity requires advanced battery technology, while for thermal processes some other means of storage must be used. Such an approach seeks to store the solar energy in the chemical bonds of the fuels, allowing for a stable, distributable, energy source that is compatible with existing infrastructures

Methods

Results

Conclusion