By 2050, 80% of the world’s population will live in cities. Cars will be discouraged and probably banned from urban areas for reasons of congestion, according to market analyst IDTechEx, which predicts car sales will peak around 2030.
Various ideas for alternative modes of city transport are emerging – from autonomous cars and taxis to scooters and bikes. Buses are also expected to play a bigger role since a large bus uses less energy and requires less space than a car per passenger kilometre. In 2027, IDTechExA estimates that 2.3m electric buses will be made, about five times the number made in 2016 - but reinvented in ‘many very new forms’.
China is also backing pure electric buses for the future – reflected in the fact the largest value market for large lithium-ion batteries in 2015 was buses, not cars. The rest of the world, in particular Europe, is also rapidly moving in this direction (see Box). Innovations in battery technology, advanced composites for stronger but lightweight bodywork and high efficient photovoltaics for the ultimate ‘energy independent vehicles’ (EIVs) will be key.Lithium technology
A typical lithium ion battery has a graphite anode and a transition metal oxide cathode, separated by a porous polyethylene film filled with a Li+ conducting organic liquid electrolyte. Lithium ion batteries used in electric buses today are mainly based on cathodes of lithium nickel manganese cobalt oxide (LiNiMnCoO2 or ‘NMC’), lithium nickel cobalt aluminium oxide (LiNiCoAlO2 or ‘NCA’), and composites of these oxides with lithium manganese oxide (LiMn2O4), or lithium iron phosphate (LiFeO4). Some buses also use lithium ion batteries with one of these oxide cathodes and a lithium titanate (Li4Ti5O12) anode. Manufacturers make their choice based on costs, the energy density per kg or specific energy of the cell, measured in Wh/kg, and how safe the cell is.
K. M. Abraham, research professor at Northeastern University in Boston, US, comments: ‘The energy density of a lithium ion battery is determined by the cathode material. The highest specific energy is shown by NCA batteries – giving over 260 Wh/kg. But safety is a big issue when dealing with the larger sized lithium ion batteries used in buses, so iron phosphate, which has a higher thermal stability but lower specific energy at 126 Wh/kg is often chosen as a compromise. In addition, the iron-based cathode is cheaper than the cobalt containing cells.’ (For comparison the specific energy of a lead-acid battery is 30 Wh/kg and that of a Ni–Zn battery is 75 Wh/kg.)
Chinese electric bus manufacturer BYD, for example, uses a proprietary iron phosphate battery for its 40 foot electric transit bus, which the company says has a range of 155+ miles on a single charge. In a recent 24-hour stress test at the company’s factory in California, the buses went further, cranking out 240 miles per charge, BYD reports. Close competitor Proterra’s Catalyst 40-ft e-bus opts for either an ‘advanced lithium titanate’ fast charge energy storage pack or an NMC extended range battery. In addition, the body is made of carbon fibre and advanced composites, making it lighter than other electric buses of similar size made of aluminium or steel and therefore less prone to corrosion. The company reports this bus is six times more efficient than a diesel bus of the same size and 15% more energy efficient than the BYD bus.
As it stands, however, lithium ion technology has just about reached its technical limit in terms of energy densities. Over the next few years, Abraham expects there to be incremental improvements in the technology – with lighter structural materials for use as the anode, and additives to improve the performance of the electrolyte. Some groups are looking to replace the anode with silicon which leads to higher specific energies in the cell.
Nevertheless, for the electric bus to drive 300+ miles per charge, innovations in battery technology will be needed – specifically, rechargeable batteries will need to have two to three times the energy density of the best currently available lithium ion cells, ie around 580 Wh/kg.
But this is certainly not the end of the story for lithium. Twenty years ago, while researching lithium ion batteries, Abraham serendipitously discovered a new family of non-aqueous metal-air batteries. ‘These batteries are unique power sources because the cathode active material, oxygen, does not have to be stored in the battery but can be accessed from the environment,’ he explains. A few calculations pointed to lithium–air battery being the best candidate for development – it has a theoretical specific energy of 5200 Wh/kg. ‘Even if the cell operates at 30% efficiency in practice, it would still be capable of producing 1400 Wh/kg, which is at least five times better than any lithium ion battery, and capable of driving up to 500 miles on a single charge,’ he says.
The basic chemistry of the lithium–air battery is fairly straightforward. The cell discharges and generates energy as Li+ reacts with oxygen to form lithium peroxide, Li2O2, which is stored in a carbon electrode; the cell is recharged by applying a current to reverse the reaction. Making these reactions take place reliably over thousands of cycles is a huge challenge for several reasons. First, the oxygen that comes in from the air must be free of moisture and of impurities like CO2 and N2. Secondly, there are rechargeability issues for both the electrodes. The lithium anode is prone to degradation after around just 100 cycles, and the lithium peroxide formed at the cathode is electronically insulating and thus eventually poisons the electrode.
