The quest for sustainable energy production has been described as ‘the moonshot for our generation’. Now mankind, instead of reaching for the stars, is striving to maintain a high-technology lifestyle without adding to the already significant environmental risk to our planet.
Hydrogen gas is regarded by many as the fuel of the future. With the highest mass-energy density of any fuel and its clean electrochemical combustion in air, hydrogen could be considered the ultimate energy carrier (Figure 1). However, the realisation of a largely hydrogen based, renewable-energy solution – the hydrogen economy – depends heavily on the development of cost effective, green production technologies.
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CLICK TO ENLARGE Fig 1: Hydrogen economy[/caption]
Hydrogen is readily produced on a large scale by steam reforming of natural gas, yet this non-renewable process is inherently environmentally offensive, producing significant quantities of carbon dioxide. Today, some 96% of the world’s hydrogen is produced from natural gas, oil or coal (48%, 30% and 18% respectively). The catalytic splitting of water to form hydrogen and molecular oxygen via electrolysis provides some 4% of the world’s hydrogen, and presents a clean, renewable and potentially cost-effective pathway to this versatile fuel.
The water-splitting process takes place in an electrolyser, which is an electrochemical substance producer (Figure 2A). Hydrogen gas is produced by the splitting of water into its gaseous constituents O2 and H2: 2H2O + energy → O2 (g) + 2H2(g). It takes place in an electrolysis reactor in which current from an external source is passed through two electrodes in contact with an ionically conducting membrane (which serves as the electrolyte).
Hydrogen gas is generated via reduction of water at the cathode electrode where hydrogen ions or water molecules accept electrons and protons from the cathode according to: 4H+ + 4e- → 2H2(g), whereas water or hydroxide ions are oxidised donating electrons to the anode electrode liberating electrons and protons to form oxygen gas: 2H2O → 4H+ +4e- + O2(g).
The protons move through a thin polymer ionomer membrane fabricated from Nafion or some other ionically conducting material (Figure 2) from anode to cathode, whereas the electrons are supplied from an external power source entering at the cathode electrode. Polymer electrolyte membranes have been developed both as proton and hydroxide ion conductors, so electrolysis can occur both at low and high pH. The electrolysis reactor is usually operated at 80°C to ensure greater electrolysis efficiency.
Photoelectrolysis and other energy sources
The energy required to drive the water-splitting reaction can be derived from any of a number of sources. One of the more attractive options is the coupling of electrochemical water splitting devices with grid-scale renewable energy harvesting technologies such as wind turbines or photovoltaics. An alternative method, in the context of solar to fuel conversion, is photoelectrolysis or light-driven water splitting.
In this promising approach, light-harvesting mechanisms, typically involving semiconductor materials, are incorporated into the electrode design so that the necessary solar energy is harvested directly by the electrode materials. Regardless of the route taken, the viability of these systems as sustainable hydrogen production technologies is, strangely enough, in the end, mainly dependent on optimising the electrochemistry of oxygen gas generation in the (photo) electrolysis reactor.
The generation of molecular oxygen at the anode is the most energy-intensive step in the overall water-splitting process. Thus, understanding and optimising the oxygen evolution reaction (OER) process is seen as one of the remaining grand challenges for energy science.
The most effective way of improving hydrogen generation efficiency – and hence, lower cost – is to fabricate new materials that are both stable and catalytically active towards the complex water-splitting reactions at both the anode and the cathode of the electrolysis cell, as well as optimising the engineering design features of the latter.
Currently, the optimal OER electrode materials are the so-called dimensionally stable anodes (DSA), which are largely based on RuO2 and IrO2 mixed with TiO2. These materials exhibit the lowest overpotentials for the OER at practical current densities. However, these platinum group metals (PGM) are increasingly scarce and expensive. Indeed, they are included in the latest 2014 EU critical raw materials list because of their manifold technological applications in sustainable energy applications, because of their economic significance and because of geopolitical considerations arising from the regions from which the PGM’s originate (mainly Russia and China).
