Alabama could become the third state to allow death row inmates to be executed by nitrogen gas -- an execution method that has so far never been used-- under a bill approved Tuesday by the Alabama Senate.

The Alabama Senate voted 25-8 to add nitrogen gas to lethal injection and the electric chair as allowable methods of execution in the state. The bill now moves to the Alabama House.

"No state has carried out an execution using nitrogen hypoxia," said Robert Dunham of the Death Penalty Information Center. He said Mississippi and Oklahoma also allow execution by nitrogen gas but have not used it.

The pace of executions has slowed in Alabama, partly because of ongoing legal challenges to lethal injection methods.

Sen. Trip Pittman, the Republican bill sponsor, said Alabama needs another execution method as lethal injection faces court challenges. Pittman had originally proposed a firing squad as an execution method, but the bill was changed in committee.

"It's an important to have another option," said Pittman, R-Montrose. "I think nitrogen hypoxia is a very humane way to implement that sentence."

Under the bill, an inmate could choose to be put to death with nitrogen gas instead of lethal injection. It would also allow the state corrections commissioner to choose another constitutional execution method if electrocution, lethal injection and nitrogen gas are all found unconstitutional.

The legislation met with pushback from some lawmakers who called it experimentation.

"It has never been tried before," said Sen. Vivian Davis Figures, D-Mobile.

Alabama senators also approved legislation aimed at shortening the time of that death penalty appeals take. The legislation, which now moves to the Alabama House, would require inmates to raise claims of ineffective counsel and the same time as the inmate's direct appeal claiming trial errors. The legislation is based on Texas' process which was recently upheld by the courts, said Sen. Cam Ward, a Republican from Alabaster.