Despite the many practical challenges to the battery’s exploitation, Abraham is upbeat: ‘The promise of this battery is so great that we need to find ways to overcome these challenges. We really don’t have any other alternatives.’The latest developments
In the past couple of years several groups around the world have started to make some inroads in this technology. For example, Clare Grey’s team at Cambridge University, UK, used a porous reduced graphene oxide electrode in place of nanoparticulate carbon and the additive lithium iodide as a redox mediator. Instead of forming lithium peroxide (Li2O2), their cell recycles via the formation of LiOH and is able to tolerate high concentrations of moisture in the cell (Science, 2016, 350(6260), 530). The researchers say that their cell data from laboratory experiments suggest new strategies for recharging the lithium–air battery that do not necessarily involve the reactive Li2O2. Grey stresses, however, that more fundamental work is necessary before these cells can be considered for commercialisation.
Meanwhile, Ju Li, professor of materials science and engineering at Massachusetts Institute of Technology (MIT) in Boston, US and his group have been working on a slightly different format for the lithium–air battery. The researchers embed lithium and oxygen nanoparticles – ‘nanolithia’ – in a cobalt oxide matrix, a sponge-like material with nanopores small enough to change the thermodynamic stability of the oxides present in the cell (Nature Energy, 2016, 1, 16111). The matrix acts as a catalyst for the transformations between Li2O, Li2O2 and LiO2. In this system, oxygen doesn’t need to be brought in from the air, thus avoiding the need for additional ‘scrubbing’ and making the electrode structure much more stable with a longer life cycle.
In addition, the researchers comment that the system reduces the amount of energy lost as heat on charging and recharging, making it safer and more efficient than the standard Li-air battery. Li adds, ‘Because these ‘solid oxygen’ cathodes are smaller and lighter than conventional lithium ion cathodes, the new design could store double the amount of energy for a given cathode weight.’ The researchers are now trying to scale up their synthesis for commercialisation.
The vision of bus transport is certainly changing. Autonomous buses with advanced sensor and battery technologies are currently being trialled worldwide. Tesla, for example, is planning ‘something like a driverless bus’ similar to the driverless trains in many airports – with the sides, front and back all capable of being raised to let passengers in more quickly.
Going a step further, some analysts predict a new generation of vehicles, including buses, which rely on harvested energy rather than on any self-contained power sources - so called energy-independent vehicles (EIVs). These purely electric buses are expected to use featherlight, highest-efficiency photovoltaics as part of their overall design.
Nanowinn Technologies in China already has an eight-seater EIV bus with ‘advanced CIGS photovoltaics and a super-efficient power train’. It’s what the industry is calling a ‘lizard bus, which wakes up with the sunshine’. And Hanergy in China has recently demonstrated EIV cars using an efficient lightweight solar cell made from gallium arsenide films it makes itself. Hanergy expects these cars will be on sale by 2020 and generate over 1kW per car – enough to charge the battery even for night time use in an urban city. As for the cost: $1m per car.
The ZeEUS 2016 Project
An increasing number of cities around the world are introducing e-buses to reduce the carbon footprint of public transport, according to a report earlier in 2017 by the International Association of Public Transport (UITP) in Brussels.
The ZeEUS 2016 report, which forms part of the ZeEUS (zero emission urban bus system) project, finds that 19 public transport, covering 25 cities in Europe have a strategy in place for 2020. By that date there should be more than 2500 electric buses in these cities, representing 6% of their total fleet of 40,000 buses. In addition, 18 European cities have a strategy for up to 2025: these expect to have more than 6,100 electric buses in service, representing 43% of their total fleet of 14,000.
The ZeEUS project, coordinated by UITP, is a consortium of 40 partners representing public transport authorities and operators, bus manufacturers, industry suppliers, energy providers, research centres and consultancies across Europe. The project is part funded by the European Commission to the tune of €13.5m. The project is testing innovative electric bus technologies with different infrastructures in 10 sites across Europe, including London. Ultimately the project seeks to increase the numbers of high-capacity – ie capable of carrying at least 55 passengers – electric buses in cities around Europe by providing evidence-based advice and support to decision makers.
In terms of scale, the report finds that Europe follows Asia, with 1300 buses delivered or on order. This figure includes battery buses, plug-in hybrid buses, and trolley buses with batteries for off-wire operation. The largest number of electric buses are seen in the UK, with over 18% of the total European fleet, followed by the Netherlands, Switzerland, Poland and Germany each with A 10% share.
Worldwide, the electric bus fleet was estimated at around 173,000 in 2015 with around 170,000 (98.3%) operating in cities in China. Government initiatives are playing a pivotal role, the report finds; the Chinese government provides generous subsidies and tax benefits to manufacturers of e-buses, including, subsidies of $81,600 per bus in 2016.
The report finds that 200 full battery electric buses were delivered in the US in 2016, predominantly in Los Angeles. In 2016, the US government announced $55m in competitive grants to deploy more zero-emission buses across the country.
In the UK the government has earmarked £390m investment for future transport technology, which includes new electric vehicles and buses, renewable fuels and driverless vehicles (C&I, 2016, 10, 7).