Furthermore, these oxides have been shown to suffer from poor chemical stability in alkaline media when subjected to anodic polarisation at elevated current density. The oxides of the first-row transition metals – in particular nickel, cobalt and iron – offer a compromise solution: although they possess inferior catalytic activity for the OER, they display excellent long-term corrosion resistance in aqueous alkaline solution and have the added advantage of being relatively inexpensive, and the metals are not deemed to be as critical with respect to security of supply as the PGM.
These oxides have been prepared from inorganic precursor materials using a wide variety of approaches, including thermal decomposition, spray pyrolysis, sol-gel routes and freeze drying, precipitation or electrodeposition from solution.
Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group
Over the last five years, members of the Science Foundation Ireland-funded Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group, which is located within the School of Chemistry and the AMBER National Centre in Trinity College Dublin, have been investigating the mechanism of, and the activity with respect to, the OER at a variety of earth abundant transition metal oxides – in particular, binary materials made from ruthenium and manganese oxides and from nickel/iron oxides.
These materials have been made via thermal decomposition in air at elevated temperature of suitable inorganic precursor salts or by electrodeposition from suitable metal plating solutions. For example, the TCD group has reported that the oxygen overpotential value recorded in aqueous base solution at a current density of 10 mA/cm2 is less than 0.2 V for an RuO2 electrode which has been diluted by 90% with inexpensive manganese oxide.
Here, the mixed oxide thin film electrode exhibits excellent stability under active water electrolysis conditions. Mixed iron/nickel oxides (50/50 composition) exhibit similar OER catalytic activity and long-term materials stability coupled with a low overpotential at a useful current density. These materials can be incorporated into existing electrolysis membrane electrode assembly reactor units, which will facilitate scale-up for long-term engineering testing.
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CLICK TO ENLARGE:Figure 2A (left): Schematic of an electrochemical substance producer. Polymer electrolyte membrane (PEM) water electrolysis cell Figure 2B (right): Schematic of an electrochemical energy producer. PEM hydrogen /oxygen fuel cell[/caption]
The hydrogen generated via electrolysis in a PEM cell is very pure and can be used as a clean fuel in a PEM fuel cell (Figure 2B). In this electrochemical energy generating device, chemicals (hydrogen fuel and molecular oxygen oxidant) are pumped in from the outside and react at the anode and cathode electrodes respectively to generate electrons and protons respectively. The electrons e- and protons H+ generated via hydrogen oxidation at the anode (2H2(g) → 4H+ + 4e-) follow different routes.
The electrons travel through the external circuit and into the cathode. The protons travel through the thin ionically conducting polymer membrane from anode to cathode. Both electrons and protons react with the molecular oxygen pumped in from outside to form water vapour as sole product according to: O2 +4e- + 4H+ → 2H2O. The transit of electrons from anode to cathode give rise to an electric current, which can power a car or a building. This process of cold electrochemical combustion has a very high efficiency, typically 60-80%.
In-house hydrogen generators coupled with a hydrogen delivery system are now a technical and commercially viable reality and will define the service station of the near future. Indeed, in May 2016,
ITM Power in the UK launched the first hydrogen-powered HYFIVE forecourt delivery system in the UK. Various auto manufacturers have embraced the concept of hydrogen cars (Hyundai, Toyota, Honda, Renault).
The time taken for a refill of hydrogen at the station is similar to that for petrol and the distance one obtains from a full tank of hydrogen is typically 300-400 km. These are attractive figures of merit when it comes to acceptance of the new hydrogen technology by members of the general public. The future for the hydrogen economy has never been so bright.
Acknowledgements:
The research described here has emanated in part from projects conducted with the financial support of Science Foundation Ireland (SFI) under grant number SFI/10/IN.1/I2969 and SFI/12/RC/2278.
Michael E.G. Lyons and Michelle P. Browne are both part of the Trinity Electrochemical Energy Conversion & Electrocatalysis (TEECE) Group, School of Chemistry & AMBER National Centre, Trinity College Dublin. For more information, email:
melyons@tcd.ie.